6259 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.

    In Memoriam: Aaron Joerger, 1991 - 2013
The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary
- Mark Joerger   
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Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

This month marks the 15th anniversary of the 'Ask Aaron' website. We celebrated the 10th anniversary of 'Ask Aaron' with a Robot Haiku Contest and had so much fun that we've decided to sponsor...

A new contest for our 15th anniversary!

Jump to our 15th Anniversary Contest Page to see what we've cooked up, and stare wide-eyed at our unbelievably huge prize. Hint: the prize has nearly as many zeros as the national debt!

I've also put together a webpage that covers The History of Ask Aaron with links to restored versions of early pages, and the astounding Ask Aaron Word Cloud.

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 Oops! Broken Link Fixed! 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: Hi Mark,

A rather confusing request, at least in my mind, but I will do my best to explain it. I have 2 Banebots 540 drive motors connected to an ESC -- currently it is a Mamba Monster, but I am very open to suggestions.

A: [Mark J.] The Mamba Monster is a bulky and complex single-channel controller intended for brushless motors but with a 'brushed mode'. If you want independent control of two motors you will need either:

  • TWO single-channel controllers; or
  • ONE dual-channel controller.
Selection of a specific Electronic Speed Controller (ESC) requires more information than just the motors you wish to control. To model the performance of drive motors and estimate ESC requirements you need to know:
  • The specific type of drive motors;
  • The number of drive motors used in the robot;
  • The voltage at which the motors it will operate;
  • The weight of the robot;
  • The gear reduction ratio used to drive the wheels; and
  • The diameter of the driven wheels.
All that you have provided me with is the number and type of motors. I can't tell you if the 540 motors will be adequate for your purpose, and I can't recommend ESCs based on only the motor type. I CAN tell you that a single Mamba Max will not do the job. Go to the Ask Aaron Combat Robot Design Tools page, read up on the Team Tentacle Torque/Amp-Hour Calculator, and walk thru the example drivetrain analysis.

Q: Using the Spektrum DX6e and subsequent receiver, I want each gimble to power the drive motors forward and reverse. Probably very easy.

A: It is very easy, if I correctly understand what you want. I believe you're asking for 'tank steer'.

  • Follow the transmitter manual instructions to set your DX6e to 'Mode 2 - quadcopter' so both control sticks 'spring-center'.
  • Plug the ESC for the left-side motor into receiver port 3 (throttle).
  • Plug the ESC for the right-side motor into receiver port 2 (elevator).
Moving the left stick forward and back will spin the left side drive motor forward and reverse, and the right stick will do the same for the right side drive motor.

Q: Would you need a v-tail mixer with this setup or an alternate?

A: No, but few robot drivers set their transmitter up in 'tank steer' mode you are requesting. Read our guide to Transmitter Programming for Combat Robots for alternative control setups. If you do decide to use a 'mix' setup, your DX6e transmitter has built-in mixing capability -- no external mixer is required for most control choices. If your controls are not behaving as you expect, our Transmiter Set-up Troubleshooting Guide can help.

Q: Then I want to power a Leopard 3650 3450kv weapon motor using an ESC. Currently it's a Mamba Max Pro, previously a eRC 85A. Again looking for suggestions.

A: Same problem here as with the drive ESCs. You haven't even told me what type of weapon you want to build -- spinner, lifter, hammer? With so little info I can't tell you if that motor is adequate for your weapon, and I can't recommend a weapon ESC. If you're building a spinner weapon, you'll want to read the Ask Aaron Spinner Weapon FAQ before you do any additional weapon design.

Q: I would like the weapon motor controlled by the "G" switch with 0 being no rpm, 1 being about half way, and 2 being max rpm. Is this possible? If so, how would one bind and where would the leads plug into the receiver?

A: Yes, and this is also easy. Follow the instructions for 'Channel Input Configuration' on page 17 of your transmitter manual to assign Switch G to port 6 (aux) on the receiver and plug the weapon ESC into that receiver port. You will bind the receiver in 'preset' failsafe mode with the gimbals centered and Switch G in the 'off' position.

Q: Many Thanks.

P.S. How would you like your site cited in a paper? Just curious because you are the best website for the topic and much of my knowledge is from your site. I just want to give credit where due. :-) [New Castle, Pennsylvania]

A: Thanks for asking. In general a source is not asked if it wants to be cited. If you are referencing specific info from 'Ask Aaron' in a paper it is correct practice to cite your source. If you simply want to list 'Ask Aaron' in a general bibliography for your paper, the choice is yours.

