Questions and Answers about Combat Robotics
from Team Run Amok

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Aaron's Wisdom  I've said this often but builders don't want to believe me:

The weapon may be the least important system on a combat robot.
If you're not winning matches it isn't because you have a poor weapon.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.


Q: Hi Mark, I've had a robot weapon idea bouncing around in my head for a little while and I wanted to bounce it off someone else to figure out if it was conceptually sound or not.
The idea is to build an overhead bar spinner, but make the bar out of a material like styrene butadiene rubber, then make teeth out of steel that would go on the ends. The theory being that when the bar contacts another robot the teeth would hit like normal but the contact time is then increased due to the deforming of the rubber.
This would lead to a greater transfer of momentum so that opposing robot would get thrown harder and the shock load on the “floppy bar” robot would be reduced, right? Is that a valid strategy, or is there something I'm missing?

Thank you for your insight, Joe [Glendale, California]

A: [Mark J.] I believe you're severely underestimating the energy level of an effective combat spinner weapon, Joe. The impacts are so violent that a solid steel bar undergoes elastic deformation of the type and magnitude you propose for a rubber bar, and a rubber bar would flex so much as to qualify as a type of 'flail'. Click here to find an earlier discussion on flail weapons. Go read that and come back. I'll wait...

In addition to being ineffective, flails require that they be positioned such that the extreme 'wobbly' deformations they experience on even a glancing impact don't send the impactors into contact with the robot itself. A robot with a flail in an overhead bar position is going to be at least as dangerous to itself as to its opponent. I'm gonna have to give this concept a strong 'nope'.

Second opinion:


Q: I know this is an odd question, but do blades with curved edges tend to get better bite or any other benefits, or does it just sacrifice a lot? For context, I'm referring to a blade with a slight curved indent into one side, somewhat like a scimitar or other related weapon. [Former Flail Idiot, Missouri]

A: [Mark J.] 'Bite' is a very specific technical term when referring to spinner weapons. As defined in the Ask Aaron Spinner Weapon FAQ:

Bite is the maximum depth of opponent insertion into the arc of a spinning weapon at a specific combination of weapon RPM and robot forward velocity.
The shape of the weapon impact surface has no place in that calculation -- however, there is a related property of spinner weapons commonly called 'grab' that does depend on the angle of the impact surface. Consider the two spinner impactors below: Two spinning impact weapons with different impact angles
  • The 'straight' impactor on the left is going to tap the forward edge of the green target, which may send the target spinning away, avoiding a more forceful impact.
  • The 'forward raked' impactor on the right is going to reach deeper into the body of the target and -- at least in this example -- produce a more forceful impact.
Such angled impactors are common on sub-lightweight robots. The only real down-side is more rapid wear on the impact surface -- which some designs counter by having a dual-faced impactor that can be removed when worn and flipped over to present an un-worn, sharp-edged face. Two-faced reversible spinner weapon impactor design

Q: I could not find any mentions of a weapon type my team is considering using in the 15 lb weight class. Our design is utilizing a spinning shell around a stationary core, with 3 portions of chain from a chainsaw with weights on the end. The chains are going to be short enough to not be considered an entrapment device.

The purpose of the weapon would be to create a nice barrier between their weaponry and our shell, while still either bludgeoning them with a weighted end on each chain, or potentially cutting into them with the chains. Do you think this would be a valid primary weapon? [Kansas City, Missouri]

A: [Mark J.] Weighted chain weapons are a type of 'flail'. The BattleBots Wiki has a category page devoted to robots armed with spinning flails with photos and links. Chain flails were fairly common weapons in the early days of combat robotics, but builders quickly learned that rigid weapons were much better than floppy weapons.

There are several 'flail' posts in the Ask Aaron Robot Weapons archive. In the box below I've re-printed an archived post that sums up their performance issues.

Q: My question this time involves flails. Simply put, what chain length to flail weight ratio is the best? Also, why has nobody ever mounted flails vertically, like on a drum or a flywheel? Finally, would a horizontal or vertical mounted set of flails do best? [Urbana, Illinois]

A: [Mark J.] If you perform a numeric analysis on flail weapon performance it's quite clear that the ideal chain length is 'zero'. An effective spinner weapon stores kinetic energy and delivers that energy in a single massive impact on your opponent. Flails swing out of the way after striking and transfer only a fraction of the energy stored in the spinning mass of the weapon -- it's like trying to hit a home run with a rubber baseball bat. Solidly mounted hard 'impactors' have proven to be much more effective weaponry.

You can't spell 'FLAIL' without 'FAIL'.

There has never been a successful flail robot -- either vertical like 'Morrigan' from Near Chaos Robotics, or horizontal like 'Chains Addiction' -- and I don't expect to ever see one. The only thing they do well is make noise. Avoid.

If you're really set on building a flail spinner, perhaps you should talk with our creative support department:

Q: I believe you just saved my team a LOT of time, I appreciate it! -Flail idiot

A: You're welcome. Don't be hard on yourselves for finding this design appealing; many teams have gone down this same blind alley.


Chinese combat robot 'Flaming Wheel Q: So here’s a question: there was a Chinese competitor called... Flame Wheel? Hot Wheel? Not for sure, it’s one of those two names. Anyways, I was wondering if there’s any advantage to having the [impactor] mass placed outwards like seen in this picture in comparison to having small, sharp teeth like Gigabyte or Captain Shrederator. [South Bend, Indiana]

Spinner impactor clear path A: [Mark J.] The robot is King of Bots season 2 competitor 'Fēng huǒ lún' or 'Flaming Wheel'. An unrelated robot fought in King of Bots season 1 under the name 'Hot Wheel'.

'Flaming Wheel' had two interchangeable spinning shells:

  1. A 'tall impactor' shell - pictured; and
  2. A slope-sided 'undercutter' shell similar to 'Megabyte'.
The choice of shell depends on the perceived vulnerabilities of their opponent. The undercutter is used to attack targets with soft surfaces or exposed edges, like tires or simple wedges. The tall impactor is reserved for hard and/or smooth surfaces where it can deliver a blunt-force 'body blow'.

The unusual design of the tall impactors does provide strong support that also keeps impact forces away from the clylindrical shell itself, but the impactors are placed in a position that is shrouded by the gradual taper of the top and bottom mounting plates. The impactor itself needs a clear shot at the opponent without any material in the rotation path that might tap the target out of the way. I don't know what the "bite" calculations for 'Flaming Wheel' look like, but there is nothing to gain by adding material that can get in the way of your impactor path.

Energy storage advantage? Minimal.

Q: This is something I’d like to tack onto the previous question, but I’ve heard people online saying “shell spinners are obsolete”. What do you think? Do you think that these designs have no place in the future? What other designs might one consider obsolete (besides hammers, but don’t let Al Kindle know I said that). [Joliet, Illinois]

A: [Mark J.] I'm gonna pass on this discussion. I already get enough hate mail for discouraging builders with new ideas. If I start talking builders out of old ideas there won't be anything left.

"Prediction is very difficult, especially if it's about the future."

- Nils Bohr 


Q: So, I've got a question...

I'm working on the designs for an antweight (US) lifter, and was running the calculations of using a Silverspark motor to power the weapon. However, the math seems to be a bit... off. Here's what I have:

Lifter Arm Length: .33 feet

Maximum Lifting Weight: 2 lbs

Maximum Torque at Gearbox (ft-lb): (.33 feet x 2 lbs) = .66 lb-ft

Max Load At Gearbox (in-oz): (.66 lb-ft x 192) = 126.72 oz-in

Motor Stall Torque: 0.64 oz-in (Fingertech Silverspark @ 6v)

Torque Overage Factor: 2

Gear Ratio Required: ((126.72 / 166) x 2) = ~1.5:1

Is that really it in terms of a gear ratio? All I need is about a 1.5:1 reduction to lift another ant with a Silverspark motor at 6v? I feel like I've done something wrong.

Also, I plan on running the motors at 12v - 14v rather than the recommended 6v. How does this affect the stall torque, or does it remain constant regardless of the amount of voltage applied to the motors? [Newton, Illinois]

A: [Mark J.] You were doing fine right up to the end. That last line should be:

Gear Ratio Required: ((126.72 / 0.64) x 2) = 396:1
It seems that you inadvertently inserted the 166 oz-in stall torque of the Banebots RS-775 18v @ 18v that you used in your earlier calculation for a featherweight lifter instead of the 0.64 oz-in for the Silver Spark @ 6v.

Stall torque, stall current, and no-load RPM of a brushed PMDC motor increase proportionally with voltage. Increasing voltage from 6 volts to 12 volts will double both the speed and the torque of the motor, so your new calculation will be:

Gear Ratio Required: ((126.72 / 1.28) x 2) = 198:1
At 12 volts, the 200:1 Silver Spark gearmotor should do.

Q: Hey there, Mark! This might be a dumb question, but I'll ask it anyways. Over on Facebook people mentioned the Fingertech vertical spinner which is apparently [expletive deleted]. I noticed, however, that somebody mentioned if it was crafted from AR400 instead of Titanium, it would hit harder. Why? Is it because of the mass increase? Is that really enough to make this spinning weapon more effective? [Kankakee, Illinois]

A: [Mark J.] Changing the material of the FingerTech vertical spinner blade from Grade 5 Titanium to AR400 Steel would result in a heavier, stiffer weapon that would in fact 'hit harder' -- but the improvement would be too small to help this specific weapon.

  • The FingerTech titanium vertical blade is a VERY small weapon blade. With a 1" spin radius and a weight of 10 grams, I calculate that it stores about 1.5 joules of energy at 11,000 RPM. A blade the same size and shape made from steel would weigh about 17 grams and store about 2.5 joules of energy. A typical antweight spinner weapon will store more than ten times that much energy.
  • Titanium is only about 60% as stiff as steel. That means that a titanium weapon of a given design will flex more on impact and will be less effective at transmitting the impact energy to your opponent -- like a fist in a boxing glove versus a bare fist. A steel version of the weapon blade would 'hit harder' than the titanium version, but there is so little energy involved here that impact flexing would be negligable for either material.
Read thru the Ask Aaron Spinner Weapon FAQ for more info on effective spinner weapon design.
Q: Hey Mark! This is probably a "cheerleader" question, but I wanted to at least try and give it a chance.

My team and I want to build a heavyweight. We know we want a strong chassis, now it's just time to figure out the weapon! We want to try a front-hinged flipper, but we aren't quite sure of where to start. Is there a "good" number to shoot for in terms of joules per pound, like spinners? We aren't necessarily trying to get the same power as bronco, as it's going more forward versus upward; we just want enough to get them over the wall.

Sorry if the hamburger is bad, we're mostly theorizing at this stage. [Akron, Ohio]

A: [Mark J.] Yes, it's a question. First I'll explain why your team shouldn't build a heavyweight flipper, then I'll point you to answers to your 'starting point' question in case you choose to make the mistake of going ahead and building it.

Why not? If you decided to take up mountain climbing, it would be a poor idea to catch a plane to Nepal and start walking up the southwest face of Mt. Everest; you should start with a smaller mountain. A heavyweight flipper is Mt. Everest -- expensive, dangerous, and not for the inexperienced. The questions you're asking make it clear that you have no experience with pneumatics and no experience with big combat robots. It's failrly certain that the selection committee for the only heavyweight robot tournament currently operating in North America will not look kindly upon your application to compete. Build some smaller combat robots, work out the finer points of pneumatic flippers on a featherweight, and put together a combat résumé that will get you to your goal.

Ignore my advice? A pneumatic flipper is not a 'one number' system (and neither are spinners). You need a system with enough theoretical force to accelerate your opponent into an arc that will achieve your purpose, but you also need to flow gas from the storage tank into your actuator as quickly as possible to get the speed 'pop' that separates a flipper from a lifter. That last part isn't easy. I have a couple of tutorials and a spreadsheet tool that you should find useful.

Q: I seemed to have forgotten one REALLY IMPORTANT word when writing my flipper question: E L E C T R I C. My team has built several robots in the past (a 4 bar lifter and some spinners) with decent success in smaller weight classes. It's true it'll be our first heavyweight, which is why we don't want to build something we aren't familiar with, as you said.

A: Heavyweight... ELECTRIC... Flipper. Woof. The cheerleader had one look at this and took the day off, so I guess it's up to me.

There are a few insect-class flippers using exotic electric servos, but the square-cube law pretty much rules out heavier flippers directly driven from an electric motor. Sub-lightweight electric-kinetic flippers store up energy in a flywheel to get enough 'pop' to launch opponents. The only remotely successful heavy electric flipper has been Team Whyachi's 'Warrior SKF' (aka 'Warrior Clan' and 'Warrior Dragon'), although many of their victories have been via direct attacks with their flywheel/spinning disk rather than attacks from their anemic kinetic energy flipper. Here are the primary problems:

  1. A flywheel flipper system requires a custom machined 'dog clutch' as explained by Dale Hetherington at his Flip-o-Matic page. Dale developed the Flip-o-Matic for use in hobby and featherweight 'tabletop' competitions that don't allow exposed high-speed spinner weapons. Dale has had good success under those conditions, but in open heavyweight competition you're going to be badly outclassed by more powerful weaponry.
  2. All electric/kinetic flippers share an unusual attribute: they have very short actuation ranges. For example, the flipper on 'Warrior SKF' moves only a few inches. This allows the flipper to expend all of the limited stored energy an a short but explosive burst. Extending this stored energy over a longer 'throw' -- like the large angular throw of a front hinged flipper -- means less energy per unit of distance travel and would require a HUGE flywheel to store enough energy to be effective. A pneumatic system can simply tap into more pressurized gas to continue the motion, but the kinetic energy stored in a flywheel depletes quickly and there won't be any more 'til it spins up again. It really isn't the right choice to power a long-throw, front-hinge flipper.
Build a nice featherweight to test your exotic design. The cost to build and perfect a feather will be less than the cost of a single redesign on a heavy. Besides, BattleBots reboot season 4 isn't guaranteed...


Q: My 1lb antweight is exhibiting some odd behavior. The weapon blade is the fingertech titanium blade. I have a V-spec 2205 motor, a 450mah 4s 70c battery, a 20a brushless ESC and two fingertech ESCs with accompanying silversparks. When I spin my weapon up to max or near max throttle one or more of these behaviors *sometimes* exhibit themselves: The power LED starts flickering, the robot starts doing donuts, power to the weapon motor gets cut and it spins down. As soon as I turn the throttle down it fixes itself and its free to spin back up again. Is this a problem with the weapon ESC not being able to handle the weapon? [St. Louis, Missouri]

A: [Mark J.] Your primary problem is not your weapon ESC; the symptom of an overstressed ESC is a puff of smoke and unrecoverable loss of motor power. You didn't mention your weapon layout, but assuming that you're running the weapon direct-drive your problem is the weapon motor.

The FingerTech weapon blade as used as an add-on weapon for the Viper robot kit is designed to spin at 8000 to 12000 RPM under the power of an 1100 Kv direct drive motor. You're attempting to spin that blade at about 34,000 RPM with a 2350 Kv motor that has a lower torque constant than the design motor. The motor simply isn't capable of spinning the weapon to that speed given the maximum current output (about 30 amps) of the battery pack you've chosen. The motor 'bogs', you get serious 'voltage sag' from the battery, and your electronics 'brown out' -- which gives you the symptoms you're reporting.

Your battery won't put up with this level of abuse for long, and may have been damaged already. If the plastic coating is at all 'puffy' and no longer tight, the battery shoud be safely disposed of immediately -- under no circumstances either charged or used.

A larger battery pack capable of supplying the 60 or 80 amps the motor might require operating bogged down to perhaps 15,000 RPM in this application would simply transfer the failure point to the weapon ESC or the the motor itself, which is rated for only around 25 amps continuous current.

Scrap your weapon motor and pick a lower Kv unit that will be able to deliver enough torque to spin the blade at a more reasonable terminal speed. Don't over-do it. You'll likely want a battery with a bit more peak current output and - depending on the selected motor - a larger capacity ESC as well.

Q: The battery seems fine and that sounds like a big overhaul that I don't really have time for, as a short term "hot fix" could I simply off-set the throttle in my transmitter settings to prevent the motor from revving up to full speed?

A: The problem is that your weapon motor can't rev up to full speed because of the aerodynamic drag of the weapon at such unrealistic speeds. Applying less throttle will reduce the current to the motor, but it also proportionally cuts torque. You might well need to cut throttle to around half to keep all the components in their safe zones. Spin-up times will greatly suffer. Try a new throttle setting in a practice session and check temperatures carefully. I'm also concerned that your battery may not have the capacity needed for a full match with a bogged weapon motor sucking down big amps. Best luck.

Q: How come I never had this problem with a 3s 65c battery and my emax 2204 motor? Similar KV to current motor.

A: Increasing voltage has rather large consequences.

Wattage increases by the square of voltage; the increased voltage also increases the current. Bumping up from 3 cells to 4 cells (a 33.3% increase) results in 1.333 * 1.333 = 1.78 a 78% increase in wattage, motor output power, and peak battery drain.

A similar effect is seen on weapon aero drag. Increasing the weapon speed by 33% means that it has to move 33% more air out of the way per unit time and it has to move each unit of air out of the way 33% quicker. That's the same formula highlighted above. The motor now has to work 78% harder to maintain the faster weapon speed -- if it has enough torque to maintan that speed at all.

Q: Do you think just stepping back down to the old 3s lipo could solve this?

A: Quite likely, yes.

Q: Just a quick update for anyone in the future reading who has a similar issue, I tried the 3s and it didn't fix the problem, it just made my robot weaker. I'm going to try offsetting the throttle so its max signal is like 75% throttle. I can't see any reason this wouldn't work, as the robot works fine if I manually keep the throttle under 80% or so, so if I do not give any future updates assume that this fix worked... or alternatively that the blade flew off and lodged itself in my trachea.

A: The failure of the three-cell battery to solve the problem casts suspicion on a new component in addition to the high current draw issue. You haven't mentioned the specifics of your weapon ESC except to say that it has a 20 amp rating, but it's possible that it's faulty or has been damaged by the current load.

  • If your weapon ESC supports calibration to your radio signal range, run that calibration sequence and try the 3s battery again.
  • If calibration doesn't help, consider replacing the weapon ESC.
From 1,734 miles away that's about all I can suggest for a true 'fix'.

Q: This is the ESC: DYS XSD 20A Brushless Speed Controller. Are you suggesting that upgrading the current rating to say 30a could help or just that it may be faulty and in need of replacement.

A: The conversation has wandered around a bit -- lets recap:

  • I'm confident that your 'brown out' problem stems from attempting to spin a 6-inch weapon bar to 34,000 RPM with a low-torque/high-RPM V-Spec 2205 motor powered by a 450mah 4s 70c battery via a 20a brushless ESC. This set-up is pulling more current than any of these components can handle.
  • Based on your prior 'bot that ran a similar set-up on a 3s pack, replacing the 4s pack with a 3s pack should eliminate the problem -- but it does not. This leads me to believe that the weapon ESC may be faulty or damaged.
  • Running the weapon motor on the 4s pack with a reduced max-throttle weapon setting on your transmitter won't not work, but it doesn't address the actual fault.
  • I'm suggesting that you replace the weapon ESC and try it again on a 3s pack largely because I don't like leaving a mystery unsolved, but also because a faulty/damaged ESC may completely fail in combat without additional warning. If it passes this test you can go back to running it on 4s at reduced throttle with increased confidence.
  • If you want to run a 4s pack with full weapon throttle, upgrading just the ESC won't get you there. Neither your motor or battery pack are likely to survive on a 4s pack with your weapon load. If you plan on upgrading the motor and battery at some point you can certainly move to a higher capacity ESC now while running at reduced throttle -- it won't hurt.

Q: It's also been suggested to me that the BEC on fingertech ESC's isn't the best and this could also be part of the problem.

A: That was my first thought when I read your 'flickering power light' symptom, but overstressed BECs on tinyESCs behave like this. BEC output and receiver current demands don't increase with raising weapon motor speed, so why would the problem only manifest above a certain throttle level? Try an alternate source of receiver power if you like, but I'm not optimistic.

Q: THE STANDALONE BEC FIXED IT! I CAN GO UP TO 100% LUDICROUS SPEED WITH NO POWER ISSUES!!!!!!!!!!!!! (at least on the 3s battery, I'll update with the 4s later hopefully I don't run into an Icarus situation)

90 minutes later...

On further consideration I've decided to leave well enough alone and run on 3s, the 4s battery is heavier and it does more to improve top speed than spin up times. Top speed is something that I don't need (It's already stupid fast at 3s, 26000 RPM if the calculations hold true) and would decrease bite, spin up is good enough already and way better than last years set up. I'll use the extra weight for wheel guards. I do feel bad for having spent the money on the bigger batteries but I think I may just need to accept that this hobby is a money sinkhole by nature.

A: Super awesome! I think that's a solid decision -- but there is a troubling question remaining: what's drawing so much 5 volt power?


Q: Hey, I'm not to sure if this has been asked, but how I should go about heat treating my s7 weopaon? [McAllen, Texas]

A: [Mark J.] It's not practical to heat treat S7 tool steel in your home workshop. Heat Treating S7 is a multi-step process requiring precise temperature control up to 1750° fahrenheit with carefully monitored heating and cooling rates. You need a specialized, fully instrumented furnace plus experience in the art. Backyard 'hillbilly' metalurgy will simply turn your weapon into a warped, brittle, useless piece of scrap.

Find a commercial heat treating company nearby (like Texas Heat Treating in Round Rock) and explain what you're doing. They just might be BattleBots fans and be willing to slip your little piece in with a larger commercial order.


Q: I know Hellachopper isnt the best bot to be asking questions about, but I was reading about two of its apparent abilities on their Facebook Page and they have me thinking more than I should:

EXTENDABLE FLAILS - They apparently have the ability to extend and retract their flails at will. First off, what would happen if they extended or retracted at full speed? How would that effect the bot, forces applied to the robot, spin speed, etc? Second, what would happen if they simply let go all of a sudden, letting the flails pop out quickly and violently?

SELF-BALANCING ARMS - How does that work? How can the arms simply balance themselves with a hammer missing? [Naperville, Illinois]

A: [Mark J.] I generally won't discuss combat robots that have never even fought a match. 'Hellachopper' is best known for catching fire during its 2016 BattleBots safety check - twice. BattleBots leads builders to try some crazy things.

The physics involved are a little beyond the usual short answers I give here. I've spent quite some time writing and re-writing the answers to keep them as brief as possible...

Extendable Flails - Extending or retracting the flails by itself will not change the total rotational energy of the weapon -- but since the Moment of Inertia of the weapon changes as the flails move in or out, the weapon speed will change.

  • Spinning at constant speed with the flails 'in': extending the flails will slow the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a larger circle. If the weapon motor has additional torque available at the lower weapon speed, it can apply that torque to spin the weapon to a higher speed. Calculations for 'Hellachopper' presented on the Reddit forum claim that aerodynamic drag with the flails extended would prevent the weapon from attaining full target speed in that configuration.
  • Spinning at best speed with the flails 'out': retracting the flails (not easy because there's a lot of cetrifugal force acting on them) would increase the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a smaller circle. If the weapon motor cannot maintain the weapon at that increased speed, the speed will fall to the level the motor can sustain.
Watch this video on Conservation of Angular Momentum to see this effect in action. The rate at which the flails extend will have no impact on the above changes -- unless the flail cables snap from the sudden load.

Self-Balancing Arms - Self balancing systems are fairly common. Some washing machines have a 'donut' shaped tube partially filled with liquid wrapped around the tub that will compensate for an imbalanced load of clothes during the spin cycle. Somewhat counter-intuitively, the liquid will shift position in the tube to position itself at the location of lowest energy, which will be opposite the 'heavy' side of the washer load as it spins.

Several combat robots with hollow drum weapons have used a product made for commercial truck tires that works the same way. Tiny ceramic beads act like the fluid in the washing machine system to redistribute mass to maintain balance, even if the weapon is damaged and balance changes.

Hellachopper's balance system is based on this same principle, but it relies on flail anchor points in sliding grooves moving to change the distribution of mass. Clever, but I'm not entirely convinced that it will work; the balancing 'fluid' needs to be free to move independent of the rotation of the weapon, and the flail anchors are not entirely 'fluid'.


Q: Hi Mark,
This is more of a theoretical question, and I've searched the archives for an answer, I think I've got it partially resolved but I wanted to ask you directly (please don't make this a Bad Hamburger):

I'm imagining two scenarios where two identical brushless motor and ESC combinations are spinning up geometrically identical rotary weapons (be it a beater, drum, bar, disk, etc.) but one is made out of hardened S7 tool steel and one is made of 6160 aluminum. If my math and intuition are correct, the two weapons should spin at the same angular speed after a sufficient amount of time has passed; the only difference is that the aluminum weapon will have a lower spin-up time (neglecting contact friction forces from bearings or anything dependent on weight).

I say this because the motors will supply the same torque, which is equal to the product of angular acceleration and moment of inertia (MOI). Of course the steel bar is going to have a higher MOI so it will have a lower acceleration, longer spin-up time. However the final speed of each weapon will occur when the torque produced by aerodynamic drag is equal to that which the motor can output. If the shapes are identical, shouldn't they have the same drag force acting on them, and then they'd have the same rotational speed?

As a side note, I'm currently reading through the RioBotz tutorial and will likely return with more questions after I finish that.

Thanks, Jack from Lake Charles, Louisiana.

A: [Mark J.] You're correct, Jack -- but I'm not sure where you're going with this.

  • The aluminum and steel bars will have equal aerodynamic drag and will have the same terminal speed;
  • The aluminum bar will spin up to that speed faster than the steel bar because of its lower MOI;
  • But when spinning at equal speeds the weapon with greater MOI will store more kinetic energy and will hit harder.
You can verify this with one of the Run Amok Spinner Weapon Calculators. Example:
For a spinning bar weapon 450mm by 75mm by 24mm in the usual spinner orientation:
  • Made of steel, the bar weighs 6.32 kilos with an MOI of 0.11 kg-m2. It stores 6200 joules of energy at 3200 RPM.
  • Made of aluminum, the bar weighs 2.24 kilos with an MOI of 0.04 kg-m2. It stores 2200 joules of energy at 3200 RPM.

Q: Why would anyone build a symmetric bar spinner? Don't asymmetric bars deliver more force at lower RPMs and save weight? [The Aether]

A: [Mark J.] You've made an incorrect inference about asymmetric weapons -- they don't deliver more force at lower RPM, they have better 'bite' at high RPM.

An asymmetric spinner bar reduces the number of impact teeth from 2 to 1 in order to improve weapon 'bite' (see: RioBotz Combat Tutorial section 6.3). Too little 'bite' results in a spinner weapon 'grinding' along your opponent's armor without delivering a solid impact. Improved 'bite' allows you to design a lighter bar and spin it at higher speed to attain the same energy storage as a heavier bar without sacrificing the ability to deliver a solid 'hit' on your opponent. If your design features a long bar that needs to be thick and heavy for structural strength and it spins slowly enough for good 'byte' (think 'Tombstone') there is little advantage to going asymmetric.


Q: Probably just an insane idea but in theory if you stacked two horizontal spinning blades on top of each other and spun them in opposite directions? What, if anything, could one expect to happen when they hit? [Pittsburgh, Pennsylvania]

A: [Mark J.] It is fairly insane, Pittsburg -- but that hasn't prevented people from building such weapons. Their combat records are not great. We've discussed both horizontal and vertical counter-rotating spinners in this archive -- search here for hozizontal spinner 'Double Dutch' and vertical spinners 'Counter Revolution' and 'CounterStryker' for full discussions. Here's a sample:

An alternative twin-spinner system design is employed by 2018 BattleBots competitor 'Double Dutch'. Their design features counter-rotating bar spinners placed above and below the robot body. Although both weapon bars can attack an opponent at the same time, it's likely that one bar with strike before the other and throw the opponent clear before the second bar can impact. Almost certainly the two potential weapon impacts will strike different locations on the opponent, which will also limit their effectiveness.

Q: on the excellent spinner spreadsheet the formula for getting a rough calculation of the stall torque is given as ((1352/Kv)*(Volts/(mOhms/1000)))/141.69

i think i must be mixed up, because it seems like this formula is producing some crazy numbers - so maybe i'm doing something wrong and you can help me out. for example, if you look at the spec of a brushless motor like the Scorpion HKII-7050-330KV and plug in the numbers:

Volts = 50
internal resistance in mOhms = 5
Kv = 330

if you use the spreadsheet formula, you get a stall torque of an incredibly powerful 289 N-m!

but if you calculate torque using some other formulas that i've learned about, like Kv/Kt where Kt is 9.549/Max Amps or using the formula PeakPower/w, where w is given by (2*pi*Max RPM)/60 then you find that both of those formulas get you a more reasonable number of 8 N-m of torque, which is way different than 289 N-m.

so am i misunderstanding something fundamental here about brushless motors and torque? did i mess up the numbers or formulas somehow? am i being dumb? i think i need some help.

-josh in salt lake city, utah

A: [Mark J.] Let's start by applying the formula from the spreadsheet to a motor with known specs. The brushed AmpFlow A28-400 will do nicely:

  • 24 volts
  • 44 mΩ
  • 205 Kv
The spreadsheet formula gives a stall torque of: (1352/205)*(24/0.044)/141.69 = 25.16 N-m. That's really close to the published 25.13 N-m stall torque for the motor -- the formula is correct.

The Scorpion HKII-7050-330KV can deliver 15,000 watts of output power while sucking down 300 amps of current. If you pay $999.99 for a brushless motor you're entitled to 'incredibly powerful' results, but does it actually deliver the 289 N-m of stall torque the spreadsheet formula says it will? Absolutely not, because brushless motors are different than brushed motors in how they operate at low speeds. The concept of 'stall torque' really doesn't even apply to hobby brushless motors. See this archived post about brushless hammer drive for greater detail on low-speed brushless torque.

So, why do the other formulas you cite give a much different torque number?:

  • Torque = Power / Angular Speed -- calculates torque at a specific speed where true power output is known. You're assuming 'Peak Power' and 'Max RPM', which never occur together in an electric motor. Stall torque occurs at zero speed, where output power is also zero, giving a 'division by zero' error; the formula cannot directly calculate stall torque. simplemotor.com/calculations/
  • Torque = Kt / Kv -- it's 2 AM and my brain has gone to mush, but this method of calculating stall torque isn't making any sense at all to me. This webpage may offer some additional help on the relationship between Kt and Kv: Learning RC - Brushless Motor Kv Constant Explained.

I don't think that you're using the most recent version of the Run Amok Excel Spinner Spreadsheet. The current version (19-D) is optimized for brushless motors and has a more sophisticated method for approximating true low-speed torque that takes motor controller software into account. Download it from the Run Amok tools page.

Q: thanks for the explanation and the pointer to the new spinner spreadsheet. great stuff (!!) but i think the new spreadsheet seems to still have a major limitation related to brushless motors, and i think it might be because there is no ability to plug in the maximum amperage (maybe?) and so instead it goes to some unrealistic theoretical maximum which is so unrealistic as to not be useful to model - or maybe i'm screwing something up.

for example, if you take the stats from that fancy scorpion motor:

50 volts
330 Kv
5 mΩ
10 magnet poles

and say we use a gear reduction of 5 to 1 and then let's say our horizontal spinner is made of steel, 0.5 meters long, 0.125 meters wide, 25mm thick, which is about 12kg (about 27 lbs).

so the results from the calculator are that this blade achieves 3,000 rpm in less than 0.3 seconds (my gut tells me that this is probably off by a factor of 10 or more) and that it will peak at 8,500 amps, which at 50 volts is almost half a megawatt of power.

so...am i doing something wrong, or is this spreadsheet unable to model a big brushless motors like this very well?

thanks again - josh in salt lake city

A: You're planning on buying a $1000 brushless weapon motor capable of 20+ horsepower output and then strangling it with an inadequate power supply? Well, to each their own...

The spreadsheet assumes that the weapon motor will be provided with all the current the brushless controller chooses to pass to it, without voltage sag. There is no adjustment for a fixed 'current maximum' because brushless controllers generally do not have true current limiting, but the spreadsheet is capable of making specific adjustments for changes in the controller firmware that limit control the motor speed at which the 'soft start' feature cuts out and the maximum power pulse percentage during that soft start period.

In SimonK firmware these features are known as 'Commutation Max' (commutation time in u-sec below which 'soft start' restriction is active) and 'Power Max' (maximum power pulse length during 'soft start') but other firmware will typically have similar features. You can 'play' with these settings in the spreadsheet to see their effect on current consumption and performance, then transfer the settings to the controller firmware to implement those power restrictions.


Date marker: June 2019
Q: Hi Mark, I'm trying to spin a bar with impactors with a moi of 0.055 kg*m2 with a Scorpion HKIV-4025-1100KV outrunner. I believe it will spec out to be 13,200 rpm with 13.9 Nm of torque at 12V. I was going to gear it down 3:1 so the weapon would spin at about 4,000 rpm maxed out. My question is about the amp peak when using the spinner tool. I read in the forums about your explanation of ohms law saying this was only theoretical but it still concerns me.

Is my design flawed and is that causing the high peak? Should I be using a brushed motor instead? What ESC should I be using? Would the current setup do - instantly fry, twitch back and forth then fry? I hate electronics so your help is greatly appreciated. This is an overhead spinner for the 12lb class. [Rock Hill, South Carolina]

A: [Mark J.] Sounds like you're in way over your head here. Take a deep breath, relax, and try to forget all about those disturbing numbers.

The Run Amok Excel Spinner Spreadsheet is a complex tool that requires the addition of a healthy scoop of 'real world' to temper its results.

The most effective use of the spinner spreadsheet is to model an existing spinner weapon system that is known to function well, then model a new design to see how the outputs differ. You can then make a reasonable judgement about the viability of the new design.
That 1400 amp current spike you're looking at is a theoretical value which assumes that the weapon motor is the only resistance in the circuit. Your ESC will add in at least as much resistance as that particular motor, and the internal resistance of your battery is somewhere in the same ballpark. Put that all together and your real world peak current tops out at around 300 amps and falls off quickly with increasing RPM. Feeling better now?

Part two: since amps equal torque and that high current spike is theoretical, that 'wishful thinking' 13.9 Nm of stall torque goes away as well. Because of the black magic any specific brushless ESC goes thru to get its associated motor to spin up from a stand-still you'd never see that amount of torque even in theory. The concept of 'stall torque' really doesn't even apply for hobby brushless motors; spin-up time for a brushless spinner will be longer than the theoretical output of the spreadsheet.

Lets take a look at some of the specifics of your design:

  • You didn't share actual dimensions, but my best guess says a bar with a Moment of Inertia around 0.055 Kg*M2 will weigh about seven pounds. That's way too much for a 12-pound robot. Spinning that heavy bar at 4000 RPM will store about 5000 joules of energy. That's healthy for a featherweight -- overkill for a hobbyweight. Excess ≠ Success
  • You've picked a $200 helicopter outrunner motor for your weapon. Nice motor, but it's designed to run at twice the voltage you intend to supply. That's like buying a Ferrari and sticking a block of wood under the gas pedal to limit output to 25% power*. If you're gonna do that you should just buy a Hyundai to start with.
* Electric motor power increases with the square of voltage: double the voltage equals four times the power.
Standard Advice If you don't know what you're doing, copy a successful design. Look around at hobbyweight spinners and find motor/ESC combinations actually in use that are powering effective bar spinners. Size your bar reasonably and make improvements over time as you find weaknesses.

Q: Hi Mark, I had a follow up question about the spinner spreadsheet. Your explanation was very helpful and brought much comfort. I know the 7lb bar is overkill but I wanted to try something like that at least once just to see what would happen in the area upon contact. Overkill is underrated. Maybe...

You said ballpark, the resistance would be about 3x the amount listed. This changes the calculation some on the battery requirements, right? Couldn't I use a much higher voltage battery with a much lower mAh? I ask because as always weight, but particularly space, is an issue. Many thanks!

A: My resistance explanation above was intended to explain why the big start-up current surge the spreadsheet shows isn't really that big in 'real world' terms. As the weapon speed starts to rise the electrical characteristics of the motor start to change. Here's a more complete explanation.

At theoretical 'stall' the combined resistance of the this particular motor and a typical controller and battery is about three times the resistance of the motor alone -- but it doesn't stay that way. The internal resistance of your battery pack and controller do not vary with weapon speed, but the effective resistance of your motor increases linearly with speed due to the rise of back EMF generated by the motor. With the weapon spinning at full speed the effective resistance of the Scorpion motor will have risen from 0.006 ohm to nearly 2 ohms and the added 0.012 ohm resistance from the battery and controller will have long been rendered insignificant.

For purposes of the Spinner Spreadsheet you should still enter the resistance value for the motor alone because the spreadsheet takes into account the rise of back EMF with increasing RPM. The additional resistance at start-up lengthens spin-up just a bit and the huge theoretical current spike at start-up is diminished, but the total draw on the battery to spin the weapon bar up to 5000 joules is essentially the same. Just remember that even though the big current spike at start-up isn't really as big as the spreadsheet shows the total current consumption is close to correct. There's no such thing as a free lunch; you still need a battery with the same watt-hours of capacity.

Q: Btw, what ever happened to the Texas bar spinner?

A: The conversation I had with Tex broke off abruptly and I have no more recent news. Here's where the 'Texas Spinner' page leaves off:

It's been about a week since I last heard from Tex. I have, however, heard from a number of applicants for the second season of BattleBots on ABC about their rejection calls. One note was from a BattleBots veteran who's design was rejected for not having enough 'bling':

ABC was picking robots based on appearance. We got thrown into the bar spinner category, and Trey [Roski] said they had a ton of entries that looked exactly like Tombstone...

Given that Tex was pitching a Tombstone clone I suspect that his 'hook' wasn't tempting enough to get a positive response from BattleBots. I hope he builds a 'bot anyhow.


Q: Hi Mark,
Probably a dumb question but I figured I'd have to ask. So you know how a 540 size motor attaches to a p60 gearbox to drive a bot... why isn't a similar set up used for powering the weapon? Attach a motor to a gearbox and use that to drive the weapon. It would certainly be easier to mount and you'd have a keyed shaft instead of a 'D' profile. It seems like a no brainer in the hobbyweight since you usually take that size or similar motor and gear it down 3/4/5 or so. Then you could do a 1:1 v belt or flat belt from keyed shaft to weapon to allow for slippage. Does the gearbox get too hot for this? Couldn't you put a fan on it if it does? I can't imagine I'm that smart and would like to know where my thinking is wrong. Many Thanks! [Raleigh, North Carolina]

A: [Mark J.] I've seen this done, Raleigh. It's not a horrible design, but you lose more than you gain:

  • Simplicity Your design still needs a belt to save the gearmotor from destructive shock loads on weapon impact. If you do the reduction with the belt drive you leave out complex parts that could become failure points.
  • Efficiency Gearbox heat comes from internal frictional losses. Leave out the gearbox and that lost energy can be spinning your weapon quicker and faster.
  • Weight A 4:1 P60 gearbox weighs in at 6.5 ounces -- plus grease. Scrap the gearbox and you can put that weight directly into the weapon where it will do some good.
  • Cost A 4:1 P60 gearbox runs about $50 -- delivered. A larger driven pulley to get that reduction costs much less.
Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.


Q: I am running a Dr. MadThrust 1700 kv motor for my weapon on my 15 lbs robot. I need to find a battery pack or make my own that will be able to run that motor and I cannot use Li-Poly batteries. I know that I need a pack around 3000 mAh but I don't know about the discharge rate. Do you have any advice? [Dublin, Ohio]

A: [Mark J.] The standard solution for robot events that do not allow Li-Poly batteries is a switch to safer and universally allowed lithium iron phosphate packs -- also known as 'LiFe' as a contraction of their LiFePO4 cathode material.

LiFe packs have lower discharge rates than Li-Polys, so you'll want to go with larger capacity packs to obtain higher amp peak drain. A pair of 3S 30C 4200mAh packs in series will get you in the ballpark. LiFe cells have different charge requirements than Li-Polys, so make certain that your charger has a specific LiFe setting.

The Dr. MadThrust 1700 kv motor is rated 105 amps of current, and the 30C 4200mAh packs are rated for 126 amps continuous current output with a 168 amp peak. The peak current draw of your weapon motor under load depends as much on weapon size/weight/gearing as it does on the motor but you gave no weapon info. I think you'll be fine -- LiFe packs are more forgiving than Li-Poly packs, and a spinner weapon will pull high current only briefly while spinning up before settling at a much lower level. Best luck.


Q: I'm looking through your info on spring flipper mechanisms and it's very well done & informative. My question is, how would I adjust a "choo choo' mechanism to make it shoot down, like for an axe? [Woburn, Massachusetts]

A: [Mark J.] Thanks. I'm happy to hear that you like our guide to spring flipper designs, but I must admit that I'm surprised that you looked at the four mechanisms on that page and decided that the 'choo-choo' would be a good choice for an overhead axe weapon.

The demands of flippers and overhead axes are quite a bit different. An axe can build energy gradually over a long arc before striking its target, while a flipper needs a big burst of energy released instantly. A modified flipper mechanism is not your best choice to swing an axe, which is why you haven't seen them used for that purpose.

  • The reaction a flipper 'bot experiences from the quick upward burst of energy just presses it down onto the arena surface where it is well supported. Everything remains stable.
  • That same quick burst of energy applied to a downward striking axe weapon will produce a large reaction that lifts the front of your 'bot and tries to flip it over backwards before the axe even strikes your opponent.
You've got plenty of 'swing' to build axe speed with a simple gearmotor powering the axe directly -- don't add the complexity of a spring load plus a mechanical linkage to wind and release it.That said, it is certainly possible to orient a 'choo-choo' to pull back and release a spring loaded hammer: see sketch at left for a simple solution. It will work, it just isn't a good idea.

Q: This isn't a techy question and it unapologeticaldly fits in the cheerleading section but I just had to ask it anyways. I've been debating asking this question on this site for weeks now and I just had to ask. If it's a garbage question through and through just toss it lol!

Okay, so I've read this website for a while now and I am aware of the cheerleading section and this belongs there but if you have the time I would love your opinion. Ima cut right to the chase and say this: Weaponized Turbojet Engine Afterburner.

