6497 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.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.

Q: I used the on-line robot drive motor sizing tool at to help me pick drive motors and batteries for my robot, but the numbers it's giving seem very small. Can I really power my 60kg combat wedge robot with a pair of 175 watt motors geared for 7 mph and draw only 15 amps total at 24 volts? [Nepal via Facebook]

A: [Mark J.] The RobotShop calculator is for hobby and utility robots that glide along smooth floors and up gentle ramps, not combat wedges that push full-throttle against opponents that push back equally hard. I'd suggest evaluating drive motor performance with a tool intended for combat robots.

The Team Tentacle Torque Amp-Hour Calculator says that a 60 kg robot with pair of typical 175 watt electric scooter motors geared as you suggest would take a leisurely 3 seconds and more than 20 feet to accelerate to 7 mph. Pushing hard against an opponent on a typical arena surface would bog down the motors near stall and rise total current consumption toward 60 amps for the pair: way too much current for those motors to handle and 1400 watts of drain on the battery.

Q: I wired up my spinner 'bot with a pair of 3-cell lipos to run each of the drive motors at 11.1 volts and my weapon motor at 22.2 volts - see the attached sketch. When I plugged in the batteries one drive motor kinda worked for a second but before I could test the weapon motor (no belt to the spinner) the receiver cables to the ESCs melted and smoked.

I checked the wiring and it's exactly as I've drawn it. What went wrong? [Columbus, Ohio]

A: [Mark J.] I've re-drawn your wiring sketch and removed the weapon components to focus on the problem. Your circuit is designed very cleverly to share the load equally between the two batteries; each drive motor draws current from only one of the batteries and the weapon draws from both batteries. If you simply had motors wired up directly with no ESCs or receiver everything would be fine; each drive motor would get 11.1 volts and the weapon motor would receive 22.2 volts. Unfortunately, the motor controller and receiver internal circuits cause a problem that isn't obvious in the block circuit diagram.

  • The black 'ground' wires in the three-wire receiver cables each connect directly to the 'ground' loops on each ESC circuit board. Each ground loop also connects to that ESC's negative power input.
  • Your two drive ESCs are connected to two different 'ground reference' potentials in the power circuit:
    • The lower ESC in the diagram has a ground at zero volts (black power wire) and power at 11.1 volts (blue power wire) so it 'sees' a net potential difference of 11.1 volts. Its black receiver cable wire connects to that zero volt 'ground' - so far so good.
    • The upper ESC in the diagram has a ground at 11.1 volts (blue power wire) and power at 22.2 volts (red power wire) and it also 'sees' a net potential difference of 11.1 volts. Its black receiver cable wire connects to that 11.1 volt 'ground' - hmmm...
  • At the receiver end, the two black wires from the two ESC cables connect to a common 'ground bus' -- that creates a direct short between the 11.1 volt blue power wire and the zero volt black power wire.
VERY FORTUNATELY the small gauge wires in the receiver cable couldn't handle that much current and melted before the battery went into runaway thermal overload and burned your shop down.
General Rule Everything in your power circuit should connect to the same 'common ground' potential to avoid unanticipated or hidden over-voltages and circuit shorts .

I hesitate to mention this, but it is possible to get this circuit to work without smoke and explosions. I don't recommend this style of circuit but I will provide a diagram in case you should decide to live dangerously. You will require an 'optical isolator' in the receiver cable to each ESC that is connected to a floating ground. Keep the red wire in the receiver cable to the isolator intact -- the isolator requres receiver power to function. I've also drawn up a proper 'common ground' circuit (minus receiver for clarity) that has all the ESCs sharing a single zero-potential ground. It does not split the load equally to the two batteries, but it is less likely to burn down your shop.

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 decribe 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 it's 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: Do you think that moist pony from [YouTube link redacted] is the best and most effective robot ever? [Omaha, Nebraska]

A: [Mark J.] By Grabthar's hammer, everybody knows who the best and most effective robot is. Now please, never mention 'Moist Pony' again.