Q: Hi Mark, it's the guy with the DX6e questions again. Sorry about not giving you all the information, I should have known better.
  • I was looking for general suggestions for the hobbyweight weight class. However, I am using Leopard 3650 at 2090kv or the banebots 540 18v motors. They are driving geared at 4:1 on 4" wheels.
  • The weapon is an egg beater with a diameter of 5" and a moi of about 0.008 kg*m^2. The gear ratio would be about 3:1.
Many Thanks!

Tentacle calculator example screen A: Given the components you've selected and the gear ratios you're looking at I'm gonna guess that you're coming at this from an R/C car background. The requirements of a combat robot are a great deal different than R/C cars.

Take a look at the screen shot from the Team Tentacle calculator at right. I've input the drivetrain details you've provided.

  • When pushing hard against another robot (a very common situation in combat) the drive motors will each be consuming 45.9 amps of current. I suspect that a RS-540 motor could keep that up for maybe 10 seconds before going incandescent and emitting a thin cloud of blue smoke.
  • The hypothetical top speed of the robot is 74.9 MPH(!) which will be of very little use in a combat arena as it takes 1701 feet and 25.4 seconds to achieve.
  • The estimated drive system battery capacity for a three minute match - without a safety margin - is a hefty 3217 mAh. Again, this is an over-estimate because your drive motors will fail in the early stages of the match.
If we change the drive gearbox to the BaneBots 38:1 ratio we get much happier results:
  • Peak current useage per motor drops to 4.8 amps, which an RS-540 can maintain for reasonable periods. You have your choice of a wide range of ESCs that can handle this power loading. I'd favor something with a bit greater power capacity to allow you to graduate to a larger bot without an upgrade: a dual channel Scorpion XL or RageBridge 2 would do nicely.
  • Top speed is just under 8 MPH, achieved in 5.5 feet and 0.7 seconds. That's entirely useable in a typical hobbyweight arena.
  • Battery capacity estimate for a three minute match - with a 30% safety margin - drops to 500 mAh.
Learn to use the Tentacle calculator!

Your brushless Leopard motor has similar problems driving your weapon. Popping your weapon specs into the Run Amok Spinner Spreadsheet with a 3-cell LiPoly battery and 3:1 gear reduction, the 80 amp max motor will pull an average current in excess of 400 amps for the first two seconds of spinup, and will still be pulling 200+ amps when it hits 85% of full zero-drag speed. With the aerodynamic resistance of a 5" beater, the motor will likely never drop under 150 amps at cruise. It will suffer the same incandescent fate as your drive motors at their original gearing. Not * gonna * work.

Sub-lightweight 'bots typically use much lower 'kv' outrunner motors to provide more torque so that they might survive at reasonable timing belt pulley reductions. The Leopard motor might survive life with your beater bar if it had about a 6:1 reduction, but so large a reduction is not easy to accomplish on a weapon. Reconsider your weapon motor choice.

Q: Just one more question... Can you give a little more step-by-step direction to assigning the RPM on the "G" switch?

A: I don't have a DX6e here in my workshop to poke at; I'm working entirely from the Spektrum DX6e manual. I can't do better than the step-by-step instructions for 'Channel Input Configuration' on page 17.

I believe that receiver channel 6 is assigned to Switch D by default. You can just leave it there if you like.

Q: Two questions.
  1. What are the principles of wedge building when you're up against other wedges and pushy bots?
  2. Do cylindrical lipo cells swell or is that limited to the prismatic pouches?
Thanks [Los Angeles, California]

A: [Mark J.] Two answers:

  1. Search the Ask Aaron Robot Design and Construction Archive for 'best angle' and 'lowest wedge' to find detailed information in multiple prior posts on this topic. Also search the Ask Aaron Toy Hacks and Kits Archive for 'Wolverine' to find a post on 'wedge killer forks'. Write back if you have additional specific questions on wedge construction.
  2. All lithium chemistry cells swell with age and heat. 'Hard shell' form factors allow some space within the enclosure for swelling, but it may or may not be adequate for a specific situation.

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 tranfer 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: What is the best weapon motor for a beetle weight small ‘motor in the drum’ spinner? (compare: Algos.) [Petersburg, Virginia]

A: [Mark J.] Nothing that you can buy 'off the shelf'. Take a look at the Ask Aaron guest commentary from Algos' builder Mike Jeffries to see what's necessary to strengthen an outrunner motor to survive in the very harsh 'motor in drum' environment.