You see all of these crazy jet engines on youtube that spit white hot flames and enough noise to wake the dead and we know from bots like ICEwave that crowds love loud bots. Now there are MANY design challenges flaws for using a jet engine in combat robotics. From the reliability of a 100,000 RPM engine when hit by a massive spinner to the weight of the jet not to mention the heat problem... In my mind the weapon would either be a automotive turbo outfitted with a flame tube or a custom made turbojet (which would package a lot better). Maybe even an EDF throwing air through to the afterburner instead of the heavy jet engine (although it would sound like a vacuum). It would be very hard to get working right, as I'm sure most non electric powered weapons are, and the robot wouldn't ever be competitive but what are your thoughts on a weapon like this?

Have a great one guys! [London, Ontario]

A: [Mark J.] The next time you're on YouTube looking for dangerous machines you might pop 'Suvival Research Laboratories' into the search box. SRL puts on destructive machine performance art shows featuring devices that include a big pulse jet very much like you suggest here. Their outdoor shows are the right place for these creations; a robot combat arena is not suited to such displays.

  1. In the very few events that allow robots to carry flame weapons there are strict fuel volume limits. BattleBots allows a 0.5 litre tank of propane or butane, which I suspect would would power a worthy turbojet for perhaps 10 seconds.
  2. A heavyweight-class combat arena is a very expensive item made largely from thick sheets of very meltable plastic. Imagine what happens to the remains of your white-hot jet engine when a spinner weapon hits it with enough energy to rip a water heater in half. No event organizer is going to allow your turbojet inside their precious arena.
Book a flight to the next SRL performance and watch their pulsejet melt a Volkswagen. That oughta get this out of your system. If not, here's the button:
Q: Hey there Mark, I was hoping to ask you about two seperate but semi-related questions concerning some bots I'm building (beetle and feather), both using HDPE.

1) All of my bots have been made of HDPE from 5 to 20mm thick, cut with a mixture of a handsaw, a jigsaw, a chop saw, and a circular/skil saw. I notice however with making curved cuts, the resulting cut tends not to be right-angled, getting more and more drastic the further you go up in thickness. The jigsaw gives cleaner finishes but bends more often, and the hacksaw gives squarer finishes but tends not to curve as well and gives a rougher finish. Do you have any tips for getting cleaner curved cuts with plastics like HDPE/UMHW that don't involve machining?

A: [Mark J.] Given your selection of tools, I think the jigsaw is your best option. Several tips:

  • Buy a brand new blade for your jigsaw and use it only for soft plastic.
  • Several manufacturers offer special blades for soft plastics that reduce heating and make for smoother cuts -- example.
  • Blade tooth count: about 10 TPI works well for HDPE.
  • For tight curves: rough cut about a centimeter from your final cut line, then come back with a second cut to trim away the thin edge.
  • A little wax on the blade helps reduce friction and heating.
  • Try lower blade speeds, and keep the material moving.
  • Square up edges with a sanding block.
2) My second question concerns a snail cam design for a spring flipper. I'm aware that you've been asked questions concerning this topic multiple times and have read through all relevant posts, but for my design I've attempted to adjust the parabolic spiral to allow for a continuous length for the final 1/4 of the rotation to allow for leeway when winding and to prevent misfiring. The motor still seems to struggle to wind spring loads it should be perfectly capable of, and while there may be other issues in the design that could cause that, I believe the cam contributes to the problem.

I've included a render of the cam in question, winding down springs 15mm from their original position. The tolerances in the drawing (eg. the cam not making contact with the spring when the flipper is fired) is to prevent shock damage to key components when the weapon is fired. The cam was also drawn by hand essentially, with me trying to compact the parabolic spiral over 3/4's of the circle. Would you know of an equation that would allow me to draw a more accurate parabolic spiral to fit these tolerances, and would a cam with it's last 1/8th being even put less stress on the motor than the current design?

A: I've been trying to avoid banging out equations for parabolic snail cams, but I suppose I've put it off long enough. Let me find a pad of scratch paper and have a go at it...

One hour passes

OK, I've got it. The general equation for a parabolic sprial cam is... wait... wait... %#!@*&%$!!!

Another hour passes

Right! It's important to get the shape of the parabolic spiral correct to even out the torque load on the motor. The spring becomes harder to compress as it compresses farther, so the cam has to provide a lot of compression at the start of rotation (half the total is in the first 25% of rotation) and taper to less compression as the load increases (the last 12.5% takes the final 25% of rotation).

The formula for the cam in polar coordinates is: Radius = k · θ 0.5 + minimum cam radius where 'k' is a growth variable that defines the lift of the cam; a 'k' of 0.0527 will give one unit of lift over 360 degrees of rotation. Here's an example for a small snail cam with a 5mm minimum radius and 15mm total lift:

Radius = (15 * 0.0527) * θ 0.5 + 5

0 degrees rotation
(15 * 0.0527) * 0 0.5 + 5 = (0.79 * 0.0) + 5 = 5.0mm

90 degrees rotation
(15 * 0.0527) * 90 0.5 + 5 = (0.79 * 9.5) + 5 = 12.5mm

180 degrees rotation
(15 * 0.0527) * 180 0.5 + 5 = (0.79 * 13.4) + 5 = 15.6mm

270 degrees rotation
(15 * 0.0527) * 270 0.5 + 5 = (0.79 * 16.4) + 5 = 18.0mm

360 degrees rotation
(15 * 0.0527) * 360 0.5 + 5 = (0.79 * 18.9) + 5 = 20.0mm

Note The mathematically correct spiral (below left) has far too steep a drop and rise at the start of its lift to allow a cam follower to drop in and correctly follow the profile. The exact needs of that 'landing area' will depend on the details of your lifter mechanism, but it will require some modification (below right) to be mechanically functional.

Mark gets himself a celebration beer

So far, so good. Now, the torque calculations I've given in previous snail cam posts assumes that spring compression occurs evenly throughout the full 360 degree rotation of the cam. You've modified that in two ways:

  1. Compression reaches maximum at 270 degrees of rotation, and
  2. There is no compression during the first ~60 degrees of rotation due to the cam follower being held away from the cam.
The combination requires the motor to fully compress the spring in about 210 degrees of rotation, which requires ( 360 / 210 ) - 1 = 71% more torque from the motor. No wonder the poor thing is struggling. Yes, reducing the no-lift zones on the cam will reduce the motor torque requirement and result in smoother operation.

How to get a proper cam profile that includes a no-lift 'flat sector' at the end of rotation? Increase the growth variable until you get the full lift you need at the degrees of rotation you want your flat sector to start and freeze the radius at that point. The smaller the flat sector, the less torque your motor will need to put out to wind the spring.

3) Finally, for a question that ties everything together, what would be the best way of cutting a cam like this out of 10mm HDPE be? I believe my first attempt was too rough, as it seems the cam gets caught on the smallest of differences during the winding. I'd love to try to make a good cam myself without having to resort to machining.

Thanks for reading. [Galway, Ireland]

A: I have concerns about 10mm HDPE being too thin for the purpose, even for a beetle. A wider cam will spread the loading over a larger area and prevent the cam follower from deforming the surface and increasing turning resistance -- that may be one of the reasons your motor is struggling. Take a look at the width of the snail cam on 450-gram antweight 'Jännä' (video) for comparison. You might also consider a larger 'flat' cam follower to provide greater surface area that would be less affected by surface irregularities.

I think you can make an entirely satisfactory cam by following the jigsaw tips I gave above. A little work with a sanding block can remove any troublesome irregularities, and a flat cam follower will be less sensitive to catching. Best luck!

Comment: Hey again Mark. Cheers for the equations, that's a great deal of useful information! I've attempted a new updated design for the cam based on the recommendations made, with a better gradual increase, less of a loss of torque, and a thicker cam with a recessed hub to compensate for the size increase (space is at a premium within the mechanism). Comparing the old cam design to the new one, I noticed that where the difference between the two was at its greatest was where the gears in my motor sheared, so your theories on load seem bang on! The fact that it's not a direct pull down is still an issue, but that would require more adjustments to equations, and the old mechanism was able to wind that part fine with a similar profile, so I should be alright there. I'll also keep your tips for HDPE in mind when working on the new cam.

Cheers again, if I'm ever in the area I reckon I'd owe you another celebratory beer!

Update In response to a bushel of off-line questions on this topic I've put together a new page devoted to all things snail cam: The Snail Cam Files. In addition to sections on cam profiles, drive motor torque, spring rates, and control circuits -- the page links to a Google Files spreadsheet I authored that will assist in designing your mechanism.

Q: I'm designing a front-hinged pneumatic flipper and I can't figure out how to calculate the initial force at the flipper (see sketch). What is the formula to calculate the red force vector? [Facebook]

A: [Mark J.] The force will change as the flipper extends and the angle of intersection changes, but the initial angle that your pneumatic cylinder intersects the flipper is 180 - 145 = 35 degrees which makes the formula for the initial force at the flipper: sin(35) * 300 = 0.5736 * 300 = 172 pounds

Q: So, the remaining force (300 - 172 = 128 pounds) goes into compressive load in the direction of the flipper hinge?

A: No. The compressive load on the flipper hinge is: cos(35) * 300 = 0.8192 * 300 = 246 pounds

Q: Wait, 172 pounds plus 246 pounds equals 418 pounds of force. How do you get 418 pounds of force from a pneumatic ram with only 300 pounds of output force?

A: We're summing 'force vectors' that have both magnitudes and directions, and you can't sum just the magnitudes. Your ram is producing 300 pounds of force in one direction and 300 pounds of force in the opposite direction as well. In a closed system like this lifter, the full set of force vectors will sum to zero, which isn't of much use in answering your question. Fortunately, trig functions are based on right triangles, and Pythagoras figured out that in a right triangle the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse. We can use that as a shortcut to verify that we are calculating the specific set of forces we're interested in correctly:

172.12 + 245.72 = 29,610 + 60,390 = 90,000

90,000 = 300 pounds at the actuator


Q: Hey guys! Its a little specific and I'm really sorry if its not the kind of question you want to receive :/

I run a 3lbs Beetleweight hammer bot and for some reason my brushless drive weapon system always has like a quarter to a half second of delay on it! For the first version of the hammer I used basically the same weapon setup as an earlier hammer bot "Dain" did. I ran a Fingertech 2838 22 max amp, 300W 22 max amp, 300W 2970kv motor hooked up to the next generation R/C car Trackstar ESC that Dain had. I powered it all by a 850mah 3S battery, a Fingertech receiver and 20AWG wires (whoops). The entire time the hammer had this bad input lag. Despite being hooked up to the same drive receiver as a lag-less drive system

Since the first iteration I've swapped to a bigger motor (650W, 50 something-ish max amps) gotten and programmed a bigger 60A Trackstar ESC, have bought a higher 80C rating battery and I have even changed the radio and receiver and upped the wires to 16AWG and yet I still swing with 0.25 to 0.5 seconds of lag when compared to Dain throughout the entire match! It's maddening!

I've never been great with electronics and I'm honestly out of ideas as to what causes this. It was the exact same problem with the 30A ESC as the 60A, the same with 2 entirely different radio and the same with a bigger C rating and lower gauge wiring!! I really feel like I've cycled out all of the electrical components and still have the same issue. I would love your opinion on this.

Again genuinely sorry if this isn't the kinda question you're looking for on this forum. I've tried to include as much info to avoid the bad hamburger. Have a good one! [Ontario, Canada]

A: [Mark J.] Your question is exactly the type of question we do like to see, Ontario. You've got a problem that has you stumped and it's a problem that other builders might run into as well.

The bad news is that you've put a lot of time and effort trying to fix the electronics when the electronics aren't causing the delayed response. The good news is that there's a little secret about brushless motors that I'm willing to tip. Follow along...

Brushed and brushless motors are very different:

  • Brushed motors are simple. Hook a brushed motor directly to a battery and it spins right up, producing huge chunks of start-up torque.
  • Brushless motors are not simple. Hook one directly to a battery and it will twitch and quickly melt; they require an intelligent external motor controller to supply each of their three input leads with power of the correct polarity at just the right instant and then change everything an instant later as the motor rotates.
Getting a 'sensorless' style brushless motor (like yours) to start rotating is a tricky problem. The correct current polarity to apply to each of the leads to spin the motor in the right direction depends on the relative positions of the magnets and coils in the motor, but with the motor at rest the sensorless controller does not know what that position is. The controller has to 'guess' and send a small current pulse thru one pair of the motor leads, then it waits to detect and analyse the electrical feedback from the motor when the motor starts to rotate in response to that current pulse.

This start sequence works well if the motor has a light load like an airplane propeller, but if the motor has a heavy load (like an overhead hammer) the small current pulse may not cause the motor to move enough to provide feedback to the controller. The controller will wait a few milliseconds and try again... and again... and again until the motor eventually twitches enough to allow the controller to figure out the correct polarity sequence and provide more current to properly spin the motor.

If there is enough free play ('slop') in the gearbox and/or belt drive between the motor and the hammer so that the motor can spin freely for about one full revolution before it takes up the load of lifting the hammer the whole start-up process takes only an instant. However, if the drive train beween the motor and load is very 'tight' and there is not enough free play to allow the motor to start its spin-up without load you're going to get the type of delay you describe.

You've given me a complete description of your hammer electronics but you have provided no details about the mechanics of the hammer drive train. Twisted Sick Robotics' beetleweight 'Dain' has the weapon motor mated to a 14:1 gearbox that then goes to a 3:1 belt drive for a combined 42:1 reduction. It's a good bet that the combined play in the two-stage gearbox along with a fairly loose timing belt allow the weapon motor enough no-load slop to get thru the startup sequence muy pronto. Given your description of the problem it's also a good bet that your unspecified weapon drive train does not have enough slop. Add some.

Reply: Hi Mark! I appreciate the in-depth tips. I run the exact same gearbox and additional reduction as 'Dain' and - credit where credit is due - WOW it works well! The only difference is that I run the additional reduction using specially cut gears, not belts. I'll be looking into a sensored weapon motor setup or swapping to a belt drive system now. Thanks for all your help! Cheers!

A: Very pretty, Ontario! It would be a pity to remove those bespoke gears; a sensored motor and controller will cure your response delay. May your opponents all have fragile top armor.


Q: I'm designing a single-tooth drum but I'm having some trouble. I started with an Archimedean spiral drum cross-section for correct bite clearance, then I shifted the rotation axis to the CAD-calculated center of gravity to balance it. What I got was a drum with a big lump sticking out opposite the tooth that will interfere with the tooth 'bite'. Is there a simple way to reduce the size of that lump and keep the drum in balance like the RioBotz 'snail drum'? [On-line Forum]

A: [Mark J.] You started out well -- the ideal profile will fit snugly inside an Archimedean spiral with nothing sticking out to interfere with maximum tooth 'bite'. Unfortunately, simply starting with a spiral and shifting the spin axis to the CG leaves that lump you described outside the ideal spiral (red spiral line in the diagrams below). You need to modify the mass distribution to pull the profile back inside the spiral.

The RioBotz 'snail drum' is optimized for that purpose, but it is highly challenging to reproduce. Fortunately, there is another Brazilian solution that's a little easier to design and make. Uai!rrior's 'Federal MT' and 'General' have interesting single-tooth disc profiles that could be used to make an effective drum with properties comparable to the RioBotz 'snail drum'.

Start your design with a basic Archimedean spiral cross section, then take slices off the bottom until the CAD center of sall mass stops getting closer to the spiral axis. A few additional design tweaks will bring the CG right onto the spiral axis.


Q: I just got my first disk weapon back from the metal shop and I think it has some problems. Builders are telling me that the sharp internal angles on the spokes are places that will start fractures. Worse, the water jet cutter overshot and left some little notches in the corners. I think I'm in trouble.

Is there anything I can do about this or do I have to start over? [Walsall, England]

A: [Mark J.] Those sharp corners concentrate and focus stress on a small area, and if that area is already weakened by a flaw caused by poor cutting technique you're begging for a failure. Yes, you're in trouble -- but it's not hopeless.

You need to smooth out those corners. The preferred way to avoid a 'stress riser' sharp angle is to carve a smooth, rounded 'fillet' curve when designing the disk -- but that ship has sailed. You can actually strengthen the problem region by removing a little material to create a smooth radius curve. Don't go crazy -- even a small radius relief cut will improve survivability.


Q: Hey Mark,

I'm putting together an antweight with a somewhat experimental weapon. The weapon would be a slightly different take on the overhead saw. Instead of a single abrasive disk trying to cut its way inside the opponent, it would be 4 or 5 abrasive disks put together to create a single thick disk. The idea would be that the thick abrasive disk wouldn't necessarily cut through, but rather take large scoops out of the opponents top armor. Admittedly, it wouldn't be the most efficient way to get to the insides, but in theory it would cause quite a bit of visible damage on the opponents' topside and hopefully create a terrific spark show...if done right.

So let's jump in on some questions about how to "do it right". For this scenario we'll assume the weapon consists of four combined abrasive cutoff wheels with 3" diameter and the motor driving the weapon is a V-spec 2205 2350kv motor.

Abrasive cutoff wheels are pretty light. I do not know the exact weight, but I'd wager that a 2205 brushless motor would have no problem spinning them up on 1:1 gearing. I'd imagine that most overhead saw bots try to find a sweet spot with their gearing where the weapon has good speed, but at the same time, enough torque to keep spinning while making contact with the opponent.

With 4 abrasive disks, that presents 4x the width of the cutting edge, and presumably, 4x the amount of friction that will attempt to slow the weapon down as it makes contact. Would I be correct in assuming that this would also require 4x the amount of torque (4:1 gearing) to pull off than a saw bot only running a single disk on 1:1 with the same specs?

Now, as I mentioned before, many saw bots try to find the happy medium of sufficient torque and good speed. With the inevitable gearing down that would have to happen, the speed would take a dive, but the torque would remain sufficient. At this rate, we could probably see a steady decline in effectiveness because we have to compensate for the extra friction that multiple abrasive disks create. Of course, we're not even getting into the amount of pressure that the overhead arm would be trying to put on the opponent as it comes down! Let's just assume that in this case it is simply "a decent amount".

Do you think the thick abrasive cutting disk would still be effective with such a sharp decline in speed, or would I be better off approaching this by using motors that tend to be a step up in terms of power than what we normally see in antweights?

Thanks, David R. [Livermore, CA]

A: [Mark J.] You've spent some time thinking about this, David. Let me see if I can redirect your thinking just a bit...

  • Consider the relationship between tires and the arena surface. Doubling the width of the tire does not appreciably increase the traction (friction) between the tire and the surface, but doubling the downward force (weight) on the tire will double the traction.
  • A similar relationship exists between a grinding wheel and the top armor of your opponent: increasing the width of the grinding wheel will not increase the friction attempting to slow the wheel, but increasing the downward force applied will proportionally increase that friction.
  • The rate of material removal is a function of the friction (applied downward force) and the rotational speed. For a given downward force and speed, a change in abrasion wheel width will give the same material removal over a larger area -- and the same number of sparks.
I think judges would be more impressed by you cutting a narrow slit thru the armor than grinding a wide but shallow divot across it.

What's the 'chicken-and-a-half' reference?


Q: When designing a heavyweight [electric] hammer (potentially applies to other weight classes) I've seen builders use a slip clutch at both the motor output or at end of the reduction the hammer shaft/sprocket connection. Is this just preference or is there a practical advantage to one vs the other? [Seattle, Washington]

A: [Mark J.] The design choice usually comes down to the availability of suitable torque limiters. If you're planning an off-the-shelf solution you'll find that affordable sprocket slip clutches are most commonly available is sizes better suited to the hammer end of the system.

There is a small mechanical advantage to placing the clutch on the motor end: it allows the inertia of the full chain and sprocket to contribute to the hammer impact rather than dissipating that energy into the slip clutch. That's typically not a lot of energy, but if you're after every last available erg of impact you might justify a custom motor-side clutch. I wouldn't bother.


Q: So I'm about to take the leap into the 30 lb sportsman's class. I've had a few designs bouncing around in my head and came up with on that I've (prematurely) gotten excited about. I thought of it last night so I don't have a lot of details but I'll pitch the basic concept.

It's a pneumatically driven rack and pinion hammer bot. As I'm sure you can imagine, the piston drives the rack, which rotates the pinion gear, which is attached to a devastating hammer that will smash the opponent into tiny little pieces (as is the case with all hammer bots).

I have a full machine shop to work with, CAD and combat experience, and a decent budget. Is this a viable design that I should tackle, or should I hit the cheerleader button? [Albany, Oregon]

A: [Mark J.] Viable? The most feared and successful combat robots to ever swing a hammer/axe have been pneumatic rack and pinion designs.

  • Pioneering hammerbot 'The Judge' is a member of the Combat Robot Hall of Fame with a record of 24 wins / 12 losses. The 'rack and pinion' assembly in 'The Judge' was actually created from chain belts and sprockets, but was functionally identical to a conventional rack/pinion. When the giant overhead hammer came down, the whole building shook and the crowd shouted "GUILTY!"
  • Overhead axebot TerrorHurtz used a true pneumatic rack and pinion to power its way to multiple UK tournament championships. Yes, they are also in the Hall of Fame.
Start with this archived post on pneumatic rack/pinion design, and follow the links therein to more info. Electric hammers are well and good, but if you want to rattle the box you need to go pneumatic!
Q: Would a fingertech chipper blade work on a direct drive overhead 1lb spinner, or would a blade that has a "bow tie" design (think bow ties made of AR400) similar to Cobalt's large impactor, except there's one on each side work? I think it will work better due to the outward weight distribution.

The blade diameter of the theoretical blade is similar to the aforementioned chipper. If its important, here is the motor: Fingertech D2822/17 1100kV Outrunner. [Charleston, West Virginia]

A: [Mark J.] You're correct in thinking that more mass out toward the outer edge is better for energy storage. A simple bar is poor, a 'bow tie' bar with flared ends would be better, and a disk would be better still. However, there are other considerations: strength, ease of construction, and 'bite' for a few.

Consider also the benefit to a 'single toothed' weapon. A weapon with a single impactor can spin twice as fast while retaining the same 'bite'. Since kinetic energy increases with the square of speed, doubling the speed increases energy storage by a factor of four! Alternately, you might spin the single-toothed weapon at the same speed and benefit from twice the bite.

The illustration shows a relatively simple modification to the FingerTech chipper blade: trimming off one impactor and drilling a balance hole near the other end, turning it into a single-toothed weapon. I think that might make for a simple and very effective overhead spinner. I should warn you that a long direct-drive spinner design is very hard on the motor. An unmodified outrunner really isn't designed to absorb the large side-loading it will take from weapon impacts. Pack a few spare weapon motors if you go that route.


Q: Planning for a almost featherweight(12KG). I got these gearmotors 555 sized from a Chinese supplier for 10$ each. They do about 800 RPM in 12V, but I plan to go with slight over-volting, 14.4v. It will be a 4 wheeled robot, powered by 4 of these motors.

Question is, Drum. I want to make a drum spinner, and taking the suggestion from riobotz tutorial book,it should weigh 3KG, just the drum alone. And drum will be powered by a 775 with a high current relay, will convert to brushless later. Does this plan have a big flaws.??

Thank you [Dhaka, Bangladesh]

A: [Mark J.] Yes, the weapon has a very large flaw -- it's a dangerous spinner weapon that will fight in an arena like the one pictured below. I answered some questions about a wedge/lifter 'bot from Bangladesh a couple months ago, but I won't offer any help with spinner weapons that will fight in open arenas. The risk of serious injury is far too great for 'Ask Aaron' to support.


Q: Hi! I was wondering if there were any advantages of 'Kraken' having a Pneumatic Crusher over a hydraulic or battery powered one. Ty and Robots ftw! [Providence, Rhode Island]

A: [Mark J.] 'Kraken' has a non-conventional weapon system that uses a heavy lift airbag instead of the usual pneumatic cylinder to power the jaw. The large area of the lifting bag can produce a great deal of force from relatively low air pressure, but requires a lot of air to pass thru a small port to inflate. That small port is a serious problem that will slow down weapon actuation. It also requires a pair of stiff plates to sandwich the bag and some custom links carry the force to the weapon jaw.

Pneumatics are comparatively simple and robust compared to other options:

  • Their solution is simpler and more reliable than a hydraulic system that requires a motor, motor controller, fluid reservoir, high-pressure pump, 5-port valve, and a bunch of pressure hoses.
  • An electric crusher requires a very heavy and expensive gearbox to survive the massive torque, and electric motors don't survive long when bogged down near stall to provide their maximum torque.
Based on performance I don't think I can recommend the airbag approach. The team claimed 10,000 pounds of force out at the piercing end of the jaws, but I don't think the weapon performed up to expectations.
Q: Hello! I just had a quick question about lifting mechanisms. I've noticed that some lifting mechanisms, like the ones on the featherweights Banana Bender (AUS) and Mad Rush (UK), as well as a number of lifting mechanisms on UK ants that use an angled piece that connects to the lifting arm rather than having the motor connected to the weapon itself.

Other than protecting the motor or servo, are there any other advantages to using this over a conventional lifting mechanism of the same size? What are the drawbacks? How does one calculate what kind of gearing ratio is needed in order to lift opponents?

((I know the last question sounds silly, but I didn't know if calculating the ratio needed to lift an opponent with a particular motor is different with these designs than with normal lifting arms)) [Jacksonville, Illinois]

A: [Mark J.] What you've got there is a '4-bar Mechanism'. The design is widely used in combat flippers and axes: 'BioHazard', 'Ziggy'. 'Pad Thai Doodle Ninja', 'Shunt' and a great many more. A search of this archive yields more than 100 hits for '4-bar'.

The calculations required to optimize a 4-bar mechanism and to determine the motor torque requirements are quite complex. There is a 4-Bar Simulator written by Adam Wrigley of T.i. Combat Robotics that will give the motor torque requirement for the common layouts of lifters using a 4-bar mechanism, but I will point out that the design used by 'Banana Bender' is a non-standard use of the 4-bar and would not be covered by the T.i. simulator.

Take a look thru the archives for discussions of the advantages of the standard 4-bar design. Briefly: the 4-bar allows greater choice in the 'arc' of the lifter, may reduce the peak torque requirements of the gearmotor, and can increase lifter speed by spreading the power input more evenly over the full sweep of the lift. A custom 4-bar design like 'Banana Bender' is something that third-year mechanical engineering students might undertake just to show off.


Q: Hello, Mark! I was thinking about the question I had a while back about how people turned mixing bowls into full-body spinners, and remembered that both Ziggo and Blendo were made from wok pots (IIRC). As with that question, I am simply wondering: how on earth do you turn a wok pot into a spinning shell of amassed destruction? I'd assume their cast-iron structures would gave them more problems than it'd be worth. [Jacksonville, Illinois]

A: [Mark J.] Not cast iron! Cast iron shatters. Iron woks exist but traditional woks are made of a sheet of carbon steel, hand hammered into a deep bowl shape. 'Ziggo', 'Blendo' and other 'wok-based' FBS use the modern equivalent of that style, with woks made of one or multiple layers of thin steel stamped to shape.

The design is the same as for the insect-class spinners using small steel mixing bowls you asked about in your earlier question; the addition of a thick bottom ring and reinforcing plates under any blades/impactors on the wok itself spread impact load over a large area of the thin material. 'Ziggo' adds a square steel cross-tube that sticks out either side of the spinning wok.


Q: Hello, I was curious as to how friction and RPM relate to each other when talking about a spinning weapon? If I increase my RPM to increase energy storage will I also loose more energy to friction in the process? Also, is this increase linear or exponential? [Grove City, Pennsylvania]

A: [Mark J.] It takes more power to spin a given weapon up to a higher speed, and it takes more power to maintain the weapon spin at a higher speed. This places a greater load on the electric drive motor: that load will reduce motor speed and increase current draw. So, doubling the calculated weapon speed by doubling voltage or halving the belt drive reduction won't really get you to twice the actual speed because the motor will spin a bit slower than expected to generate enough torque to handle the increased load.

Back to your question -- you have several sources of energy loss in a spinning weapon system, and they don't all behave in the same fashion. Without getting to deep into engineering exceptions:

  • Bearings Frictional losses from bearings are essentially linear when operating inside their design parameters.
  • Belt Drive If used, a properly selected and set-up belt drive is quite efficient -- upwards of 98%. Losses are fairly linear. See this PDF on belt efficiency if you want to nitpick.
  • Aerodynamic Drag Unlike bearing and belt drag, aerodynamic losses are exponential. When moving thru the air at twice the speed your weapon must move twice the volume of air out of the way and it must move each unit of air twice as quickly, meaning that aero drag increases with the square of speed. If your weapon is a thin disk that displaces little air as it rotates this isn't too much of a problem, but if the weapon is a big and aerodynamically ugly beater bar you're going to run into a great big wall of resistance.
Energy storage in a spinner weapon is also exponential. If you do succeed in doubling the weapon speed you will store four times as much kinetic energy -- but the power needed to maintain the weapon spinning at double the speed will be more than doubled because of exponential aero drag.
Q: Hey, it’s the idiot lifter guy back again. What material would have the best kinetic energy storage? I saw some things about energy storage in riobotz and the archives. [Tampa, Florida]

A: [Mark J.] For a given size and shape, the Moment of Inertia of an object depends on mass -- so denser materials have greater kinetic energy storage. The densest metals are:

MetalDensity (grams/cm3)
Uranium19.05
Tungsten19.25
Gold19.32
Plutonium19.84
Platinum21.45
Iridium22.56
Osmium22.58
None of these metals are commonly used in combat robot weapons.
  • There is more to a good spinner weapon than energy storage. A pound of osmium would cost you about $5800 and a spinner weapon made of that very brittle metal would shatter on impact. It makes more sense to use a more durable metal and make the weapon a bit thicker/longer/wider to compensate.
  • Look at build reports for successful spinner robots in the weight class you're building and see what's commonly used.
  • But first, finish building your antweight wedge. 'Ask Aaron' answers questions from builders.

Q: Hi Mark.

What are some of the advantages and drawbacks of multi-motors setups for HW Horizontal weapons? [New York, New York]

A: [Mark J.] Good engineering practice calls for a design solution to be as simple as possible. Abandoning the traditional single-motor solution to a spinner weapon adds to system complexity and increases the number of potential failure points in an already highly stressed system. If you're going to do that you'd better have very good reasons.

One common reason for resorting to multi-motor weapon drives is satisfying dimensional restrictions imposed by the robot design. The current incarnation of 'Son of Whyachi' abandoned the motors used in previous versions because they were too tall to fit into the desired very low profile of the robot chassis. Team Whyachi's final weapon design relies on a circle of eight(!) 'Mini Magmotors' driving a central spur gear. The ring of small motors allowed the height of the robot to be reduced by several critical inches.


Q: For my first robot, I was thinking of building a finger tech robotics kit and adding a lifter like the add on like Whiplash’s without the spinner powered by a weapon motor. Any good?

8 minutes later

Q: For my first robot, I was thinking of building a finger tech robotics kit and adding a lifter like the add on like Whiplash’s without the spinner powered by a weapon motor. Any good?

3 minutes later

Q: I’m sorry, I send a message about my lifter idea, where do I find it? I will look for this question in the archive, will that work? [Tampa, Florida]

A: [Mark J.] You appear to have mistaken 'Ask Aaron' for 'Instagram'. Should you submit another question here, please do take a moment to read the explanatory 'thank you for your question' page that appears after your question is submitted.

Since you're expecting an immediate response and you're paying nothing, perhaps you'd like to go to the Frequently Asked Questions and read section #8 -- which gives quite specific advice on the design for your first combat robot.

Q: For my post, I pick the second. Also, I did not mean to re send the exact same post eight minutes later. I know that makes me sound like an idiot, and.. I kind of am. Is there any major flaws with the lifter idea? Is it too complicated? I don’t want to make a wedge.

A: The major flaw with the 'lifter idea' is that you are a first-time builder. FAQ #8 refers you to this list of reasons why a first-time builder should not build a 'bot with an active weapon. That's my "good and free" answer. If you'd rather have "The answer the guy asking the question wants to hear" just click this button:


Q: Hello, it's Jacksonville lifter guy again! I was sitting around, minding my own business, when I suddenly got an idea: rather than having a huge plow that can lift up and down, what if I used a small lifting arm in-between two hinged wedges? Any advantages and disadvantages to having a small lifter like that rather than a wide plow? [Jacksonville, Illinois]

A: [Mark J.] It's kinda dangerous to let ideas sneak up on you like that. Stay vigilant. Some random lifter observations:

  • A narrow lifter arm as you describe can let an opponent 'fall off' to one side or the other. It's kinda like trying to eat peas with the flat side of a knife, except the peas have wheels and they're trying to drive off the knife.
  • You can get away with a narrow arm on a 4-bar lifter that is moving upward and toward your opponent (think 'BioHazard') but the combination of a short and narrow arm with a single pivot lifter that is pulling away from your opponent has a very limited number of real-life circumstances where it might be useful.
  • A wide plow lifter has the added advantage of 'getting out of its own way' as it rises, allowing your 'bot to move forward and keep the lifter under the edge of your opponent rather than pulling the lifter away. If either of the hinged wedges in your design is not 'under' your opponent, they will block and push them off the lifter as it rises. Bah!

Spring Powered Flipper Weapons

Pneumatic flipper weapons are awesome but their complexity can be troublesome, particularly in smaller combat robots. Flippers powered by mechanical energy stored in springs or elastic bands could offer attractive alternatives to pneumatics if designs that use electric gearmotors to trigger and reset spring flippers were better known to builders.

I've put together animations and discussion of four poorly known spring flipper designs and wrapped them up in a new webpage: the 'Choo-Choo' overrunning clutch winder, the constant-torque 'Snail Cam', the compact 'Slip Gear' ratchet, and the elegant and stable 'Servo Latch' reset.

Ask Aaron: Four Spring Flipper Mechanisms

Q: Hey, Mark. It's time I show the idea I've been throwing about. While I had admittedly planned on a FBS, I'm leaning more towards a lifter for simplicity and the fact I've grown to become a fan of the DUCK! method of using my face to break your fist. I made a post in regards to the lifter designs themselves here:

[Facebook permalink removed]

Please share any comments or critiques you have in regards to both designs! Thanks again! [Jacksonville, Illinois]

A: [Mark J.] I don't split comments across 'Ask Aaron' and other forums/venues. If you ask a question here I will comment here. If you post on the Facebook Combat Robotics group I may comment there. However, I will not comment here on an active discussion elsewhere, and I will not link to an active forum thread.

If you want my opinion, ask here. If you want a grab bag of forum comments, ask there.

Q: OK, Mark! RESET I have a question regarding a featherweight lifter.

I wanna note that I would've built a wedge as a featherweight, but EOH requires a weapon that's at least 6 pounds in order to qualify unless the design is approved with a weapon that's less than that.

I was planning on using a 182:1 gearbox and a Banebots 775 motor for the lifter. However, I've come across a big thing in regards to their designs. One of them is inspired by 'DUCK!', with a short, beefy lifter on the front while the other is based off of the UK heavyweight 'Shockwave', with a lifter that can rotate 360 degrees around itself. I'd assume that one version is better at doing something than another, I just wouldn't happen to know what that is.

  • What are the pros and cons to both designs?
  • What kinds of problems might be encountered with each weapon design?
Another thing I should mention is that I was thinking of adding a 1:1 gear connecting the lifter to the motor as a simple way to alleviate shock from directly hitting the lifter, or perhaps using a chain system. Would something like this work? If not, what is a way I can reduce the shock felt on the lifting arm, especially during a heavy collision with a spinner?

A: [Mark J.] How did you decide on a 182:1 gear ratio for your lifter before you settled on a lifter design? The gear ratio determines the torque available to the lifter, and the torque required for a simple lever-arm lifter depends on the length of the arm from axle to tip. Examples:

  • A featherweight lifter with a one-foot lifter arm will require one foot * 30 pounds = 30 foot-pounds of torque to offset the weight of a featherweight opponent and start to lift it. I recommend gearing for twice that level of torque to allow the lifter motor to quickly lift that amount of weight: 60 foot-pounds of torque. A 12 volt BaneBots 775 motor delivers 61 ounce-inches of stall torque at its rated voltage; 61 in-oz = 0.18 ft-lb. That makes the required gear reduction = (1 foot * 60 pounds) / 0.18 foot-pounds = 333:1.
  • Shortening the lifter arm length to half-a-foot reduces the torque requirement and required gear reduction by half: 30 foot-pounds, achievable with a 167:1 ratio gearbox.
  • Shortening the lifter arm farther to a 'Feather Duck!' range quarter-foot reduces the torque requirement and required gear reduction by a factor of four: 15 foot-pounds with an 83:1 ratio gearbox. Maybe even less because such a short lifter can really only hope to lift one end of your opponent.
Your next consideration is the torque rating of the gearbox. The maximum recommended torque rating of the BaneBots P60 gearbox is 35 foot-pounds. Given the shock loading a lifter gearbox may have to endure from weapon impacts, it is wise to stay well under that figure in your gearing. Many lifters use two gearmotors -- one on each side. This is not because a single gearmotor could not provide adequate power, but because a single gearbox might not survive the torque loading.

Shock Reduction

Your proposed 1:1 geartrain connecting the gearbox to the lifter will not reduce loading on the gearbox -- a shock load on the lifter will be passed on at a 1:1 ratio the box. You will often see additional gear reduction between a lifter and a gearbox, and that DOES reduce shock transmission! An external 4:1 reduction will increase the torque from the gearbox to the lifter by a factor of four, and will reduce the torque of shock transmitted back along the lifter toward the gearbox by the same factor.

Pros and Cons

  • A 'Shockwave' style long-arm lifter can lift your opponent higher and can also self-right your 'bot. The long lifter arms require more torque to operate and transmit greater shock from a spinner hit back to the gearbox.
  • A 'Duck!" style short-arm lifter can really only lift one end of your opponent off the floor and break their traction to make them easier to push. The short lifter arms are by nature stronger, require much less torque to be effective, and will transmit less spinner shock to the gearbox. If you're building a 'brick', this is your preferred approach.
You'll be interested in scrolling a couple posts down this archive to a question about 'Sandstorm' for more info on lifter calculations. That's a lot of info in a short package. Write back with specific questions if you need greater detail.

Q: Hey Mark. Let's talk theory.

I've been more or less dubbed the "horizontal drum" guy due to my choice of over-sized and thick horizontal weaponry that I tend to favor. While most builders in the ant and beetle class tend to use horizontal weapons with a thickness ranging between 3mm and 3/8", my weapons range between 3/4" and 1" in thickness. The basic idea is to increase the impact zone, much like a drum, instead of a smaller, more focused zone of impact.

My initial theory on this was that because the area of potential impact is increased, the energy in those hits would be spread out over a larger area and would actually be weaker, but covering more ground. A bit like throwing a stiff jab when you were born with hands that would make Andre the Giant blush.

However, I'm finding the reality to be a little different and I'm not sure why. It appears that the impacts are actually NOT weaker, but instead far more powerful than anticipated. It's as if the larger area of impact is increasing the percentage of energy transferred because it has more area to transfer TO.

But then again, I'm just a guy with a few crazy ideas and a garage full of tools. Do you have a little bit of insight about what might actually be happening here?

David Rush [Livermore, CA]

A: [Mark J.] OK, let's get theoretical. A spinner weapon needs a combination of things to land a powerful impact:

  1. good 'bite';
  2. high kinetic energy; and
  3. unyielding impact surfaces.
Assuming the same diameter, speed, and profile in comparing your 'drum' to a more conventional STD, I think we can exlain the advantages you have in impact.
  1. Bite - Your tall impactor sweeps a larger area on your opponent and has a better chance at running across and grabbing an exposed screw head, sharp edge, or hard wheel hub in a foamy tire than does a shorter-height impactor. That gives a significant advantage in 'bite'.
  2. Energy - Odium's 'drum' appears to be made of aluminum, but given that it's three to eight times as thick as a typical steel STD you've got at least as much kinetic energy storage potential as a thin disk and potentially a fair amount more.
  3. Unyielding - Have you ever seen a road sign that some frustrated hunter shot with a rifle? The energy went into making a nice clean puncture and transferred very little energy into the sign as a whole. Contrast that with a road sign hit by a shotgun -- it's in much worse shape because the energy was applied over a larger area and transferred to the sign material more effectively.
At the end of his stand-up act Gallagher smashes a watermelon with big flat mallet and sprays fruit mush over the first several rows of the audience. If he hit the watermelon with a drywall hammer he would just punch a hole in it in the same way the rifle bullet punctured the road sign. Spreading out the impact energy over a large area assures that the whole melon gets its share of the impact energy.

Granted, your opponent is not made of melon -- but the various materials from which they are made can and will absorb energy by deformation. If you're trying for localized damage by tearing into your opponent to grab or rip away material, a small and possibly sharpened impactor will serve that purpose. If you're trying to 'swat' your opponent and send it flying with as much energy transfer as you can manage, a large impactor area will minimize localized deformational energy dispersion and transfer more of the kinetic energy to your opponent as a whole.


Q: Hey, Mark! Back once again with another question. I noticed in a recent post that 'Sandstorm' used two individual gearboxes connected jointly in order to give its [lifter] weapon power. Is this, in any way, more effective than using one gearbox with more power? If so, why? I was thinking, for example, would there be any benefit from using two Banebots gearboxes with 129:1 ratios over one Banebots gearbox with a 182:1 ratio.

As an off-topic question, if using two motors has any advantage, would using two Banebots gearboxes with 64:1 ratios be a good option, or would they be too weak for a decent lifter? [Close to Champaign, Illinois]

A: [Mark J.] Don't confuse 'power' and 'torque', Champaign. Mechanical power is expressed as rotational force (torque) multiplied by its rotational speed (RPM). A reduction gearbox will increase output torque and decrease speed in equal measure -- there is no increase in 'power' regardless of the gear ratio.

Why use two gearboxes/motors rather than one gearbox with a motor twice as powerful? Electric lifter weapons require a great deal of torque, but there is a limit to how much torque a given gearbox can handle without destroying itself. BaneBots gives this warning for their P60 gearboxes in all ratios:

MAXIMUM TORQUE RECOMMENDATION

We recommend maximum torque not exceed 35 ft-lb for all P60 Series Gearboxes. It is possible to mount motors that will exceed this in higher gear reductions. Higher reduction gearboxes should be utilized primarily for speed reduction. Designs utilizing a P60 gearbox / motor combination that will exceed 35 ft-lb should include a method of limiting torque to prevent damage to the gearbox.