Q: What do you think of brush wheels for antweights? Too flimsy? [Syracuse, New York]

A: [Mark J.] Are we talking 'bristlebots'? Calling them 'brush wheels' is misleading -- wheels spin, bristle brushes just hop up and down.

Combat bristlebots like 'Vibrant' are novelties: poor speed, awful pushing power, no reverse. What part of that do you like? Some builders enjoy making weird 'bots just to be different. Avoid if you're trying to win.

Q: Would an HDPE unibody work for a beetle wedge if it was protected by grade 5 titanium plates? I was hoping the unibody would work as a sort of shock absorber to protect against high energy spinners. [Oak Harbor, Washington]

A: [Mark J.] Polyethylene unibodies are common and successful in insect class combat robots, but UHMW polyethylene is preferable to HD. An image search for 'UHMW beetle robot' will provide many examples. No metal cladding is needed -- save the grade 5 titanium for your wedge.

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, achieveable 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 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 five posts down the page 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: I want to build a switch! I'm looking at Team Whyachi's pictures and drawings on their website, and I'm confused as to how how they're able to use a metal socket screw to close the circuit. Even if there was a nylon washer separating the cap from the copper "bridge" and the two contacts, wouldn't the thread/screw body still touch and you'd get shocked if using a metal allen key? And how do they know how many turns it takes for full contact without overtightening the screw? [New York, New York]

A: [Mark J.] The Whyachi switch socket screw is completely isolated from the electrical circuit by a nylon 'top hat' grommet that extends thru the contact bar hole. The screw is isolated only to prevent the Allen wrench from accidentally shorting to another exposed electrical component. Direct current at normal robot-level voltages will not give you a 'shock' on touching a terminal -- you won't even feel it.

The Whyachi switch uses spring pressure to make contact, not screw pressure.

  • Tightening the screw pulls the contact bar down against spring pressure to break contact;
  • Loosening the screw allows the contact bar to rise up by spring pressure to make contact.
  • 'Four turns' is a recommendation, not an absolute.
Take a look at Charles Guan's 'Overhaul 2' Build Log - Part 9 for details on the simple and compact switches he made for the purpose. Search that page for 'master power'. Keep it simple.

Q: I asked '@the_guy_who_has_a_robot' and he recommended you to me. Can you design an antweight 18mm thick 50mm long 5mm shaft hole monotooth vertical spinner (just spinner) for me and send the stl to [email address redacted] please? [Camarillo, California]

A: [Mark J.] I don't know '@the_guy_who_has_a_robot' and he apparently does not know what we do here. From the Ask Aaron mission statement:

"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."

Q: I am '@the_guy_who_has_a_robot' and I would like to apologize. I told the guy who had a question about the mono tooth spinner to go to you if with his question. I didn’t know how to awnser it and I told him about you. I told him to give him lots of details. Not to ask you to make it. I apologize. Please accept my apology [Omaha, Nebraska]

A: Thanks for the explanation, Omaha. Getting caught in the middle requires no apology.

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: So I’m making a new 1lb and 3lb competition in the midwest. As a builder what do you suggest I should do. ( box size. Large sheets of lexsan and where I can get is) thank you very much [Omaha, Nebraska]

A: [Mark J.] What would I suggest? I'd suggest that you attend combat robot competitions and talk with event organizers until you no longer need to ask me to sumarize in a couple of paragraphs everything you need to know to put on a safe and well-run competition.

Unwilling to do that? Head over to SPARC: Standardized Procedures for the Advancement of Robotic Combat and read every word in their 'Event Resources' section - top of page on the right. The last item in their event resouces section is the 'SPARC Arena Construction Best Practices' which provides construction considerations, cost estimates, material recommendations, and example arena photos.

Check local plastics suppliers for polycarbonate ("Lexan") sheet. If you cannot source locally, a quick web search will turn up multiple suppliers -- but shipping on large sheets can get expensive.