You may also find this article from Servo magazine helpful in detailing the evolution of the 'motor in a drum' design for 'Version 2' of the 'Saifu' antweight kit.

Q: Weapon belts fall off a lot. Why use belts when you can use direct drive?

A: I think the answer to your first question gives you the answer to this question as well. Brushless outrunner motors are designed for model aircraft. Their structure is far too weak to survive the direct impact shock transmitted by spinner weapons.

It's not uncommon for direct-drive weapons using unmodified motors to requre multiple motor replacements in a single tournament. A belt drive helps to isolate the motor from direct shock loading, and a properly designed belt drive will rarely have the belt come off. There are several posts about the correct design and construction of weapon belt drives in the Ask Aaron archives.

A belt drive is also useful in reducing motor loading and decreasing spin-up time with larger spinner weapons. Designing a weapon for optimum spin-up time and terminal speed is more complex than just bolting a weapon onto a motor shaft. Take a look at the Ask Aaron Spinner Weapon FAQ for more info on this topic.

Finally, belt drive spinner weapons simply work better in the real world -- it's not just theoretical. Take a look at the top ranked insect 'bots at You'll find more belt-drive weapons in the top slots than direct-drive weapons.

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: Channels 5 and 6 on my FlySky T6 transmitter can each be assigned to a knob or a 2-way switch. I'm using one of the knobs for channel 6, but it would be more convenient to have a 3-way switch for my purpose. Can I replace one of the knobs with a high-mid-low switch? I can solder. [Sarasota, Florida]

A: [Mark J.] Yes, it's fairly easy if you know your way around a circuit board. We can replace a knob's potentiometer with a 3-position ('center off') toggle switch and a pair of resistors that will give high/mid/low signal control.

Open the back of your transmitter and locate the potentiometer on the back of the knob you wish to replace. The ones I've seen have been 5k ohms. It has red, brown and black wires attached, as shown at right. Note: there may be a white wire attached with the black wire; keep them together. Transfer the wires to the toggle switch with resistors attached as shown at far right. The resistors should each be half the value of the potentiometer -- in this case 2.5k ohms.

Would you like to see it done? There's an overly long, excruciatingly detailed, and very shaky YouTube video.

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

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 surprizes 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 unfamilliar small brushless motors.

Q: Hi, my application is not for a war robot, but I'll ask here all the same.

I am in the process of playing with a design for an autonomous agricultural robot, and am attempting to use a mobility scooter chassis as the platform to base a prototype on.

The unit is an Australian made twin motor machine, with both motors run as one rear drive and front wheel control arm steering. I will pick it up tomorrow and don't know the spec of the motors/gearing yet, other than it is 24vdc with a single Curtis controller and has a brake and regenerative charging. I'll remove the brake to make room for an encoder, and because I don't want it on there..

My plan is to run it in reverse with dual controllers giving skid/dynamic steering through the motors, and letting the steering rack trail like castors for now, and possibly fitting a motor/actuator to the steering rack at a later date for added control.

The robot will carry quite a payload as it will be used for selective spraying and will have a 50-80L tank on board, along with associated controls and array. I will aim for 200kg but allow for 300kg absolute max, more likely 250kg max (size of a medium quad bike).

The robot will need to travel over open pastures, and I'd say the 20 deg inclines would be the max when loaded but will attempt to make it negotiate as steep as possible land as it can handle without rolling over.

It's on about 330mm OD tires/wheels right now, but I'd like to run 400-450mm tractor style tires at the front and would like a nice strong 4-5km hour continuous working speed, with possibly a bit faster for moving it around between jobs.

I'm quite keen on a 36-48vdc system for increased efficiency between battery charges, and will run as large a battery bank as my weight/space limits will allow, and I may even go with Lithium batteries as they are much lighter (and dearer).

My questions would be:

  1. What gearmotor and ratio would you recommend for the rough description above? Heavy industrial worm drive or something else?
  2. Will 36 or 48v be a better option for longer range and battery life?
  3. Can you recommend a gear motor for this application with a 36-48v motor?
  4. If not, can I overvolt a 24v worm drive motor to 36v and choose a reduction ratio to suit my application? I did notice the slow rpm of the motors (14-1500) and wondered if it would be OK to take them up to 21-2200 rpm at 36v, given that I'll be using soft starts and steady navigated operation (not robot wars). The dual motor controller I'm looking at using has advanced programming options including current limits.
  5. If I am running the machine in reverse with the motors trailing as pictured, am I likely to be running an internal cooling fan in the wrong direction? Would it be better to swap the motors to the other side with the motors out front (I realize you may not be familiar with the Preslite motors pictured)?
  6. Any other advice? Encoders for autonomous control?
Please don't worry about battery capacity other than to pick the most efficient setup possible and estimate power usage per hour, I'll then look into the amount/type of battery used once I'm closer to taking off the existing 24v system if that's what we're doing.