It's very easy to exceed a 35 ft-lb gearbox requirement, particularly when impact loads are added in. The lifter in Ellis Ware and Giles Ruscoe's heavyweight 'Sandstorm' starts with two BaneBots P80 gearboxes and adds on external spur gear reduction to take some of the torque load off the P80s. A pair of smaller gearboxes are often easier to pack into a low-profile 'bot than a bulkier single gearbox, plus twin drives offer redundancy should one motor fail.

The suitability of any gearbox or gearbox pair requires analysis of the loads that will be placed on the gearbox shaft(s) by your specific lifter design. I can't make a general comment about specific gearboxes without knowing much more about the lifter design. Several posts in this archive give examples of lifter analysis -- search here for "calculate how much torque" for a start.


Q: Hi Mark, Had a conceptual question about impactors on horizontal spinners. Traditionally, the inserts come to a triangle point with the hypotenuse facing bot. Is there any sense in making a vertical ramp impactor to get under opponents instead of a straight line to hit anywhere in the height of the weapon?

Many Thanks. [Pittsburgh, Pennsylvania]

A: [Mark J.] I'm confused by your question. I'm not sure why you'd want to use a horizontal spinner to try to 'get under' your opponent when the weapon is designed to tear them apart. If you want to get under them, build a wedge.

The impactors and inserts I typically see on horizontal spinner weapons are typically flat, hardened, vertical surfaces designed to transfer destructive impact energy against an opponent's hard surfaces. Some spinners have special blades with sharp leading edges to swap-in against softer-armored opponents, but hitting a hardened surface with a sharp blade blunts it immediately. See the comparison of 'Tombstone' bar weapons at right.

For a reader question (now in our Ants, Beetles, and Fairies archive) I asked builder Jamison Go why he sometimes replaces the single-tooth disk (STD) with a saw-edged impactor in his beetleweight horizontal spinner 'Silent Spring'. His answer:

Everyone is hype about single tooth blades for "bite" but why? What does bite actually do for you? It is heavier engagement on a piece of material which is arguably more energy dissipated just because it has nowhere to go.

What if the opponent has no such grabbable surfaces? Say for example, a robot made of rubber or foam? The traditional STD would be ineffective unless sharpened every match and even then its likely only good to the first few hits because it is THE singular wear point.

The saw-blade is for whittling opponents who have only compliant armor or soft things at the hitting surface. Instead of going for one big impact which would normally be absorbed, I flake material away at a high rate. What happens is I end up grabbing the same amount, but only after several milliseconds of tearing deep into them.

[My opponent was going to] use his over undercutter attachment which meant the only hitable surfaces were his wheels, hence the decision for that type of blade.


Q: Hi Mark! I was wondering if there were any robots with instead of a single drum, Had two or three disks next to each other to make more power from all three motors? Thanks! (Also go RunAmok) [Lynn, Massachusetts]

A: [Mark J.] Thanks for the 'Run Amok' shout-out, Massachusetts.

There have been several multi-spinner combat robots, but the spinners are usually located on opposite ends of the 'bot. A current example of that design is BattleBot 'Rotator', armed with large independent horizontal disks on front and back. The flaw in this design is that only one of the two disks can impact your opponent at a time, limiting the damage the system can inflict.

An alternative twin-spinner system design is employed by 2018 BattleBots competitor 'Double Dutch'. Their design features counter-rotating bar spinners placed above and below the robot body. The two bars are powered by a single electric motor. Although both weapon bars can attack an opponent at the same time, it's likely that one bar with strike before the other and throw the opponent clear before the second bar can impact. Almost certainly the two potential weapon impacts will strike different locations on the opponent, which will also limit their effectiveness.

A different double-spinner approach is seen in antweight 'Not So Free Hugs' which has twin saw blades on articulated arms. The idea here is not impact damage but the ability to trap and saw into soft wheels or plastic side armor. Here the ability to access different locations on the opponent is a benefit as it gives a greater chance to find something vulnerable to saw damage.

You'll note that all of these designs are horizontal spinners, and that none of them are terribly successful. The potential benefits of multiple vertical drums or disks are even more difficult to imagine. Why bother with three small independent disks and three small motors when you can have one larger drum and a large motor three times as powerful? A hit anywhere on the drum would transfer full impact energy to the opponent, whereas a less-than-perfectly-aligned hit from a triple disk might only transfer impact energy from one of the disks. Sorry, but I'm seeing added weapon complexity and weight with no real advantage. Simple robots win.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: I've had difficulty sourcing tool steel for my beetleweight bar spinner, so I've been looking into methods to harden mild steel. Have you heard of Robb Gunter's "Super Quench" method? It sounds like you can get a good facsimile of tool steel out of mild steel fairly easily (well, as easy as other home blacksmithing techniques anyhow).

Have you heard of this method, or do you know of any teams who have tried this? Would it be effective for a bar spinner? I think I'm going to give it a shot, unless I hear a good reason not to. [Mark from Vancouver - again]

A: [Mark J.] What do the things in this list have in common?

  • Apricots cure cancer.
  • Global warming is a hoax.
  • Chain letters are like free money.
  • Goat pheromones will make you irresistible to women.
  • Elvis is alive in a nuclear submarine under the polar ice cap.
  • Detroit suppressed a device that attaches to your car's air filter and doubles gas mileage.
  • Hillbilly metallurgy is awesome.
Give up? They're all things that you'd really like to believe are true but absolutely aren't.

There's a long list of desirable and required metal properties for spinner weapons, and if Billy Bob's mystical backyard hardening provided a reasonable mix of those properties you wouldn't have to search the back alleyways of the internet to find out about it. Don't waste your time.

Tool steel and abrasion resistant steel are fine spinner materials, but if you can't lay hands on them a nice bar of 'cheap and widely available' 4130 steel (AKA 'chromoly') is your go-to substitute. You can harden chromoly using standard practices and when you're done you'll know what you've got.

A little more about 'Super Quench'

The lower the carbon content of steel, the faster the cooling temperature drop has to be in order to create the crystalline structure needed to harden that alloy by heat treating. 'Mild steel' has a very low carbon content (0.05% to 0.30%) and is generally not hardened by heat treatment because the techniques needed to chill mild steel quickly enough to achieve barely significant hardening are simply not worth the effort. It's ever so much simpler and more productive to start with a higher carbon alloy.

So far so good, but then some backyard metal bangers dredged up work done by a metallurgical researcher who was able to add a little heat hardening to "unhardenable" mild 1018 alloy steel as kind of a parlor trick. The internet has built this 'super quench' mild steel up to mythical proportions, touting it as a substitute for tool steel. In truth, the stuff isn't even a match for common low-alloy steels.


Q: Hey Mark, question about insect-class pneumatics:

I understand that a larger cylinder bore equals more useable force, however, how do you calculate the limit of this, when the slower piston speed due to larger bore becomes a greater limitation than the additional force?

For example, say my valve has a flow rate (scfm) of 2.1469; I am thinking of switching from a cylinder with 5/8" bore to 3/4". Could you point me in the right direction, even external links on the math behind calculating this?

Thanks! [Utrecht, Netherlands]

A: [Mark J.] It's very difficult to model the speed of pneumatic systems, in part because of the interaction of multiple system elements on gas flow and in part because of the compressible nature of gasses. Ultimate force is easy, but actuator motion starts as the pressure first builds past the force preventing system movement and may be complete before the force even approaches full theoretical force.

You can find a discussion of factors effecting pneumatic system speed at the Machine Design: Pneumatic Speed page, and you can poke thru the Team Run Amok: Pneumatic Tricks page for tips on improving speed.

In wildly over-simplified terms both your speed and force are related to the cross-sectional area of the actuator bore: double the area = double the force and double the time to extend.

Your existing 5/8" cylinder has a p * (5/16)2 = 0.307 in2 cross-section area and your proposed 3/4" cylinder has a p * (3/8)2 = 0.442 in2 cross-section area.

Switching to the larger actuator should yield (0.442 / 0.307) = 144% of the original theoretical force, with full extension in 144% of the original time -- but that's not what you'll see in actual practice. There is no simple math to get real-world numbers.

If you're like most builders you picked your current components because they 'looked' about right given what you've seen in other 'bots. In general it's simply too difficult to quantify a combat robot pneumatic system design. From a practical standpoint, improvements to your system are best approached in an entirely experimental fashion: try something and see if it works.
Q: Hi Mark, I'm working on a hobbyweight horizontal spinner and wanted your opinion. There are spinners like 'Last Rites'/'Tombstone' that ping pong like a fbs after impact and others like 'Hazard' that barely turn. I looked in the forum and read up on Hazard's faux clutch and don't fully understand it. So really 3 questions:
  1. Is a clutch needed for a 12" bar spinner in this weight class?
  2. Does it make a difference if the bot is 4wd?
  3. If a clutch is needed, how do you make one?
I looked on McMaster Carr and they have clutches, but I do not know if they are suitable.

Thank you for your time and insight. [New York]

A: [Mark J.] There have been a lot of questions from builders about weapon clutches here at 'Ask Aaron'. A search of the Ask Aaron Robot Weapons archive shows 51 hits for 'clutch', including a few that discuss the odd pressure clutch used by 'Hazard'. You'll be interested in reading thru those earlier posts for additional depth on the subject, but I'll summarize important points here.

  • 'Hazard' had a weapon clutch, and recent versions of Ray Billing's big bar spinners also have weapon clutches. The presence or absence of a clutch isn't what accounts for the difference in their reaction on impact.
  • 'Tombstone' has its huge bar spinner way out in front of the 'bot. The point of impact is a good distance away from the robot's center of mass, which is not far ahead of the two drive wheels. That distance gives a lot of lever advantage to the reaction force from a weapon impact to spin the 'bot around. Combined that with the enormous kinetic energy of the monstrous weapon and you've got the reason for the 'hockey puck' reaction of Ray's 'bots on impact.
  • 'Hazard' had the axle for its spinner centered in the square formed by its drive wheels. The point of impact was much closer to the robot's center of mass, which was surrounded by tires that would have to be dragged sideways across the floor to spin the 'bot around. That makes for much less reaction to a weapon impact.
  • The purpose of weapon clutch is to protect the weapon drive components from damage from an abrupt deceleration when the weapon strikes. The weapon motor and belt/chain drive build up a lot of kinetic energy of their own and the shock-loading from a sudden stop can overstress shaft interfaces, bearings, and hubs. The larger the robot, the more of a problem this is, and I'd put a hobbyweight into the 'probably need' category. Even a little 'slip' in the system can go a long way toward saving the weapon drive.
  • The simplest and most common method of providing that little bit of 'slip' is to use a belt drive to transfer power from the motor to the weapon. A V-belt is common for large 'bots, and 'round' belts are common in sub-lights. An alternative is to use a widely available toothed 'timing belt' and pulleys, but to machine away all or most of the depth of the 'teeth' on one of the pulleys to allow slippage under high load.
  • Do Not Even Think about direct driving a 12" spinner with an unmodified brushless motor -- with or without a clutch. The side-loading from a weapon impact would immediately destroy the motor. There are a very few hobbyweight direct drive weapons, but they use custom fabricated hub motors with huge axles designed to take the load.
For what it's worth, I don't understand Hazard's clutch either. The weapon bar is squished in place on the shaft between two phenolic washers by a shaft collar. I've got a really good picture of the collar, but I can't figure out how you get enough 'squish' pressure from the collar to hold the weapon bar firmly enough. If anybody happens to run into builder Tony Buchignani do me a favor and ask him, wontcha?
Q: Hi Mark, it's drum spinner guy again. Here is my weapon design thus far. The length is fixed, the radius is flexible, but the larger the better obviously for energy storage. I fell it's overbuilt just because of its weight, but wanted your take. This is for a 15# robot.

Mostly, I want to know if I can use a thinner drum as it is currently 3/8 aluminum. I am trying to maximize bite so I was only planning on spinning it to 5000 rpm and have it store around 2000 joules while having a max bite between 12.5 and 25 mm. Any advice is, as always, much appreciated. [Pittsburgh]

A: [Mark J.] Looks like a classic drum -- no problems with what I can see, but your comments raise a couple points:

  • The photo of the drum attached to your earlier posts a bit down the page is from a 12-pound robot that fought more than ten years ago -- before LiPo batteries and brushless weapon motors raised weapon energy levels to the high levels seen in current competition. It spun much slower than you plan to spin your drum and faced opponents with much less damaging weapons than are seen today. Comparing your CAD to that photo and updating to current standards of combat, I don't believe your proposed drum is 'overbuilt' at all.
  • You say the drum will be built from aluminum -- but there's aluminum and then there's * A L U M I N U M *.
    • If you're making a drum of nasty, soft aluminum alloy like '3003': the drum is gonna get gashed/slashed/crunched and will need to be very thick to survive.
    • If making a drum of heat-tempered 'aircraft' alloy like '7075': the drum will be harder, stronger, and can be made with a thinner wall.
    The question of "How much thinner?" will be answered in combat.
  • If you're designing to get a realistic 25mm 'bite' you'll want an impactor tall enough to take advantage of that bite: about 25mm. You want to have the 'reach' to impact your target before it can penetrate far enough to impact your drum!

Q: How can you design a single impactor or snail style drum weapon and assure it is balanced? Every time I start to design one in Autodesk Inventor, I keep making it an egg beater or really asymmetrical, and doubt its balance. [Pittsburgh, Pennsylvania]

A: [Mark J.] The Autodesk Inventor CAD program can display an object's center of gravity. Hover over the CG to display its coordinates.

  • If the center of gravity is centered on the axis of rotation of your drum, it's balanced.
  • If not centered, move the axis or add/move some mass until it is.
Here's a video of builder Marcus Quintilian designing a Single Tooth Disk on Fusion 360 -- the process is similar in Inventor.

Q: Hi, single tooth drum guy again. What screws are recommended for attaching an impactor to a drum? It is bad design practice as the force on the screws is sheer force, but I do not see another way around it. Just looking at alternatives to a unidrum as it gets very complicated very quick.

A: You haven't mentioned the weight class of your 'bot, and impactor strength issues increase rapidly by weightclass. I'll need to be a bit general.

Impactor failure from shear at high weapon energy levels is what led to the development of unidrums. Machine screws are made for optimal tensile strength at the expense of shear resistance, and you REALLY can't expose screws to impactor shear forces anywhere above insect level weapons.

If you're going to attach drum impactors with screws you'll want to machine a recess into the drum such that the lateral impact force is transferred directly to the drum. The retaining screws are isolated from shear and serve only to pull the impactor down into the recess. See diagram and photo at right.


Q: Hi. I am building a 15 lb combat robot, and I have a 1700Kv Brushless Inrunner motor by Dr. Mad Thrust. I am running a 25v system with a reduction of 1:1.5 so the weapon will be spinning at around 28,000 rpm. Do you think this is too fast for the weapon? I have seen many beater bar robots spinning insanely fast, and the downfall of my last robot I believe was the weapon speed. I have attached my weapon so you can base your answer off of that as well. Thanks :) [Dublin, Ohio]

A: [Mark J.] Your drawing/CAD failed to 'attach' so I don't have the details of your weapon design. I'll have to be general in my comments.

  1. It's unlikely that your actual weapon speed will approach 28,000 RPM even with a very powerful weapon motor. The Kv 'speed constant' value is accurate for an unloaded motor. As you add load to the motor the speed drops, and the aerodynamic load on a spinning weapon increases with the square of speed. Spinning a weapon at 28K RPM requires four times the power needed to spin the same weapon at 14K RPM -- sixteen times the power needed at 7K RPM. Your weapon will fall farther off the calculated speed as aero drag increases, and will pull greater current in direct proportion to the increasing load.
  2. Read thru the Ask Aaron Spinner Weapon FAQ for considerations in evaluating spinner speed. Note in particular the section about drums/beaters going 'weapon-to-weapon' against similar designs.

Q: So I heard that 'Warrior Clan' or now called 'Warrior Dragon' used kinetic energy from their spinner. Is this true? If so how? By the way I'm a big fan of your website! [Turks and Caicos Islands]

A: [Mark J.] Team Whyachi's kinetic spinner/flipper has a long history, having first fought in 2009 at the non-televised 'BattleBots Professional Championship' as 'Warrior SKF'. There are multiple posts about Warrior SKF in this archive: start here.

Note Some sources incorrectly claim that 'Warrior SKF' is a rebuild of Team Whayachi's earlier rambot 'Warrior'. The name was recycled, but the two very different robots are structurally unrelated.


Q: Hey, Mark! Apologies for sending you a question right after my last bunch, but it wasn't until this morning that I actually remembered it. Okay, so I noticed 'Wrecks' uses a vertical flywheel that is much thinner than conventional flywheels.

What material did they use (I'd assume S7 steel, but I didn't know)? Other than less weight, is their any other advantage to having a thinner flywheel? Are there any notable disadvantages? [Champaign, Illinois]

A: [Mark J.] 'Wrecks' relies on gyroscopic precession generated by its vertical spinner (made of AR400 steel) for it's 'walking' motion. A larger diameter weapon has a greater mass moment of inertia (MoI) and generates greater precession force at the same RPM. Cutting the flywheel width in half allows for a 40% increase in diameter at the same mass, and doubles the MoI.

Advantage

  • Thinner allows you to make the spinner larger in diameter for the same weight. The larger spinner makes it better at generating gyroscopic precession -- which makes 'Wrecks' a better walker.
Disadvantages
  • More precession force is good for a precessional walker, but it causes problems for a wheeled robot. See this earlier post on 'gyrodancing' for details.
  • A thinner vertical spinning weapon is more vulnerable to damage from horizontal spinner strikes.

Q: About how large a brushless motor do I need for a beetle spinner weapon? [Cincinnati, Ohio]

A: [Mark J.] Specifics will depend on your design, but popular beetleweight spinner weapon motors run about 150 watts per pound of robot and about 6% of the total robot weight. For a beetle, that typically translates to outrunner motors in 28mm or 36mm diameters.

Q: Weapon BLDC motor: 6% of total mass; this rule applicable to FW and LW weight classes? [Paris, France]

A: [Mark J.] The guideline (not a rule) was specific for beetleweights, and extrapolating to larger robots is non-linear. The Square-Cube Law requires larger robots to devote a greater percentage of their mass to structural elements, leaving less for other components. As robot weight increases you find proportionally smaller spinner motors and those motors are pumping out less power per unit weight because they are also subject to square-cube issues for structure and heat dissipation. A heavyweight spinner motor might typically produce 50 watts per pound of robot at about 3% of the robot mass. Fitting this to a log curve gives the chart shown -- but it's only a guideline.

Current successful beetle spinners have brushless weapon motors that cluster around 6% of the robot weight -- but you can't run that backwards to say that any motor that is 6% of robot weight will make a good beetle spinner motor. Different BLDC motor versions of the same size and mass will have quite different performance figures. Combined with the '150 watts per pound' guideline it's a quick check to see if you're in the ballpark. If you're considering a beetle weapon motor that weighs 3% or 12% of your total weight allowance you'll want to make sure you have good reason to use it.


Q: Do you have any general suggestion for a good saw for a 6 lb vertical saw? [Reading, Pennsylvania]

A: [Mark J.] Weapon design doesn't start with the hardware. First consider the function and execution; what will the saw do and how do you plan to do it?

  • Will you attempt to cut into your opponent, or will you snag and toss?
  • Do you want to have a saw in a fixed mounting, or will it be on a moveable arm?
  • Is the saw your primary weapon, or is it secondary to a clamp or dustpan?
  • Are you building a great big weapon to shove around the arena, or a weapon balanced with a strong/fast chassis?
Tell me about your weapon design and I'll try to recommend some hardware.

Q: It will cut, not snag and toss. It will be on a fixed mounting. The saw is the primary weapon, and has a wedge leading up to it. The blade will be balanced with a strong and fast chassis. Can you recommend a weapon motor with this info too?

A: That's a tough combination. The blade will take hard impacts from front and side, which leaves out ceramic cutoff wheels. You'll have no control over the 'feed rate', so a toothed blade is likely to jam. Blades of a size suited to a primary Mantis weapon are also uncommon.

My choice would be a Dremel Saw-Max 3" multi-purpose carbide blade. It's metal, reasonably sized, and uses carbide grit instead of teeth. It will cut thru a variety of materials, and make pretty sparks against metal too tough to slice. I think it's probably your best option, given your weapon design.

Motor recommendation I hope you don't plan to direct-drive a saw blade off an unmodified outrunner and go crashing into things with it. If you're going direct-drive you're going to need very serious modifications, similar to the rework required for a motor-in-drum design.

Direct-drive or indirect, you'll want about 700 watts of motor power and 12K to 16K RPM blade speed. Something like the Turnigy Aerodrive SK3 3542-1000kv would be about right.

Q: I am the 6 lb saw guy and... That blade allegedly can’t cut plywood! I think the motor choice is good, and i’m going indirect drive.

A: It's true; if your opponent is made out of eight feet of 3/4" plywood you're screwed. There are much better blades for that purpose. But assuming that...

  • Your opponent is made out of more conventional combat robot materials; and
  • Your design requires that you have to chew on whatever part of your opponent gets jammed into your blade;
...your best overall choice IMHO is a metal blade with carbide grit. You can thank me later.
Q: Greetings! I asked a question here awhile ago about downforce/suction system ideas for a sumo bot. I never really got around to testing anything, because I've had near perfect success with the wedge I already had (funny how these things work).

Anyway, my new question is about a saw attachment for a beetleweight. I'm bolting a servo-powered arm to my weird modular beetle-thing, and I've got one rather important question left before I can complete this. Should I go brushless or brushed for a saw weapon motor? I've never used an active weapon more complex than a servo before, so I'm not sure where to start. I know that with a brushed motor I'll get more torque on start up, that seems like it'd be good to prevent stalling the blade. I also know that I could control it with just a simple relay, leaving the throttle channel free to control the arm. However, brushless is obviously lighter and faster, and the more obvious choice for conventional spinners. How should I approach this?

Info:

  • The 'saw' is a 4-inch diameter cut-off disc (couldn’t find a real sawblade).
  • Will have a 3s battery with enough capacity to run a spinner.
I have enough weight for either option. [Toronto, Canada]

A: [Mark J.] This is a very big step in complexity from a simple servo weapon, and there are a lot of pitfalls. A saw is very different from a conventional 'impact' spinner:

  • An impact weapon stores energy in the rotational inertia of the spinning mass and releases that energy in one burst when it hits the opponent, greatly multiplying the continuous power output of the weapon motor.
  • A saw weapon must continuously supply power to overcome the frictional drag of the cutting process. That requires MUCH greater power output than spinning up an impact weapon for a single hit.
You have no real option except brushless motor power. No brushed motor of reasonable size is going to have nearly enough power to operate a saw blade.

Some other design considerations:

  • A motor and saw blade on the end of a simple arm directly attached to a servo forms a lever that greatly multiplies force placed on the blade end and focuses that force on your servo. The servo isn't going to be able to exert enough force to apply cutting pressure on the blade, and the blade is going to kick back enough force to destroy the servo. At very least your saw arm should be strongly hinge-mounted to your chassis, and your servo should attach to the arm via a link and servo-saver to isolate it from side forces -- a simple 4-bar linkage.
  • Some event organizers (the smart ones) do not allow ceramic cut-off discs. They produce a cloud of abrasive dust in operation and have a tendency to shatter when subjected to side-loading from a moving target. Arena debris is bad enough without adding grit and shards.
  • You're not going to have enough power to effectively operate a 4" blade on a beetle. There are small, high-speed, fine-toothed metal blades available that would be much better for your purpose: like this Proxxon 2" 100-Tooth circular blade.
  • Since you made no reference to a belt drive system I assume you're planning direct-drive from the motor to the saw blade hub. You'll want to keep as much of the load created by the saw on the more strongly supported bearing nearest the motor mounting. Do not hang the whole assembly off the end of the motor farthest from the mount!
  • A brushless motor requires a brushless motor controller, but that controller can be assigned to a transmitter channel activated by a switch for simple on/off control -- just as your brushed motor relay could be switch controlled. Your 'throttle' stick can still be free to control the servo.
Saws are cool, but they're more of a show-off weapon than a competitive design. I'll strongly suggest that you consider another weapon design for your first spinner weapon. If you do build it, pack extra weapon motors and servos -- you're gonna need 'em.

Q: Thanks for the help so far! I am happy to take a leap in complexity if it results in a cooler bot, putting on a good show is my #1 priority.

It took some searching, but I was able to find small, fine-tooth saw blades in my area for a reasonable price. They're all listed as 'slitting saws', and seem to be a perfect choice for this. Good to know I don't need to use that cut-off disc.

I forgot to mention the belt system I was planning on using to drive the blade. I didn't even know direct drive was an acceptable option, but it's certainly one I'll consider if (when) weight gets too tight.

A: I'm not sure I'd call it 'acceptable', but direct drive weaponry is fairly common in insect class 'bots. A nice belt drive with a little 'slip' will repay itself in performance and motor longevity.

Q: For controlling a brushless motor via a switch, it's as simple as just plugging the ESC into the correct channel, right? That is a huge relief if so.

A: If you're running a single direction 'forward-only' controller, all you need to do is plug it into a switched auxiliary channel.

  • The 'off' setting on the switch sends the same signal to the controller as a throttle stick pulled all the way down.
  • The 'on' setting on the switch sends the same signal to the controller as a throttle stick pushed all the way up.

Q: Hi, I'm designing a mantis weight vertical spinner using a pair of fairly low kv outrunners (run at around 15v, 330kv) I had lying around and have been using your spinner calculator to figure out the kinetic energy of the weapon I had designed. I have settled on an 85mm diameter by 10mm thick steel single toothed disk as the weapon but am struggling with figuring out the right RPM to run it at.

I can get more than enough KE running the disk on a 1 to 1 ratio where it spins at about 5000 rpm, and I know crazy high rpm is not good, but looking around I have seen other small robots with crazy fast spinners and so was wondering if 5000 rpm is an OK speed to run at, or if the faster spinners know something I don't. I know this isn't a design service but I was hoping if you could tell me if I was in the right ball park. [New York, New York]

A: [Mark J.] I'll assume that you've read our Spinner Weapon Design FAQ and plugged your weapon design numbers into the 'Bite Calculator' therein. Your weapon speed is actually conservative for an STD insect spinner. The calculator says that with a 5 MPH closing rate you'll have more than an inch of 'bite' -- crazy good! If you're happy with the energy storage and spin-up times, you'll be fine. Have fun at 'Bot Blast'.


Q: I know this will vary greatly due to design, but for a 1lb drumbot what's a good amount of KE to shoot for? [Charlotte, North Carolina]

A: [Mark J.] "How much KE" is the first question at the top of our Spinner Weapon FAQ. Read the rest of that FAQ while you're there.


Q: Hello Mark!

Does LEM 130s make good weapon motors for a heavyweight overhead spinner?

and how's does their power compare to an ETEK-R? [a suspicious server in San Jose, California - at 4:46 AM local time]

A: [Mark J.] The LEM 130-95 or 95S are not generally used for heavyweight spinners. They are about 1/4th the weight and power of the ETEK-R (aka Motenergy ME0708), cost 3 times as much, and are considered by builders to be more fragile.

Ray Billings has tried all the big pancake motors in his spinners; his current choice for a balance of power, durability, and cost is the ETEK-R. If Ray likes it, I like it.


Q: Heyo! I had another quick question: I noticed in a video with the UK featherweight flipper Legion that they put the cables for their flipper on a small hinged arm. Why? Is there any sort of advantage when compared to putting it on a different part? [Champaign, Illinois]

A: [Mark J.] I'm unable to find photos or videos showing the weapon structure you're referencing, Champaign. Perhaps you'd be kind enough to share a link?

Q: Hey, it's me again! I found the video where I saw Legion's Flipper! You can see it at 1:58 when it lowers and you can see it again better at 2:04.

A: Legion's flipper arm is not attached to the pneumatic ram (visible in picture 1). I assume that the 'cable' (seen in picture 2) is an elastic bungee cord that passively pulls the flipper arm down to assist the ram's retraction. The 'small hinged arm' it wraps around is the bottom stop for the flipper arm -- it does not appear to be hinged.

The bungee must continue around the underside of the 'bot and anchor near the rear of the 'bot, allowing enough length to apply a retraction force for the flipper arm all the way down to the stop. Attaching the bungee far forward on the flipper arm gives it a leverage advantage to pull the ram downward and to hold it down against bouncing.


Q: Is there any history of horizontal spinner bots, large or small, using a slip clutch on the weapon bar to minimize stress on the motor/axle? It seems to me that *some* rotational slippage of the weapon would be acceptable if not even desirable, as during an impact the energy released is built up from the momentum of the spinning bar itself rather than the force being immediately supplied by the motor.

I was thinking of something maybe even like maybe a rubber tube that slides over the axle then a clamping hub is mounted around that, the rubber might provide some give. No ideas set in stone right now, just shopping around possible ideas. [Ballwin, Missouri]

A: [Mark J.] Some form of slip clutch is very common in all types of spinner weaponry, but it's rarely located at the weapon/shaft interface. Radial and axial impact loading at that location is much greater than the torsional loading on the axle -- great enough to destroy most clutch designs.

The most common way to get some impact slip protection for the motor is to use a round or v-belt drive to the weapon. The inclusion of a belt tension mechanism allows for adjustment of the amount of slippage: tight enough to keep the spin-up quick, loose enough to slip on a big impact.

Larger 'bots may use slip clutches from industrial equipment or a clutch of the builder's own design. I've used automotive clutch material squashed between jumbo washers with lock nuts and spring washers. Other builders have tried clamping hubs on finely polished shafts with collars above and below for location. Lots of designs are possible. A web search for 'torque limiter design' may give you some ideas.

Q: I think the torque limiter might be too complex (read: heavy) for my application (antweight). I guess that’s not really a question so much as a statement, so here’s my question, what sort of slip mechanism if any would scale down well to my weight class?

A: You did ask about "large or small" applications. If you'd specified 'antweight' I could have tailored a better response.

Insect class weapons typically isolate their motors from impact shock with belt drives. Custom pulleys used with round belts are effective at power transmission with a good slip response when the weapon is abruptly decelerated on impact. See the photo of 'Silent Spring' above. However, the small motors used in antweights are not generally sensitive to deceleration. Although they do not have much 'slip' capability, weapon drives with commercially available timing belts and toothed pulleys adequately protect antweight motors from impact shock.

If you're thinking about a direct drive weapon, the radial and axial impact loadings are a much greater concern than the torsional loading a slip clutch will deal with. Extreme reconstruction of the motor to include a much larger shaft and bearings is required for such an application. See this post in the Ask Aaron Robot Weapons archive for an overview of the preparation required. You may also search the Ask Aaron Ants, Beetles, and Fairies archive for "direct-drive" to find other posts on the topic.


Q: Heyo, guess who's back again! This time around, I had several questions regarding a post I made a while back about a featherweight FBS. I had a few more questions that came to mind as I came up with ideas for the design:
  1. I've noticed that there are usually two kinds of full-body spinners: those that are low and flat, like Capt Shrederator, and those that are big and square-ish, like Megabyte and Gigabyte. What pros and cons come with each of those designs or, more specifically, those shapes?
  2. Flat vs Angled sides: which is better to have and why?
  3. Why are "Tuna Can Spinners" like Mauler's Season 3.0 appearance not very common or popular anymore?
Thanks again for your help and advice! [I-74 near the Illinois-Indiana border]

A: [Mark J.] It will make more sense if I answer your questions in a different order than you asked them:

  • The tall and flat-rimmed 'tuna can' spinner design as used by the 'Mauler' series of robots encountered stability problems as energy storage levels rose to higher levels. 'Mauler 51-50' in particular suffered from instability caused by sensitivity of the shell to "polhode motion" that caused the entire 'bot to flip up on edge! With ever-higher levels of energy storage in rotary weaponry, the tall 'tuna can' is no longer a viable design.
  • To make a full-body spinner more stable you can reduce the height of the cylindrical shell and thicken the outer wall to make it more compact and less flexible while maintaining the same spinning mass. This is the approach the 'Shredderator' series takes -- but it is difficult to cram a weapon shell support and drive system into a short structure. Because of this difficulty many builders take a different approach...
  • Going to a domed/conical/slope-sided shell gives a more rigid and stable mass distribution while still allowing ample room for weapon motor(s) and a tall, strong, well supported weapon axle. 'Gigabyte' falls into this design class.

Q: Why are ramming spikes so bad? [Richmond, Virginia]

A: [Mark J.] What exactly do you expect a spike to accomplish? Unless your opponent is made of balloons, a pointed metal spike traveling at 'bot speed won't even scratch their armor.

Even worse, spikes help your opponent to beat you by giving their wedge a spot to slip under your bot and/or handing their spinner weapon an easy-to-grab target. If you're unlucky you'll get the spike stuck in a wooden arena bumper and be a sitting duck. Leave them off!


Q: Hi Mark.

Most of us have heard that if two vertical spinners go weapon to weapon, the faster tip speed of the two will usually win that exchange. However, I've recently heard a counter-argument that if one of those weapons has a much higher RPM, but slower tip speed (example: Drum vs Vertical disk) it might actually have an edge over the faster weapon. Is there any validity to this claim? Are there other exceptions to the "faster weapon wins" rule?

Thanks, David [Livermore, CA]

A: [Mark J.] I've heard that same theory. It's based on the statistical likelihood that the weapon with the higher RPM (but lower tip speed) may land the first impactor. While true, that impactor will land on a surface that is moving upward faster than it is, so the smaller weapon has no opportunity to dig in and 'throw' the larger weapon. The best that can be hoped for from such an impact is that the larger weapon may be 'bumped' up and back by the glancing blow to a curved surface with little bite.

Take a look at this post for a discussion of deciding factors when dissimilar verticals go head-to-head.

I think it's safe to say that tip speed wins with similar sized weapons but adding in design variables like ramps and major size variation clouds that advantage.

Example a large vertical spinner has a disadvantage to a small drum if the big vert has no ramp to elevate the smaller drum weapon up into the 'upsweep zone'. The big vert will likely strike the smaller drum without much upward vector to the impact. This results in an impact that will throw both 'bots back and away from each other. The smaller weaponed 'bot will likely come out of this type of confrontation better than the taller and less stable big vert. It may look as if the small drum won the impact, but it's actually just the big vert's unfortunate reaction to its own hit vector.


Q: I'm very curious as to what actually goes into hits as far as transferring energy into the other team's chassis without self destructing yourself. I have been playing with vertical spinners for the last few years and I still have alot to learn with this.

When I first joined [the team] our director essentially told us that a higher moment of inertia [MoI] is the key to the kingdom as far as making a weapon that hits hard. In the past few years I've played with making high moment of inertia weapons to try to hit hard but never quite getting the results I want. I only really seem to hit hard and 'out' and send way too much shock back into my own chassis and end up bending my weapon shaft and chassis considerably. I've seen several bots with tiny weapons like 'Knockout' and 'Disko' hit ridiculously hard and almost directly vertical. As far as throwing 'up' goes, they tend to roof the other teams quite often and never seem to wreck themselves too much.

Is there something simple I'm missing? Is there a way to graph bite, RPM, and moment of inertia to find the optimal setup? Is there any help with this in general? [Warren, Pennsylvania]

A: [Mark J.] We've discussed the elements of your questions previously, but they're scattered broadly around the 'Design and Construction' and 'Robot Weapons' archives. Let's see if I can pull those prior posts together to get your answer in one place.

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when your kinetic energy weapon strikes your opponent the impact on your weapon and its support structure is as great as the impact on your opponent. There isn't any escaping this 'kickback' force: your weapon has to be strong enough to withstand this impact. The advantage you have is that you know the place and direction of this force, while your opponent may have to take this force pretty much anywhere.

If your weapon and shaft are bending they simply aren't strong enough.

Hitting 'up' versus 'out' is a function of where in the weapon arc it strikes your opponent. A large diameter vertical weapon will tend to strike the opponent low in the upsweep zone and will hit more 'out' than 'up. This is why you see so many vertical spinners with a wedge/fork ramp leading up to the weapon. By elevating your opponent as they approach your weapon you will get more of an 'up' angle on your hit and as a bonus will often get greatly improved 'bite' by exposing a beautiful sharp edge at the underside edge of your opponent.

There is no magic equation to balance weapon speed, bite, and MoI because the optimal balance of RPM and MoI is entirely situational.

  • Against an opponent with nice sharp edges for your weapon to dig into you will want high weapon speed to build maximum energy storage because the importance of 'bite' and MoI are greatly reduced.
  • Conversely, facing a carefully smoothed opponent that presents only gently curved surfaces (like any of Russ Barrows' "Dark" series of robots) will require maximum 'bite' and a high MoI to compensate for the reduced weapon speed.
If you aren't getting good hits you'll need to examine what it is about the design of your opponents that is preventing your attack from being successful. Adjust your weapon and attack strategy to adapt to their design. Remember, a faster closing rate on your opponent increases 'bite', so don't be timid in your charge toward them.

If you haven't read thru our Spinner Weapon FAQ you will be interested in doing so. There are some more detailed expansions on the topics I've mentioned above. You may also wish to read thru section 6.5.2 of the RioBotz Combat Tutorial.


Q: So I was wondering how they make flames shoot out huge bursts of flame like Shamen did but there would not be a lot of room for lots of flame? ty! [Lynn, Massachusetts]

A: [Mark J.] We answer no flamethrower questions here at Ask Aaron. See Frequently Asked Questions #28.


Q: Looking at the previous questions and answers, what are your thoughts on bots like Brutality in the modern era Mark? Recently just watched a match of it with Last Rites back in the late 2000s, the wedge seemed to have made it under Ray quite easily. [San Jose, California]

A: [Mark J.] "Bot's like Brutality"... meaning top bar spinners? Top spinners (like tradional bar spinners and full-body spinners) are a design that is very effective if you get everything right. People remember the successes of specific examples of these designs ('Hazard', 'Last Rites', 'Ziggo'...) but overlook the myriad copies that failed miserably. None of these designs are as simple to get right as they appear.

More specifically to your question, I think the 2007 ComBots Cup fight between 'Last Rites' and 'Brutality' marked a turning point in weapon design. It became obvious that long and relatively thin weapon blades like Brutality's were subject to breakage from the 'modern era' hyper-energy thick and robust weaponry; compact fast-spinning single-toothed drums/drumettes like 'Touro' and big bars like Last Rites/Tombstone had a strong advantage.

If you specifically want to beat 'Last Rites/Tombstone' you would do well to take note of the design and tactics of the robot with the best record against Ray's big spinners: 'Original Sin' has won 6 of its last 7 matches vs. 'Last Rites' and is 12 of 19 overall against the big bar spinner. Simple robots win.


Q: Hello Mark.

Our team is currently designing a heavyweight bot with a Hazard/Brutality style overhead bar spinner. The bars will likely be between 75-90 lbs ranging from different materials depending on matches.

Right now we are looking to drive the weapon with four 3" long mags, or four 3" AmpFlow a28-400 motors, with Whyachi M3R2 right-angle gearboxes joining the two motors and two belt/chain drives to join the two pairs. This was designed so we could keep our bot chassis rather low to the ground for an overhead spinner, hopefully keeping most components in a 4-5" height constraint excluding the bar itself, instead of using the big and bulky ETEKs.

Are there any disadvantages or advantages to this setup other than parts costs? We have heard from builders that describe Ampflow motors as inconsistent and unreliable.

Are there any better alternatives to this? Two or three LEM-130s perhaps?

And are there any sufficient ways to synchronize the motors for these kind of setups? [New York, New York]

A: [Mark J.] Have you noticed that Team Whyachi isn't using their own gearboxes to power their weapons? Take a look at this recent post about the 2016 version of 'Son of Whyachi'. Their weapon drive solution meets your design goals with much less weight, complexity, and expense. Team Whyachi has been running a common spur gear weapon drive across multiple design revisions with good results.

AmpFlows/Magmotors become unreliable when loaded heavily for prolonged periods or overvolted on robot drive trains or lifter weapons. Their brushes are not up to sustained operation at very high amperage. They are quite reliable when running at nominal voltage with reasonable gear reduction in a spinner weapon.

All the 'cool kids' are running brushless weapon motors, but that's a high risk power solution. I consider brushed motors to be a very viable weapons drive option in heavy robots. If SoW can spin their huge weapon with eight Mini Mags I suspect that you could get away nicely with a bit less power -- like a couple LEM-130s.

I know it's a bit counterintuitive, but you really don't need to worry about motor synchronization with multiple similar motors driving a common load. The math is more than I want to write out on a nice spring day like this, but buy me a beer the next time I'm in New York and I'll explain. In the meantime, don't sweat it.


Q: Hey, Mark! Just had a quick question: I noticed a lot of eggbeaters don't have leading wedges and instead use flat fronts, such as 'Bigly' and 'Conker 3'. Is there an advantage to having a flat front on an eggbeater? Are there any drawbacks? [Champaign, Illinois]

A: [Mark J.] Take a closer look. Those 'flat-front' beaters actually slope back underneath -- kind of a 'negative wedge'. It is deliberate, and it's done to take advantage of the huge 'bite' available from a large diameter beater bar.

  • A fast-spinning mini-disc with small 'bite' has to play the 'lowest wedge' game to get under their opponent and find an edge to grab.
  • A beater can store the same level of energy as a disc of the same mass while spinning much slower. That gives it enough bite to be effective against flat surfaces.
The attack plan is to ride up the opponent's wedge and beat the stuffing out of whatever it finds up there. If there is no wedge, just hit whatever you run into. It often works quite well.

Disadvantage Going weapon-to-weapon against a smaller diameter drum with a faster tip speed is pretty much instant death.


Q: So I heard "Wrecks" uses his weapon to power his legs. How does this work? Ty! :) [M] [Lynn. Massachusetts]

A: [Mark J.] You don't have that quite right. 'Wrecks' is a 'precessional walker' -- its legs aren't powered at all, but the weapon still makes it walk. See this post in the Ask Aaron Design archive for a description of the principle.


Q: What weapon esc would be appropriate for a a28 150 ampflow motor running an ar400 8 inch diameter 2 inch thick disc at 24 volts with a 2:1 chain reduction? [Philadelphia, Pennsylvania]

A: [Mark J.] Calculating ESC requirements for a spinner weapon is much different than finding a suitable drive train ESC. There is a strong surge of current at startup that falls away to a low maintenance current level. Attempts to use current limiting ESCs or adding in a 'servo slower' to the signal line can severely reduce spin-up performance, so you should seek out an ESC with a high surge capacity.

The first step is to model the performance of the weapon system to find out how quickly the start-up current surge lasts. The A28-150 AmpFlow has a stated armature resistance of 64 mΩ, which would give a stall current of 375 amps -- but there is additional resistance in the battery/wiring/ESC. Adding in 20 mΩ (my default 'guess' factor) for those other circuit elements brings the real-world 'stall' current for the A28-150 to about 285 amps @ 24 volts.

Plugging 84 mΩ into the Run Amok Excel Spinner Spreadsheet along with the other motor and weapon details (thanks for providing those) gives a chart of spin-up performance and the current draw. It looks like you've done your homework and you have a good balance of energy storage and spin-up time. Current draw drops from 285 amps at hard start to about 100 amps at the one-second mark.

There's a good ESC match for this level of current and voltage. The Robot Power Vyper has a 120 amp continuous rating and a 'greater than' 250 amp peak current rating. It's good to 36 volts, and has over temp protection. Size is 3.125" x 2.875" x 1.375" with a 185 gram weight. I think that's your winner.

The only potential problem I see is possible stalling of your weapon. Replacing your chain drive with a very slightly slippy v-belt might prevent a full-scale meltdown if your weapon stalls in combat and you're too busy to tend to that immediately.

There are other options for control of your weapon motor. I mention them here for completeness:

  • I mentioned current limiting and adding a 'servo slower' to the receiver signal line to bring the peak current down for a mid-range ESC, but either of these options lengthen spin-up time and waste the big torque of the AmpFlow. You might as well just swap in a less powerful motor.
  • You could go overkill on the ESC with a Vex Pro Victor BB. That will certainly do the job, but your pocketbook will ache.
  • You might consider a power solenoid/contactor for simple on/off weapon control. 'Spinner god' Ray Billings doesn't like solenoids with high-performance AmpFlows, but your quick spin-up time gets you out of huge amp draw quickly, so the motor should survive the big power dump - particularly if you switch to a v-belt with a little slip.
  • And then there's the old trick with four-brush AmpFlows: split the wiring for the two brush pairs and power each set with a separate ESC. Yes, it's a little Mickey Mouse and you can fry the ESCs if you don't know what you're doing. You might just get away with splitting the brushes and running two VEX pro Talon SRX ESCs. The Vyper is a better option, but some builders like to be crazy/different.

Q: If the amperage rating on the ESC isn't 'real' then how do you find the 'true' amperage rating?

A: There are too many design, component, and construction variables to get a 'true' number from a physical examination of an ESC, and you certainly can't rely on any given manufacturer's advertised ratings due to huge inconsistencies in how that number is determined. Your choices are:

  • Precision Buy one, instrument it, and test to failure.
  • Reputation Consult with builders who have used that ESC in applications similar to the one you have planned.
  • YOLO Stuff one in and see what happens.
The last two options are the most popular. The 'Vyper' is widely used in combat - particularly in the UK - and has a good reputation. I would not hesitate to use it in your application.

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Q: Would the ampflow a28 400 running a small disc be appropriate for a lightweight? If so, would the RageBridge 2 be a good weapon ESC? [Arlington, Virginia]

A: Several comments:

  • I cannot make a weapon motor recommendation based on a weight class and a general description of the weapon type. We offer a collection of design tools so that builders may perform their own evaluations of combat robot system. These tools include two different programs that will model the performance of specific spinner weapon systems. You will do well to learn to use these tools.
  • You will also benefit from reading our Spinner Weapon FAQ to gain an understanding of the principles of spinner weapon design and performance.
  • The RageBridge 2 is a dual-channel brushed controller designed to control drive motors on opposite sides if a tank-steer robot. The special features it offers are intended to support drive train applications. Using it to run a single weapon motor would be a waste of its features.
  • The A28-400 AmpFlow motor is a big, heavy brushed motor -- less than ideal for a 'small disc' on a lightweight robot. Current weapon design favors much lighter brushless motors that are well suited to spinner weapon applications.
So... I'm gonna say 'no' and 'no'. Stop guessing and do your design homework.
Q: Hey, man! It's been quite a while since I last posted here, huh? Welp, I'm back with a bit of a featherweight conundrum. To give you the long story short [You call this 'short'?] I went to EOH [the Engineering Open House] at the U of I [University of Illinois] in March and watched the featherweight competition [Robobrawl] they had. After doing so, I've been thinking about getting a team together to compete for next year. However, there's a catch: I don't want to pitch anything at them, guns blazing: I want to have most, if not, all of the specs down and ready to present to a group of people as a document and a presentation (if needed) in order to heavily streamline work or maybe even get it done before the competition (seeing that the next competition isn't until March 2019, I can say I've got a bit of time on my hands). Not only that, but it will help our team get the money to participate both through iRobotics [website] (which gives a starting budget of $2500) and through local, smaller businesses that may want to help out.

My idea is to make a featherweight full-body spinner similar in shape to that of 'Ziggo' (the idea was that it would be themed around a land mine) or the only FBS that was a featherweight I could recall off of the top of my head: 'Badger' (from Australia).

There are five things I came here to ask, which I will lay out for you below:

 
First What would you recommend for weight distribution? Given that a FBS has the ability to add more weight to its weapon because the weapon is also the armor, what percentage should I try to aim for? Following along the 30-30-25-15 rule, it would result in the robot being able to use up to 55% of its weight towards the weapon and armor. However, that would mean the shell would weigh in at 16.5lbs, which I fear would be WAY too much for the robot. Using only 30% seems to small for a FBS because that means the shell weighs in at only 9lbs. The largest spinner at EOH had a bar made of S7 Steel that was 8" in diameter and weighted in at 8lbs, and since I assume a full-body spinner would be a bit larger than a bar, that 9lbs would be a bit on the weak side. 40%-45% would put it at 12 and 13.5lbs, respectively, and I feel that those numbers are better than the other ones mentioned. However, I still wanted to ask you in advance on what you thought I should do.
Second I was trying to calculate the minimum amount of energy storage the robot should be able to hold. Going off of your statement that for every Kg of mass, the robot should be able to store 60j of energy, I crunched the numbers for the approximate weight in Kg of the robot: 13.6, 13.63, and 13.7Kg, respectively. I found that the ABSOLUTE MINIMUM (without multiplying by 2, as you recommend) would be 816j and the ABSOLUTE MAXIMUM would be 1644j. Here is the question that I have about these numbers: if my robot is, for example, storing 1644j in its shell, will the amount of force itself affect driving capability or would it have more to do with the shape and mass of the robot? Also, do you think 1644j is too much or too little for a featherweight FBS?
Third I was wondering what material you'd recommend to use. From what I was told, the main building block of robots that compete at EOH is Aluminum of a wide variety of grades. I was thinking to use something along the lines of either 6061-T6 Aluminum, 5083-H131 Aluminum, or 7075-T6 Aluminum. Which grade of Aluminum do you think would be the best bet? If, by some chance, we were able to stretch our budget and were able to buy Titanium or had enough weight to go for Steel rather than Aluminum, which grade(s) would you recommend?
Fourth Do you think it would be better to have bolted or welded teeth? I know this would vary depending on what material was chosen for the shell, but I'm afraid of having teeth that constantly need bent back into shape. I know the newest iteration of Captain Shrederator uses bolted teeth, while Megabyte uses welded teeth. Under what circumstances should I use one or the other, and which one do you think I should go for?
Finally How should testing be done? I'd want to be able to test driving with the robot both with and without an active weapon, but I have no clue what kind of area would be suitable for a 30 pound FBS.

I know that that's a lot of info, and some of the info is missing because an exact size hasn't been chosen yet, but I hope you can help me out! As always, if there's anything else you would recommend I am always happy to hear your advice! Thanks again for the advice and all of your help! [Champaign, Illinois]

A: [Mark J.] New question length record - 847 words I think I can answer your questions more concisely.

The featherweight FBS that you should 'recall off the top of your head' and emulate should be:

Combat Robot Hall of Fame - Honorable Mention 2015

Tetanus / Triggo - Team Brain Damage owns the featherweight class in the northeast US. Full-body spinner 'Tetanus' went 21-2 with five titles before being updated with a stronger shell, renamed 'Triggo', and adding on a 20-6 record and three more titles to date.

Go to the Team Brain Damage website and read all three of the build logs for 'Tetanus'. Zac understands FBS design very well and his build logs will answer most of your questions. Update - the links to the 'Tetanus' build logs on Zac's website are down, and they are not available on the Internet Archive. I've written to Zac to see if he has that info available someplace. Stay tuned.
  1. Full Body Spinners commonly place 50% or more of their weight into the weapon shell and drive system. It isn't too much. If you're gonna go for a big weapon you should go REALLY BIG!
  2. FBS are all about energy storage. I would say that 1600 joules for a feather FBS is conservative. Keep the center of gravity low and you should have no driving problems.
  3. Shell material is not critical - 'Ziggo' used a modified cooking pot. For a feather I'd suggest making it easy on yourself and use steel; chromoly is tough, widely available, and inexpensive.
  4. At this energy level you can get by with welded teeth if using a steel shell. For an aluminum shell, recess and bolt steel teeth thru the shell to a backing plate.
  5. Safety first! Check with iRobotics to see if they make their Robobrawl arena or another facility available for testing -- they should.
  6. Zac's first FBS was unsuccessful. Most FBS have losing records -- they are not an easy style of 'bot to build. I don't recommend an FBS for your first move into heavier weight classes.
  7. Landmines are not amusing. They maim and kill innocents. Pick another theme.

Q: So I had a simple (or I think simple) idea of a bot that had a rotating spike. Is this a good design or does it need some work? By the way ty for wandering all of my questions! :) [Lynn, Massachusetts]

A: [Mark J.] I'm certain you have a clear idea in your mind of 'a bot with a rotating spike', but your description leaves too much to the imagination for me to share in your idea. Read: The Hamburger is Bad.

Q: Heres a cad of the idea. The spike is the spike that spins. Is there anything you would add to make it better? Thanks! :)

A: So... it's a spike... on the front of the robot... that spins like a drill bit.

I don't understand what you expect the spike to accomplish, and I don't understand why you think spinning the spike helps. The spike WILL help wedges to slip under your bot, the spike point IS highly vulnerable spinner bait, and a sharpened point - spinning or not - will do no damage to any 'bot armor.

My suggestion to make the 'bot better: remove the spike and rotating mechanism and use that weight to make a stronger wedge.

Here's the cheerleader button if you need to press it:


Q: Hi Mark,

I was doing calculations for our competition and was wondering about the relationship between bite and energy storage. What is the tipping point for bite to energy storage? For our 5 inch radius spinner we can spin it to 16,000 rpm with a bite of 5 mm at 12,000 joules stored. We can also spin it to 8,000 rpm with a bite of 11 mm with 3,500 joules stored. At what point is bite more important than energy storage, and in what situations? Does this change for bot designs and weight classes?

Many Thanks. [Pittsburgh, Pennsylvania]

A: [Mark J.] Take a look at the 'Rotational Speed' section of the Ask Aaron Spinner FAQ. The 'bite' required is highly situational, and it does depend on your weapon design, your opponent's design, and the influence of the strategy employed by both you and your opponent as it effects the true maximum closing rate of the 'bots. As it says there:

There is no single 'right balance' of bite and speed for a given weapon. The balance is situational:
  • Fighting a hard-surfaced opponent with no sharp edges calls for all the bite you can muster.
  • A drum head-to-head against another drum requires maximum speed and can dispense with bite.
  • Small arenas and close fighting call for big bite, while larger arenas and higher closing speeds need less.
  • When your opponent has only soft exposed surfaces it may be better to ignore 'bite' and switch to sharp 'shred'.
  • If you have an effective ramp to help your vertical spinner get a shot at your opponent's sharp front under-edge you need very little bite.
Stay flexible. Design for ample bite and adequate energy storage, keep some extra RPM available for special cases, and be prepared to throttle back your weapon if it's just 'skittering' across the hard surface of your opponent. Consider swapping in a sharp edge blade for 'soft' opponents.
You'll also want to read this post farther down in this archive that discusses design effects and special situations for 'bite' adjustment.
Q: Does the shape of the bar on a bar spinner make any difference? Thanks! :) [Lynn, Massachusetts]

A: [Mark J.] Yes, shape and size make a huge difference in spinner weapon performance. Read the Ask Aaron Spinner Weapon FAQ and then play with the Spinner Weapon Kinetic Energy Calculator. You might also be interested in the extended discussion I had with a builder designing a large bar spinner: Texas Bar Spinner.



'Original Sin'

Q: So you said simple robots win. What are some examples of simple robots that did well? Heres one I like... [Lynn, Massachusetts]

A: [Mark J.] 'Tombstone' is likely the simplest 'bot at the BattleBots reboot and it has the best record at those events -- but the producers of the show only accept complex robots with big weapons that generate a lot of destruction for the cameras. Truly simple robots aren't allowed to compete, so this can't really be considered 'open' competition.

The 250-pound version of 'Tombstone' does very well against the artificially complex robots at the new BattleBots: a 91% match win rate! However, Ray Billings' other bar-spinner robots don't have the same success rate against more competitive 'real world' combat robots.

NameWeightWin Rate
Tombstone250 lbs91%
Tombstone340 lbs64%
Last Rites220 lbs61%
The Mortician120 lbs60%

Why not as good? Because simple robots are tougher competition than complex showboat 'bots. Which heavyweight robot has the best record against 'Last Rites'? That would be an even simpler robot: the fearsome wedgebot 'Original Sin'.

  • 64 wins and 15 losses overall (81% win rate);
  • 12 wins and 7 losses vs. Last Rites (63% win rate);
  • An unmatched seven RoboGames heavyweight titles;
  • An unmatched four ComBots Cup championships; and
  • Full membership in the Combat Robot Hall of Fame.

    Simple 'bots win


Note - 'Tombstone' is actually a deceptively simple robot. The design appears to be straightforward, yet numerous builders have tried to copy Ray's bots with very little competitive success. There is more to the Team Hard Core 'bots than meets the eye.
Addendum - 'Original Sin' just added another championship at the 2018 RoboGames. That makes eight!

Q: What if a bot had a hammer but the blade could split in half and turn into a spinner? Would this be a good idea? What are some of the pros and cons about this? By the way thanks for answering my Bronco pneumatics question! Thank you! :) (M) [Lynn, Massachusetts]

A: [Mark J.] You may not have thought this through. Perhaps I don't fully understand your design, but it makes very little sense to me.

  1. A conventional hammerbot swings an arm with a weight on the end in a half-circle arc from the rear of the 'bot to impact (hopefully) your opponent out in front. If you miss your opponent, the hammer hits the arena floor. How exactly do you split that hammer arm in half and have enough clearance for the resulting weapon to spin without hitting the floor?
  2. Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

    Design Philosophy

    A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

  3. The three primary types of active weapons have very different design priorities to accomplish their tasks. Compromising the weapon drive system to bridge the needs of two weapon types will result in a weapon that is not good at either task.
    • A spinner stores kinetic energy in rotational inertia that is transferred to the opponent on impact. That energy is accumulated by the weapon as it spins up to speed over some period of time. More energy storage is generally better, but there is a trade-off between increasing rotational speed and the ability to obtain enough 'bite' to effectively transfer that energy.
    • A hammer also stores kinetic energy as it accelerates from its resting position toward impact, but it has much less time-distance to accumulate that energy. It's nigh impossible to get levels of energy storage in a hammer weapon comparable to a spinner weapon. Get as much as you can and hope its enough. Heavyweight hammerbot 'ßeta' manages about 7 joules per kilogram.
    • A flipper does not store kinetic energy to impact the opponent. The flipper mechanism applies force generated from stored potential energy directly to the opponent to accelerate them upward. Measuring the 'static' force of the system does not equate to the net force applied while the system is in motion.
  4. Finally, do not overestimate the importance of the weapon system.

Aaron's Wisdom  I've said this often but builders don't want to believe me:

The weapon may be the least important system on a combat robot.

If you're not winning matches it isn't because you have a poor weapon.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: So I had an idea that I have no idea if it is good bad or dumb. A spinner that can change from vertical to horizontal. If this were to compete, what would its advantages and disadvantages be? Thanks!

A: Let me say this again:

Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

There have been many attempts at robots with multiple/interchangeable weapons, and very few have done well. You may be interested in reading up on 'Bombshell' -- a heavyweight robot designed around a modular interchangeable weapon set. 'Bombshell' was constructed by a large, well-financed, very experienced team -- this is not an undertaking for an inexperienced builder.

Build a simple robot, strong enough to hammer down a door. If a part breaks, make it stronger for the next fight. Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose.


Q: How does 'Bronco' make his pneumatic flipper so powerful? I heard pneumatics are normally slow and not that powerful. Thanks! :D (M) [Lynn, Massachusetts]

A: [Mark J.] You heard wrong. Inertia Labs is one of many teams on both sides of the Atlantic that have been building lightning quick and terrifying pneumatic flippers, hammers, and crushers for the last 20 years.

I'll warn you that performance like you see in 'Bronco' does not come from 'off-the-shelf' components. Most if not all of the pneumatic components in the twin Broncos (yes, there is a complete, ready to run, spare 'Bronco') are custom made at no small expense. Pneumatics are dangerous even if you know what you're doing. Don't try pneumatics on your first robot... or your second... and probably not your third.
 

Q: I intend to build an underweight beetleweight lifter. The mechanism at the moment is a 4 bar lifter powered by a servomotor. Judging by the T.I. tool, it appears that the front bar 'H' should be driven in a frontbar setup. Is it alright if I attach my servo to the rear bar 'F' though? [Paris, France]

A: [Mark J.] I understand your confusion.

Early versions of the T.i. Four-Bar Simulator were only capable of modeling lifter designs with powered front bars, and the diagrams all showed lifters with that layout. From version 2.0.6 onward the tool could model either front or rear powered bars, but the diagrams were not updated.

You may power either the front or rear bar, but in most cases it is preferable to power the shorter of the two 'upright' bars as that approach requires less torque. That's usually the rear bar.


Q: I'm presently designing a horizontal bar-spinner attachment for my fairyweight (150 gram) robot to better combat the vertical-disc weapons I'm beginning to see more frequently. I've run the numbers through the Run Amok spinning weapon calculator and they seem to line up reasonably well energy-to-weight wise, but want a second opinion seeing as this is the first time I've designed a serious spinning weapon. I have 55 grams of weight to make these modifications.

I'm presently powering it with an E-flite Park 250 motor:

Voltage Supplied: 7.4 volts
Speed: 16280 rpm
Kv: 2200 rpm/volt
Torque: 0.103 N·m
Ri: 250 milliohm
The bar specs are as follows:
101.6mm long x 20mm wide x 1mm thick Steel
Steel bar weighs 15.8 grams
Steel bar reaches 12 joules at 16,270 RPM in one second
The bar has no impactors or anything of note, it's just a solid rectangular chunk of metal. Is there any blatant rookie mistakes I'm making with the design of this blade? [Calgary, Alberta]

P.S: Love the Cheerleader button. I've definitely used quotes from it before when talking to prospective builders who don't want my advice

A: [Mark J.] I think you made a typo in your question as I get 21 joules rather than 12 joules, but your overall calculations are good. There are, however, a couple caveats:

  • The Run Amok Javascript Spinner Weapon KE Calculator makes a few simplifications in order to run on your laptop instead of a NASA supercomputer. The simplification in this case assumes spherical chickens in a vacuum -- that is, no aerodynamic drag on the weapon. At 16,270 RPM your weapon would have a tip speed very close to 200 MPH. The aero drag at that speed is well beyond what your adorable little outrunner motor could provide -- you're not gonna see that speed and your motor may bog down and pull a lot of amps trying.
  • You can get away with direct-drive on a small diameter fairy drum weapon at 16K RPM because the tip speed is reasonable and the impact will take place at a small radius from the motor shaft. The greater the weapon radius, the longer the lever arm the impact has to transfer bending force back to that 2mm weapon motor shaft. Take a look at this post in the Ask Aaron "Ants, Beetles, and Fairies" archive for a discussion of structural considerations on mounting motors for direct-drive weapons.
I like the 'horizontal vs. vertical' matchup, but a direct-drive horizontal fairy spinner has some serious design challenges.
Q: Hey, Aaron! I have a question this time regarding RPM and tip speed. For example, 'Hobgoblin' ran at 2500 RPM, but only had a tip speed of 90 MPH. At the same time, PP3D ran at 2500 RPM, but was much more powerful. Does RPM correlate to weapon [power], or does the weapon [power] correlate more with the weapon design and the motor used? [Decatur, Illinois]

A: [Mark J.] Many terms and measurements are applied to spinner weapons, but two measures are particularly important:

  • Moment of Inertia (MOI); and
  • Rotational speed (RPM).
Moment of Inertia is a measure of the resistance of the spinning mass to changes in speed. A weapon rotor with a high MOI will require more power to spin up to speed, but will deliver a more powerful 'hit' because it more forcefully resists attempts to slow or stop its rotation. MOI is calculated from the mass of the rotor and how far each bit of mass is located away from the center of rotation (weapon axle).

Weapon RPM should be obvious -- the faster something is moving, the greater the impact when something tries to stop it. What may not be obvious is that the relation between speed and impact strength is not linear; the kinetic (motion) energy of an object increases with the square of its speed. An object moving at 80 feet per second will strike with four times the impact of the same object moving at 40 feet per second.

Combining MOI and RPM gives the energy storage of a rotating mass, and the amount of energy stored by a weapon determines the amount of damage that weapon is capable of doing. Too much weapon speed can reduce the ability of the weapon to deliver a forceful blow because a very fast weapon may 'skitter' across the surface of your opponent without getting any 'bite' to transfer destructive force. Tip speed is generally only a factor when two similar spinner weapons attempt to strike 'weapon-to-weapon'.

The Ask Aaron Spinner Weapon FAQ covers all these topics in greater detail and offers links to calculators for MOI, bite, and tip speed. It also discusses proper motor selection for specific weapon designs. If you have an interest in spinner weapons I would suggest that you study this FAQ. An effective spinner weapon involves more than just getting something to spin.


Q: Hi Mark. Is there a practical way to determine a rough percentage of how much stored kinetic energy is actually being transferred to an opponent?

Thanks, David [Livermore, CA]

A: [Mark J.] I enjoy estimating energy transfer from the pitch and volume of my opponent's screams, but I suspect that you're looking for something more objective.

If your radio gear is set up for telemetry (a FlySky FS-i6, for example) you can add a compatible optical tachometer to read the RPM of your weapon from the transmitter. Convert the RPM of the weapon before and after a good 'hit' into stored energy levels; the difference in energy was transferred by the impact.

With a horizontal spinner a good part of that transferred energy may go into throwing your 'bot across the arena in reaction, but with a vertical spinner the greater portion of the energy should go to your opponent.


Q: Is there a calculator to determine if a bot will flip over based on the MOI of the weapon, the distance from the wheels to the weapon shaft, and the diameter of the wheels? Can you calculate how fast you would be able to turn based off the information from the spinner spreadsheet? Does overall width matter in stability of the weapon and in the turning ability of the bot - are wider bots more stable?

Many Thanks! [New Castle, Pennsylvania]

A: [Mark J.] Yes there is a calculator, but the variables used by the calculator are a little different. It will tell you how fast you can rotate without wheel lift, and if you know the weapon MOI you don't need the spinner spreadsheet. All the design factors are explained on the calculator page. So... where is this calculator? If your question starts with 'Is there a calculator...' your first stop should be the 'Combat Robot Design Tools from Team Run Amok' page:

...I realized that there was another tool that should be salvaged from the wreckage of the T.i. Combat Robotics site. Their 'Designing Around the Gyroscopic Effect' page has a full explanation of the forces at play when a vertical spinning weapon exerts a lifting force on a turning robot. That page is now safe here at RunAmok.tech.

After walking you thru the math, the page offers a simple javascript calculator that will model the stability of specific robot designs to let you know how serious the gyro effect will be on your planned robot. If you're designing a big drum or vertical disk/bar spinner you'll want to make use of this calculator to avoid unpleasant surprises in the behavior of the finished robot.

Follow the link above to our design tools page and scroll most of the way down the page to find the link to the 'Designing Around the Gyroscopic Effect' page. I'd give you the direct link here, but you'll benefit from looking over the other tools on that page.
Q: I noticed SOW uses eight Mini-Mags for its weapon. Is there any advantage to using eight Mini-Mags over, say one ETek or two Rotomax 150cc's? [Decatur, Illinois]

A: [Mark J.] There are advantages, but they aren't obvious.

  1. Team Whyachi has learned the benefits of a low body profile for overhead spinners. At 4" height, the Mini-Mags are shorter than the options you mentioned, and easier to 'package'.
  2. Team Whyachi has a long history with brushed motors. With a short BattleBots build schedule I suspect they didn't want to take time to sort out a cluster of unfamiliar small brushless motors.

Q: I've read thru the Team DaVinci pneumatics guide, but I've seen pneumatic flipper 'bots that have much different layouts.

  1. Some large 'bots have something called a 'QEV'. What is it, why is it there, and how does it work?

  2. I read a comment in an on-line forum about a 'bot that failed to pass safety inspection because they couldn't de-pressurize the pneumatic system without causing the weapon to fire. Huh? How is that possible? Was there some advantage to setting the system up like that?

  3. I've seen video of a 'full pressure' UK beetleweight flipper. There are very few pneumatic components and they don't look anything like what I've seen in larger 'bots. How do these systems work, and where can I buy the parts? [Voices in my head]

A: [Mark J.] My answers to this set of questions kept getting longer and longer, so I moved them onto their own page in the 'Team Run Amok Tips and Tricks' collection: Tips and Tricks for Robot Pneumatic Weapon Systems.


Q: What is the purpose of 'True Grit'? Its weapon seems dumb. [Williamsport, Pennsylvania]

A: [Mark J.] With all of the 'Wedge Industries' designs to choose from, you wanna pick on 'True Grit'? Not 'Cone Army'? Not 'Spongetron Roundwheels'? Not 'Pizza Party'?

'True Grit' is a featherweight 'Sportsman' class 'bot, currently ranked #8 by Botrank. You can't judge sportsman weaponry by open class standards:

"Another goal of this [Sportsman] class is to eliminate the high energy destructive spinning weapons and to encourage robots dedicating their weight allowance to more complex and creative weapons that typically cannot survive in the traditional weight classes. Weapons that have portions that are exterior to the robot envelope and rotate more than 360 degrees are limited to rotating at tip speed of less than 21ft/sec or storing no more than 200 joules of energy."

A 200 joule impactor spinning weapon by itself isn't going to do much damage against 30-pound opponents, so Alex Horne designed a set of electric lifter forks to work with the toothless (Alex calls it 'infinite toothed') sandpaper drum to form a lifter/flipper system.

How well does it work? Well enough to have a winning record, but sportsman class is more about fun than cut-throat competition and Alex seems to have ample fun with this design.


Q: I think the spinner spreadsheet might not be correct in its current ratings. I simulated the beetle weapon from Weta with a NTM-3536 1400KV and it says I'm drawing 700 amps peak at 4s. I got the Ri value from the Hobbyking website and cross checked it against a similar hacker motor so I'm pretty sure it's correct (or at least close-22mOhm). I got the MOI from the CAD model of the drum after applying materials to the bodies (~180,000 g-mm^2). Commutation max is 1024 and power max is 25%. It says it spins up (95%) in 0.28s. [Ontario, Canada]

A: [Mark J.] Thank you for your concern. Allow me to explain the current calculations used by the spreadsheet.

Ohm's Law is used to calculate current flow thru a conductor of known resistance. When power is first applied to a permanent magnet DC motor, the wire coils act as simple resistors. Entering the values you have provided into the equation gives the theoretical current flow:

Current = Voltage / Resistance = 14.8 volts / 0.022 Ohms = 673 Amperes

Does the motor actually pull that many amps? No, because:

  1. It is unlikely that your battery can supply that much current;
  2. There is resistance in the circuit in addition to the motor coils; and
  3. The brushless motor controller limits current at low RPM to prevent huge start-up current peaks.
Once the motor starts to rotate the interaction of the rotating magnetic field with the wire coils generates an electro-magnetic backforce that adds effective resistance to the coils. The faster the motor rotates, the greater the backforce. By the time the motor reaches full no-load RPM, the current will have dropped to just a few amps.

The Run Amok spinner spreadsheet does not perfectly model a spinner weapon, but it offers a good estimate based on a reasonable amount of input data. Real-world data has validated its performance estimates.

Now... why are you spinning that tiny little spinner with that big motor?

Q: That's interesting and makes sizing an appropriate ESC a weird question. Is there a good way to figure it out other than go with trial and error or intuition (or copying what others do for similar motors)?

That big motor is actually the recommended motor for the Weta drum. If you're doing it anyways, you might as well overdo it I guess.

A: ESC selection is always a problem and the output from the Spinner Spreadsheet isn't going to be any help in this area. In truth, the specifications given for motor controllers are generally fictional and meaningless. One might assume that a '60 amp' motor controller would be capable of continuously sourcing 60 amps of current at whatever voltage the controller is rated. Attempting to pull that current thru a random Chinese ESC labeled 60 amps will quickly give you a puff of smoke and charred remains (for reasons given previously here at 'Ask Aaron'). Yet the 60 amp ESC that comes with Weta kits will survive a momentary surge many times that large.

Let other builders fry controllers with testing. See what's actually working in similar designs and start there.


Q: Hello Mark,

Thank you for providing such a useful online resource for robot builders. It can be a challenge to find robot combat specific content sometimes.

I am trying to design a horizontal bar spinner that can be made predominantly with basic tools, manufacturing processes, and off the shelf components where possible (ie. drill press, water-cutting) for cost, reparability, etc. This is very early on in the piece, not a lot of calculations have been done yet. Any dimensions below are just for the sake of the discussion/question.

I think I want to use a chain drive for the weapon, in large part because off the shelf components are much more readily available, but I have a couple of questions regarding the implementation.

  1. Would it be reasonable to attempt to mount a sprocket to a brushless motor in a similar style to the riobotz DIY friction clutch (p.142 of riobotz manual) using a stock mounting fixture (pic at right)? I'm not sure how else to go about it before biting the bullet and getting something machined for purpose.

  2. In a system similar to 'Suitcase Nuke', is there a rule of thumb for clearance around the weapon chain? Currently it's sitting around 2cm above the bar and 0.5 - 1 from the top plate.

  3. To mount the sprocket on the weapon side of things I intend bolt (/permanently mount) the 2024 T3 bar (20mm thick, 60mm wide) to a round 2024 piece (20mm thick, diameter same as bar width) which the sprocket will mount to. This 'hub' would be drilled/watercut small and reamed to size as one piece then have a bronze bushing pressed in. UHMW spacers above and below to keep bar vertically in place.

I think it will work but I was wondering if I could get some feedback on the idea. The appeal is that it should be very simple to make, and avoid needing to get specific parts machined. Any help/feedback would be greatly appreciated.

Cheers, Matt [Australia]

A: [Mark J.] Hi, Matt! You didn't mention the weight class you're building. I'll assume from given dimensions and popular classes in Aus that you're going featherweight.

  1. I'm a believer in Trantorque keyless hubs for functional and robust off-the-shelf hub solutions. They expand their outer diameter and decrease their inner diameter when tightened to create in interference fit to the shaft and sprocket/pulley bore simultaneously. This is much more secure than clamping the sprocket between compressing plates in the manner of a prop mount.

    If you use the pictured mount I'd suggest drilling off-axis thru the sprocket and hub to add a hardened pin for rotational locking and use the prop-nut purely to hold the sprocket onto the shaft.

    Incidentally, I've had no success with DIY friction clutches like those depicted in RioBotz. I used a similar design to torque limit the 'sidewheeler' blade weapons for Robot Wars heavyweight 'Run Away'. Keeping a constant torque slip setting proved unworkable -- it rapidly loosened in operation in spite of my best efforts. If you want to limit torque in your system I'd recommend a simple V-belt instead of a chain drive.

  2. There is very little off-plane movement to roller chain. I think your clearance values will be fine.
  3. So... a dead shaft with a bronze bushing pressed into the aluminum blade/hub with a sprocket on top. Sure -- that works. Leave a lubrication hole to add a squirt of oil to the bushing once in a while. I'd be tempted to use axial thrust needle bearings in addition to the UHMW spacers -- UHMW is slick and will flex a bit to absorb axial shock, but the loadings could be great enough to cause binding.
Overall, I think your design is practical and appealing. Keep me up to date on your progress.

Q: Thank you for the quick response and feedback. Yes, this robot is for the featherweight class and will be my first combat robot. As a follow up to each part;

  1. I'll look into the keyless hubs, they look like a good option if/when I give up on trying to friction limit the system. Just to clarify, I assume you mean to make a key way in the prop-pin and sprocket when you suggested the off-axis hole and pin. My original plan was to use a v-belt but I found it quite difficult to find appropriate pulleys that didn't require significant modification to use. I was just considering that it may be an option to use some steel rod + plate to make a stronger mount (similar in shape to a prop-mount) using the same mounting points. At least there are some options to explore, thank you.

    I am probably reading too much into this but your responses seem to indicate that friction limiting is nice to have but not as necessary as I am currently thinking. Might be a good idea just to buy a brushless motor and test it to see what it can deal with.

  2. Thank you.

  3. I'll look into some needle bearings, it's a good idea. When I eventually get to making this robot I'm happy to test both just spacers & spacers + needle bearings against a concrete wall (in a safe and controlled environment of course) to see how they compare and let you know the results.
Anyway, thank you for your feedback. I'll be sure to send some updates your way although I expect it to be a fairly slow process. Cheers, Matt.

A: My suggestion about an 'off-axis hole and pin' was a poor-man's keyway and key substitute. I'm unable to guess at the shaft diameter of your prop mount, and a small shaft made of soft material (aluminum) isn't a good candidate for a keyway. A steel pin pressed deeply into the aluminum base has a better chance of surviving without weakening the shaft -- see sketch.

The reason for torque limiting in a spinner weapon is not to reduce stress on spin-up -- that can be done thru your motor controller firmware. The torque limiting comes into play when your weapon is abruptly decelerated from hitting your opponent or the arena wall. The weapon is made to survive this deceleration, but your off-the-shelf weapon motor is another matter. The larger the motor and the faster it spins, the greater the risk of damage from impact shock loading.

Featherweight 'Suitcase Nuke' has no torque limiting in its weapon drive, and I suspect that your weapon can do without as well. Hobby brushless outrunners have a fairly small rotational inertia compared to the heavy armature of the larger brushed motor used to spin Nuke's weapon. You might want to pour an epoxy film around the magnets in the rotor (rotate it slowly as the epoxy sets) to 'battle harden' it against shock, but that's easier than a torque limiter.

Best luck!


Q: Hello! It's me again! I had a question after watching 'King Of Bots'. So, I'm sure you're aware of the Russian entry 'Stingray'. Well, I was wondering what you think about the design for the drum?

I know you said in an early post of mine that a beater bar is pretty ineffective for heavyweights. But, what about this design? Do you think it's a viable alternative to a circular drum? What would be some advantages to this shape other than weight reduction, and what would be some disadvantages? [Champaign, Illinois]

A: [Mark J.] What I said about 'beater bars' was: at heavyweight size they are not structurally sound. Given what happened to Stingray's drum at KoB I think we can agree that it had structural problems as well.

  • Disadvantages The weapon is too long for the diameter of its support shaft, and the cutout sections focus high loading at specific 'stress raiser' locations on that shaft. Cutting away from the outer radius both weakens the structure and removes mass from areas critical to maximizing energy storage. When removing material on a spinner weapon to save weight you should choose areas close to the center of rotation -- both to preserve strength and retain as much energy storage as possible.
  • Advantage It's relatively simple to machine.

Q: I hope this was not asked before but if I missed it, I could not find it.

How can I best determine the maximum ratio between spinner and body weight on the Robot? I have a concept and it does involve more than 50% of the robot mass to be actually spinning and I am concerned it will reduce handling by too much.

- Alex [Noord-Brabant, Netherlands]

A: [Mark J.] The archives have become so large that it can be a challenge to find a specific topic therein, but I do appreciate your effort.

  • The Gyroscopic Effects section of the Ask Aaron Spinning Weapon Design FAQ has a short discussion of the maneuverability issues associated with high energy storage spinner weapons.
  • Handling issues are much more severe in vertical spinners than in horizontal weapons, but they are also more predictable. The T.i. Combat Robotics Designing Around the Gyroscopic Effect page is invaluable for estimating gyroscopic problems in vertical spinners, and their calculator lets you modify design elements to reduce those effects.
  • There have been several horizontal shell spinners with more than 50% of their mass rotating, including some 'melty brain' spinners that twirl 100% of their mass. Turning motion is not an issue with horizontal spinners, but rotational stability is a serious concern. There are several articles in the Weapon Archive on this topic -- start with this post on Polhode Motion.
If you would be willing to share more specific details about your design I might be able to give more specific advice.
Date marker: January 2018
Q: Is the Saifu kit a drum or an eggbeater? [Beckley, West Virginia]

A: [Mark J.] People like to put things in neat and tidy categories, even when it doesn't matter. I'll answer your question as soon as you tell me whether a photon is a particle or a wave and whether a frog is a lizard or a fish.


Q: Hey, man! Back again! I was watching a few videos of Ol' Nightmare fighting, and I noticed that it kinda looks like the weapon bends a bit as it turns, almost like it's twisting the body. Watch the rumble between it, Witch Doctor, and Overhaul: there are several instances that show what I'm trying to explain to you. Why is it doing that? Is it supposed to do that? Am I just going plain insane and seeing things?!? [Champaign, Illinois]

A: [Mark J.] Yes, Nightmare's huge weapon does twist the chassis when the robot turns. This is the same gyroscopic force that causes the gyrodance you asked about in this earlier post (now in the 'Ask Aaron Robot Weapons' archive). Nightmare's minimal and weak chassis visibly twists a bit before the gyro force is great enough to lift the outside wheel off the arena floor. Physics.


Q: What is the difference between a crusher and a hammer [Arlington, Virginia]

A: [Mark J.] Same as the difference between a hammer and a vise:

  • A hammer hits things - quickly;
  • A vise squeezes things from two sides - slowly.

Q: What are the physics of average wedges beating most average spinners? [Fairfax, Virginia]

A: [Mark J.] Physics... Do you mean 'probability'?

The findings were essentially the same for both studies -- you may be surprised by the results...
Q: Thanks for the new brushless spreadsheet, it helps immensely in planning. Your calculations are based on the SimonK firmware, but how different is that from the default firmware most chipsets come with? Or is flashing to SimonK such an essential part of getting a bot ready for the arena that I shouldn't even think about using off the shelf firmware? Still learning the process, I'm in the midst of designing two beetleweight bots and I just learned about SimonK a couple of days ago (just when I thought I had all the requisite bases covered).

Is there a good SimonK tutorial you can recommend that explains the settings for weapon motors? All I've found for robotics is that you can set the ESC up with a reverse mode for drive motors (the Robert Cowan video you linked to in the FAQ), but I don't know what settings to tweak for a weapon motor. [Vancouver, Canada]

A: [Mark J.] I'm glad to hear that you're finding the new Brushed / Brushless Spinner Weapon Spreadsheet useful. The newly released version 19c includes input for the SimonK 'soft-start' parameters so that you can model the effects of these settings. All the un-sensored brushless ESC firmware flavors have a common approach to motor start-up and low-speed operation:

  1. There is a brief period where the ESC responds to R/C input and applies a little power while it sorts out the electrical responses it receives from the motor to make sure the motor has started rotating and is spinning in the right direction.
  2. Next, the firmware increases the power available a bit more and monitors the rising motor speed. This 'current restricted' period protects both the motor and the ESC from overcurrent until the Back EMF (video) rises high enough to restrict current on its own.
  3. Once this 'safe' speed is achieved, the firmware ends the current restriction and as much power as the R/C commands can flow to the motor.
Note The firmware 'restricts' current flow by reducing the current pulse width to a set percentage of full power. This is not the same as true 'current limiting' which cuts off current above a set amperage limit. You can still exceed the current capacity of the controller.

The nice thing about SimonK firmware is the large number of parameters that are user adjustable. If you know what you're doing you can match the firmware to the performance needs of your application. If you don't know what you're doing you can smoke both the motor and the ESC. Very, very few people know what they're doing.

Fortunately, brushless ESCs are widely available that have SimonK or BLHeli firmware already installed -- no flashing required. A brushless weapon is MUCH less demanding on firmware than is a brushless drive system, and the default settings for either of those firmware packages will work well with a reasonably designed spinner weapon. I highly recommend this approach. If you really want to tinker with setup, you can play with the parameters listed in this earlier post that determine when the current restriction is lifted -- at your own risk.

If you do want to dig deeper into setting SimonK parameters, get a fresh cup of coffee and read thru 'How to Be a Brushless Hipster'. Charles knows what he's doing.


Q: Okay, I read through Charles Guan's treatise on brushless drives. It made my head hurt, but I think I've absorbed most of the important information.

A: Charles excels at making heads hurt.

Q: The brushless motors I'm looking at as weapon motors (ACK-3510CP-630KV) list a max current of 22 amps and recommend a 40 amp ESC. I am intending to direct-drive the weapons. I know that's a risk and has downsides, but I see so many belts and chains come off on insect weights, and recently there have been several highly-successful insect weight bots with direct drive (Sgt. Cuddles, Weta2 kits, Margin of Safety).

A: 'Sgt. Cuddles' and 'Weta' kits have low moment of inertia weapons supported on both ends, and 'Margin of Safety' has a custom motor with a HUGE bearing mounted in a large aluminum plate to absorb impact. A little bird tells me that your design has a large moment of inertia weapon at one end of the stock motor and the single point of motor support at the other end. I'm gonna guess... three hits before the motor comes apart and that 4mm shaft bends.