Please let me know you thoughts, and thanks in advance for your advice. [New South Wales, Australia]

A: [Mark J.] I don't think I can be much help to you. 'Ask Aaron' has a collection of design tools for weight-restricted robots that operate for three minutes at a time across flat smooth surfaces, pushing at full capacity against immoveable opponents, all the while balancing on the edge of thermal failure while their opponent is unleashing kinetic energy attacks entirely capable of ripping them to scrap. Typical longevity of our drivetrains is measured in minutes. None of these things apply to your robot project.

I do have a few comments that may help.

  • Worm drive gearboxes are inefficient. It's common to lose 20% of motor power to friction thru a worm drive. Combat robots don't use them for drivetrains.
  • Operating voltage has no direct effect on battery efficiency. A 4000 watt 48 volt motor will drain the same watt-hours of power from a battery system as a 4000 watt 24 volt motor performing the same task. What you gain in higher voltage is a reduction in amperage for the same power output. This makes things easier on your motor controller, assuming it can handle the greater voltage.
  • Overvolting motors does more than increase RPM. A 50% overvolt will bump speed and stall current by 50%. It will also increase peak horsepower, thermal loading, and peak power consumption by 125%. Some motors will survive this additional loading and some will not.
  • Internal cooling fans are not common on motors in the class you're looking at. Where present they are typically radial-flow fans designed to operate in either rotational direction.
  • Current limiting motor controllers are good for motor longevity, but limiting amps also limits torque. Given that you're attempting to pull 300kg up a 20 degree slope at 5 km/hour across uneven terrain you may need to supply serious current.
I'm unable to make a gearmotor recommendation for you as none of the gearmotors I work with would be suitable for your application. All that I might suggest would be to start with the mobility scooter drivetrain and see how it performs -- you'll have a better idea of how much improvement is needed.
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: Hey hey! I was just lazing around today and I was randomly wondering: what is a good width for a heavyweight V-Belt for a spinning weapon? I was thinking 1/2", but IDK if that's overkill or if that's too small. [Decatur, Illinois]

A: [Mark J.] 'Heavyweight' covers a wide range of weapon and motor sizes. A 20 HP motor spinning up a huge bar spinner will have different requirements than a 4 HP motor spinning up a compact diskette. There are also considerations other than belt width. See this guide to belt selection for a starting point. I'd advise finding a comparable weapon on a successful robot and finding the belt solution they use. The hamburger is bad.

Q: Hey, it's me again (apologies for not sending this with my last message. I remembered this just as I had hit send. (-3-) Anyways, I heard about the "toothless" drum spinner that used Colsons instead of an actual drum. I was wondering this: if a row of Colsons were combined with a sloped wedge or plow and the robot was given good torque, would it be able to throw robots easier?

A: Several things:

  • Spinner weapons are not about 'torque' -- they are about kinetic energy storage delivering a huge impact that greatly exceeds to power directly available from the weapon motor.
  • A toothless weapon depends on a high closing speed to crash the drum into your opponent to get grip. They are useless in close quarters, and a wedge doesn't help like if does for a toothed drum that relies on 'bite' to grab a bottom edge.
  • Can you name a successful 'open' class toothless spinner? If they were effective more teams would be using them.

Q: I have returned (again)! First off, I wanted to apologize for the bad Hamburger I gave you yesterday. At least you won't get salmonellae from it. Second, I was cleaning out some old files and found my old collection of questions I asked several months back, along with a few I never asked. I didn't intend on sending you another message 24 hours afterwards (I didn't want to annoy you because I feel like my constant questions can be a bit annoying sometimes), but I wanted to give these questions before I forgot. OKAY, so, here we go:

1) Sewer Snake's Lifter: are there any significant advantages (outside of being able to do cool flips) or disadvantages to having a front and rear-hinged lifter connected to one motor rather than using just one or the other?

A: In Sewer Snake's standard configuration the black 'rear hinge' fork is primarilly offensive while the red 'front hinge' fork is almost entirely used to self-right or otherwise get the 'bot out of awkward positions. Together, they make great use of a single electric lifting system.