1) Does using a larger ESC make any sense? Will it, for instance, let me push the drives to slog through that low-rpm phase faster without exploding the capacitors?

2) Is it equally practical (or, perhaps even better) to do as Charles does and purchase some really good capacitors to replace the ones on the ESC? I have pretty good experience with soldering and electronics, so I figure I can probably handle that. As far as I can tell, this is one of the big differences between say a 40 and a 60 amp ESC, and if I'm reading his blog right it's the capacitors that balance the surge of energy required as they ramp up to speed.

A: Better caps and more/better MOSFETs are seldom a bad idea, but I suspect that the weak link in your direct-drive high MOI weapon will be the extended time your motor will be pulling amps beyond rated capacity during spinup. With your current motor design you may need to extend the ESC current restriction to a higher RPM range to keep the motor from smoking.

Q: I know I'm probably getting way over my head as a newb, but for me the learning and experimenting aspect is one of the most exciting aspects of the sport.

A: I understand perfectly -- but efficient experimentation requires a baseline condition against which the results may be compared. I recommend starting with the stock set-up and finding your weak points. Team Juggerbot had a saying that should be better known: 'Damage is weakness leaving the robot'. Break it, then make it stronger.

Q: Also, I know it's early, but are you thinking about attending the Seattle Bot Bash this spring? I'm hoping to get there with at least one working bot. I'll be the guy watching as his 10 year-old daughter drives (see, this is why I want two bots).

Thanks again for all your help and for maintaining this fantastic resource!

A: You're very welcome. No promises about SBB -- best luck to you and your daughter.


Q: So I'm straining the motor too much, plus its 4mm shaft and bearings can't stand the rigors of direct-drive to the weapon for long. Clearly some kind of pulley system is necessary unless I'm going to build my own motor (some day, maybe, but not on my first bot). I think something with a slim motor up front with a very short belt between weapon and motor pulleys would probably be best (similar to Silent Spring). Then the question becomes, what kind of ratio do I need?

I hope I'm correct in assuming that the delay where the ESC is restricting current to the motor can be used as a proxy to guesstimate "motor strain". If so, what do you think would be a safe(ish) level of motor strain, measured by the amount of time it takes before the motor kicks into full power?

Thanks again, this is all immensely helpful!

A: You're on the wrong track, Vancouver. The proxy for strain on a permanent magnet DC motor is current consumption. While the ESC 'soft-start' function is active, the stress on the motor is greatly reduced. The newest version of the spreadsheet features plotting of amperage consumption -- let me show you a few graphs:

Vancouver sent me some specs and drawings for his weapon that he would prefer not to share. These graphs are based on that info.


1) Here's your direct-drive weapon as it stands:

Your weapon motor is highly stressed, pulling WAY above its 22 amp max rating for almost two seconds. It won't last long and neither will your ESC.


2) Here's your weapon with a 2:1 drive reduction:

Better. There's still a big power/stress spike when the soft-start function stops, but the total time your motor spends frying is greatly reduced.

We can reduce the peak current consumption by tinkering with the SimonK parameters to extend the soft-start RPM range.


3) Here's your weapon with a 2:1 reduction and the TIMING_RANGE3 parameter in the SimonK firmware reduced from '1024' to '256':

Now we're getting into the survivable range. Your spin-up time is down around 1 second, the peak current has been cut by half, and the blade speed is at a reasonable level to get some 'bite'. Your energy storage drops from 900 joules to about 225 joules, but that's entirely adequate for a beetleweight.

The need to tinker with SimonK firmware settings indicates to me that the selected motor is simply too small for so large a spinner bar. My recommendation is to either shrink the weapon or grow the motor. I'd shrink.

Hi, it's Vancouver.

Again, can't thank you enough for all the help you've provided. This is my go-to resource for bot building information and you've compiled an amazing collection of tools and advice over the years.

Here is the latest CAD of my beetle bar spinner. Pretty much a total redesign of the chassis. I hope it puts enough weight on the wheels for traction now. The blade... well I know it's a total beginner move, but I want a big weapon, so let's assume that'll stay.

The new motor I've found is an AX-4008CQ, stats: 600KV, 84 Ri, 24 (as far as I can tell from the pictures) magnets. I have it on a 1/4" timing belt going from a 30 tooth to a 40 tooth pulley for a 1.5:1 reduction. As far as I can see without your fancy new tool (any ETA on when it'll be available for the public btw?) it should handle the load without much difficulty. I've also found room (and weight) for a 60A ESC.

The bar is .254m long x .038m wide x 4.8mm steel.

Chassis is 10mm UHMW, electronics box will be covered in either garolite or lexan, weapon motor cover 2mm Aluminum.

Do you see any more major problems, or is it looking reasonably competitive now? [Vancouver, B.C.]

A: [Mark J.] Wait no longer, Vancouver -- the full release of the new Brushless/Brushed Spinner Spreadsheet is live! Details and download link. I think you'll like it.

I've learned not to argue with a man about the size of his weapon. I think you'll be fine. I'll point out that a 30 tooth pulley driving a 40 tooth pulley is a 1.33 to 1 reduction.

The only worry I have is with chassis flex. Your rails are all thick and well braced, but UHMW flexes a lot with impacts -- maybe enough to pop off a belt with a weapon that large. Do you have any weight left for a carbon fiber A-frame stiffener piece to sandwich in between the UHMW weapon supports? Garolite would probably do. If it seems a little 'flexy' when assembled keep that fix in mind.


Major Update to the Spinner Spreadsheet - Now Live!

The new Run Amok Spinner Weapon Spreadsheet is now live in full release!

  • I've integrated the brushless and brushed versions of the spreadsheet into a unified version. Radio buttons let you pick the familiar brushed calculations or the new 'soft start' modeling for brushless motors.
  • The output chart now includes current consumption for the weapon motor to assist in choosing the gear reduction ratio and SimonK soft-start settings.
  • A new 'SimonK Soft Start' parameter section allows modeling of brushless motor performance with modified soft-start settings. This feature can show how to reduce or delay an excessive current consumption peak.
  • A new layout places all the spinner rotor bits in one section, and the motor/controller inputs in another.
Thank you for your helpful comments on the beta release -- I hope you find the new version useful! You may download the new Spinner Weapon Excel Spreadsheet from the Ask Aaron Combat Robot Design Tools page.

Q: Hey there,
Just wondering if there is a good baseline for flipper force to weight ratio? I'm shopping for pneumatic cylinders for a 30 pounder I've designed (think Wheely Big Cheese with guarded wheels) and the rams im coming across have a force rating of like 450-500 pounds. This number seems to be too low to really throw an opponent with much authority. Is there a force number I should say "Ok it can't be less than this" or am I over thinking it? [Adrian, Michigan]

A: [Mark J.] 'Force' is useless in a flipper unless it comes with 'speed'. Force times speed equals power, and it is power that you need for a flipper. Here's how you get power...

There are four primary factors that must work together for an effective flipper:

  1. Ram force: calculated as gas pressure times the piston area;
  2. Ram extension: over how great a distance can the force be applied;
  3. Lifter geometry: the physical lifter mechanism usually decreases force and increases speed; and
  4. Gas flow efficiency: how quickly the gas can get into the cylinder to provide effective force.
That last factor is the critical factor in flipper effectiveness. Pressure regulators, hoses, ports, and valves must be selected to flow the gas as quickly as possible from the pressure tank thru the system and into the cylinder in order to extend the ram at maximum speed under the load of your opponent's mass.
  • A featherweight ram weapon that provides a maximum 500 pound force but which extends slowly because of poor gas flow will only be a 'lifter', but...
  • A smaller diameter ram delivering only 200 pounds of force may fill quickly enough from the same valving system to produce an effective 'flip'.
The calculations involved in modeling flipper performance are not trivial. Typically, flipper builders will simply obtain the pneumatic components with the greatest flow capacity available and hope for the best. If you wish to model your flipper design, take a look down at the bottom of our Combat Robot Design Tools page for links to the 'Team Da Vinci: Understanding Pneumatics' page and the 'Hassocks Hog: Pneumatic Flipper Spreadsheet'.

Q: What exact purpose (other than serving as a directional tool) does the arm of Megabyte serve? [Decatur, Illinois -- close to Champaign]

A: [Mark J.] The tube sticking up thru the center of 'Megbyte' serves as the dead shaft for the rotating shell, is bent to provide a directional cue, and aids self-righting by preventing the 'bot from flopping all the way onto its back.

Q: How does one go about adding safety switches and locks to full body spinners? [Champaign, Illinois]

A: Use a hollow tube for the weapon dead shaft, run the power wires up the inside of the tube, and mount the kill-switch at the top. The lock can be a carabiner run thru matching holes in the shell and chassis.


Q: Do you think putting a motor inside a drum for a heavyweight would be a good idea or a bad idea? '841' is a modified Weta kit, and the weapon inside the drum is really good, especially for making the robot more compact! However, I have a feeling that it wouldn't work so well with bigger bots... [Champaign, Illinois]

A: The high energy storage levels of modern robots combined with the 'scale factor' (FAQ #17) have rendered hollow drums generally unsuitable for large combat robots.


Q: Hi Mark - feedback on your beta brushless weapon calculator.

 Density:
Length:
O/D:
Wall:
7800 kg/m3
250mm
200mm
35mm
I think that there is an error in the kinetic energy output, although I'm not sure how to correct it. I've attached a couple of screenshots showing the differences between the beta spreadsheet and the Team Cosmos KE calculator using a drum with the following specifications:

Your spreadsheet gives the correct weight of 35.38Kg, whereas the Team Cosmos calculator gives the weight as 31.48Kg. To eliminate the weight as the variable, I have changed the Team Cosmos calculator to show the correct weight.

In terms of kinetic energy at 50% (4271RPM), the Team Cosmos calculator shows 25,243 joules. The beta spreadsheet shows 132,045 joules. I'm not quite how much this is off by, but the numbers don't stack up.

Regards, Rob K [Trumpington, England]

A: [Mark J.] You've been caught in a dimension input difference between the calculators, Rob.

  • The Team Cosmos calculator asks for the diameter of the tube;
  • The Run Amok spreadsheet asks for the outer radius of the tube.

Plugging the input values from the Team Cosmos calculator screenshot you sent (a steel tube 250mm long and 100mm in radius with a 35mm wall) into the Run Amok spreadsheet gives essentially identical output: 35.38 kilos storing 25,306 joules @ 4271 RPM.

Thanks for checking with me and for testing the beta spreadsheet. The new Brushed / Brushless Spinner Spreadsheet is now in full release.


Q: Hey Mark! The new brushless spreadsheet looks good. How exactly do you figure out the speed where a SimonK ESC switches from startup into full power mode? [Philadelphia, Pennsylvania]

A: [Mark J.] Thanks. The power level and switchover process was a deep riddle. Configuration variables and their explanations on the SimonK Home Page turned out to be outdated or just plain wrong. With some help from brushless gurus Cosmin Gorgovan (who confirmed my suspicions in the firmware) and Charles Guan (who did his best to confuse me) I worked my way to the bottom:


1) In spite of what the documentation says, the default controller firmware settings will limit the power pulse length to 1/4th [PWR_MAX_RPM1] of the full power pulse length [MAX_POWER] until the motor speeds up enough to complete one 'commutation' in under 1024 µ-sec [TIMING_RANGE3]. That's [60 seconds per minute / 0.001024 seconds per commutation] = 58,594 commutations per minute. Great... but what's a commutation?

2) A hobby brushless motor (three phase) requires six electrical power switchings (commutations) to complete one 'electrical revolution'. That makes [58,594 commutations per minute / 6 commutations per e-revolution] = 9766 eRPM at switchover to full power. Interesting... now what's an eRPM?

3) An electrical revolution is the mechanical rotation of the motor required to move thru one pair of permanent magnet 'poles' and return to the electrical starting point in the commutation sequence. Switchover to full power happens at [9766 eRPM * 2 magnet poles per pair / number of magnet permanent magnet poles]. Awesome... how many poles does my motor have?

4) The motor specs will tell you how many poles a motor has; it's always an even number. A typical outrunner might have 14 poles, where a typical inrunner might have 4.

  • For a 14-pole outrunner motor: cutover to full power comes at [19532 / 14] = 1395 RPM.
  • For a 4-pole inrunner motor: cutover to full power comes at [19532 / 4] = 4883 RPM.

Commutation Speed to RPM Conversion
Commutation: µ-sec RPM:  
Magnets: poles

In general form: 20,000,000 / (commutation in µ-sec * magnetic poles) = RPM.


Q: Hi, I have seen somewhere that a robot used a banebot wheel for a spinning disk. What are the advantages and disadvantages of using a wheel for a spinner? [Great Falls, Montana]

A: [Mark J.] There have been a few 'toothless' spinner 'bots. Pictured is 15-pound 'Mad Cow Disease' that uses multiple Colson wheels as a spinning drum weapon. The robot has a 4-4 record at BotRank, so it can hold its own.

Since toothless spinners rely on contact friction rather than 'bite' they can be spun very fast to store big energy without penalty and they are effective against smooth surfaces that standard spinners just skitter across. They are also durable, inexpensive, and easy to replace.

Primary disadvantage: their hits are friction-limited, so they aren't effective at delivering really huge impacts. I suspect a high-speed toothless disk could be a very effective weapon in a fairyweight 'bot...


Q: This is regarding the selection of motors for weapon system.

I've noticed that selection of a brushed motor is simpler since the RPM and torque characteristics are linear. However, brushless motors do not follow this linear nature. I've also noticed that selection of a brushless weapon motor is primarily based on experience, brand and the stator diameter for a specific kv rating. Also the ESC plays a major role.

How do I select a BLDC for my drum with 0.01 kg·m2 moment of inertia, weighing 4.8 kg?

Can you recommend some BLDC motors with a kv rating in the range of 300-400. I've been looking at Alien Power System and Scorpion motors. I have smoked a Propdrive 5060 with 380kv running on 6s earlier. The ESC accompanying this was a Redbrick 200amp model. [Braunschweig, Germany]

A: [Mark J.] I'm told that there aren't many combat robot builders in Germany and that the German Roboteers Association doesn't know of anyone building a drum robot such as you describe. They'd like to get in touch with you.

While you've noticed quite a bit about brushless motors you may not have noticed that these very compact and lightweight motors do not like to be bogged-down in their lower RPM range. Hobby brushless motors are designed to spin very light propellers quickly into their upper RPM range before encountering significant loading.

I've tried modeling the performance of your weapon powered by the Propdrive 5060 380kv. I suspect that you're using the low-RPM outrunner to direct-drive the weapon drum -- a very poor idea for a weapon this large. The Propdrive has a maximum current rating of 90 amps, but my model shows the motor drawing nearly three times that amperage for the first three-quarters of the fairly long spin-up. I'm not surprised that you smoked it.

Featherweight drum weapons are typically powered by a belt reduction from a brushless inrunner motor. This design allows the motor to build RPM quickly for greater efficiency. Read thru the build report for featherweight 'Big Ripto' for the general layout. When in doubt, copy a successful example. 'Big Ripto' uses a KB 44-74 1500KV brushless inrunner weapon motor rated 2600 watts at a maximum 120 amps.


Q: Remember flail = fail? Well 'Hellachopper', if he was more reliable could win a lot of fights! How does flail equal fail if he succeded? [Fairfax. Virginia]

A: [Mark J.] That's very grand speculation about a robot that never fought. Come back and tell me about its success when it has a winning record. "If" is a fantasy:

If your aunt had bollocks, she'd be your uncle.

Q: I've thought about putting a Continuously Variable Transmission into a weapon system so that it can always have optimum gearing for combat. I never see any other bots with CVT's (probably for a good reason) and I want to know what you think of using a CVT in a weapon system? Thanks! [Nashville]

A: [Mark J.] Scroll down one post.


Q: Concerning spinners who don't need to worry much about tooth bite or are drums-fighting-drums and so have reason to be capable of reaching absurd RPMs: Could one use a changing ratio (shifting gearbox, pulley variator, etc.) to start with high torque to spin up to a decent speed quickly, then shift down the torque to up the speed and start a longer climb to a higher RPM?

Am I missing some physics something? Is the idea simply only worth anything in theory; it isn't possible to implement it to any effect IRL? [Troy, New York]

A: [Mark J.] Your physics are fine, but the weight required for a variable gearbox capable of reliably surviving combat loadings exceeds that of a more powerful motor that can attain the same time-to-speed performance. It's very tempting to add complexity to a combat robot, but simple robots win.


Q: Hi Mark, I'm currently building my first robot which is a beetleweight vertical spinner. I have several questions about my weapon I'm designing:

  1. I used your Spinner Weapon Kinetic Energy Calculator to determine that with the motor I have picked out the disk can store about 450 joules of energy after only 4 seconds. I would like to know on a scale of 1-10 (1 being extremely weak, 10 being overkill) what you think of my weapon?

  2. Using the same parameters I used for the Weapon Calculator, my robot has a maximum bite of only 4mm. I don't think this is nearly enough bite and I want to know what your opinion on how much bite is enough for a robot of my size (beetleweight)?

I absolutely love your website and I have learned many things just from reading questions. Thanks! [Nashville. Tennessee]

A: [Mark J.] I'm glad to hear from you, Nashville. Energy storage is one of several factors to consider in evaluating potential weapon performance. You need stored energy, but you also need to gather that energy quickly and deliver it to your opponent in an effective hit. A careful read of the Ask Aaron Spinner Weapon FAQ will answer most of your questions. Some specific comments:

  • You don't have four seconds to build energy in a small insect arena. Your opponent will be on top of you in a lightning sprint over a few feet -- you need to be ready.
  • You're right to worry about 'bite'. Most of your weapon impacts will be made in close quarters at low closing speed, and a weapon with poor bite will just 'skitter' across your opponent unless it can find a sharp edge to grab. There is no magic number but 4mm is very poor. As it says in the Spinner FAQ:

    Bite is good, and more bite is better.
  • The gyroscopic forces produced by a 450 joule beetle weapon are likely crippling to your mobility -- you'll be unable to turn rapidly to stalk/avoid your opponent. Read the T.i. Combat Robotics Gyroscopic Effects article and plug your design numbers into the gyro calculator at the bottom of that page to see how much trouble you're gonna have.
  • New builders find it hard to believe but the weapon may be the least important system on a combat robot. Drivetrain, radio set-up, general construction practice, reliability, and balance are all much more important. You might be surprised to learn what type of weaponry has the best win/loss record.
In short I don't know enough about your robot design to comment on how effective the whole robot may be. From what little you've told me my best recommendation is to gear down that spinner weapon for a quicker spin-up, better bite, and improved mobility.

Q: Hello Mark it's Nashville again.

I took your advice and changed the gear ratio for my weapon and entered the same values as before into the Kinetic Energy Calculator. Now the disk can spin up to maximum speed (about 200 joules) in only 2 seconds, but it still only has 5mm of bite. Is there any good way to increase bite without sacrificing kinetic energy or mass?

A: [Mark J.] You haven't shared details of your weapon except for energy level and spin-up time. Not knowing more about your design, I can't make intelligent suggestions on what you could change. The Hamburger is Bad. General notes:

  • For a given mass, larger diameter weapons spinning at lower speeds will have greater bite at the same energy storage level;
  • Reducing the number of impactors benefits bite -- which is why asymmetric single-tooth weapons have become so popular.

2. I am competing in the BotsIQ team for our high school and we would like to design a 'Ciclone-esque' bot but are having difficulty with one rule in particular:

"[The Switch must] be positioned in such a way that it can be operated without placing any body part in the path of any weapon system or other powered movable part of the Bot."

...in conjunction with:

"The Master Switch (or Switches) shall be accessible without the need to lift or turn the operating Bot or to remove any cover or grill."

Any thoughts how one could place and securely fashion the switch and where it would be located? Many thanks as always. [New Castle, Pennsylvania]

A: Have you wondered why 'droop end' bar spinners are uncommon? Have you noticed that RioBotz retired 'Ciclone' with a disappointing record? There is a reason! Read the post on polhode motion in this archive -- droop bar spinners are unstable and will cause big trouble at high energy levels. Pick another design.

If you insist on going with this design, mount the spinner on a large-diameter hollow dead shaft, run the power wires up the inside of the dead shaft, and mount the kill-switch at the top.


Q: Hi again Mark, couldn't the facebook guy use iPhone's slow-motion record then count how many spins from the video to estimate RPM? It goes up to 240 fps so up to 14k RPM should be fine. [Bristol, England]

A: [Mark J.] Ah, you're referencing the oddball speed estimations post farther down in this archive. Slow motion video has two components: frame rate and shutter speed. Apple doesn't give out shutter speed info for its iPhone slow-motion mode, but even assuming a speedy 1/500th second shutter in good light you're still gonna have nothing but a blur at several thousand RPM. Without an accurate shutter speed you can't even estimate RPM from the blur length.

You can purchase a digital laser tachometer that's good up to 99,999 RPM for less than the price of new antweight hubs and you would 'know' rather the 'guess'. An enterprising builder could make lunch money renting one out at tournaments to resolve bets.


Q: Hi, I am entering a competition soon that has a 45 second spin down time. I am concerned that what I'm looking at with my weapon right now will not meet this spin down requirement, is there any way I can calculate this in advanced to be sure? Thanks, Aiden. [Bedminster, New Jersey]

A: [Mark J.] Sorry, Aiden -- we can calculate the energy stored in your weapon that must be dissipated, but there are too many variables in the mechanical/electrical/aero drag during spin-down to get a calculated time estimate. I can provide a few tips to decrease your spin-down:

  • If your weapon uses a speed controller: make sure the selected controller has 'dynamic braking' available and that it is turned on.
  • If your weapon motor is brushed and activated with a relay/solenoid: see the Ask Aaron Solenoid and Relay Guide for info on setting up a DPDT solenoid for dynamic braking.
  • If you have a reversing ESC: a VERY gentle application of reverse current toward the end of spin-down is useful.
  • A small servo can be used to provide mechanical friction by directly applying a pad to a belt/disc or by tightening a strap around a shaft/can/drum.
If all else fails and your design allows, you can drive the weapon into the arena wall to stop it quickly.
Q: Hi mark an old builder from [someplace]... I was off the work from some time... leave it... I was thinking about to build a wedge bot (actually tired of drum and disc in [someplace]... yah here are too much of them)...

I was thinking of solid triangular wedge than a metal sheet... 'cause I have seen wedge bot they get easily knocked out from weapons 'cause these wedges have less inertia... what if I put a strong hardened solid wedge to take weapon bites on it... I want your views? [Western Part, Someplace]

A: [Mark J.] It isn't a secret: counter drums and mini-disks with a THICK - HARDENED - STEEL - SCOOP


Q: Alright, here's a question: I am looking to determine the amount of force that an impact will bring from an overhead thwackbot when I have the following variables:

* The robot has 3-inch wheels that are .5 inches wide
* The robot's body is a 2-inch square with a width of 6 inches
* The swinging arm is 5.5 inches long and 1/4 inches wide
* The arm is 6 inches wide and 1/8 inches wide and holds 5, 3 1/4 inch nails (so it looks like a rake)
* The robot's overall weight is 1lb
* The speed is 5mph
* The robot can accelerate to top speed in about .5 seconds

Knowing this, how much force is my rake on a stick going to deliver? [Champaign, Illinois]

A: [Mark J.] You aren't catching on, Champaign.

  1. You're gonna have a very hard time trying to accelerate zero to 5 MPH in 0.5 second in a dusty, greasy combat arena while your thwack hammer (rake) is flopping back and forth.
  2. Assuming that you have magnet wheels and enough torque to accelerate that quickly, the attack strategy for an overhead thwack is to accelerate toward your opponent and slam full reverse on the motors at just the right instant to bring the hammer (rake) over to strike your opponent -- who will not be just sitting there waiting for you to pull off this tricky maneuver.
  3. What you effectively have here is an electric hammer (rake) with the available torque limited by robot traction. Electric hammers without this limitation aren't competitive, so you cannot expect a hammer with this handicap to be effective.
  4. Bullet point #1 in the Ask Aaron Mission Statement says:

    The Ask Aaron site exists to support builders of combat robots with information, design tools, and advice based on our robot competition experience. We are not a free engineering service, and we won't do your homework for you.

I'm not going to run the numbers for you because:

  • Bullet point #3 above entirely kills the possibility that the energy for this weapon is significant;
  • Bullet point #4 above notes that "we won't do your homework for you"; and
  • I'm tired of proving to you that the reason old combat designs aren't used anymore is because they don't work.
If you have time on your hands, the Ask Aaron Electric Hammer Spreadsheet (one of the 'design tools' mentioned in our Mission Statement) allows you to find the number for yourself. Use the combined torque of your two drive gear motors as your input, but limit the torque to traction breakaway. You won't like the number.

Now, go build a contemporary design!


Update This question sat in my mind until curiosity made me pull up the hammer spreadsheet and run the numbers. I found the weight of five 3.5" common nails (0.34 ounce times five), assumed that the rake arm was aluminum, and fitted a pair of FingerTech Silver Spark gearmotors @ 7.4 volts. With best case traction and a perfectly timed reversal, the rake stores a bit less than 4 joules of energy on impact. The rake couldn't crack open a peanut with 4 joules.

The question asked about 'force' rather than energy storage, but the translation from joules to impact force depends on the nature of the impact, which depends on the properties of the object struck. See Hyperphysics: Impact Force.


Q: Is there a calculator that helps you find the right balance of bite and speed for a spinning weapon? [Great Falls, Montana]

A: [Mark J.] There is no single 'right balance' of bite and speed for a given weapon. The balance is situational:

  • Fighting a hard-surfaced opponent with no sharp edges calls for all the bite you can muster.
  • A drum head-to-head against another drum requires maximum speed and can dispense with bite.
  • Small arenas and close fighting call for big bite; larger arenas and higher closing speeds need less.
  • When your opponent has only soft or resilient exposed surfaces it may be better to ignore 'bite' and switch to 'shred'.
  • If you have a ramp to help your vertical spinner get a shot at your opponent's sharp front under-edge you need very little bite.
Stay flexible. Design for ample bite and adequate energy storage, keep some extra RPM available for special cases, and be prepared to throttle back your weapon if it's just 'skittering' across the hard surface of your opponent. Consider swapping in a 'sharp' blade for 'soft' opponents.
Q: Hi, so when two vertical spinners hit weapon to weapon, from my understanding the one with higher tip speed wins. But what if they each have the same tip speed but one is symmetrical and the other asymmetrical, would the symmetrical spinner win? [Great Falls, Montana]

A: [Mark J.] The 'higher tip speed wins' rule assumes otherwise similar weapons. If there are significant differences in weapon size or design the advantage analysis gets complicated.

Tip speed offers an advantage because the faster tip can catch the opponent's drum tooth from behind and throw their 'bot upward while remaining safe from the same attack. Two identical drums with the same tip speed will interlace the teeth of their drum weapons and bounce off each other with harmless glancing impacts.

If the drums are not identical the advantage in a same-tip-speed battle goes to the weapon with the greater bite and tooth-depth:

  • The weapon with a longer tooth (or teeth) can reach in to strike the opponent's drum body while keeping its own drum body out of harm's way.
  • The weapon with greater bite has a better chance to strike the opposing weapon drum with better than a glancing blow.
In your scenario, the winner is likely the asymmetric (single-tooth) weapon.
Q: What would be the most appropriate motor for my robot idea to use? Yes, a flipper and axe (or more rather a hammer, or a plank of wood with a nail in it) as one weapon don't go together, but I want to have one single 180 degree weapon.

I also want a robot with a protective and defensive chassis, and a Tornado-esque 4 wheel drive even though very few recent successful robots in UK/Europe have more than two wheels these days and, 'Bronco' aside, successful US robots with more than 2 wheels today are also hard to name.

What is the best motor for the arm, and how should it be connected to the rest of the robot interface? [London, England]

A: [Mark J.] I believe you answered your own question when you wrote, "a flipper and an axe as one weapon don't go together." Several prior posts in 'Ask Aaron' have discussed the disparate requirements of the two weapons: explosive torque and acceleration thru a short arc for the flipper, and efficient acceleration to maximum speed at the end of a long swing for the axe.

There is no electric motor solution that can simultaneously cover both of these requirements regardless of the weight class for which you are designing. This is the answer to the question you failed to ask, "Why are there no robots like my design?"

Mechanical linkages for hammers and flippers are extensively discussed in the Ask Aaron archives. Start with this post about the electric hammer used by 'ßeta', and you may be interested in this archived post about an electrically wound snail-cam spring flipper.


Q: I'd like to find the actual speed of my spinner weapon but I have no measuring tools and no budget. Are there any oddball tricks? [Facebook]

A: [Mark J.] A few, but I can't vouch for any of them...

  • The eyeball test [Paul Gancitano] -- download one of many free stroboscope apps to your phone and put a daub of white paint on the side of the weapon tip. Spin up in a darkened room and find the fastest strobe speed that 'freezes' the white spot in place. Result may be revolutions per second (Hz) or direct RPM depending on the app. Note: I can't find a working app that is fast enough for our purpose.
  • The earball test [Derek Reihe] -- put the edge of a playing card in the path of the tooth and match the tone it makes to a known source (frequency generator app). Matching the tone (Hz) gives revolutions per second. Divide by two if two teeth hit the card on each revolution.
  • The tongue test [Anonymous] -- spin it up, spin it down, then quickly lick it to see what it tastes like:

Olives
Cheese
Oranges
Salt
  Egg Salad
Beer
Dirt
Blood
-
-
-
-
-
-
-
-
  2k RPM
  4k RPM
  6k RPM
  8k RPM
10k RPM
12k RPM
14k RPM
Let it spin down some more.  

If you actually want to know how fast your weapon really spins you can buy a digital laser tachometer online for less than $20.


Q: Hi, I have a tooth on a antweight vertical spinner that is 5mm thick, 30mm wide and 13mm long. Would O1 tool steel be suitable for this use? [Great Falls, Montana]

A: [Mark J.] Given only that information I really can't make a specific comment, Montana.

  • No competent engineer would spec material before knowing a great deal more about the design than you have told me. You didn't mention how the tooth is supported, the weapon speed, or the weapon moment of inertia.
  • It's poor engineering practice to specify dimensions for a heavily stressed part and then try to find a material that will survive in that application. Material and design must work together to perform a specific function.
  • See Frequently Asked Questions #17 and the Hamburger is Bad.
I can say in general that tool steels are commonly used for weapon teeth, as are various grades of abrasion resistant steel. Shock resistant S7 tool steel is a common choice for its great toughness. Your O1 steel may be an adequate choice, depending on your design details. Remember that tool steel is only as good as the heat treating it receives. I'd suggest tempering O1 to no more than 60 HRC to retain toughness.
Q: Hi Mark! How would you calculate the gear ratio of a friction drive weapon system on a ring spinner? Would it be like the calculation of a v-belt pulley?

A: [Mark J.] Two design elements that don't work -- let's put them together! Yes, the reduction calculation is similar to a V-belt system:

Reduction = Inner Diameter of the Ring ÷ Diameter of Drive Wheel (to 1)

Q: Because of your dislike of the design as a whole (for legitimate reasons) I also have to ask (as I cannot find it in the archives): how would you calculate the gear ratio of a ring spinner using a ring and pinion gear style or simply inter-meshed gears? Many Thanks! [New Castle, PA]

A: That calculation is the same as for any system of two gears:

Reduction = Number of Teeth on the Driven Gear ÷ Number of Teeth on the Motor Gear (to 1)

Gear reduction diagrams and more detailed explanation at wikiHow.


Q: I was watching Battlebots and Robot Wars (again) and noticed that robots such as 'Ringmaster' and 'MR Speed Squared' could gyrodance. I saw it firsthand as well with the Antweight 'Spinook' in Peoria this year. I know any robot with a spinning weapon is capable of feeling some kind of gyroscopic force, but how exactly does this happen to horizontal spinners? Do you think there is a way to harness this power to make a gyrobot like Gyrobot? If there is, I'm going to assume it would be a better option to try to build a combat robot out of a potato and a rubber band. [Champaign, Illinois]

A: [Mark J.] I can post formulas and diagrams about gyroscopic forces that go on for pages, and in fact I have: search the Ask Aaron 'Robot Design' and 'Robot Weapons' archives for 'gyroscopic'. If you really want to understand gyro forces I strongly recommend that you purchase a small toy gyroscope and play around with it.

Any time you apply a force to change the direction that the axis of a spinning mass is pointing, gyroscopic resistance will attempt to redirect that force at a right angle to the original force direction.

  • A vertical spinner must deal with this force redirection every time it turns. A quick pivot with the weapon at full speed results in that familiar 'gyro dance'. Check the Total Insanity Gyro Effects page for the math behind the dance.
  • Horizontal spinners don't change the orientation of the spinner axis when they turn, so are easier to drive. In order to 'dance' a horizontal spinner has to be tilted, either from a reaction to a hit by their own weapon, from an opponent attack, or from instability in their weapon design.
  • Horizontal spinners do not work for a Gyrobot-style 'walker'. With a vertical spinner the gyro force that resists axis deflection causes the unsupported edge of the robot to rotate forward and 'walk', but with a horizontal spinner the gyro force only causes the front or back of the robot to 'lift' which provides no useful walking rotation.

Q: I've seen several Antweights ('Odium' from Bot Brawl 2017, 'Debacle' from TeamVelocity's YouTube Channel, and a green spinner in the Team Velocity video of the rumble at NG Conference 2017) that get thrown onto their backsides and manage to drive on their back ends. Is the force being made from the spinner essentially acting as a front-mounted propeller?

A: Horizontal spinners standing on their tails are stabilized by the gyroscopic forces of their weapons, not an aerodynamic 'propeller' effect. Think of it as a very slow gyro dance and then go back to my answer to your first question.


Q: I am exploring three options for a horizontal spinner. All three assume a Team Whyachi TWM3R gearbox with a blade storing up to 50,000 to 100,000 joules of energy. The vibration mounts have a 258 lbs max static compression load, 15072 lbs/in spring rate in compression, 86 lbs max shear load, 1102 lbs/in spring rate in shear.
  • Option one mounts the gearbox to the frame rigidly and the blade to the 1" TI 6AL-4V threaded spin shaft of the gearbox. This design will transfer a big portion of the hit back to the frame and gearbox, but minimal energy is wasted.
  • Option two mounts the gearbox to the frame with vibration mounts and the blade to the 1" TI 6AL-4V threaded spin shaft of the gearbox. This design will absorb part of the hit energy on the vibration mounts, thus wasting part of the hit energy.
  • Option three mounts the gearbox to the frame with vibration mounts, has a belt and pulley or sprocket and chain between the gearbox shaft and a 1" TU 6AL-4V live axle with the blade. This design is more complicated and heavier, but the pulley and belt provide a clutch to protect the gearbox.
My tendency is to go for option two as the least likely to break. Option one may be too efficient in not absorbing energy fed back to my bot when it hits. Option two is simple. Option three is used by multiple bots, but isn't simple, is heavier, has more components that could fail and it would seem like the energy fed back to the frame would be almost as much as option 1.

I am looking for very sturdy, simple and able to deliver a big hit.

Which option would you choose and why (or is there a better fourth option)?

Can you point me to any sources for the equations to calculate the feedback energy, vibration mount sizing, component failure points? I'm trying to figure out what the weakest link is in the weapons chain to see if I can strengthen it.

Thanks [Denton, Texas]

A: [Mark J.] For bar spinners in this energy range I defer in design expertise to Ray Billings and Team Hardcore. He has been working big bar spinners for many years and has uncovered all the weaknesses and engineering tweaks the hard way.

Combat robots have such unpredictable force vectors that the standard engineering calculations for strength are largely useless; the process is to build it, break it, then make it stronger next time. Ray has cycled thru this process countless times, and he gives away too many of his secrets in this Reddit Q&A session. Read it carefully and treat it as gospel.

To your designs: my suggestion is to scrap the twin-Magmotor/M3R2 combination. That 1" titanium shaft won't survive impacts for long at those energy levels. Ray kept upsizing his shaft diameters and materials until he settled on 1.5" 4340 chrome-moly hardened to 45rc. Take your needed gearing reduction in the pulleys/sprockets instead of a heavy and (at these energy levels) fragile gearbox.

I'd follow Ray's choice of a big pancake motor - he likes the Motoenergy ME0708 - isolated from the weapon shaft. He tried belt drives but found them unreliable, so for years Ray's big spinners used simple chain and sprocket drive from motor to weapon. Recently he as added a slip clutch in the drive to give the motors a bit of protection. Ray estimates that it costs him about $1000 per match in ruined components and general wear to fight his big spinners. Yes, that's per match, not per tournament. The slip clutch helps a little with expenses. With the motor isolated, no shock mounts are used.

Also of note is that Ray likes bushings rather than bearings for the weapon. There is a litle more drag, but they will survive the huge impact loads more reliably.

You may be interested in an extensive and detailed heavyweight bar-spinner design discussion I had with another Texas builder. I gave it a dedicated section in the archives: The Texas Bar Spinner Papers.

Q: The is the three options for horizontal spinner guy. I was doing some research with the Rio Botz tutorial on the issue and came across this picture. Looks like you are right about not trusting the Team Whyachi shaft. I'm going with a 1.75" Ti 6AL4V ELI shaft (including bearings and external shaft to hold the sleeve bearings 2.375") and a chain drive (ala Tombstone).

Thanks for the advice.

A: RioBotz bent that shaft in a middleweight spinner about ten years ago. I think your upgrade is wise, although I'd prefer a suitably tough steel in place of 6AL-4V titanium. Keep me up to date on your progress.


Q: For a pneumatic weapon what is a good benchmark power to aim for? I understand from a spinner perspective the you recommended [a minimum of] about 60 Joules per kilogram but how would this translate to a pneumatic flipper or hammer weapon? Thanks [Baltimore, Maryland]

A: [Mark J.] Your question involves an 'apples to oranges' comparison.

  • A spinning weapon stores kinetic energy in the rotational inertia of the weapon that is transfered to the opponent on impact. That energy is accumulated by the weapon as it spins up to speed over some period of time. More energy storage is generally better, but there is a trade-off between increasing rotational speed and the ability to obtain enough 'bite' to effectively transfer that energy.
  • A pneumatic hammer also stores kinetic energy as it accelerates from its resting position toward impact, but it has much less time-distance to accumulate that energy. It's nigh impossible to get levels of energy storage in a hammer weapon comparable to a spinner weapon. Get as much as you can and hope its enough. Heavyweight hammerbot 'ßeta' manages about 7 joules per kilogram.
  • A pneumatic flipper does not store kinetic energy to impact the opponent. The flipper mechanism applies force generated from stored potential energy directly to the opponent to accelerate them upward. Measuring the 'static' force of the system does not equate to the net force applied while the system is in motion. Calculating that force by modeling gas flow thru the regulator, valves, and actuator ports is difficult and is further complicated by the geometry of the flipper mechanism.
The traditional answer to "How much lifter speed and force is enough for a pneumatic flipper?" is "All you can get." If you do want to model some potential flipper configurations, the 'Hassocks Hog' team webpage features an Excel spreadsheet that attempts to do just that: A guide to designing a pneumatic flipper.
Q: Hi Mark. I was doing some research on ways to prevent bolts and screws from loosening during vibration or stress. Two common solutions I am aware of include nylon insert bolts/nuts and thread-locker. Generally speaking, is one method better than the other? Also can both be used simultaneously or will the thread-locker mess up the nylon? In my specific instance I am building a weapon arm with through holes at the ends that holds 2 S7 tool steel impactors approx. 1" by 1 3/8" which will be tapped and have bolts run into them. Many Thanks... [Pittsburgh]

A: [Mark J.] There is a good discussion of the benefits and drawbacks to threadlocker vs. nylon inserts at eng-tips.com. Briefly, threadlocker holds better than nylon IF properly applied to clean threads and allowed to fully set, while nylon inserts are foolproof and immediately useable. If there is a chance that I'll need to remove the bolt and replace it between matches I'll use nylon, otherwise a carefully applied threadlocker is my choice. Don't use both -- most threadlocker compounds don't mix with plastics.

Important You requested that I not use the drawing you sent -- but you may have noticed that there are no current robots running similar designs. There is a very good reason: your weapon is unstable. Take a look at the post about polhode motion in this archive and watch 'Secto' in this video. Your design is not identical but it has the same instability! Also the fastening method you propose for the impactors puts a huge shear load on the bolts, which is very poor engineering practice. The bolts will fail. I'd strongly suggest starting over on the weapon design.


Q: I was thinking of using a drum 6" in diameter (4" drum with 1" teeth on each side) and 6" wide. However, several people said that having a drum with square dimensions was a bad idea. But why? Is it a bad design choice, and if so, is there a way to counteract or reduce its flaw(s)? [Champaign, Illinois]

A: [Mark J.] I don't know who 'several people' are, but they were paying close attention in their physics classes. Take a look at this post about polhode motion in this archive.

Long rotating drums are pretty 'stable'. Very short rotating drums (disks) are stable as well. However, drums close to 'square' dimensions may have very similar moments of rotational inertia in all three possible rotational axis and become unstable -- possibly flipping the entire robot in unpredictable directions.

The flaw comes from the distribution of mass in the rotating drum and there is no 'fix' other than redistributing the mass into a less-square shape. The instability effects are less of a problem in a robot with a weapon at one end, as most drum weapons are, than a full-body or overhead spinner. I'd still be worried about possible instability and change the weapon dimensions.


Q: I have a question more about a certain segment in the Riobotz book. I was reading it and came across section 6.5.5 regarding Full-Body Drum Spinners. I get the part regarding how what they refer to as a "shell" drum spinner, but I don't get the next two parts. They read as follows:
  1. "If the drum was a cylinder mounted on rollers, it would be called a ring drum."

    What does that mean? The whole body is a ring shape inside with the drum rolling on top of it?

  2. "A full body drum, which would be like an overhead thwackbot but without a rod. The robot would use the power of its own two wheels to spin its entire chassis (and not just its armor) as if it were a big drum."

    Okay, I get that.

    "...The challenge would be to implement at each wheel an independent braking system that would allow the chassis to spin up without moving the bot around. After reaching full speed, the braking system would release, and the robot would be driven by slightly accelerating or braking each wheel motor."

    Um... What?

Can you help me with some kind of translation or breakdown? I have a feeling I'm overthinking it. [Champaign, Illinois]

A: [Mark J.] The RioBotz Combat Tutorial was written in Portuguese and loses a bit in translation. The descriptions are also a bit brief in places. To be fair, 'Ask Aaron' probably doesn't translate well into Portuguese.

  1. They're talking about a ring spinner like 'The Ringmaster'. The spinning 'drum' is a ring around the outside of the chassis. The ring is entirely supported by rollers around the chassis edge leaving the top of the robot chassis clear of any drum supports. The advantage being that inverted operation is possible if large wheels stick all the way thru the chassis or - like 'The Ringmaster' - an additional drive system is made for the top.
  2. A conventional 'thwackbot' cannot move when spinning -- it has to stop spinning to move like a normal wheeled robot. By carefully timing very brief independent decreases in the speed of the drive wheels while the 'bot spins, the rotation is disrupted in a way that moves the robot in a controlled direction rather than just spinning in place. This is called translational drift, 'melty brain', or 'tornado drive'. It's horribly complicated to implement and not terribly effective.
These designs have not proven successful in combat. They are 'show off' technology to impress other competitors with your mad building skills.
Q: Why do saws have less bite than discs with less teeth? And why do hyperspeed weapons not get bite? Yes, I did read the spinner weapon FAQ. It said the reason was longer insertion time. Over speed weapons should do the same thing as mid-speed weapons except faster. Bots like Saifu, Weta, Algos, and DDT all use high speed weapons and they have no problems. [Arlington, Virginia]

A: [Mark J.] I believe you misunderstand what is meant by weapon 'bite'. A faster weapon, or one with more teeth, allows less time for the opponent to be inserted into the arc of the spinning weapon before the next tooth rotates around to strike. The Spinner Weapon FAQ points readers to Section 6.3 in the RioBotz Combat Tutorial for an explanation of the relationship between weapon speed and bite, as well as the formulas for calculating bite depth. If the explanation in the FAQ is unclear to you, I suggest reading the RioBotz tutorial.

The robots you list do have effective weapons - it is possible to have an effective spinner that has poor bite if you compensate with other design elements.

Examples:

  • 'Algos' uses a wedge to lift the opponent up and into the drum, exposing a sharp angle on the underside of the opponent that requires very little bite to grab and throw.
  • 'DDT' does not direct-drive the big weapon blade -- the large diameter disk gives a high tip speed at a lower RPM for better bite.
  • 'Weta' has a reduction belt drive to keep the RPM reasonable, plus offset impactors for adequate bite.
  • 'Saifu' is a direct-drive high-speed drum, but you don't need or want full RPM against all types of opponents -- you can and should throttle back for 'hard to hit' opponents.
Opponents with sharp edges to grab do not require good bite, but better bite allows effective attacks on flat or rounded surfaces.

Q: Does bite apply to hammers?

A: 'Bite' as described here applies only to continuously rotating 'spinner' weapons. The concept of 'bite' does not apply to 'strike on demand' axe or hammer weapons.


Q: Hello, Mark! Back with some questions: this time, these questions are regarding a heavyweight build. The robot would be heavily based off of Electric Boogaloo:
  1. My original idea was to use a spinner similar in shape to Deathroll's spinner (I even asked them if they could sell me a spare! Of course, the dimensions would probably change), but around 12" tall. However, I may be better off with using a 12" bar seeing as it will be easier to put together and get machined, especially since this would be my first heavyweight bot. Which would you recommend and why? Also, which is better for self-righting, like with Aftershock? I'd like to be able to do so with this design.

  2. I was thinking of mounting the bar or disc with two motors. Why? First, if one motor gives out, I got a spare. Second, I like symmetrical designs or designs with singular motors centered (motors to the side that make the bot look asymmetrical always bugged me for some reason lol). Of course, this would mean extra batteries and more money (which I probably would have if I used a bar rather than a custom disc). What do you think? Should I use two motors or one? What would the major perks of this, and what would be the biggest flaws or concerns be?

  3. Depending on the parts I use and disc's dimensions (as well as personal taste), I may be able to split the chassis entirely in half (EB had a small but reinforced portion in the back if I recall correctly, so it wasn't split in half). Only one problem: this adds onto the whole "expense" and "complexity" issues of having two motors (When I say "expense" and "complexity", I'd prefer not to spend over $7500 on the bot). I'd probably need two receivers, and coding would probably be a PITA. More importantly, it would be hard to keep it in one piece if it's being held together by one small piece of aluminum. What do you recommend as solutions?
As with all of your responses, your response and help will be greatly appreciated! [Champaign, Illinois]

A: [Mark J.] Having conquered the lower weight classes you have decided to move up to the heavyweight class?

  • How many combat robots have you built?
  • What prior experience have you in mechanical design and construction?
  • In how many tournaments have you fought?
  • How many battles have you won?
  • The last set of questions you asked had you attempting to turn metal salvaged from the scrapped drinking fountain at your school into an antweight -- how did that go?
Very seriously: You have no business building a heavyweight combat robot. If you're actually planning a heavyweight build you should reconsider. If this is simply an exercise in fantasy design, you should stop wasting my time. I will be pleased to assist you in building an insect or sub-light robot, and when you have learned the painful but relatively inexpensive lessons to be overcome in those weight classes I will assist you in moving up to a suitably heavier robot.

Your questions are well thought out and nicely presented. I'm not taking this position to be cruel, I'm attempting to do you a favor by giving you the best advice I can provide.


Update I've reflected on my reply to your questions. I still believe I gave you my best advice, but it does not sit well with me to leave your specific questions unanswered. What follows is as much for my benefit as yours:
  • Keep your design as simple as possible -- simple 'bots win. As you noted, a bar spinner is simpler and less expensive than an oddball custom single-toothed cutaway disk. It's also more durable.
  • A bar is not good for self-righting. Your best choice there is a full-circle disk like 'Aftershock'. An inverted circular disk in contact with the arena floor is still easy to spin, even if slowed by the impact that flipped the 'bot. When the tooth strikes the floor it has a chance of popping the 'bot back upright (if you're lucky).
  • One motor, please. Let it bug you. It's more efficient, less complex, less expensive, and - in spite of your hopes - more reliable. If one motor quits you probably took enough damage to take the whole weapon out.
  • A split chassis is a weak chassis. Dual receivers would be the least of your problems. It's very embarrassing to have your robot torn in half. One... solid... strong... box... chassis.
Still, do not build a heavyweight!

Q: For a heavyweight horizontal spinner weapon system, is the perm PMG 132 a better choice than the E-tek Motoenergy ME0909 PMDC motor? [Portsmouth, Ohio]

A: [Mark J.] The preferred brushed heavyweight weapon motor - for a balance of power and durability - is the Motoenergy ME0708.


Q: Hello, I have 2 questions. Bot info:
  • A 12-Pound Tombstone clone
  • Two wheels: 2.375" diameter, 0.5" width
  • Two RS395 Banebots motors
  • 4:1 P60 Banebots gearboxes
  • Hobby King ACK-5312CP 330KV weapon motor

1) I'm very new to this, could you double check my gearbox choice (or even motor choice if necessary)?

2) I will be using A123 LiFe batteries (3.2v, 1100 mah 30 amp discharge). If my calculations are correct (which I doubt) 1 of these batteries should be able to power the weapon and drive for 25.714 minutes.

How many batteries do you think I need? Thanks for the help. [Hicksville, New York]

A: [Mark J.] I can't see all your input fields in the image you sent from the Team Tentacle Torque Calculator, but it's clear that you have not entered the correct values for the RS-395 motor. Let's start over with the correct values:

The 20:1 ratio P60 gearboxes with the 2.375" wheels give the best performance from the RS-395 motors in a moderate size arena: around 5.5 MPH, reaching that speed in about 6.5 feet. That's ample speed and power to push around a big spinner weapon. The 26:1 gearboxes would be a little easier on the motors, but the 20:1 should be fine.

The RS-395 motors run well between 12 and 15 volts -- you'll need four LiFe cells wired in series to provide 12.8 volts. The drivetrain will use about half of the capacity of the 1100 mah cells in a 5 minute match.

Your weapon motor is a powerful choice for a hobbyweight. It operates in the 22 to 30 volt range, so you will want a separate battery to operate your weapon. That battery will require between 7 and 9 LiFe cells wired in series. You have not given me enough information about your weapon to calculate the battery capacity (mah) needed. Heavier, longer blades will use more battery power to spin up, and the reduction ratio between your weapon motor and blade will also impact current usage. Example, courtesy of the Team Run Amok Excel Spinner Spreadsheet:

A steel bar 15" long by 2" wide by 0.5" thick running a 2:1 reduction from the ACK-5312CP motor at 28.8 volts (9 LiFe cells) would spin to about 1300 joules energy storage in roughly three seconds. Assuming six spin-ups from a dead stop in a 5 minute match, total weapon current consumption would be about 520 mah.

So, you'd need one four-cell LiFe battery to run your drivetrain and one 7 to 9 cell LiFe battery to power your weapon. Combined, they should run your 'bot in combat for about 10 minutes on a full charge.

Are you sure you wouldn't like to build a nice wedge robot instead?

Q: Thanks for the quick response. My weapon's current plan is a 5 pound 12 inch diameter 1/2 inch thick S7 steel asymmetrical "disc", shaped somewhat like a teardrop. The thickness is to avoid vertical spinners from breaking the blade. None of this is set in stone but the 12 inch diameter, and somewhat the weight.

My fights will only be 3 minutes long. The bot does strategically not need to be fast - in fact it should be as slow as reasonable to save weight and power consumption. The 26:1 gearboxes look best to me to that effect.

I hope to have as little power left over after a fight as reasonable, perhaps only enough to last 4 minutes in total. That being said, I still think I can squeeze 12 LiFe A123-18650 batteries into my bot, in 2 groups of 4 and (with slight design alterations) 2 groups of 2. But of course the fewer needed, the better.

Funny you should mention wedgebots, as I eventually hope to build a bot to take down Original Sin! Anyway, do you have any blade alteration or battery number suggestions? Thank you.

A: A 12" diameter, 1/2" thick steel disk weighs more than 16 pounds, so your 'teardrop' design must have a LOT of material cut away from the full disk. A simple steel blade 12" long, 1/2" thick, and 2.875" wide weighs 5 pounds. A 5-pound weapon is heavy for a hobbyweight, so be careful with your weight calculations.

The number of cells I'm suggesting for your batteries are not there for current capacity, but simply to obtain the voltage needed for your two motor systems running at differing voltages. You can't run your drive motors at 30 volts, and running your weapon motor at 15 volts would drop it's power output from 1000 watts to 250 watts. 'Tombstone' has this same problem and solves it the same way that I suggest you do: two battery packs with differing voltages.

I'm a bit concerned about the ability of the cells you have selected to provide the start-up amperage needed by your monster weapon motor. The motor is rated 40 amps continuous, but can briefly draw as much as 140 amps under starting load and will draw over 70 amps for as much as two seconds while staining to pull that heavy weapon up to speed. Given that your cells are rated for 30 amps continuous draw, pulling more than twice that current may damage the weapon battery. You may want to reconsider your LiFe cell choice.

Q: Thanks for the advice. I'm happy to lock in 4 1100 mAh batteries (the yellow ones) for the drive power with 26:1 gearboxes, if you think that's not too many.

A: My earlier explanation was not sufficiently clear. A battery pack for a specific application must supply three things:

1) Adequate capacity to power the device for the required time (mAh).
2) The current needed by the device -- without damage to the battery (amps); and
3) A suitable voltage for the device (volts);
We have calculated that the drivetrain for your robot will use about 330 mAh of total current in a three-minute match, so your 1100 mAh cells have that well covered.

We have also calculated that the peak current draw of the drivetrain is about 10 amps, so your 30 amp continuous output cells also have that well covered.

That leaves voltage. To get adequate performance from your chosen drive motors you need a battery pack that will supply 12 to 15 volts. Each of your cells produces 3.2 volts:

One cell = 3.2 volts;
Two cells wired in series = 6.4 volts;
Three cells wired in series = 9.6 volts;
Four cells wired in series = 12.8 volts.
You require four cells wired in series to supply adequate voltage to your drive motors. Fewer will not do. You could get by with smaller cells, but you need four of them.

Q: Now for the weapon power. Apparently the bigger A123s (green) are back in stock. They have a very slightly lower voltage (3.2V) but a higher amperage(50A), plus a maximum impulse discharge of 120A.

However, as I understand it, the same number (7-9) of these green batteries would be needed for the weapon motor's voltage as of the yellow batteries. Since the greens weigh more and are slightly bigger, this is less ideal. LiPos are not allowed at my upcoming event. Is there another battery you recommend instead?

Not using 100% of my motor's capability seems ok to me, as it might help prevent breakage (I think). Obviously I want to use as much as possible, but I'm okay with using less. Perhaps not filling up the voltage requirement completely but satisfying the corresponding Amp requirement would do the trick. The green A123s might be better at that. I currently have 1-1.5 pounds to spare for batteries, and the fewer cells I can reasonably use the better. What do you think?

1 yellow = 0.088125 pounds
4 yellows = 0.3525 pounds
8 yellows = 0.705 pounds
12 yellows = 1.0575 pounds
1 green = 0.18125 pounds
4 greens = 0.725 pounds
8 greens = 1.45 pounds
I plan on ordering the parts this week so I can experiment with a prototype before I finalize the bot.

A: Your problem is that your weapon motor is, as I mentioned before, a powerful choice for a hobbyweight. It requires both high voltage and a high peak current capacity. Having a big weapon motor and not running it at full voltage is a serious waste of power. Power varies with the square of voltage, so dropping a couple of cells cuts your power by almost half:

9 cells = 920 watts
8 cells = 728 watts
7 cells = 558 watts
6 cells = 410 watts
With each drop in power your spinup time increases and your weapon power storage drops. That big weapon motor is causing you more problems than it solves. If you want to run at a lower voltage you'd be much better off with a smaller motor designed to run at that voltage.

If you ask around the on-line forums, someone is going to suggest that you build a 9-cell battery pack to power the weapon and 'tap' the pack at 4-cells to power the drivetrain. This is possible, but I cannot recommend it. You'll be drawing more mower from some of the cells than others, and the draw from that big weapon motor may 'brown out' the drive and electronics. Proceed on that path at your own risk.

I think I'd go find a more reasonably sized weapon motor that would run well at four or five cells, downsize the weapon rotor, and run the whole bot off a single battery pack.


Q: Can you make a permanent page for those series of questions about crazy weapon designs? I think you'd be doing a great service to dissuade new builders from doing something too crazy. [Waltham, Massachusetts]

A: [Mark J.] I generally assume that builders writing in to 'Ask Aaron' are interested in combat robots that will win matches, but an increasing segment of builders are interested in impressing audiences and other builders with 'show-off' designs. While our focus remains on supporting simple and effective designs, I also recognize that one man's crazy is another man's awesome.

The recent long series' of questions about... unusual... weaponry and design has found a place in the Ask Aaron Design and Construction archive and I'll keep trying to set reasonable expectations for any design thrown at me.


Q: In the weapon FAQ or something it says discs are better than bars. Why do bots like tombstone or last rites use bars if discs with carvings such as in Nightmare or DDT are better?

A: [Mark J.] A spinning disk stores more energy at a given RPM than does a spinning bar of the same mass and swept diameter. This is because the disk has more of its mass located farther from the axis of rotation. The Ask Aaron Spinner Weapon FAQ features a comparison of the energy storage capacity of four spinning weapon designs with the same mass and swept diameter: bar, disk, eggbeater, and hollow drum.

Better energy storage is a good thing, but it is not the only factor to consider when designing a spinning weapon.

  • Simple designs like bars and solid disks are much easier to construct than eggbeaters and drum weapons.
  • At large sizes and very high energy storage levels the durability of thin disks, eggbeaters, and hollow drums becomes a serious concern.
  • Some consideration must be given to the vulnerability of the weapon to attack by other weapon types. For example, the large edge area of a horizontal disk is vulnerable to a fast-spinning vertical drum weapon.

Q: And if vertical spinners supposedly generate more kinetic energy then why do people use horizontals? [Baltimore, Maryland]

A: Who said anything about verticals storing more energy? You didn't find that here. The orientation of the spinner has no effect on energy storage.


Q: Is there a rule of thumb for how much battery capacity to have for a spinning weapon? [Manassas, Virginia]

A: [Mark J.] Too many variables for a 'rule of thumb' -- how about a calculated solution? The Team Run Amok Spinner Excel Spreadsheet calculates the mass, moment of inertia, stored kinetic energy, tip speed, spin-up time, AND the approximate battery capacity requirement for your spinner weapon. You'll need Microsoft Excel to run the spreadsheet.

In general, the capacity needed for your weapon will be much less than the capacity needed for your drivetrain.


Q: Is there any advantage in hooking 2 brushless motors to a single spinning disk weapon?

A: [Mark J.] Two motors = twice the power -- but because each brushless motor relies on it's controller for commutation as well as current, each motor requires its own ESC. It's generally preferable to use a single larger motor with twice the power.

Q: Also, can I put a custom aluminium pulley around an outrunner brushless motor retained by a couple of set screws with plenty of loctite in a hobbyweight? [Quebec, Canada]

A: Set screws are best avoided for this purpose: they apply large point-pressure that may deform the rotor and are failure-prone even with a threadlocker. If you're making a custom pulley you should size the bore for an interference fit and press the rotor into place.

A word about threadlockers: some builders recommend applying Loctite to shafts and other sliding assemblies to secure them, but that's well outside the intended use of the standard blue or red Loctite. There are special formulations of green Loctite designed for holding loose-fitting components to shafts, but I can't recommend them for torque carrying applications in combat.


Q: I saw some of your old answers about 'Beta' and I thought I would investigate how much better 'Beta' is than just using regular gears, but I couldn't figure out if I was doing it right.

So anyways, what I was doing was putting some motor statistics into your hammer spreadsheet and figuring out the best gear ratio for maximum joules as a starting point. Then I doubled the gear reduction and wrote down the number of degrees to reach 60% speed and what the speed was. Then I reduced the ratio a little, and took the speed from the previous step as a starting point, and found how many more degrees it would take to reach 60% again from that point and repeated this out to 180 degrees.

I ended up with almost the same amount of power as without all the fancy gearing! Did I do something wrong? The only thing I could think of was maybe it is only worth it if the hammer is really heavy and the motor would really struggle or something. [Quebec, Canada]

A: [Mark J.] For readers unfamiliar with John Reid's 'beta', the heavyweight British hammerbot uses an intricate chain 'snail cam' (technical term: 'fusee') that decreases the motor reduction ratio as the hammer accelerates thru its arc in order to keep the motor close to peak output power. This previous post in the Ask Aaron Design and Construction archive discusses both the design theory and how to use the Team Run Amok Excel Hammer Spreadsheet to find a snail cam design solution.

You aren't doing anything wrong in your calculations; the approach you used is quite similar to the one I outline in the above referenced post. You are also correct that the snail cam results in only a small increase in weapon strike force -- I make it about 20%. There are, however, other benefits to the snail cam. In particular the lower starting load on the weapon motor increases motor efficiency and reduces current consumption, making life easier on the motor, battery, and motor controller. This is very important when your expensive motor is stressed very close to its limits! Think of it as an investment in longevity.


Q: I'm planning out a hobbyweight Tombstone-esque horizontal spinner. I was recommended to use an outrunner of 1000-1500 Watts for the weapon motor, and I found one that's 70mm (2.75591'') tall when oriented appropriately. Meanwhile, I found another outrunner that's not much heavier, but has over 2000 Watt power. The problem is, it is 83mm (3.26772'') tall.

The heights of these motors would be added to the thickness of the top and bottom plates, each .25'' (as of the current plan). That for the big motor makes a bot almost 4 inches tall. I'm finding it difficult to locate the dimensions of other hobbyweights, but I have a feeling that this is too tall, and I know a horizontal spinner should be short as possible. Mainly I worry that the required chassis would be too heavy, and my rough calculations put V1 above the ideal 3 pound range.

Any structural advice? Thanks. [New York City]

A: [Mark J.] Building a hobbyweight and you've never been to a live robot fight? Not recommended. I'll get to the structural advice, but I have a few more suggestions first...

  1. Don't let your choice of weapon motor dictate the design of the rest of your spinner weapon. Start with the required energy storage and available weight allowance, find weapon dimensions that will meet those goals at reasonable RPM and 'bite', and then select a motor that will spin your weapon to that RPM in a reasonable length of time. Adding more motor power to a correctly designed weapon will not make it more destructive -- it will only shorten the spin-up time. See the Ask Aaron Spinner Weapon FAQ.
  2. There are very short multicopter motors that deliver ample power for your hobbyweight weapon. Take a look at the 'Hobby King ACK-5312CP' as an example.
  3. Your proportions are off. The weapon system in a design such as yours should weigh about half the total robot weight, and a large diameter weapon will store much more energy than a smaller diameter weapon of the same weight. The spinning bar weapon in pictured hobbyweight 'Fiasco' is 16" long.
  4. Weapon drive can be 'outside' the chassis to reduce the chassis height/weight and improve strength.
  5. Structure: the entire top and bottom plates of the robot should be single pieces. Do not tack the weapon support onto the body of the robot and expect it to survive. The weapon support structure has to survive the same impact that the weapon deals to your opponent. 'Fiasco' uses 0.375" thick 7075 aircraft alloy aluminum for it's chassis, but solid 0.25" thick aircraft aluminum without the fancy cutouts is likely alright. Much more 'meat' around the weapon hub is needed for adequate support.
Don't go crazy with weapon motor power. 'Fiasco' had well less than 1000 watts, but its weapon was nearly as dangerous to itself as to its opponents.

I've edited the text of this next question to remove identifying details at the request of the builder. I've also trimmed some text to get to the question I think is important to share with 'Ask Aaron' readers.

Q: I'm having trouble with the design for my featherweight drum weapon.

My drum is 3-1/2" diameter by 4-3/4" long with two full-length steel impactors. Your Spinner Weapon Calculator shows that my drum has a moment of inertia of 0.0025 kg·m2 and that my 2400kv inrunner brushless motor (14.4 volts, 3:1 reduction) will spin it to about 1800 joules at 11,500 RPM. Drive is a pair of 1300 RPM brushed gearmotors with 5" diameter wheels spaced 13" on center. Plugging all these values into the T.i. Gyro Effects calculator gives a huge gyro lifting force of 57 Newtons on the raising wheel!

I know that featherweight spinners with more stored energy than my design are drivable, so why does my design have such a strong gyro lift? What can I do to reduce that lift besides making the chassis crazy wide? [location withheld]

A: [Mark J.] One of the problems with the weapon design calculators at 'Ask Aaron' is that they're a little too easy to use. It's a simple matter to plug numbers into the input boxes, pick the numbers out of the boxes at the bottom, and run with them. I think I've failed to draw the full picture of what those values mean and how to best use them for modeling a weapon. I'll work on that, but for now let's clear up your concerns.

Weapon speed: You aren't going to get your drum up to 11,500 RPM. Your motor's 2400 kv assumes zero load, but your drum will generate significant aerodynamic drag at high RPM. Aero drag increases with the cube of speed, so while the drag at 4000 RPM is modest, drag at 12,000 RPM is 33 = 27 times as great! This is one reason I recommend weapon designs with a high moment of inertia and modest speed. A wild guess: the weapon might spin to 8000 RPM with your current gearing before drag stops acceleration.

Gyro lift: Buried and inadequately highlighted in the text of the T.i. Robotics Gyro Effect page is a description of the conditions assumed for their calculation of gyro lift:

"...how fast the robot is rotating when you have one side of your drive in [full speed] forward, and one in [full speed] reverse..."

That's spinning in place at full speed! It's an unlikely scenario for a drumbot during combat, and I advise against setting up your R/C transmitter to even allow such a condition. Smooth control at speed in combat is best achieved by limiting the max turning rate at the transmitter. I discuss setting the turn rate in the 'Dual Rates' section of our Transmitter Programming Guide.

The best use of the T.i. Gyro Calculator is to play with the numbers to see how fast your design CAN turn before the gyro effect becomes a problem. My 'back of napkin' calculation shows that with an 8000 RPM weapon speed your design can rotate 90 degrees in 0.4 second without wheel lift -- more than fast enough.

I'll get to work on a new output box for the gyro calculator that will give a maximum spin rate without wheel lift.

UPDATE: I've completed my additions to the T.i. Gyro Effects calculator. The calculator now has two new output fields:
  • 'Max Robot Spin Rate': the time needed for the robot to make one full revolution at the calculated maximum rotation speed of the robot; and
  • 'Max Flat Turn Rate': the fastest the robot can make one full revolution without wheel lift.
Together these values allow the builder to evaluate how much the turn rate of the robot must be slowed to keep it flat on the arena floor, and whether the 'flat turn' rotation speed is adequate for combat. I think this will simplify use of the calculator allow builders to better interpret the output.

Q: Hi Mark, what is an EDF brushless motor, and is it a good idea to use them on lighter bots to power spinning weapons? [Sydney, Australia]

A: [Mark J.] There's nothing special about an 'EDF' motor; they're just brushless inrunners suitably sized to power an 'Electric Ducted Fan' in R/C 'jet' aircraft. In general an 'outrunner' style motor of similar power will spin at lower RPM and will be easier to adapt to a combat robot weapon in a light combat robot.


Q: What is tip speed? [Kansas City, Missouri]

A: [Mark J.] The outermost edge (tip) of a spinning weapon travels a circular path around the axis of rotation. With each rotation the edge of the weapon travels a distance that can be calculated by a simple formula:

Distance Traveled = 2 * Distance from the Tip to the Axis * Pi

If we then multiply this distance travelled in one revolution by the number of revolutions per second we obtain a distance travelled by the weapon tip in one second -- the tip speed.

Example: a simple spinning bar weapon has its impact tips located 250mm from the spin axis. If this weapon spins at 4000 RPM the tip speed of the weapon is:

Tip Speed = 2 * .250 meter * 3.1416 * (4000 / 60) = 105 meters per second = 235 MPH

Tip speed is often used as a 'brag number' by builders, but it is a critical number for drum spinners. When two drum spinners strike 'weapon-to-weapon' the weapon with the greater tip speed has an enormous advantage.

The Team Run Amok Spinner Weapon Kinetic Energy Calculator will calculate your weapon's tip speed, as well as the weapon mass, moment of inertia, stored kinetic energy, and spin-up time.


Q: Alright, I'm here with the last question for a while (to let other people ask questions) - I've been really interested in HUGE drum spinners. However, I'm needing to look for some inspiration for a design. I currently have a few "big drum spinner" designs in mind, like REDRUM, Fluffy 2, and Barber-Ous. However, I'm looking for some other large drum spinner designs as well. Note when I mean huge, I mean freakishly huge (larger than Yeti's). Also, I do know the pros and cons with drums that are much, much bigger than normal (I did do a bit of research already on that). Any names you can think of to look up and research in terms of HUGE drum spinners?

Thank you for responding to all of my questions! [North Carolina]

A: [Mark J.] I do recommend studying successful designs related to your own ideas, but it's also important to look into the challenges created by pushing design elements beyond conventional scale. For a 'freakishly huge' drum you'll want to explore the gyroscopic effects caused by trying to turn a robot with a big vertical spinner. Some robots' gyro-dance is so extreme as to render them useless in combat. I've reprinted a fairly recent post from the Ask Aaron Weapons Archive to give you a start:

Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator

Run a few huge drum designs thru the calculator to see if you can find something stable and effective.


Q: Hello (again). I am here (again) with a question! This one isn't about designs regarding my bot or scaling. My question this time involves flails. Simply put, what chain length to flail weight ratio is the best? Also, why has nobody ever mounted flails vertically, like on a drum or a flywheel? Finally, would a horizontal or vertical mounted set of flails do best? [Urbana, Illinois]

A: [Mark J.] If you run the numbers it is immediately obvious that the ideal chain length is 'zero'. A rotary weapon stores kinetic energy and delivers that energy in a single massive impact on your opponent. Flails are entirely ineffective at transference of stored kinetic energy in that manner.

You can't spell 'FLAIL' without 'FAIL'.

There has never been a successful flail robot -- either vertical (like 'Morrigan' from Near Chaos Robotics) or horizontal -- and I don't expect to ever see one. The only thing they do well is make noise. Avoid.


Q: Hello! I'm here with a new question. I was watching the newest season of Robot Wars (for the second time) and couldn't help but notice Hobgoblin being the only Beater Bar. Not only that, but I can't think of any other beater bots of that size or even close to that size. My question is this - why aren't there more beater bars in the big competitions? [Urbana, Illinois]

A: [Mark J.] A beater bar weapon is simple and very efficient at storing energy in a small 'bot (see the 'energy storage' section of the Ask Aaron Spinner Weapon FAQ) but as the size of the weapon increases the Square-Cube Law takes hold and the required cross section of the support arms grows rapidly in order to provide the strength required to survive impact. See also this post in the Ask Aaron Design and Construction archive from 'Buzzards Bay' about scaling up small 'bots to larger sizes.

Compare the weapon proportions of heavyweight 'Hobgoblin' (below left) to those of beetleweight 'Wave of Mutilation' (below right). The thickened support arms and impactor bars of Hobgoblin's weapon are necessary for strength in a heavyweight-scale beater, but they move mass toward the center of rotation and greatly reduce the energy storage efficiency of the design. A mini-disk or drum of the same weight and radius would be stronger and would store much more kinetic energy at heavyweight size -- which is why you don't see heavy class eggbeaters.

Incidentally, the RioBotz Combat Robot Tutorial section 2.2 spends three pages arguing that the square-cube law doesn't much apply to combat robots. Given that RioBotz tend to design large robots and then scale them down to lighter classes, the result of their ignoring the square-cube law is not disastrous -- but have you noticed that their larger 'bots do better than their smaller clones?


Q: Hi, my team is planning to build a [heavyweight] vertical spinner as shown in the image. The diameter is about 16 inches, and the total mass is about 43.358 Lbs. The bite is 2 inches. What would be the optimal RPM? [Buzzards Bay, Massachusetts]

A: [Mark J.] Notes:

  1. Listing weight to three decimal points implies precision much greater than 'about'. Let's round it off to 40 pounds, OK?
  2. 'Bite' is a theoretical calculated figure based on weapon speed, the number of impactors, and the closing rate of your robot and the target. It is unrelated to the shape or height of the impactor.
  3. See the Rotational Speed section of the Ask Aaron Spinner FAQ for a full explanation of bite and how to determine a reasonable RPM.
  4. There is no single 'optimal' RPM. Speed helps determine bite, and the needed bite depends as much on the design of your opponent as it does on your weapon. Again, this is explained the the Spinner FAQ.
  5. Those sharp 90 degree internal angles adjacent to the impactors are stress risers -- you'll want to smooth those out.
Read the whole Spinner FAQ while you're there -- it will save you from writing back four or five more times.

Then this missive from ChaosCorps arrived in my mailbox:

Hi Mark, figured I'd write in quickly on the post from the builder in Buzzards Bay, Mass.

Not sure if the disk image shown is the one he provided or not, but if it is I can explain a bit of what's going on with it, since it's the disk we used on Bombshell during BattleBots season 2. The disk itself is only part of our disk setup, the softer steel main body. The disk was designed to use S7 tooth inserts that key into the unusual cutouts and are retained with a high strength bolt. We also cut the disk on a waterjet which means those sharp corners actually did have a small radius that wasn't necessary for modeling the part. Between the disk being 2" thick and a softer steel which minimized the chances of a brittle failure it proved to be sufficient in our application.

As a side note, we released the full set of CAD files for the season 2 version of Bombshell.

Thanks, ChaosCorps! That image was provided by Buzzards Bay, and although I thought there was something familiar about it I failed to recognize its source. Knowing that the material is soft steel with impactors that drop into those cutouts makes much more sense. Made from hardened steel those sharp cutout angles would be trouble.

It's generous of you to post the CAD files; not many teams offer design details to help other builders. So... how fast do you spin it?

We run the vertical disk on Bombshell with two A28-150's on two series 4s lipo packs (29.6v nominal) with a 2.5:1 reduction. That puts the no-load speed at around 3,000rpm.

Thanks again! Buzzards Bay: you can watch the videos and see the effectiveness of this weapon at 3000 RPM.


Q: I am looking at building a 30 pound robot this year, and had a question for the lifter which will be around a foot long.

I was looking at using a banebots 256:1 or maybe more gearbox and a brushless motor for the lifter, but reading they say to not expose the gearbox to more than 35 ft-lbs. I'll gear the gearbox down more with a chain to the lifter arm so lifting 30 pounds should be fine, but if I run into the wall or the other robot or a weapon with the arm, how do I keep the gearbox from breaking on the impact when all that force goes back into it? [Quebec, Canada]

A: [Mark J.] Plug 'torque limiter' into your favorite search engine. There are a wide range of industrial devices that will limit torque to the range you seek. If you want to keep it simple (and cheap) a V-belt/pulley system in place of your proposed chain/sprocket can be set to slip as the torque approaches your 35 ft-lb limit.

Alternately, consider a lifter design similar that used in heavyweight 'Polar Vortex'. Their lifter mechanism presses down against the arena floor and is isolated from impact forces acting on the wedge.

Note: brushless motors have very poor low RPM torque compared to brushed motors Consider a brushed motor for your lifter.


Q: Hey there Mark,

I've been pondering a concept for a while and am wondering if it would even do anything different, or in fact, would create a new world of problems.

Many bots armed with spinning weapons use some sort of gear reduction in order to take a high RPM motor and bring it down a bit in order to increase torque and spin up time. But what if it were reversed? Instead of using a 1000kv motor with a 2:1 reduction, would anything change if we used a higher torque 500kv motor with 1:2 "reduction"?

David R. [Livermore, CA]

A: [Mark J.] Builders generally choose a weapon motor with the lowest RPM for a given motor weight and power so that the smallest gear reduction ratio possible is used. This assures that the reduction is efficient, using light and compact components. Consider:

  • Power is calculated as the product of RPM and torque. Amongst motors of similar size and weight, motors with lower kv factors will tend to have both lower RPM and lower torque, which equals lower total power. You can't make up for reduced power with gearing.
  • A motor with half the kv rating of another motor would need to produce twice the torque to output the same output power. The motive physics of electric motor design effectively say that isn't going to happen. You'd need to go to a larger and likely heavier motor to make that work, and weight is a precious commodity in combat robots.
  • Your math is a bit off. If you reduce motor RPM by 50% you need to decrease the gearing by a factor of [1 / 0.5 = 2] to maintain the same output RPM -- your example 2:1 reduction would become a 1:1 reduction rather than a 1:2.
Builders are already carefully selecting components to maximize the performance of their weapon systems for weight, power, and stored energy. You're not going to find significant improvement by further tinkering with motor kv ratings.
Q: I want to replace the brushed drillmotor in my hobbyweight bar spinner with a brushless motor. I'd like something with comparable RPM so I can use my existing 2:1 belt drive without ruining the 'bite'. In my last event I was beaten by another bar spinner with a much heavier bar and what I think was a Turnigy D2836 brushless motor. Could that motor work for me if I increase the bar mass and keep the original drive? [Rather Not Say]

A: [Mark J.] I suspect your opponent was running something larger than a Turnigy D2836. Motors that size are commonly used for beetleweight weapons and even some antweights. Power is comparable to the drillmotor you currently use, whereas a typical hobbyweight brushless weapon motor is well into the 1000 watt power output range.

The favored weapon motor in any given class changes frequently. I suggest that you check in at the 'Combat Robotics' group on Facebook for advice on the current 'best' motors and ESCs for your weight class.

Your current weapon speed is conservative for a sub-light spinner. If you're worried about bite, you can shorten and counter-weight one end of your blade to make your weapon a 'single-tooth' bar. The shorter end will never hit, giving you twice as much bite or allowing greater RPM. See the Ask Aaron Spinning Weapon FAQ for more on single-tooth weapons and bite.


Q: I have a robot idea which involves a 4 wheel drive and a weapon that is both an axe and a flipper at the same time (ie, it is a flipper which is rear hinged but able go 180 degrees). Which would be the best motor for the weapon, and which would be the best motor for a 4 wheel drive if it is a heavyweight? [Uxbridge, England]

A: [Mark J.] Axe and flipper go together like anchovies and ice cream. An axe head needs to be out at the end of a long handle so you can swing it up to speed, but you wouldn't deliberately design to lift the entire mass of your opponent way out at the end of a long arm. Consider:

  • The power profile of an axe weapon is very different from a flipper. An axe requires a drive that will accelerate the relatively light axe head on the end of a long arm to very high speed over the full span of the weapon throw, while a flipper requires enormous thrust to accelerate the entire mass of your opponent upward over the span of just a few inches. The same mechanism simply isn't going to do both jobs.

  • While an electric motor can drive the axe head on a long arm up to speed in 180 degrees of arc, truly huge torque is needed to lift (let alone flip) a 100 kg mass way out at the end of that same long arm -- enough torque to shatter a reasonably scaled gearbox. You can build an electric axe, but a decent flipper requires the explosive power release available from pneumatics.
As for the drive motors: selection depends on multiple factors about your design, pocketbook, skill level, and expectations that you have not shared with me. See Frequently Asked Questions #21. We gladly provide tools to assist builders in component selection but we won't do your homework for you.

Note: According to BotRank, there has never been a combat robot named 'Anchovy Ice Cream'. Help yourself.


Q: How to calculate spinup time for a weapon running on a brushless motor. I'm building a combat robot with an asymmetric single tooth disc running on two brushless motors. [Maharashtra, India]

A: [Mark J.] -- Why?

Your question has been previously answered here at 'Ask Aaron'. You are free to search for it -- should you find the determination to do so. Look here for a start: brushless motor power curve.


Q: Hey Mark can you comment on the vertical spinner of professor chaos. [Madhya Pradesh, India]

A: [Mark J.] -- Why?

You are welcome to search the archives for answers to your question. There are a multiple posts about 'Professor Chaos' in this archive. Search for 'Professor Chaos'.


Q: how does a spinneris attach on a ring spinner? i saw the ringmaster build photo... but how do it fit up there? [Quebec, Canada]

A: [Mark J.] The spinning ring on 'Ringmaster' is assembled in three parts. The impactor ring simply drops into position where it meshes with the drive gears. Two 'cowls' (dark grey in the photo) are then positioned to rest on the guide wheels above and below the impactor ring. The cowls are bolted to the top and bottom of the impactor ring, sandwiching the impactor in place and positioning it.

Q: But is there a better way to do it? like in a hobby or beetle?

A: Ring spinners aren't effective in any weight class. They have too little mass in the ring to be damaging and the structure is too complex to be reliable. People build them to show off their shop skills -- they are pretty!

The 'better way' is to build a conventional weapon.


Q: After some thought I think I want to make an undercutter type spinner either 12 or 30 pounds. I came up with the attached sketch, nothing is to scale, which is my question.

First does this make sense? I was thinking that making the part as close to the blade as strong as possible was the way to go, and tapered bearings are stronger than regular ones as long as the bolt is tightened right.

Second, how do you figure out the proper size of everything? I have no idea how big of a shaft I should be using, how big of bearings to use, how close I am to the blade cracking because I put too much stuff through it, ect. [Baton Rouge, Louisiana]

A: [Mark J.] Multiple suggestions:

  • Scroll down the page a few posts to find the undercutter layout question with an attached diagram. The two weapon bearings must be well separated to eliminate destructive 'moment loading'.
  • Tapered bearings are not commonly used for spinner weapons. The primary bearing loading when the weapon 'hits' is radial -- the lesser axial loading is accommodated by the freedom of the robot to move along that axis in response to the impact force. Standard ball or roller bearings will do nicely.
  • Find some 'build logs' for robots of your proposed style and weight. Like it says in the FAQ: "Look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger."
  • Grade 8 bolts are very strong in 'tension' -- stress applied along the length of the bolt -- but bolts in general are not designed to withstand lateral 'shear' forces of the type your design places on the weapon shaft. An undercutter in the sub-light weight classes should use a 'tougher' steel shaft that can absorb shock loading without breaking.
  • The hub and pulley must be attached to the shaft in a manner that prevents rotation. Set screws suck. Ideally the pulley, hub, and shaft are all broached for a keyway. Keyed shafts @ Robot Marketplace. Examples of keyways and other attachment options are available in the Ask Aaron archives.
  • The fewer holes you put on your weapon blade the better, and minimal localized stress on any hole that does exist is critical. The preferred blade hub design is a 'clamping' style that holds the blade in place without adding bolt holes thru the blade itself. There are examples of clamping hubs in the Ask Aaron archives.
Don't try to re-invent the wheel. Find examples of successful weapon mounting designs, study them, and base your design on what you have learned.
Q: I have desgined (not yet built) a beetle weight robot with a drum that lights up (with LEDs for entertainment purposes) but what if I spin my drum like so fast that it wont cause damage? I'm sure I'm going to get 'The Burger Is Bad' or 'Check the Archives'. [TCRM, Malaysia]

A: [Mark J.] The hamburger is good and you don't need to check the archives. Read thru the Ask Aaron Spinner FAQ. The 'Rotational Speed' section of that FAQ discusses the factors involved in getting the weapon to 'bite' into your opponent rather than just 'skitter' across them without doing any damage.

Read the whole Spinner FAQ while you're there -- it'll save you a lot of time and trouble.


Q: do you have a drawing on how an undercutter blade is fixed to a bot? i know u did for a fbs and a horizontal spinner. [Quebec, Canada]

A: [Mark J.] The mounting is pretty much like an FBS turned upside-down. The weapon shaft is rounded on the ends and usually sticks out both below the blade and above the chassis to allow the 'bot to invert.


Date marker: January 2017
Q: Why does RioBotz choose to have 13 radii and 18 sections [in the design of their integrated single-tooth 'snail drum' weapon]? [Dublin, Ohio]

A: [Mark J.] The selection of 18 initial sections was arbitrary -- it came from dividing the 360 degree polar coordinate plot into twenty-degree sections. That seemed to be a reasonable number of facets to machine into the drum in the final design. Two of the sections (40 degrees) were combined for the impact tooth at full radius and one section is the curved 'tooth notch' designed to reduce stress concentration that has no single radius. Then:

"After observing the nearly flat shape of the optimal solution in the regions between 220o and 320o... the algorithm [was] re-evaluated considering only 2 flat sides in such region. This new optimal solution is very similar to the previous one, but it is easier to machine due to the reduced number of facets." -- Drum Shape Design and Optimization Using Genetic Algorithms

The creation of just two facets ('k' and 'l') in the region from 220o and 320o reduced the number of radii to thirteen.