2) I've noticed that some robots that use a connected drivetrain are connected in the center, with one sprocket leading to the right side of one wheel and one sprocket leading to the left side of the other (Examples I could come up with off the top of my head include Deathcap (Beetleweight) and Otis (Featherweight), but there are many more). Why not have one chain connected on one side of both wheels? What advantages does one receive from making their drive chain in the way explained above?

A: Multiple benefits:

  1. Simplicity - a single belt/chain would require one or more tensioners to keep adequate contact with the center pulley/sprocket.
  2. Redundancy - if one of Deathcap's drive belts fails (belts and chains DO fail) it will lose drive to one wheel, but the 'bot still has mobility.
  3. Compactness - twin belts/chains to the same side of the drive wheels adds extra width that is saved by this inside/outside design.

Search the Robot Design archive for 'Bombshell' to find a previous post discussing other facets of this design choice.

3) S.L.A.M is a robot that, for some reason, I keep thinking about. I keep thinking about the fact that it used wedges instead of bludgeons or teeth. Is there any pro to making a spinner that would utilize a similar concept? (My idea was to make one that was much shorter in order to allow for more weight to be used in the sides.) (NOTE: I have a very small feeling this one may have been answered or a similar question may have been answered before. If so, feel free to skip this question)

A: Career competition record for 'S.L.A.M.' (1999 - 2000): 1 win/3 losses. The one robot it beat had a 0 win/2 loss record. I think that says enough. Emulate successful robots -- or you can always click the button.

4) Does the leg placement on a shufflebot matter all that much? If I were to have a shuffling design similar to that on Pitter Patter, would it matter which part was touching the ground so long as one part on each side was touching the ground at one time?

A: I don't think I understand the question.

  • The shuffler modules on each side of the robot are not synchronized with each other, so different shuffler 'foot' elements will be in contact on opposite sides of the 'bot at any given time. If everything was synchronized the 'bot couldn't turn.
  • Within a single shuffler module, the order that the 'feet' make contact to the ground makes no difference, as long as the contacts are evenly spaced.

5) In your opinion, how high is too high for an angled spinner? 45 degrees? 22.5 degrees? 5 degrees? Or, do you think angled spinners should be avoided?

A: Do not angle your horizontal spinner. An angled spinner will strike downward on the leading side of the spinner arc, and a hit on that side will throw your 'bot upward. Keep it flat.

6) Do you think "Driscs" are viable alternatives to drums? What advantages and disadvantages do they have over a normal drum (one that's crafted from a single piece of metal)?

A: See my answer about drumbeaters. Call them anything you like -- if the weapon energy storage numbers work and they're strong, they're viable.

7) There's a competition every year at the U of I called Robobrawl. It's an event held at the U of I's Engineering Open House. After talking to the people behind the event, they said that iRobotics supplied them with up to $2500 to build a robot and had access to aluminum machining tools. However, it seems to be that they only allow machining on Aluminum. Some robots have had things like Polycarbonate panels, but these are usually not available through iRobotics. I know Aluminum is great all around, but is an all-aluminum robot a good idea?

A: No one material is 'best' for all parts of a robot, but not everything has to be 'machined'. I'd bolt on a few flat steel armor plates and impactors.

8) Lastly, I linked a picture of a flipper idea I had. I wanted to create something that was similar to designs in the US, but not as stiff, and the UK, but not as free-flowing and reliant on cables to keep it from breaking. Is this a good idea in terms of a good mechanism? What would need to be changed or are things of potential concern?

A: It looks entirely classic US style to me -- nearly identical to Team Run Amok's The Gap. I'm not sure what you mean by 'not as stiff'. You will want a clevis mount to locate the bottom of your cylinder -- you cannot just leave it hinged on one end and unattached on the other. The disadvantage to the design is the height of the structure; see similar but more compact design examples at the Team Da Vinci 'Understanding Pneumatics' page.

As always, I thank you for responding to my questions! I appreciate it very much!

Q: Why are bigger motors (example: Perm 132) slower in rpm than smaller motors? (example: Park 150) [Arlington, Virginia]

A: [Mark J.] Think about it this way:

  • A large diameter brushed pancake motor spinning at 3000 RPM may have the outside edge of the armature moving relative to the stationary permanent magnets at 24 inches per revolution: 3000 * 24 = 72,000 inches per minute.
  • A small diameter brushless outrunner motor spinning at 36,000 RPM may have the inside edge of the rotor magnets moving relative to the stationary coils at 2 inches per revolution = 36,000 * 2 = 72,000 inches per minute.
These two motors have the same 'linear speed' at the interface between the coils and the permanent magnets, but the smaller motor has a higher RPM because each revolution takes less linear distance to complete.