Q: Hi Aaron, could you tell me, for a spinning drum that stores 10,000J [30 Lbs class] what is the difference between a 10mm bite and a 20mm bite? Thanks. [Valle del Cauca, Colombia]

A: [Mark J.] Bite (what's bite?) is calculated as a maximum depth of opponent insertion into the arc of a spinning weapon at a given weapon RPM and forward velocity. You'll get that maximum bite rarely, just like 13 black only comes around rarely on a roulette wheel. Sometimes your luck will be very poor and you'll hit your opponent just as an impactor is facing them and get no bite at all! On average, you'll get half the max bite -- less as your attack speed drops.

  • Consider an opponent who was wise enough to avoid exposed sharp edges in their robot design. Given a nice sharp angle to attack you don't need a lot of bite, but if you're forced to attack a flat or gently curved surface you need all the bite you can get.
  • Greater bite also is a bonus when your attack velocity drops. A weapon with a lot of bite can still be effective in close quarters when you don't have a chance to back away and take a high-speed run. Watch some combat videos and pay attention to the speed at which most weapon hits are made.
What's the difference between 10mm and 20mm bite? A few more match wins. Bite is good -- more bite is better.

I've been writing quite a bit of JavaScript lately, so what's a little more? Take a look at the new 'Bite Calculator' in the Spinner Weapon FAQ.


Q: Why doesn't 'Witch Doctor' have gyroscopic forces acting on it? One side doesn't lift. [Reston, Virginia]

A: [Mark J.] See this post on gyroscopic forces a little farther down in this archive.


Q: Hey Mark,

How does the flipper on Lock jaw in Battlebots Season 2 harness the power of the springs? From what I could tell it was winched back but how was it able to fire then reset again? Additionally, could this method of flipping be utilized in all other weight classes as an alternative to pneumatics? Thanks in advance! [Straight Outta Facebook]

A: [Mark J.] Donald H. doesn't divulge much about his robot designs, and I can't see enough detail in the photos of 'Lock-Jaw' to understand the clutch mechanism. Fortunately there are builders who do share their spring-powered flipper designs:

  • Dale Hetherington has built pretty much every exotic robot design there is. Take a look at his very detailed build log for 'Dead Air'.
  • I'm personally very fond of the snail-cam spring loader for it's simplicity and adaptability to small weight classes.
That will get you a start.
Q: Hello Mark,

I've heard it said that Wrecks' vertical disk (30-35 lbs) has a much higher Moment of Inertia than Electric Boogaloo's vertical spinning weapon (~70 lbs). With the assumption that Wrecks is using a similar motor (big assumption), how is this possible?

-David R. [Livermore, CA]

A: [Mark J.] A little clarification:

The Moment of Inertia (MOI) is NOT a direct measure of how much energy a spinning weapon stores; the motors or speed of rotation have no bearing on the moment. MOI is a measure of the energy needed to change the rate at which the weapon is spinning. Its value depends on the mass of the weapon and (most importantly) on how that mass is distributed relative to the rotational axis.

Consider:

  • A small chunk of matter is rotating around an axis at a given RPM at a distance of six inches. With each rotation the chunk of matter travels a distance of ( 2 * 6" * pi = ) approximately 37.7".
  • That same small chunk of matter is now rotating around an axis at the same RPM at a distance of 12 inches. With each rotation the chunk of matter travels a distance of ( 2 * 12" * pi = ) approximately 75.4".
  • The chunk of matter 12" from the axis must be travelling twice as fast as it did when only 6" from the axis in order to complete a revolution in the same length of time.
  • The kinetic energy of a moving object increases with the square of its velocity (K = ½mv2), the mass 12" away from the axis has four times as much kinetic energy as an equal mass 6" from the axis when rotating at the same RPM.
  • Doubling the distance of the mass from the rotational axis doubled the speed of the mass, which raised the energy stored by the spinner at any given speed by a factor of four (velocity2). This also increases the MOI of the spinner by a factor of four, even though the mass of the spinner has not increased.
Now, let's take a look at the spinner weapons on 'Electric Boogaloo' and 'Wrecks': 'Electric Boogaloo' has a bar spinner weapon - closer to 50 pounds than 70. A bar has a lot of its mass close to the rotation axis and relatively little mass far from the axis. The formula for the MOI of a rectangular bar spinner is:

MOI Bar = (mass/12) * (length2 + width2)

'Wrecks' has a large diameter spinner with most of the mass concentrated in a ring at the outer edge -- far away from the axis of rotation. The formula for a the MOI of a thick ring (discounting the supporting spokes) is:

MOI Ring = (mass/2) * (inner radius2 + outer radius2)

Let's use the new Run Amok JavaScript Spinner Weapon Calculator to compare the MOI of the two designs based on rough estimates of their sizes:

  • A steel bar similar to the spinner weapon used by 'Electric Boogaloo': 455 mm long by 160 mm wide by 40 mm thick weighs 22.7 kg and has an MOI of 0.440 kg·m2.
  • A steel ring similar to the spinner weapon used by 'Wrecks': 300 mm outer radius, 200 mm inner radius, and 12 mm thick weighs 14.7 kg and has an MOI of 0.956 kg·m2.
The ring weapon is less than 2/3 the mass of the bar weapon, yet has more than twice the Mass Moment of Inertia. That's a much more efficient use of weapon mass for energy storage.

Q: How do electric hammers not burn out [...just down the road from Ashburn, Virginia]

A: [Mark J.] If you aren't careful they do burn out.

The motor for an electrically powered hammer weapon needs to be powered off at either end of the weapon swing to avoid an extended 'stall' condition where the motor would consume damaging current levels. This can be done a couple of ways:

  • Most builders control the hammer with one of the spring-centered joysticks on the transmitter and release the stick at either end of the weapon swing. Just don't forget to release the stick!
  • Some R/C relay boards and a few ESCs (like the Talon SRX) have provisions for 'limit switches' to sense when the controlled device has reached the end of its travel and automatically shut off power in that direction. This allows an electric hammer to be actuated by a simple single-throw switch on the transmitter -- typically channel 5.

Q: What is the best motor to use for a horizontal spinner [Los Angeles, California]

A: [Mark J.] The Hamburger is Bad.

See Ask Aaron Spinner Weapon FAQ.


Q: Hello! I am a high school student that I building a new robot. I am building a drum spinner, and that spinner will be operated by a Brushless motor with the specs of:
  • Turns: 10T
  • Voltage: 12S Lipoly
  • RPM/V: 560kv
  • Motor Poles: 10
  • Internal resistance: 0.017 Ohm
  • Max Loading: 100A
  • Max Power: 4200W
I currently do not have the dimensions of weapon itself, but I am interested in knowing the "equation" in finding the speed of the weapon at full speed. I have the Wh of the battery, Volts, and most of the other specs. Also, where can I find the equations to calculate torque, and stall torque. [Dublin, Ohio?]

A: [Mark J.] What's wrong with this group of questions?

Given the syntax, grammar, and language structure, I have trouble believing that the author is from Ohio. Further, the motor and weapon design are unusual for a combat robot that might be constructed by an American high school student in the mid-west.

If the author is a high school student in Ohio and they are building a combat robot this size, they should have a local mentor to guide them in design, construction, and safety. The mentor should be providing the answers to questions like these.

Either way, I'm not comfortable answering your questions. The best I'm willing to do is to point you to this Wikipedia article on 'Motor Constants' and warn you that stall torque on brushless motors is much less than the calculated value due to the software in the motor controller limiting current at low motor speed.


Q: How to make 2 [pneumatic] cylinders work in sync? Use 2 buff tank and 2 valve or 1 buff tank with 1 valve [to] supply 2 cylinders? Thank you (just like Subzero) [Yunnan, China]

A: [Mark J.] I see that you've just asked the same question of Subzero's builders on their Facebook page. The better question might be, "Why use two pneumatic cylinders instead of just one of larger diameter?"

If I had to use two cylinders I'd want both to fill from a single gas supply thru a single valve to avoid any pressure imbalance. Why don't we wait a few hours and see what the builders say?


Subzero's builders did reply a few hours later:

Team Hammertime / Teamxd: That version was one supply tank with 2 rams, one valve and no buffer tank.

Q: What's the best & safest way for someone who's only done non-weaponed bots to do their first weaponed one? [New Jersey]

A: [Mark J.] I'm not sure how to respond to 'best' but I can offer some safety guidance. You didn't mention how large a robot you are interested in building or the type of active weapon you are considering, so I'll have to keep this general.

  1. Review your basic workshop safety practices:
    • Wear eye protection when there is danger of flying chips, abrasive dust, or irritants.
    • Remove rings and other jewelry before operating machinery.
    • Keep your workspace and the floor around it clean and un-cluttered.
    • Securely anchor materials being drilled, ground, or machined.
    • Cover and secure sharp edges and points in the work area when not in use. Use gloves where appropriate.
    • Do not wear loose clothing around power tools -- a work apron may be appropriate.
    • Monitor your lithium battery charging, and use a suitable charging container if needed.
    • If it's gonna be loud, wear hearing protection.
  2. Keep safety restraints on weapons that prevent them from operating unexpectedly until you are ready to test/use the weapon.
  3. Do not test or operate your weapon without containment suited to your weapon and robot.
  4. Be cautious of mechanism pinch-points: articulated levers, chains, hinged surfaces.
  5. Keep your design and expectations at a level appropriate for your building skills and resources.
  6. If you aren't sure you understand the safe operation of specialized equipment, fittings, or assemblies -- ask!

Q: Dear Aaron, which horizantal spinner has more effect on the other bot? And to you? Thx, [Google Fiber ISP]

A: [Mark J.] I'm not sure I understand your question. The base physics of a horizontal spinner are the same if it's a bar or disk -- or a top/mid/undercutter. The effectiveness depends on other factors such as energy storage, 'bite', and chassis stability. Suggest you read the Ask Aaron Spinner Weapon FAQ and then send in a more focused question.


Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator


Q: Hi again, I've stumbled upon a problem, the snail cam spring reloader [needs to stop] after one full rotation, I cannot seem to find a suitable solution to do so with a gear motor. Do you think a stepper motor is better for this or is there a way to make motor start and stop after one rotation and a push of one button if you will. [Bristol, UK]

A: [Mark J.] When you turn off the windshield wipers on an automobile have you noticed how they continue for the rest of the wipe stroke and then stop in the park position? That's what you're looking for and here's how you do it:

  • The circuit shown at right allows the gearmotor to run until the flipper arm (not shown) reaches the fully loaded position and presses down to open the 'interrupter' micro switch -- stopping the gearmotor.
  • A momentary closure of the normally open R/C switch re-starts the gearmotor long enough to fire the flipper, which closes the interrupter switch.
  • The gearmotor then continues to run thru the rest of the reload cycle to an automatic stop.
The positioning of the micro switch is simplified if you use a lever-style switch that can be bent to fine-tune the switch point. The micro and R/C switches must have enough current capacity to handle the gearmotor, and the micro switch must be wired 'normally closed' (NC). For larger robots the micro switch can trigger a relay with the capacity to handle the motor load.

Q: I was actually thinking to have snail cam consist of 2 shapes, 1) the main cam and 2) a smaller cam with sudden increase in radius where reloading needs to stop that way there is no need for finest tuning. [Bristol, UK]

A: You have lots of options on the interrupter. A micro switch is simple, but you can certainly use other sensor types: infrared emitter/detector, inductive proximity, magnetic... whatever you're comfortable with.


Q: I was wondering if you have heard of or made any progress on getting the T.i. 4 Bar Simulator ported to a newer version of Windows. Thank you for any information and providing a valuable service. [Kansas]

A: [Mark J.] The author of the T.i Four Bar Simulator wrote the code back in 2007 when he was still in school. He tells me that he's not sure where the source code is, but that it may or may not be on an old desktop which he believes is stored in his parent's attic. The best he could offer was to take a look in the attic the next time he went home for a visit. In my mind I picture a dust-covered computer leaning up against the holy grail and partially covered by a lost da Vinci manuscript.

I keep an old Windows XP desktop next to my 'Super Nintendo' console in a dark corner of my basement just so I can run 'Four Bar' and play 'Donkey Kong Country'. A similar set-up may be your best option as well.


Q: Hi there,
I've got an idea for a spinner robot where the rotating ring spins inside a circular chassis, and once it reaches a sufficiently high speed, two 'teeth' on the ring are extended (to protrude beyond the chassis) by centrifugal force. On impact, the ring is slowed, meaning the teeth retract again (using a spring), leaving the ring to spin up again without external resistance. This would mean the ring could spin up and inflict damage on an opponent, even if the opponent was continuously in contact, since the teeth would be shielded by the chassis until the ring was at full speed.

I haven't been able to find any examples of a robot which uses this system, so my question is, are there any, and if not, why not? I feel there must be some fundamental reason why robots don't use such a technique - do you have any suggestions?

Many thanks, M [Bournville, England]

A: [Mark J.] I know of no examples of such a weapon system in use, and I think I know why there are none.

Spinner weapon teeth take huge abuse. The entire force of the weapon is transferred thru them to the opponent. Typically they are made of hardened tool steel and set into well braced recesses in the weapon body where they are secured by the best quality bolts obtainable. Still, an impactor tooth's life is short. A good impact can and will shear them away.

Your proposed design places the impactors on pivots which would themselves bear great impact forces, as would the stops required to restrict the tooth's outward motion. Moving parts subject to high loading are bad. Adding more parts subjected to high loading is worse. Simple is good.

Consider the situation just after your weapon impacts your opponent. If your opponent is still there to restrict your weapon from spinning back up, then your weapon isn't doing its job. Your opponent should be flying away from you at great speed, unable to prevent your weapon from spinning back up.

I don't believe that the benefits of a retracting-tooth weapon would offset the added complexity and fragility. The mass of the circular chassis shield would be better put to use in the mass of the spinner and the weapon motor. Keep it simple; simple robots win.

I just got more curious than normal and ran a calculation on the force needed for a spring to hold an impactor in against the outward centrifugal acceleration on a typical weapon. At 3000 RPM an object 250 mm from the axis of rotation sees 2500 gravities!

Q: Hey, M's weapon sounds like 'Greenspan' that used a flywheel with free spinning hammers. [Dublin, Ohio]

A: [Mark J.] I don't think that's what 'M' has in mind. There were a lot of 'flail' and 'pivot hammer' weapons in the early days, but those designs have been replaced by fixed impactors that are better at transmitting the full energy stored in a flywheel as a single big hit on the opponent.

The 'M' weapon shields the flywheel from direct contact with the opponent which allows it to spin-up even if the 'bot is in contact with the opponent or another obstruction. The impactors are held inside the protective bumper until they approach full speed, and then they either slide or pivot outward beyond the bumper in a fashion that locks them against lateral movement. See my sketch of a (poor) pivoting impactor design at right.

These types of retractable impact teeth would not swing out of the way on impact in the way Greenspan's hammer did -- they would deliver an unyielding blow. That's good, but my objection is that the sliding or pivoting mechanism would be a weak point subject to failure. Complex is bad -- simple is good.

Comment: To back your statement on M's weapon, I remember there was a Beyblade battling top kit called "Wing Attacker" which had that very setup. It... wasn't very good. [Arden, North Carolina]

Reply: [Mark J.] I'm not sure that Beyblade performance transfers to combat robots, but someone might appreciate that data point.
Q: After doing the calculation for my featherweight horizontal spinner, i got 1484 joules of energy. Is it enough? [Quebec, Canada]

A: [Mark J.] "How much energy should my spinning weapon store?" is the first question in the Ask Aaron Spinner Weapon FAQ. You can read your answer there. I suggest that you read the rest of the FAQ as well.


Q: I saw in my beta question you don't like electric hammers, what about pneumatic? in my first look at the math, it seems like pneumatic hammers are pretty tame too. Unless I did the work math wrong, you'd need a gargantuan cylinder like chomps to even break 1 KiloJoule which even some nastier 3 pound spinners beat. [Dublin, Ohio]

A: [Mark J.] Since you didn't include your calculations I can't check them, but a pneumatic system can provide much greater force than an electric system of comparable weight.

  • Are there any electric flipper robots out there? There are lifters, but there are no flippers -- an electric motor/actuator/solenoid cannot provide anywhere near the explosive release of power available from a comparable pneumatic system.
  • Also consider the great complexity, expense, and effort expended by Team Hurtz to construct their electric hammer. It is a beautiful piece of work, but a comparable pneumatic weapon could be built from off-the-shelf components at a fraction of the cost.
I'm not a fan of hammers in general, but if you're going to build one it makes sense to use pneumatics. Assuming perfect gas flow, a 3" diameter pneumatic actuator at 250 psi provides 3.14 * (1.5^2) * 250 = 1,766 pounds of accelerating force. Try to match that with an electric motor. The trick is in getting that 'perfect' gas flow...
Q: I have a third grader trying to answer a science fair question about the effectiveness of vertical vs horizontal spinners. He's built a bot out of a Thames and Kosmos building kit but he can't get enough power out of the motor to get spinners to do any damage to a piece of styrofoam. Spinner just stalls when it hits the foam. That's our problem to deal with, but:
  1. Are there equations we can run to determine the answer to this question?

  2. Does the angular velocity of a spinner change if it's in the vertical position, if all other things are equal?

  3. Does gravity assist or impede?

My engineer-son is asking questions his English-major mother can't answer. [Raleigh, North Carolina]

A: [Mark J.] You want to run some angular momentum equations for your third grader's science fair project?!?! My third grade teacher was still trying to get us to stop counting on our fingers. I guess things have changed.

It was my third-grade son who led his biology-major father into this mayhem. I'll be pleased to assist as best I can.

  1. Read thru the Ask Aaron Spinner Weapon FAQ. The entire FAQ will prove instructive, but you may find particular interest in the large, friendly blue text box near the top of that page describes the principle of spinning flywheel weaponry:

    General Principle

    Spinning weapons are flywheels. They rely on rotational inertia to collect energy from a continuous power source (electric motor, internal combustion engine...) over time and store it as rotational kinetic energy. On impact, the flywheel releases the stored energy in a blow that far exceeds the energy directly available from the continuous source.

    From your description of your son's spinner stalling, it is apparent that it does not have adequate rotational inertia to store sufficient kinetic energy from the small motor powering it. You could use a more powerful motor, but as a display of physics it would be much more interesting to increase the rotational mass of the weapon and note the change in the performance of the spinner.

    Here are your equations: How to Calculate Rotational Kinetic Energy, and I think this explanation of Kinetic Energy and Mass Moment of Inertia in Combat Robot Weapons might fill in some of the gaps.

  2. If you look thru the equations referenced above you'll discover that nowhere in the calculation of momentum or velocity is there a mention of horizontal vs. vertical orientation; the energy of the spinner system is not changed by its orientation.

    However, going from a horizontal to a vertical orientation does effect the performance of the weapon in another way. When a spinner weapon impacts the opponent there is both an action on your opponent and a reaction on your 'bot.

    • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. Since your 'bot is supported by the arena surface, it does not move and most of the impact energy is transferred to your opponent.
    • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of the impact is split between moving the two 'bots in opposite directions. The desired transfer of damaging impact energy to your opponent is much less efficient.

  3. A balanced spinning mass is neither assisted nor impeded by gravity. In a vertical orientation the effect of gravity on the rising side of the mass is perfectly offset by the gravitational effect on the descending side. In a horizontal orientation nothing is rising or descending.
I threw a whole lot of information at you, but I think you can pick thru it to find answers that make some sense to you. New questions will arise -- write back as needed.
Q: I am in 150g weight competition, I was wondering if its possible to make a spring loaded spike/ram with ability to reload it, do you have any resources I could have a look at? [Bristol, England]

A: [Mark J.] Take a look at this archived post describing a spring-powered flipper reset by a rotating snail cam. With a little imagination it could be oriented to reload a spike, although a flipper is a more effective weapon.

Comment: Thank you, i was in fact going to do a flipper inspired but that very video, there is a lot of useful info in that other answer!


Q: "Melty Brain" robots do not count as having an active weapon under current Battle Bot rules. Have you ever seen a melty brain style robot that was paired with another weapon? I imagine that the robot's rotation could add even more energy to a hit from a spinner. [Westerville, Ohio]

A: [Mark J.] No, and it won't.

The concept of a 'melty brain' spinner is that the entire mass of the robot becomes a spinning weapon. Stealing mass and energy from this very efficient primary weapon and trying to add it back with a secondary weapon will do no better than break even on energy, and will add undesired complexity. Use all your weapon weight allowance on a single weapon. Simple robots win.


Q: Has there ever been a counter rotating vertical spinner in a robot? I'm thinking of two large disks parallel and close to each other on the same dead shaft. Only one would have teeth and they would have one or more small perpendicular drive wheels between them causing the counter spin. The reasoning behind building this would be to lessen gyroscopic effects. Thanks! [Minnesota]

A: [Mark J.] I know of two big 'bots that were designed to nullify the annoying gyroscopic forces associated with vertical spinners. Both used mechanically simpler solutions than your proposed co-axial counter-rotating disks:

  • Team Boilerbots built 'Counter Revolution' to compete at BattleBots. The twin counter-rotating vertical disks are not co-axial, but the counter rotation largely cancels the net gyro effects when the robot turns.
  • Richard Chandler campaigned superheavyweight 'Strike Terror' at BattleBots 4.0 and 5.0 with a vertical spinning weapon that was free to pivot in the longitudinal axis. This allowed the weapon to twist near-horizontal when turning without effecting the chassis, and then re-establish a vertical spin when turning ceased.
Neither 'bot was particularly successful. I'd recommend against adding the mechanical complexity your design requires. Simple 'bots win.

'Wedgemaster Wedge' writes in to remind us of 'CounterStryker' -- a 6-pound 'mantisweight' with counter rotating vertical disks built for Bot Bash:

Comment: Zac O built and documented this bot which is pretty close to what that dude wanted.

Thanks, Wedge.

Comment: Thanks Mark and Wedge that is exactly what I was thinking about. I found a video of CounterStryker fighting and it seemed to handle the turns well.

A: 'CounterStryker' has a good record: 2nd at Bot Bash '15 and 3rd at Bot Bash '16. I'm not a fan of friction drive for weapons -- Zac took care with the design and it works well in this insect class 'bot, but I wouldn't try it in a larger 'bot.


Q: How do horizontal spinners keep their weapons off of the frame? I know for example the most iconic horizontal spinner Last Rights/Tombstone has an adjustable height blade, meaning it isn't riding on the bottom frame. Is the friction of the bearing on the shaft enough to keep the blade from shifting during big collisions or is there something more to it that I am missing because that doesn't seem adequate? [Cleveland, Ohio]

A: [Mark J.] Typically the weapon hub and pulley/sprocket fill the entire space between the frame members. They ride against the inner bearing races or against 'thrust bearings' that take displacement loading during a 'hit'. The diagram shows a 'live shaft' arrangement -- in a 'dead shaft' design where the shaft does not rotate the bearings are incorporated into the weapon/pulley hub and the spacer is part of the hub assembly.

If the design allows additional space between the weapon bearings for blade height adjustment, tubular spacers (orange in the diagram at left) slide over the weapon shaft above and/or below the weapon hub to raise or lower its position.


Q: How does a horizontal full body spinner like 'Barber-ous' work? Is it a shell spinner like 'Ringmaster' on its side? [a server in California]

A: [Mark J.] First, a little terminology clarification:

  • Spiners are classified by the direction their impact, not by axis orientation. 'Barber-ous' is a vertical spinner and 'Ringmaster' is a horizontal spinner.
  • A 'shell spinner' has the entire exterior of the robot spinning. 'Ringmaster' is not a shell spinner -- it is an example of the rare and complex 'ring spinner' where only the outer edge of the body spins, leaving the wheels exposed at top and bottom to allow inverted operation.
Team Rotractor's original Barber-ous webpage (archived) has the worst build report I've ever seen, but I'm still grateful that it exists. The chassis photo at right came from another source. The shell and electricals have been removed and you can see that the layout is not nearly as complex as Ringmaster's:
  1. The weapon motor is mounted on the central chassis that also carries the batteries and electronics. The entire central chassis is concealed by the weapon shell when assembled.
  2. Non-rotating stub axles come off each end of the chassis. This version of 'Barber-ous' has worm-drive gearmotors bolted to the ends of the stub axles to power the drive wheels. The gearmotors are obscured in the photo by the wheels and hubs.
  3. Laying on the floor is one of the two large weapon hubs. With the gearmotor removed, the free-spinning hub slides onto the stub axle and a drive chain connects the hub sprocket to the weapon motor. The weapon hub on the far side is already in place. With the weapon hubs in place, the weapon cylinder slides on over the hubs and chassis and is bolted to the hubs. The drive-wheel assembly can then be re-mounted.
'Barber-ous' went thru many revisions and updates to the chassis and drive motors, but the weapon drive principle remained the same.

Q: What are the best motors currently for horizontal spinner type robots? Our weight limit is 85 pounds, and we're looking for something with relatively high torque and low spin-up time for the weapon bar. Also not sure whether to use brushed or brushless. [a server in Illinois...]

A: [Mark J.] The hamburger is bad. I cannot match a weapon motor to a weapon based only on the robot's weight class and weapon type. See the Ask Aaron Spinner Weapon FAQ to find out why and to learn what information is needed to calculate weapon performance.

If you don't know whether to use brushed or brushless, use brushed. Brushless adds a level of complexity and many pitfalls for builders unfamiliar with their quirks -- particularly in a heavier weight class such as this. Brushed is simple. Simple is good.

Now, it's possible that you just want me to take a blind guess and recommend an affordable and reliable old school motor that will make your entirely undescribed heavier-than-lightweight bar spin-up and look like a combat robot. If that's the case, Robot Marketplace has found a few of the classic EV Warrior motors. Run one at 24 volts thru a 2:1 pulley reduction to your bar weapon and you're probably in the ballpark.

What event runs an 85 pound weight limit? That's not a standard US weight class.


Q: Why can't 'Warrior Clan' launch bots into the ceiling like its previous form 'Warrior SKF' could? [Massachusetts]

A: [Mark J.] There are no significant changes to the 'bot, so the capacity is still there. I can only speculate that they have not had opponents that are particularly vulnerable to their flipper weapon. The SKF weapon is powerful but it does not have a great deal of vertical motion; their ideal opponent would have a low structural edge close to its center of mass.


Q: Sorry sir for posting a question but can you please clear my doubt " heavier the drum,Oppenent will find difficult to Topel you" is it true???? [Maharashtra, India]

A: [Mark J.] Stop asking about weapons and start building Indian arenas to contain the weapons you have.


Q: Hi There

First off, a huge thanks for keeping this site going, it's a hugely valuable source of information and by far one of the most comprehensive sites on combat robotics on the web. I'm currently designing a heavyweight (110Kg) robot after an extended break - I last built autonomous antweight/sumo bots in 2005.

My question is about spinners - namely, getting a large bar - al la Tombstone/Last Rites - up to speed in a respectable fashion. The current Robot Wars arena is 22m x 22m, and allows for around 2.5s of spin up time (on average) before first impact occurs.

Bar specs:
1300mm x 125mm x 30mm
Weight is roughly 38Kg

Motor Spec:
8500RPM, 42Nm Peak Torque (stall torque isn't stated, nor is the internal resistance, so I've used the peak torque figure in the Run Amok spinner calculator)

Results using a 4:1 gear ratio:
1339 RPM in 7.18 seconds, storing 53397 Joules.

Now, obviously this isn't [quick] enough - is this a case of me just not working out the stall torque correctly, or a case of choose a different motor? (If it is, which ones would you reccomend?)

Thanks [Fulwood, England]

A: [Mark J.] Welcome back to combat robotics, and thank you for the kind comment.

The Team Run Amok Spinner Spreadsheet - like any modeling software - is only as good as the data that goes into it. A motor with uncertain specs or a brushless motor with a non-linear torque curve results in questionable output, but in this case I don't think the motor specs are the problem.

If we conservatively assume that 'peak torque' is the same as 'stall torque' as you have done, the power numbers for the motor are still very impressive. Let's run a comparison by replacing your un-named motor in the spreadsheet with the weapon motor 'Tombstone' currently uses -- the mighty 'E-Tek-R' at 56 volts. We'll set the reduction to 2:1 to get comparable weapon RPM from the slow-spinning E-Tek:

E-Tek-R with 2:1 ratio: 1360 RPM in 6.04 seconds, storing 55,120 joules

Not a whole lot different from your results, so motor power isn't the problem. The real issue is that you're spinning up a big, heavy chunk of steel with a huge moment of inertia and the motor needs more help on low end torque to get a quicker spin-up. Let's try modeling your weapon with increased reduction ratios at meaningful time intervals:

Reductionjoules @ 2.5 secjoules @ 5.0 sec
4:112,00033,600
5:115,00037,000
6:118,00038,000

Energy storage in the kilo-teens range is plenty to warmly welcome hard-charging opponents, and potential energy storage that takes more than 6 or 8 seconds to obtain is effectively useless for anything but 'showboat' hits on an already-beaten opponent. Dial in some additional reduction. It will put less stress on your motor and battery, and will give you a better balance of spin-up time to useable peak energy storage.


Q: Hello Mark! It's said that the flipper of Warrior Clan (Warrior SKF) is powered by its spinning ring, that's amazing! Can you show me how it works? [Guangdong, China]

A: [Mark J.] Team Whyachi's 'Warrior SKF' has been around for several years and we've had quite a few questions about its design and function. Search this archive for "Warrior SKF" and "dog clutch" to find those posts. You'll also discover what 'SKF' stands for.

The best explanation of how a flywheel flipper works is the well-illustrated build report for Dale Hetherington's 'Flip-O-Matic'. If flywheel flippers were a good idea you'd see lots of them; you don't see lots of them.


Q: I've been thinking about this one weapon design that I haven't really seen anywhere. Normally, pneumatic "poking" weapons tend to not be very good in terms of effectiveness, but I was thinking about taking this weapon design to a logical extreme by making the entire front the robot a heavy pneumatic battering ram/plow.

The idea would be to push the other robot at full speed towards an arena wall, much like the typical strategy with a simple wedge or rambox, but then use the pneumatic ram to shove the other robot into the wall with even greater force than simply slamming it into the wall under conventional drive power.

Coupled with a powerful drivetrain and a sturdy supporting structure, this could end up doing some damage (though not necessarily as much as a spinner) while also being a potential counter to spinners, essentially putting the paper back in rock paper scissors without violating the active weapon requirement. Your thoughts? [That one guy from Asheville, NC, who occasionally also posts from Chicago, IL, and has a battlekit drum spinner and a Fingertech Viper with a ridiculous and excessively long name]

A: [Mark J.] Newton's third law is working against this weapon:

For every action, there is an equal and opposite reaction.

You're pushing an opponent of equal mass across the arena. When you fire the weapon it will shove your opponent forward and shove you backward with equal force. The energy of the system of the two 'bots has a net gain of... (wait for it)... zero. The impact of the two-robot system impacting the wall is unchanged, and no additional damage is done to your opponent. Not much of a weapon. Better to spend the weight used on the weapon on extra drive power.

It is an active weapon, but it is not a 'damaging' weapon. It adds complexity and weight with no improvement in performance. I suspect that's why you've never seen such a weapon.


Q: I am looking for slipping clutch on a horizontal spinner. The speed is going to be about 8,000 RPM to 12,000 RPM and the torque is from 1.8 Nm to 4 Nm and I will be happy if I can use the clutch on a dead shaft kind of system. I am using RS 40 chain to spin up the spinner disk and it is for Lightweight Robot. So where can I buy the clutch suitable for this job and usually in combat robots what brand(s) are used and who supply them? [Kuala Lumpur, Malaysia]

A: [Mark J.] Torque limiting clutches like you describe are not off-the-shelf items. Commercially available clutches are bulky, heavy, expensive, and poorly matched to the speed and shock-loading found in robot combat applications. A very few builders construct their own slip clutches thru a trial and error approach -- a great deal of error.

The standard method to limit torque in robot spinner weapons is to use a v-belt drive and set the belt tension to slip at your required loading. Don't make it complicated if you don't have to.

Q: I have now 2 questions that I would like to ask:

1) Is there any tutorial on making slipping clutches, any guide or someone who can help me on this issue that you know of?

2) I checked couple of places for V belt design. The Rio tutorial is not very detailed on the V part section. So is there any reference for designing V belts systems and describing the types of the belts and things like that. I am not looking for a straight solution, I prefer to dig and read to understand what I am doing.

Also if I understood correctly, the torque limitation using the belt is going to cause a force on the shaft of the motor. Since I am planing to use an R/C Brushless (On this part I know what I am doing so please do not wipe it out of the question :) ) without a gearbox, doesn't the extra force slow down the motor significantly and reduce the lifespan of the motor? (I think it does and pretty sure about it, but I want to know your opinion too and how significant you think the difference is)

Thank you for helping many people including me.

A: Slip clutch design is a rare engineering specialty. I can point you to a NASA Tech Brief on Slip Clutches for an overview, but so few people are involved in the actual design of such equipment that there is no tutorial. As I said above, trial and error would be your instructor.

In contrast, there is a great deal of V-belt design help out on the web. Here's a good place to start: Machine Design: V-belt selection.

Chains, gears, and belt drive systems all create a side loading on the drive motor shaft. The side load places stress on the motor bearing but does not directly place a drag on motor output -- a lateral load is not 'work' and does not subtract from output power. A properly tensioned V-belt is quite efficient at energy transfer; it is better than 95% efficient in many cases. Given that the expected lifespan of a combat robot is measured in minutes, I believe there are more important design issues you should be tackling.

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

R/C brushless hobby motors have become the standard for combat robot spinner weapon power. While I consider their use in drivetrain systems to still be experimental, I certainly would not question your selection of a brushless motor for your weapon.


Q: Can you tell me which motor is best for cylindrical fly wheels like the Minotaur bot who compete in BattelBots Against Blacksmith [Azad Kashmir, Pakistan]

A: [Mark J.] I have several problems answering your question:

  • I don't think they build 250 pound robots in Pakistan, and you didn't tell me how heavy a 'bot you are building;
  • I have no idea what motors are available to you in Pakistan; and
  • I am reluctant to assist in weapon design for competitors in the sub-continent due to poor arena safety in the region.
I suggest you read the Ask Aaron Spinner Weapon FAQ for weapon design help and evaluate the motors available to you according to the guidelines given there.
Q: Hi, I have a hammer robot and I choose to directly attach the ram 3 inches below the fulcrum. The pivot point is also just a pin though an aluminum bar. The ram has a 6 inch throw so the hammer has a roughly 100 to 120 degree angle cocked. My question is, would it be worth it to add gearing to get a 180 degree swing and bearings for better efficiency or keep the rugged, less component design. Thank you for you help - Team Humphrey [West Virginia]

A: [Mark J.] The amount of 'work energy' available from your actuator is not increased by adding gearing to expand the range of motion, but a rack and pinion can increase efficiency in converting the linear motion of the actuator into the rotary motion of the hammer. However, careful attention to the hammer linkage geometry can keep the thrust vector favorable without the added weight and complexity of a gear system -- see example at right. I recommend keeping it simple.

Your pivot is a weak point in the structure of your weapon arm. You didn't share any details of your arm design, but you should be very cautious about enlarging the hole in the arm at this highly stressed point. Without knowing more about the design I can't make a recommendation on the benefits. If you've got a big, meaty chunk of aluminum around that pivot I'd recommend boring the hole just enough to press in an oilite bushing to avoid steel-on-aluminum purely for reliability. If well lubricated your simple pivot has minimal frictional loss, but the bushing will prevent spalling and wear that can lead to failure.

Q: Hi, again thank you for your suggestions on my hammer bot. For the new Battlebot show design I am trying to upgrade my old 120 pound bot design to the new 250 pound. So my question is 250 psi components instead of my old 150 psi components. I have found a 250 psi 3.25 inch bore 6 inch stroke cylinder. But a 24-48v 5-port solenoid and quick exhaust valves rated at 250 psi are very hard to find in McMaster Carr and Grainger. Do you have any suggestions? Thanks!

A: The old standard 5-port valves no longer deliver the performance expected from high-performance pneumatic weapons. The current standard uses individual high-flow solenoid assisted valves to pressurize and vent your actuator. The R/C controls are more complicated than the simple bang-bang switch control for the 5-port as you have to control each individual valve in the correct order, but it's worth the effort.

The preferred valve is the Burkert Type 5404. They're expensive, but if you're gonna play with the big boys you're gonna need big valves, big actuator ports, and a high-flow regulator.

Q: Hi, I used to compete in battlebot season 5, battle at the beach, RFL nationals etc. I had a middleweight robot Major Punishment it was a 150 psi pneumatic hammer sort of. I actually used 2 quick exhaust valves one was set up normal for exhausting co2 out but the other was arranged to exhaust my buffer tank into the firing side making a cheaper(smaller 5 port solenoid) lighter and high cv flow (3/4 inch quick exhaust). I may do the same at 250 unless you see a flaw? [West Virginia]

A: [Mark J.] Your use of the 'quick exhaust' valve to dump your buffer tank is functionally identical to the way Burkert valves (mentioned in your previous post) work. The retract on you hammer can use smaller valves or even a simple spring return. I still don't have a source for a workable 250 psi 5-port valve.

You mention CO2 -- BattleBots rules no longer allow CO2, so you'll have to go with High Pressure Air (HPA) or nitrogen.

Q: Also I used a Flail Medieval style spiked ball and chain which I think gives me advantages like hitting spinners and drums and more likely keeping my hammer head attached and not bent. It also separates me from some of the rebound force into my bot weapon and arm. My question is what do you think are the good and bad to my weapon? and my bot weapon design. Thankyou!

A: I'm familiar with 'Major Punishment'. Tough competitor with good maneuverability!

The small mass of the spiked ball makes for poor energy storage, and the chain separates the ball from the additional energy stored in the arm. I suspect those are reasons why chain-flails are no longer seen in robot combat.

Your points about durability and rebound are well taken, but you're losing a great deal of attack energy in trade. BattleBots is looking for competitors with weapons that can cause massive damage. The solid hammer weapon on 'Beta' claims an impact energy of 3000 joules from their 11 kg hammer, and you're going to have a great deal of difficulty matching that with a small flail weapon. Think bigger -- much bigger!

Aside: unless the ratings for ABC BattleBots second season pick up there may not be a third season. Consider a 220 pound main 'bot and a 30 pound 'assistant' so you'll have something in a standard weight class if the 250s evaporate.


Q: Hi again and thankyou for your good knowledge of many robot tech subjects. Do you know how and why Beta's hammer is so powerful, my previous thought was pneumatics was much more powerful for the weight? [West Virginia]

A: [Mark J.] 'Beta' does have a powerful electric hammer, but your belief that a pneumatic hammer can be more powerful for the weight is entirely correct. Go back and look at old video of 'The Judge', or run the power calculations and you can satisfy your beliefs. But 'Beta' is an effective competitor where no previous electric hammer has been. What has changed? Batteries have changed.

With a PMDC brushed motor: amperage equals torque and torque equals hammer power. In the past it simply wasn't realistic from a weight perspective to have a weapon battery pack that could deliver somewhere in the neighborhood of 1000 amps that the Briggs & Stratton E-Tek motor could turn into torque. Modern battery technology has made that entirely feasible, and electric hammer 'bots are now a competitive option -- particularly in a competition where pneumatic systems are limited to 250 psi.


Q: Dear Mark,in the new season of Battlebots lots of vertical spinning weaponed-bots are seemingly getting smaller to give weight allowances for better armour,like Poison Arrow and Witch Doctor.But from Witch Doctor's rather shocking loss to Red Devil in the round of 32 I think being small is not a really good idea,which makes them become perfect targets for clampbots to get a hold of,do you agree? [Chinese Forum]

A: [Mark J.] I've known Red Devil's builder Jerome Miles for many years. He is a fine young man, a great builder, and a talented driver. He also got very, very lucky in his fight against 'Witch Doctor'.

Improvements in battery and brushless motor technology have made it possible to shrink the mass and size of effective spinner weapons. Robots with these more advanced weapons are quick, maneuverable, and deadly efficient in deploying their weaponry. Any weapon system has weaknesses against specific counter attacks but the high-speed single tooth disks can certainly hold their own in a tournament. Don't form your opinion on the outcome of a single battle.


Q: How is T-minus's flipping device so effective, since the ram is nearly horizontal when actuated. Wouldn't this initially direct the majority of the force horizontally instead of vertically? I would have thought a flipper would become more powerful the closer to vertical the ram is oriented; how does the T-minus design allow such force upon actuation? I'm trying to see it in terms of the statics behind the design. Thanks! [Grand Rapids, Michigan]

A: [Mark J.] You're entirely correct; from a statics point of view the design of the 'T-Minus' flipper is terribly inefficient. To efficiently convert the linear action of the actuator to rotational motion of the single-pivot lifter, the actuator should pivot to remain perpendicular to the lifter arm motion. This was a primary design consideration for my heavyweight lifter 'The Gap'.

The complication is that all of the robot systems must work together, and concentrating on the efficiency of any single system leads to design compromises in other systems. Inertia Labs elected to concentrate on a well-armored compact and maneuverable low-profile design. That design required a 'lay-down' initial position for the actuator. The actuator never gets close to perpendicular alignment with the flipper arm motion and the force vectors are horribly inefficient.

So, back to your original question: how is T-Minus's flipper so effective? BRUTE FORCE! Inertia Labs made up for inefficiency with a big actuator, huge valves, large ports, and scary high gas pressure. If you have enough power you can get away with inefficiency, and the overall robot design is brilliant.


Q: I had an idea and I wanna know if it could work, i dont think I saw this design anywhere. Imagine a hammerbot a little like terrorhurtz or the one i send you videos a in aquestion below. But, the rack and pinion isnt connected directly to the shaft of the hammer. The pinion is on a dead shaft, bolted or weld on a sprocket. Above, we have a smaller sprocket, wich is bolted on the hammer. In theory, i could get more speed out of the same actuator, by gearing it with a ratio of 4:1, for example. Do you think it could work? It's not for any weight class in particular, just a design i had in my head that i thouh was worth sharing to you.

Thanks a lot for all you do for the combat robot community, you inspired me to build robot, you showed me it wasnt only reserved to pros :) [Quebec, Canada]

A: [Mark J.] I'm glad to see you're enjoying combat robotics and spending some time thinking about design improvements. In the gear train you describe, the 'pinion' on the dead shaft is called an idler gear. An idler gear has no effect on the gear ratio -- you would get the same gearing if your 'smaller sprocket' rode directly on the rack without the added complexity and weight.

About Gearing: Your pneumatic actuator can produce only a certain amount of power as defined by the cylinder bore, the gas pressure, and the rate at which the gas can flow thru the valves and ports into the cylinder. Power is a function of time and is described by the formula:

Power(t) = Force * Velocity

Gearing changes the ratio of force to velocity, but does not change power. You can 'gear down' to get greater force (torque in this case) and reduced speed, or you can 'gear up' to increase speed with reduced torque.

To be effective your hammer weapon must accelerate to as great a speed as possible in only half a revolution -- it is torque that creates that acceleration.

  • If you reduce the torque by gearing up too much the hammer will accelerate slowly and not achieve its best speed in the distance available.
  • If you reduce the speed by gearing down too much the hammer will accelerate quickly to a peak speed that is much lower than it might have if it were geared to use the full distance available.
What you're looking for is the gearing that provides the torque needed to accelerate the hammer to peak speed just as it impacts your opponent, maximizing the power from the actuator.

Q: ok, but if i use a chain instead of a gear? would it still nt affect the gearing of the hammer?

A: I don't see how the system you described could be implemented with a chain, but regardless...

...no number of intermediate idler gears, idler wheels, or idler sprockets in a drivetrain will impact the overall gear reduction. The reduction ratio is calculated from only the sizes of the first and last elements in the sequence.

Q: Mark, why does 'Lucky' and 'Son of Ziggy' take a lot of time to make their weapons ready before they can use weapons again? That's a deadly drawback! [Jiangsu, China]

A: I haven't noticed any particular delay on weapon reset for 'Son of Ziggy' (video). I believe SOZ uses a spring powered return on the flipper and it takes just a moment for the high pressure gas vent from the pneumatic actuator so the spring can pull the weapon back down.

'Lucky' was rushed into battle before the flipper could be fully sorted and a lot of problems surfaced at BattleBots. Scroll down two posts to find a report.


Q: does some kind of rack attachment exist to put on a pneumatic ram? I would want something like the one terrorhurtz use. Thanks a lot :) [Quebec, Canada]

A: [Mark J.] The 'Terrorhurtz' weapon rack is all custom machine work. There are industrial pneumatic actuators that perform a similar function, but they're expensive and heavy [example]. Google: 'pneumatic rotary actuator'.

Q: how did john reid make the rack and pinion system? did he just put grooves on a longer shaft he then put in the pneumatic ram? it seems like it in the picture. or what about the robot in this video? or this video?

A: John Reid cut precision gear teeth into an extended length shaft on the pneumatic actuator. He has also provided a roller to support that shaft from the underside. I would think that the gear teeth would interfere with the front seal on the actuator, so there's more going on than is explained in the photos.

The robots in the other videos have their hammers driven by rack gears that have been attached to the pneumatic system. The attachment and support of the racks is critical, and the method used by the builders in the videos is not clearly shown.

British builders are famous for scrounging parts from scrapyards and repurposing them, so I suspect that those racks were scavenged from a discarded mechanical assembly. I would point out that neither of the bots in the videos are delivering what I would consider 'damaging' blows.

There are other designs for pneumatic axes: see this post for a discussion of the pneumatic weapons on 'SlamJob' and 'The Judge'.


Q: how does Ziggy's flipper seems so powerful compared to Lucky's? Aren't they built by the same guys? [Quebec, Canada]

A: [Mark J.] In my original answer to this question I attributed the reduced performance to BattleBots rules prohibiting the use of custom pneumatic components. It seems that I may be misreading the BattleBots Design Rules. I thought section 10 was quite clear on pneumatic components:

"There are no specific restrictions on the system design; however, the pneumatic system must use best practices and commercially available components that are rated for the operating pressures used."

I received a note from a reader in Massachusetts who was on-site at BattleBots 2016 and who offers a better explanation:

I have to disagree with you on the Lucky vs. Ziggy question. If Battlebots rules prohibited custom pneumatic components, how do you explain Bronco's black-box variable pressure system or Chomp's custom everything-except-the-tank? Also, having been at the event I know that there were custom components inside of Lucky, although not to the same degree as the other pneumatic bots.

You can feel free to ask Mark or Rob for the specific details, but the story is that the short time period they had for building prevented testing until the last minute. At that point, it was shown that the spring-retract and release valve system simply wouldn't do, and firing the system on full pressure (which was significantly greater than Ziggy's) would guarantee the arm being jammed or unable to cycle. Even operating at a greatly reduced pressure, the arm still got jammed repeatedly, and was only occasionally able to cycle back down.

There were other fundamental design issues in Lucky's flipper that kept it from being as potent as Ziggy's, but they're of a more mathematical nature and since I wasn't privy to the whole design process I can't really answer in good faith. Really, all of Lucky's issues came down to their status as a last-minute addition and the extremely short time that they actually had to build in.

Thanks, Massachusetts. I look forward to seeing what 'Lucky' can do with the bugs sorted.


Q: what kind of ICE engine people use to power spinner? i know that Icewave uses a fireman saw engine, but i dont seem to be able to find one anywhere (ebay, mcmaster carr). Is there other type can use? i know it might not be the most efficient way or the most simple way to power a spinner.. I just want to see if i can make one.. like i always love hammerbot, even if they are not really that efficient. [Quebec, Canada]

A: [Mark J.] Chainsaw engines are popular choices for ICE spinners -- light, powerful, and easy to obtain. Outputs around one horsepower for every 20 pounds of robot weight are about right. Check carefully with event organizers for rules specific to internal combustion engines at their event. Many events simply do not allow ICE. Current BattleBots rules (Rev. 2016.2):

Internal combustion engines are allowed, but with the following requirements:
  • The engine must use a self-starter that is activated by remote control.
  • Any electric fuel pumps must be able to be shut off by remote control.
  • If the engine uses a separate fuel tank, the tank and fuel line must be well protected.
  • The fuel tank must be vented (no pressurized tanks) with a vent system that will not continuously leak fuel if the bot is upside-down.

ICE weapons are temperamental, unreliable, and have a poor record in combat. They are most certainly not a sane choice for novice builders.


Q: I am building a hobbyweight with a small (~2 lb.) vertical spinning bar sticking out of the front wedge. I recently attended an event in which a couple of the other competitors were running Turnigy brushless motors for their belt-driven weapons (both of them did well). I am basically just trying to replace my heavy brushed motor with a lighter brushless motor while keeping the belt drive. What should I attach to a brushless motor (5mm shaft) to spin that 2 pound bar on my hobbyweight? [Albany, Oregon]

A: [Mark J.] I need more info:

  • Dimensions of your spinning bar (length, width, thickness);
  • Diameter of the pulley on the bar;
  • What brushed motor you are replacing, and at what voltage; and
  • Which Turnigy brushless motor you want to swap in.
I suspect you'll need a larger reduction ratio than you have with your brushed motor, and so will need a smaller motor pulley. Send me the info and I'll run the numbers.

Q: The steel bar is 5" x 3" x 1/2" (roughly, a couple of the corners are taken off a little bit). It is attached to a 3" pulley. I was running a Kawasaki 21.6V circular saw motor with a 7s lipo. I haven't yet selected which Turningy motor but was thinking something along the lines of the Turnigy XK3665-1200KV and running it with a 3s or 4s.

A: Hmmm... I have no clue about the power output of your circular saw motor, and I suspect you don't either. How did you decide on a 2" to 3" pulley ratio for the weapon?

The Turnigy XK3665-1200KV is an inrunner motor that would spin at close to 18,000 RPM on a 4-cell battery, but running it on 4 cells rather than its rated 7 cells reduces the output power by almost 70% [1 / (7 / 4)^2 = 33% of max power]. Pick a motor rated for the number of cells you want to use. For 4-cells something like the Turnigy Aerodrive SK3-3548-1050kv would be about right.

Your weapon bar is puny. At 8000 RPM (too fast) it stores less than 570 joules of energy. Consider adding thickness, increasing the diameter, or going to a full disk. Changing out the bar for a 6" diameter steel disk 1/2" thick bumps the 8000 RPM energy storage to nearly 1700 joules -- no longer puny.

Running a 1" diameter motor pulley to a 2" pulley on the weapon could work nicely for this set-up. Make sure the belt width is adequate to carry this amount of power.


Q: What are the benefits of an asymmetrical spinning blade versus a symmetrical one? [California]

A: [Mark J.] Briefly, you can spin your weapon twice as fast and store four times as much energy without losing critical weapon 'bite'.

From the Ask Aaron Spinner Weapon FAQ:

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read. It turns out that [weapon bite at a given weapon speed] depends on the spacing of the impactors and how fast your 'bot moves forward during an attack. You can effectively use greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to obtain if you have multiple impactors and a timid attack.

There are also multiple posts in this archive discussing single-tooth weapons. Search here for 'asymmetric'.


Q: hi do u have a way to calculate how much torque my lifter will produce? what gearing should i use for the gearboxe in a 30 lbs robot? [Quebec, Canada]

A: [Mark J.] You asked a question a couple months ago about chain driving a featherweight lifter. My reply to that question featured a link to the formulas needed to calculate torque for a simple lever arm lifter (not a 4-bar lifter). Suggest you re-read that post and follow the link.

You should gear the lifter motor so that maximum lifter load requires only about half of the motor's stall torque (torque overage factor = 2). That assures the fastest lifter speed when fully loaded. Here's an example:

  • Lifter arm length (pivot to tip): 1.0 feet
  • Maximum lifting weight: 30 pounds
  • Maximum torque at gearbox (ft-lb): 1.0 feet * 30 pounds = 30 lb-ft
  • Max Load at Gearbox (in-oz): 30 lb-ft * 192 = 5760 oz-in
  • Motor Stall Torque: 166 oz-in (BaneBots RS-775 18v @ 18v)
  • Torque Overage Factor: 2.0
  • Gear Ratio Required: (5760 / 166) * 2.0 = 69.4:1

In the example given, the BaneBots 64:1 P60 Gearbox would do nicely. Run your own design numbers thru the same process to get your ratio.

Note: although a torque overage factor of 'two' provides the fastest lift at maximum load and keeps motor loading reasonable, some builders prefer a smaller torque overage factor to give a faster lift when the lifter is only raising one end of the opponent rather than the entire 'bot. This places a greater load on the lifter motor, but is an option. As long as the torque overage factor exceeds 'one' the lifter will function without stalling.

Q: hi srry if im annoying.. but do the lifter have to be a straight bar for the math to work? because my idea was to use something similar to the one nyx has [pictured at right]. I have done very basic physic at school, since i am only 17 so pardon my ignorance.

A: I'm always pleased to get follow-on questions. I have to make assumptions about most questions to keep the answers short, so don't be hesitant to ask for clarification. Curiosity and persistence are virtues.

The math works for any single-pivot lifting arm: measure the arm length as a straight line perpendicular to the pivot axle -- from center of axle to far tip of arm. Bends and angles along the way don't count.

Note the big gears 'Nyx' uses to handle the large torque loads on this long lift arm. A small BaneBots gearbox used as the pivot is unlikely to survive torque forces so great as seen in this design.

Q: so. can i use like a 16:1 ratio gearboxe then use gears to acheve the right ration, i assume thats how nyx work. Im i right?

A: Yes. That will reduce the torque load on the BaneBots gearbox. Chains require less precise positioning than gears and are more forgiving of minor misalignment -- easier for a novice builder.


Q: do u have a diagram on how an horizontal spinner like tombstone or mechavore is mounted? [Quebec, Canada]

A: [Mark J.] There are two common methods to mount spinner weapons; 'Tombstone' uses one, and 'Mechavore' the other:

  • 'Tombstone' uses a non-rotating 'dead shaft' solidly attached to the chassis as a structural member. The weapon blade and drive sprocket are attached to a tubular hub with bearings on each end that ride on the shaft. You can see spare weapon blade/sprocket assemblies for 'Last Rites' ready to slide onto the shaft in the picture at right.
  • 'Mechavore' has its spinner weapon and drive sprocket attached directly to a rotating 'live shaft'. The shaft itself rides in bearings attached to the chassis.
There is a diagram of live and dead shaft designs with a discussion in the Ask Aaron Weapon Archive: Live Shaft vs. Dead Shaft.

Q: hello im the guy that asked the question about the horizontal spinner mounting option. Do i need to put something between the bushing and the frame under the spnner? it seems like the friction at this point will be very high. Im sorry if my english was not really good ; )

A: Vous parlez mieux l'anglais que moi le français. Je parle le français comme une vache espagnole.

I assume we're talking about a 'dead shaft' design? Some bearings come with 'extended' inner races that will space the weapon hub up away from the chassis to prevent rubbing. If the selected bearings do not have an extended inner race, a small washer placed on the shaft will rest against the race and rise the hub up for clearance. I left the washers out of the linked diagram for the sake of simplicity, but then I left out the chassis as well.


Q: Hi Aaron,
First of all thanks for making your Excel Spinner Spreadsheet, it's amazing! There's one thing I'm not sure about it though.

I've got a Scorpion HKIII-4020-890KV brushless motor with a 6S LiPo battery to power my weapon. My horizontal bar is made out of 4340 stell (around 7.83 g/cc) and has Lenght = 0,42m Width = 0,04m and thickness = 15mm so my moment of inertia is 0,29269 KgM^2 and the mass is roughly 2Kg

So if I use a 3:1 gear ratio and consider that the no load speed would be 80% of the KV times the voltage (24V) I will get at my maximum energy (4724 J) and speed (5411rpm) in less than a second (around 0,85s) Would you say that's correct? I think I've mess up the no load speed.

Thanks in advance! [Brazil]

A: [Mark J.] Brushless motors in combat robots remain more of a black art than a science, but I can assure you that you're not going to spin up that bar in 0.85 second with that motor.

The Team Run Amok Excel Spinner Spreadsheet works quite well to estimate the performance of spinner weapons with brushed motors. As warned (perhaps inadequately) in the current version of the spreadsheet, brushless motor startup torque depends a great deal on the brushless motor controller. Differing controller hardware, firmware, and user settings will greatly impact the low-speed torque of a brushless motor. In particular, a sensorless motor like the Scorpion HKIII-4020-890KV will not come close to the 'best case' torque calculated by the simple brushless torque estimator included with the Spinner Spreadsheet because a sensorless controller must take an educated guess at the stator position relative to the magnet array. Also, the motor would need to pull over 1800 amps at start-up to produce that 'best case' torque, and your controller most certainly isn't going to allow that much current even if your battery pack could supply it.

I'd de-rate the stall torque of the Scorpion motor by a factor of at least six -- call it 3.3 Newton-meters. Aerodynamic drag will slow total spin-up time as well as reducing top RPM. Assuming 20% speed loss due to aerodynamic drag, I 'eyeball' weapon speed around 5200 RPM with 4200 joules in something like 6 seconds. That's still very nice performance for a featherweight.

Now the bad news: if that motor is bogged down to continuous operation at 80% of free running RPM it may pull in excess of its 65 amp continuous current rating. So much depends on who the motor controller behaves in situations like this. Brushless motors do poorly when bogged down -- you might consider increasing the reduction ratio.

Again, that stall torque rating could vary a good deal from the estimate and the performance figures would vary with it.

Q: I have a couple more questions about the size of my bar and the axis that supports it. This is all for a featherweight horizontal spinner, and if the model from above is correct I have 157 J per pound of my class which is pretty nice. I'm thinking about using the bar I've talked before with an axis of 10mm, both of them made out of 4340 Stell at 34HRC. Do you think that's enough thickness for my axis? How would you simulate that in a CAE program? I have experience with PTC Creo Simulate academic edition, I'm just not sure how to model this.

A: See Frequently Asked Questions #17. 'Ask Aaron' is not a free engineering service -- and if it was, no competent engineer would spec that weapon shaft without knowing a great deal more about the support the shaft would be given in your design.


Q: Hey Mark,
Are bearings absolutely necessary for a spinning disc/bar weapon? I was looking at Team Nightmare's build of Backlash (1st version) and nothing was mentioned about bearings. It's kinda hard to tell in the pics, but it appears that the disc is attached to the pulley and they both spin on a dead shaft. Are there some type of bearings in there that I'm not seeing or is it possible to spin a weapon on nothing but a greased up shaft? [A few miles south of here, Oregon]

A: [Mark J.] Take another look at the weapon pulley in the 'Backlash' build report -- that's a sealed bearing at the hub. The loading at the hub of a high-energy spinning weapon far exceeds the capacity of a 'greased up shaft'. On a good hit it would spall and seize. You can (and a fair number of builders do) use oilite bronze bushings in place of ball/roller bearings to support weapon shafts. They have a bit more friction but can survive enormous shock loadings. Keep them well oiled.

Q: So if I wanted to use something like a needle roller bearing, I would put it on the dead shaft and then attach the weapon/pulley to the outside of the bearing? If so, what is the best attachment method? I know set screws aren't great but could they work for a 12 pounder?

A: Not a great idea to apply significant point force (like a set screw) to the outside race of a bearing. A needle bearing would typically be installed in the weapon/pulley as an interference press-fit with thrust bearings and spacers taking up any lateral motion on the shaft.

Alternately, flanged ball bearings can be simply inserted into the sides of the weapon/pulley assembly to be held in place by the structure supporting the dead shaft. Washers or tubular spacers can take up any excess clearance.


Q: I've been noticing that some spinners that use outrunners to mount the motor pulley to the can of the motor and not the to shaft. Best example I can think of is the Australian featherweight 'Decimator'. Other than reducing height, does this offer any other advantages? Any significant disadvantages with this? [A few miles north of here, Oregon]

A: [Mark J.] A design like Aussie 'Decimator' (photo) mounts the weapon pulley on the can and tucks the entire weapon drive inside the chassis without requiring an internal bracket to support the weapon motor. It's clean and simple, and it does save a bit of height while still protecting the weapon belt. Simple is good.

The drawback is that the pulley is now located at the far end of the motor from the motor mount. This violates the general engineering rule of providing support as near as possible to the point where lateral force is applied to a structure. Decimator's design places great mechanical loading on the motor's internal bearing support compared to mounting the pulley in the conventional manner. The short height of Decimator's weapon motor keeps the load from becoming too great -- stresses on a taller motor would be worse.


Q: How can I install a pulley on a brushless outrunner motor? [Quebec, Canada]

A: [Mark J.] Well, you could the obvious and slide a nice FingerTech pulley onto the output shaft of the outrunner and tighten down the Allen screw. That works. I'd file a flat spot on the shaft to give the Allen screw a better bite, and I'd put a drop of threadlocker on the screw to keep it from loosening.


Q: how could i give a blue, energetic look to my flamethrower like th one used in complete control? [Quebec, Canada]

A: [Mark J.] We don't discuss flame weapons here at Ask Aaron. See Frequently Asked Questions #28.


Q: We're competing in a junior high battlebot competition. 50 pounds maximum on robot. We're wanting to do a spinning weapon like tombstone. Any thoughts on motor, maybe a cog belt. and pulleys? [Amarillo, Texas]

A: [Mark J.] Start by reading the Ask Aaron Spinner Weapon FAQ. The FAQ outlines the key elements in spinner weapon design and points you to tools to match a specific weapon design to a suitable motor and weapon drive.

There are also a great many posts in this archive about weapon motor selection.

As for drive components, browse the Mechanical and Drive Components selection at Robot Marketplace.

I'm pleased to provide the tools needed to design your weapon, and when you have your design roughed out I'll be pleased to look it over and offer suggestions. I won't design your weapon for you, but I'll give you a start:

An AmpFlow F30-150 motor @ 24 volts spinning a 24" x 3" x 1" steel bar via a 4:1 belt drive reduction. The bar weighs 20 pounds, spins up to more than 1600 RPM in 4 seconds, and stores better than 4200 joules of kinetic energy at that speed.

Q: Is this Park 370 Outrunner EFL-370-1360 Brushless Motor powerful enough to spin a 2.5 lb steel bar (overhead spinner curved down like that in one of the designs in the riobotz book) or a fbs shell? Live shaft most likely but dead shaft is possible. Using Timing belts and a gear ratio of about 3.5:1 with and rpm of about 4000. I would like to know if it is strong enough to spin the weapon up to speed in a reasonable amount of time under combat conditions (being hit while attempting to spin up). 11.1v lipo.

Or should I use this Feigo inrunner motor?

Thank you. I know my questions could probably be answered in the excel spinner spreadsheet but I am not very good at using that particular tool. Using the lighter motor would be amazingly helpful for weight but I fear it is too small to spin that much mass effectively under combat conditions and will get burned out. [Pennsylvania]

A: [Mark J.] I'd suggest you start by reading the Ask Aaron Spinner Weapon FAQ, wherein you will find out why a spinner weapon cannot be evaluated based only on the weight and material of the spinner, and why the dimensions of the spinning mass are critical in the calculations.

It's also not reasonable for me to comment on adequate performance of a spinner weapon without knowing in what weight class the robot will compete. Is this for a mantisweight, hobbyweight, BotsIQ, or maybe something else?

Neither of the motors you ask about are suitable for your purpose:

  • The Park 370 has a power output around 100 watts -- suitable for an antweight class spinner, but way too small for you.
  • The Feigo does not list any specifications except RPM per volt. All I can say is that it spins way too fast for your design.
If you aren't willing to do the calculations to pick out a suitable weapon motor, look at successful robots in your weight class with designs like yours to see what weapon motors they use. That will get you close.

Important note: you don't see any successful examples of that 'overhead spinner curved down' design in actual combat because that particular rotor shape in unstable! See this post elsewhere in this archive. Pick another design.


Q: Do you have any thought on the PERM PMG 132 motor vs a Motenergy ME0708 or ME1003? This will be for use in spinning blade or drum robots.

I understand the performance differences but wonder if there are any strengths or weakness in either of them that would be specifically important to combat robots. [Orange County, California]

A: [Mark J.] I'm going to defer to Ray Billings on this one. Ray and Team Hardcore have a whole lot of experience torturing big pancake motors in heavyweight weapons. They've gone thru a BIG stack of original Eteks, PERM PMG-132s, and Motenergys. None of them hold up for long, but if you need big weapon power you have to live with short lifespans. Ray estimates it costs him about $1000 a match for equipment replacement -- mostly batteries and weapon motors.

His current choice - for reliability and power - is the Motenergy ME0708, also known as the 'Etek-R'. If Ray likes it, I like it.


Q: Would a alien power system c8080 brushless motor work for a heavyweight vertical spinner? If not, what brushless motor would you recommend for a heavyweight vertical spinner? [State of Tennessee Department of Education]

A: [Mark J.] It's poor design practice to start with a weapon motor and build the rest of the robot around it. Design your robot to do what you need it to do, then build to meet that specification. All of the components of the weapon (and the robot) must work together.

That said, a 5000 watt brushless weapon motor like the C8080 is underpowered by current heavyweight standards. Design a suitable heavyweight-class weapon (think 'Tombstone') and plug in the performance specs for different motors until total weapon performance meets your goals. There are heavyweights that use Alien Power weapon motors, but not the cute little C8080.


Q: Hello,
Why are all full-body spinners' drive systems so slow? I know they need good stability especially with the gyro effect, but why can't they be fast? And would making the top part spin the opposite direction as the bottom tooth part eliminate gyro and maybe even help with the "throwback" con of those spinners?
Thanks, Missouri

A: [Mark J.] Let's talk a bit about the 'gyro effect':

  • The gyro effect -- more correctly 'gyroscopic precession' (video) -- is a force acting at a right angle to a force that's acting to change the direction the axis of a rotating body is pointing.
  • The axis of a vertical spinner weapon points left/right in relation to the robot. Turning DOES change the direction that axis is pointing. A vertical spinner (like a drum spinner) DOES exhibit a 'gyro effect' when turning that can lift one side of the robot off the arena floor.
  • The axis of a horizontal spinner weapon points straight up/down in relation to the robot. Turning DOES NOT change the direction that axis is pointing. A horizontal spinner (like a full-body spinner - FBS) DOES NOT exhibit a 'gyro effect' when turning.
Horizontal spinners don't suffer from the adverse 'gyro effect', but they can have trouble with simple torque reaction. Newton's third law says that for every action there is an equal and opposite reaction, so when you apply torque to spin-up a big horizontal spinner, an equal torque is applied that attempts to spin the robot in the other direction. This force greatly diminishes once the weapon is up to speed, but at weapon start-up it can be quite troublesome. Now that we have the terminology sorted out, let's get back to your questions.

Not all FBS are slow, but most builders skimp on the drive system in order to put additional mass and power into the weapon system. Let's face it, a FBS is all about the weapon. The attack strategy doesn't depend on speed, and any side of the robot is as dangerous as any other so maneuverability isn't an important factor either. All you need to do is move toward your opponent and wait for contact. Adding a powerful drive system steals power from the weapon and doesn't significantly add to the effectiveness of the robot.

Splitting the shell into two counter-rotating components adds significant complexity and fragility to the weapon system and results in less energy storage in the part that actually hits your opponent. It could eliminate the torque reaction on spin-up, but the 'throwback' on impact is a consequence of Newton's third law and the horizontal impact vector. You can't cheat Newton.

Q: So the rotational energy from the counter-rotating top wouldn't stop the bot from spinning away from impact?

A: That's right -- Newtonian counter-reaction trumps counter-rotation every time.

Q: What I'm trying to figure out is how to not get beat up from super-low wedge spinner bots. If they come in slow enough to move the gyro they can get under you. Unless you have a fast drive you can't get where they can be hit.

A: Like I said, not all FBS are slow. If you're willing to trade off some weapon power for robot speed there's nothing in the design to stop you.

Q: How do you defeat a bot that has a knife edge wedge and titanium spinner on top of that?

A: People beat wedges all the time. Suggest you watch some video of matches with this type of opponent to see how it's done in whatever weight class you're building. Modify your design and strategy accordingly.


Q: Greetings:
I was wondering how effective the following weapons would be on a robot. Note that I've ordered these roughly from "least likely to work" to "most likely to work", and am only seriously considering building the last three.
  1. A vertical spinning weapon on a pneumatic piston
  2. A horizontal spinning weapon on a pneumatic piston
  3. A drill/'Niterider'-style weapon on a pneumatic piston
  4. A pneumatic piston on a horizontal spinning weapon
  5. A lifter that deposits the opponent onto a spinning weapon on top of the robot (if you need a better description I can link you to a picture.
  6. A horizontal spinning drum weapon
  7. A "face spinner" (think 'Invertibrat' from Season 4)
  8. A front-hinge flipper a la 'Firestorm' or 'Cassius', except the lifting arm is replaced by a blade to potentially damage the opponent (obviously this would have to be a high-powered pneumatic flipper)
And yes, I know your "efficient, simple, easy to use" mantra, I just wanted your opinion on this. [Portland State University]

A: [Mark J.] I don't consider any of the weapons to be practical, so let's talk about the designs purely from a conceptual standpoint.

  1. A spinner weapon gets much better 'bite' if the closing speed on the opponent is greater. In close quarters, having the ability to quickly thrust the spinner forward is conceptually valuable.
    Conceptual score: A
  2. Same benefit as #1, but horizontal spinners throw both the attacker and target in opposite directions. Less desirable.
    Conceptual score: B
  3. 'Niterider' had a 'disemboweler' spike intended to penetrate another robot then twirl some stiff wires around to chop up internals. Would work really well if combat robots were armored with cardboard and filled with eggs. Adding pneumatics won't help.
    Conceptual score: F
  4. Whaaatttt?? How does that accomplish anything except sudden imbalance?
    Conceptual score: F
  5. I really hate dual-weapons. Decide whether you want to flip 'em or shread 'em, then put all your weight allowance into a single effective weapon.
    Conceptual score: D
  6. OK, maybe this one is practical. The drum stores more energy than a bar or disk, so it has some promise. Keep the drum fairly short and with a large diameter and you might have something.
    Too practical for a conceptual score
  7. 'Invertabrat' was an unsuccessful 'flipper' robot that violated the 'two-weapon rule' and added an ineffectual milling head to the back of the robot. Spinning at full speed the milling head might possibly leave a nasty scratch on aluminum or plastic. Enlarged, one edge goes up (good) and the other edge goes down (bad). Hit with the wrong edge and YOU fly upward.
    Conceptual score: F
  8. Blades can't cut modern robot armor, and a sharp edge makes it easy for your opponent to slip off to one side or the other. If you wanna flip 'em, flip 'em. If you want a pneumatic pickaxe, build that. Points for visual appeal.
    Conceptual score: C

Q: Hi Mark! I have a quick question on drumbots. How is a drum usually mounted and supported on a dead shaft, such that it can handle direct impacts and keep the shaft from bending? [Midvale, Utah]

A: [Mark J.] Usually? Sturdy endplates, great big bearings and a big hardened steel shaft anchored securely to the chassis as close to the bearings as possible.

Q: Hi Mark, I'm following up on my drum spinner mounting question. What if the drum was split in half with the pulley mounted in between? In my case this would be necessary given the compactness of the bot and motor mounting restrictions. So the shaft would either be 1.5" thick titanium or 1.1" thick steel anchored on the far left and right sides of the bot, and the drum itself is about 2" thick with a 6.3" OD not including teeth. It just has that vulnerability in the middle where the drum is split to be able to mount the pulley in between. Is this a viable option if built as robust as possible? If not, another option could be using the drum itself as a pulley? (Assuming the gearing could still be made correct)

A: I really dislike splitting the drum right at it's point of greatest structural weakness. Adding additional bearings near the center to support the split would load the shaft at its weakest point -- a poor engineering choice.

Your drum is amply thick to machine a v-belt groove (or two) into the drum. That will weaken the drum a bit, but not nearly so much as splitting the drum to add a central pulley.

Q: Hi Mark. That sounds good. I was leaning towards a built-in V-groove as well. Tolerances will have to be very high on the machining though, since the plan is to have a single tooth and a counterweight spanning across the entire drum. So the V-groove will have to go through those two AND the drum, misalignment between the three could cause troubles. It should work out fine though.

I could also just split the tooth and the counterweight in the middle to allow the pulley to go around just the drum. Is there a disadvantage to splitting a tooth into two pieces? When the drum hits it can potentially put the entire load on just one tooth.

A: You haven't mentioned the length of your drum. Assuming that it's fairly long I'd split the impactor tooth and the counterweight. At the energy levels a large drum weapon generates even a tool steel impactor will elastically deform enough on impact to localize the loading, so there is little benefit to a long-span tooth.

Q: Drum guy here again, thanks so much for your help so far! The total drum length is 11", but it actually won't be split directly down the middle, based on the latest design... more like into a 3.5" piece and a 7.5" piece. By the way, just wanted to mention that with the current materials and dimensions the drum will generate just under 20KJ at 90% RPM.

A: That's a lot of energy for a weapon that size. An aluminum drum with the dimensions you give plus steel tooth & counterweight weighs about 33 pounds and needs to spin close to 8000 RPM to store that much energy. Balance is going to be critical. Best luck.

Q: Drum guy here again. Thanks for the input! You are exactly right about the total weight and RPM. I'm curious though, what dimensions did you estimate the steel tooth would be, and how much it extends out of the drum? I've been trying to balance how far it extends out vs how far it is supported in the drum, and have yet to determine the sweet spot.

A: I didn't assign specific tooth dimensions. I just added a bit of mass to the outside diameter of a drum with the dimensions you gave to bump up the weight and get the RPM for 20K joules to come out a round number. There is a formula to calculate the maximum useable tooth height for a specific weapon at a specific closing rate:

Tooth Height (inches) = Closing Speed (inches per second) * 60
Weapon RPM * Number of Impactors

The formula requires some assumptions on closing rate, but does at least provide some guidance. How deep to set it into the drum? Best engineering judgement.


Q: Is there any difference between a pneumatic setup for co2 and nitrogen use? [Third Tube from the Right, Internet]

A: [Mark J.] Yes. CO2 converts to a liquid form under pressure and is stored at about 850 PSI. The storage tank must be mounted to assure that only the gaseous CO2 from the top of the tank enters the pressure regulator and actuator. As CO2 changes from a liquid to a gas it gets VERY cold -- cold enough to 'freeze up' a regulator designed for other gasses, so you need to make sure your regulator is made for use with carbon dioxide.

Nitrogen can be stored at pressures up to 5000 PSI and does not convert to a liquid form under pressure -- you'll need a larger tank to hold the same quantity of gas as the liquid CO2 system, but you don't need to worry about tank orientation or the problem with extreme cold. The higher pressure requires that the storage tank and regulator both be rated appropriately.

See the 'What a gas!' section of the Team Da Vinci: Understanding Pneumatics page for more details, and read the whole page while you're there.


Q: I can't think of one, and there may not be one just because I haven't seen many bots with it; but is there any pros to having a non circular kinetic weapon? Like Tillah from Team Juggerbots' square drum? [Spring, Texas]

A: [Mark J.] Can't think of one, you say?
  • Bar spinners like 'Tombstone'?
  • Asymmetric mini-discs like 'Witch Doctor'?
  • Cutaway discs like the 2015 version of 'Nightmare'?
  • Snail drums like 'Touro Maximus' (image right)?
  • Monotooth drum hybrids like 'HyperShock'?
Plenty of examples, some of them requiring a great deal of engineering and construction effort to produce. As covered in the Ask Aaron Spinner FAQ the best energy storage does not come from these designs, but builders don't generally waste effort so it's a safe bet that there is an advantage to be gained. The advantage varies with the style of weapon:
  • Bar spinners are simple to construct, robust, and durable;
  • Asymmetric mini-discs are single-pieced, durable, and can spin at high RPM while retaining 'bite';
  • Cutaway discs are... Well, they're pretty;
  • Snail drums are durable and show that you have serious design and machine skills; and
  • Monotooth drum hybrids have the advantages of asymmetric mini-disks but can store more energy.
Q: I think I worded my question wrong lol. I meant are there any advantages to a polygonal kinetic weapon like a square (or cube like I guess) drum. Then again, there are eggbeaters that are apparently good

A: Like it says in the 'Energy Storage' section of Ask Aaron Spinner FAQ an ideal design places as much of the mass as far from the rotational axis as possible. A circular cross-section achieves this goal for a given diameter, while something like the weapon on 'Tillah' (photo at left) has a good portion of the mass located at less than the max diameter and loses energy storage efficiency. As I recall, Ron Ender found that big square steel tube in a scrap yard and just really liked the look of it. It does look awesome, and that counts for something.

Eggbeaters can store a great amount of kinetic energy for their mass because they place a lot of the material a long way from the rotational axis where it does the most good. They are a viable design choice for insect class robots but they don't scale up well to heavier weight classes; they're too fragile at larger sizes.


Q: Is making a counter-rotating saw blade a good idea at all? I did a little test on some aluminum with my dad's saw (counter-rotating). It didn't have any throwback, and it made a clean cut just touching it. Using a single blade, it had a LOT of throwback, made not very deep but nasty looking cuts, but wasn't good at cutting from the flat side. Any thoughts? [Dearborn, Michigan?]

A: [Mark J.] I know that the new batch of 'BattleBots' are all complex examples of 'machine porn', but that's only because the audition process for the show demands that type of design. If you want to build a robot to win matches in open competition you need to keep the design simple and robust.

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

So, no counter-rotating saws. In fact, no saws at all. Effective use of a saw requires that you immobilize your opponent, and your opponent has other ideas about that. Keep it simple.


Q: How can u self right using a 4 bar lifter? i saw this video on youtube and even after watching it i just cant figure it out..

P.S. i really love ur site, its the best out there and u gave me the inspiration the build bot thank u :) [Quebec, Canada]

A: [Mark J.] Thanks for the props, Quebec.

The video of antweight 'Pad Thai Doodle Ninja' self-righting is taken from an awful angle to actually see what's happening. I think you'll get a much better idea of the process by watching this video of 'BioHazard' self-righting. Getting a 4-bar lifter to flop back upright requires extensive pre-planning and a fair amount of tinkering. You'll notice small extension 'claws' on the back of PTDN's lifter that I'm sure were added to get the self-righting to work.

Charles Guan's 'Equals Zero' website has an archive for PTDN that includes the design requirements for getting a 4-bar to self-right:

"[Self-righting] is kind of tricky with 4-bar lifters. You really have to take into account the center of gravity of the bot, and the length and extension of the arm, in order to facilitate this. Generally, 4-bar lifter bots flop onto their backs and come to rest on the arm whenever it is then deployed, as the CG is too far forward, and no self-righting is possible. [The classic video of former Battlebots heavyweight Biohazard shows how a 4-bar can self right.]

Notice how [BioHazard's] center of gravity is far enough back that the bot hinges on its rear edge and does not come to rest on the arm. The arm’s retraction then keeps the CG within the line drawn between the arm’s contact point and the bot’s rear edge, and it gathers enough momentum to push back over. Making it able to do this meant making the arm extend all the way back across the bot. Notice also how Biohazard had a ‘tang’ at the very back of the arm, a part that sticks up – this aids in the maneuver by making the contact point with the ground further forward, so the ‘line’ is longer.

This goal meant that I was continually watching the bot’s center of gravity in autodesk Inventor, and also continually modifying the linkage to suit. The arm had to have a certain amount of extension to make sure the CG was in the right place, and that extension had to jive with everything else’s placement."

The full text, with some pictures, is way down at the bottom of the Equals Zero archive for Pad Thai Doodle Noodle. When you scroll down to the part of the archive where Charles is doing a brake job on his van you're about half-way there. Keep going. WAY DOWN. Seriously.
Q: Hi Mark,

First of all, I know. You heavily advise against ring-spinners. However, there is one that catches my eye due to what seems to be a pretty solid design. This upcoming season of ABC's Battlebots will feature The Ringmaster. A Single-tooth ring spinner with the weapon system being driven via gears rather than friction wheel. You can see all how it all works on their facebook page.

I'm heavily considering using a similar drive design in a lightweight ring-spinner that i'd like to build (not a ripoff, I have standards.)

Now, you've almost completely crushed my dreams of building a ring-spinner. But if I would be wasting my time by building one with this type of drive system and ring support, I would indeed like to know.

For knowledge-sake, lets assume that I built a lightweight clone of The Ringmaster. Good RPM, Quick Spin up, "perfectly" Counter-balanced weapon, and so on. What deal-breaking issues or concerns do you think I'd most likely run into?

Thank you sir, David R. (Livermore, CA)

Quick addendum

I know that I am speculating that The Ringmaster works well even though we haven't even seen it in action, and we probably won't get a good idea of that until the new season airs. In fact, just about ALL of my question involved speculation. So I understand that nothing from the scenario I gave you is completely concrete. So upon your response, I'll be sure to have a pinch of salt ready. Thanks :)

A: [Mark J.] 'Crusher of Dreams' -- maybe I should put that on my résumé?

Where to start...

  • First: run a Google search for Ringmaster's team leader 'Hal Rucker'.
  • Second: examine your budget and technical skillset to see if they match Hal's.
  • Third: no-spoilers, but check in on the historical success of ring spinners.
  • Fourth: consider the wisdom of the Team Run Amok Design Philosophy:

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

Here's the problem with shell and ring spinners in general: the impact vector is lateral, parallel to the floor, and uncomfortably close to the center of mass of the robot. You hit your opponent and Newtonian physics throws you off in the opposite direction like a high-powered hockey puck. They are nearly as dangerous to themselves as to their opponents. A ring spinner has the advantage of being able to operate when inverted at the cost of much greater mechanical complexity. An extended center pole of the type used by lightweight champion shell spinner 'Ziggo' is a much simpler solution to this problem.

If you want to build a ring spinner as an example of 'machine porn' go ahead and do it for that reason -- and yes, gear drive it. Just don't expect it to dominate.

Quick addendum

1999: people watch BattleBots, see some frat boys build a robot out of a beer keg, say to themselves 'I can do that!', hundreds of teams do just that and combat robotics as a popular hobby takes off.

2016: people watch BattleBots, see robots that appear to be built by NASA contractors under DARPA funding, say to themselves 'I can't do that!' and combat robotics as a popular hobby dies.

Just a theory.


Q: Hello Aaron:
   The
[pneumatic] cylinder has two holes: inlet and vent. As we know the speed of cylinder depend on air pressure and flow, So I got a idea: if we make cylinder have two "Inlet holes" the flow of the cylinder will be larger, so the cylinder speed will be faster? [Yunnan, China]

A: [Mark J.] A pneumatic system has multiple restriction points: the pressure tank valve, the pressure regulator, the solenoid control valves, and the actuator cylinder ports. Typically the speed of the system is limited by the performance of the regulator and valves. Gas flow at the actuator port is typically quite good; improving the flow there will not provide noticeably greater speed.


Q: do u have a diagram on how is actuated an axe like on shunt? can u explain how the linkage works? thx [Quebec, Canada]

A: [Mark J.] 'Shunt' uses a complex pneumatic powered multi-bar linkage to provide a full 180 degrees of axe motion. You can see Shunt's designer explaining and demonstrating that linkage in this video.

The diagram at top right is from Chris Hannold's book "Combat Robot Weapons". It shows a much simpler overhead axe/hammer weapon linkage.

Additional linkage options and a full explanation of pneumatic systems can be found at Team Da Vinci: Understanding Pneumatics.

Q: hi axe guy again in the picture u put, is the hammer able to do a 180 degree rotation?

A: No -- see the small diagram at right. When the axe retracts the axe bracket runs into the pneumatic actuator, and when the axe extends the actuator shaft runs into the axe pivot axle. It's only good for about 90 degrees.

You can play with the design and get a little bit more, but a simple single-pivot design like this is limited to an efficient swing of about 120 degrees. That's why Shunt's designer went to the complex linkage to get 180 degrees.

Q: axe guy again, do u have any diagram or image of the multi-bar linkage u were talking about?