General rule - everything else being equal, smaller diameter armatures/rotors spin faster, but provide less torque. There are other factors involved, but I'm not going to write a treatise on motor design in this short-answer format: more info.

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 that 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: Mark, with the booming scene of Chinese robot combat, here's one short question: How long do you think it will take for a Chinese bot to be introducted into the Combat Robot Hall Of Fame?

Also: I would still like to thank you and Aaron's kind help to us from 10 years ago, without your useful information I could not be here today and commentating fights for the teams I can dream of fighting in China, thanks. ['Chinese Forum' guy that now has became the commentator of FMB in China]

A: [Mark J.] Ahh... I remember the 'Chinese Forum'. Many interesting questions came from that group of builders. I appreciate your gracious note of thanks, and I'm pleased that you are involved with 'Fighting My Bots'.

The explosive rise of robot combat in China took me by surprise.

  • The early robot combat history of the US, Europe, Brazil, India, and Australia all started with local and regional tournaments attended by enthusiastic novice builders who developed their own designs and styles of combat.
  • In contrast, the first Chinese tournaments I learned of were large, televised tournaments attracting international competitors. This stiff competition may have pressured local builders to adopt successful existing designs rather than create novel designs of their own.
Chinese builders will certainly develop their own style, but their path to a Hall of Fame robot will be different from what we have previously seen.

The website attracts a very limited readership from China. The attached map shows how our typical web traffic strongly skews to the US and Europe, with China trailing behind India, Malaysia, and Australia. I wonder how many builders in China are even aware of the Hall of Fame?

Reply: Mark, please be sure that the Combat Robot Hall Of Fame is still known in China, the web traffic from China might be a bit low, but we are certainly aware of it! I really hope in the future that I can have a chance to meet you in person, for all the help Aaron and you have gave to me over the years. [Chinese Forum, or should I now be called FMB commentator guy?]

Q: I found this in the BattleBots thread on Reddit. Thought you'd enjoy it. It's from 'InquisitorWarthIncom Technologes Robotics Division', giving advice to a potential new builder:

In general, there are a few resources I recommend:

Team Run Amok: Robotica Season 1 Champions. Lots of good insight into the sport, though admittedly Mark Joerger can be a little cold and unfriendly at times - usually when he's frustrated with someone.

A: [Mark J.] A little cold and unfriendly? I prefer to think of myself as quite warm... and unfriendly.

Q: Hi, I don't have any question regarding the combat robots, but want to know how to build a safe arena for combat robotics in India and what weight category do you think we should play in? [India, just outside of Chicago?]

A: [Mark J.] See Frequently Asked Questions #38 - How do you build a safe robot combat arena? Fight a weight for which you can afford a safe arena.

Q: Hello there, first off a bit information, at my work we are starting a small competition in which we build fratherweight (13,6kg since it seems featherweight is different in other regions). Anyway your website is listed as one of the better sources for information and questions. So u might get a bit more questions fron the Netherlands.

Anyway my question, between all my coworkers plans to build very destructive robots, I would like to try to win my battles by a bit more control. Taking inspiration from 2 Robots I liked, being Dr Inferno Jr and Tazbot. The idea is to combine a base with good protection from wedges and deflection for spinners with a turret mounted arm to control and work as a srimech. For it to work I think grip is very important. Any advice on which tires provide best grip? I was thinking of using polyurethane wheels from a palletloader that i have but I do not know if they provide good grip in a normal arena. [Zuid-Holland, Netherlands]

A: [Mark J.] You've failed to mention what flooring you will have in your "normal arena". It matters. What I would recommend for a steel floor would be different from a painted wood surface, which would be different from a dusty concrete floor. My choice also varies with the size of the wheel you want to use. Traction is also highly dependent on the percentage of your robot's weight that is supported by your driven wheels.

Search for "traction" in the Ask Aaron Materials and Components archive and you'll find many posts about tire compounds and traction tricks. You'll also want to read the Ask Aaron guide on Optimizing Robot Drivetrains for tips on getting motor power efficiently to the ground.

Q: The first idea was combining the simple forks from panic attack with a reversible shuffling to gain a small weight advantage, but looking for shuffling robot I only find a lot of a guy with a cardboard robothead, so I let that one go.

A: If you search for 'shufflebot combat robot' you'll get results more relevant than 'cardboard robothead guy', but don't build a shufflebot -- Simple Robots Win.

Remembering Aaron... 

Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.

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