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:


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 the Ask Aaron Robot Weapons archive give examples of lifter analysis -- search there for "calculate how much torque" for a start.

Q: Do you still go to fights as a spectator; if so, where? [Hagerstown, Maryland]

A: [Mark J.] I'm reluctant to discuss my travels. I don't even tell Google.

Q: Do you know if BaneBots P60 gearbox's gears are interchangeable? I have a 4:1 that I want to change to 5:1. What would I need to make this work? [Hagerstown, Maryland] 111

A: [Mark J.] Yes, you can turn the 4:1 P60 into a 5:1 P60. Parts cost is about $25 - shipped.

The planetary gears and carrier plate just slide out when the gearbox is opened. Demount the motor and use a gear puller to take the old pinion gear off. Take care in pressing the new pinion onto the shaft. Reassemble the gearbox with fresh grease and you're good.

Q: Apparently there is no 5:1 pinion for a 5mm shaft, Any ideas?

A: A few...

  1. Scour the 'net for a gear with these specs:

    Number Teeth:11
    Gear Specs:0.6 Modulus, 20 Pressure Angle
    Installation:Interference (press) fit
    Bore:5 mm
  2. Make friends with a good machinist willing to bore out a 3.2mm pinion to 5mm.
  3. Drive over to your local R/C hobby shop and dig thru their gear bins.
  4. Switch to a VEXpro gearbox.

Q: I have a brushless motor that is rated 12 to 15 volts and two drive motors that are rated 3 volts. How do I power them without burning up the drive motors or not giving the brushless motor enough power? [Danville, Illinois]

A: [Mark J.] The only '3-volt' gearmotors I know of are made by Tamiya, and they are not nearly strong enough for the current level of robot combat. The motors are weak, the plastic gearboxes shatter, and the skinny little axles bend. Worse, I don't know of any electronic speed controllers that will work from a 3 volt power supply -- most won't operate below 6.5 volts. You'd be well advised to dump those motors and go with something more commonly used in combat robots.

If you insist on running Tamiya gearmotors, or if you've got a motor I don't know about, you can use a 4-cell LiPo battery (14.8 volts) and program your radio transmitter to limit the drive motor controller output to a maximum 25% of full throttle -- effectively limiting motor voltage. Twenty years ago we ran Tamiya motors at more than 7 volts without trouble; if you're afraid of burning up drive motors you're in the wrong sport.

You'll need an R/C transmitter that has 'Adjustable Travel Volume' (ATV), sometimes called 'Travel Adjustment'; check your radio manual. Most combat-capable radios have this feature. Set the travel volume on the drive train throttle and turning channels to 25%, and leave the weapon channel at 100% so your brushless weapon motor will get the full 14.8 volts. This technique is a little hard on the drive motor controllers, but those Tamiya motors draw so little power that it won't be an issue.

I'd much rather see you get proper drive motors and a weapon motor that can all run on at the same voltage -- they're certainly available.

Electrical Engineers: don't bother to write in to tell me that electronic speed controllers aren't really voltage limiters. You're right - but with the duty cycle set to a max of 25%, the current limiting will keep the drive motors healthy.
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: Is there a reason those who use brushless only have used the ones made for airplanes? Why not cars? Something like this: Turnigy 1/8th Scale 4-pole 2100KV.[Hagerstown, Maryland]

A: [Mark J.] There are two broad types of brushless motor design:

  • Outrunners have a stationary core (stator) and a magnet-lined 'can' (rotor) that spins around the outside.
  • Inrunners have a stationary outer 'can' like a brushed motor with a spinning magnet rotor inside.
The outrunner-style motors are commonly used in smaller combat robots because they spin more slowly and with greater torque than inrunners. This makes it simpler to get the correct reduction for weapon use, and keeps input RPM low enough for inexpensive drive gearboxes.

As robots get larger you see more inrunner designs in use. Larger inrunner motors spin more slowly than the tiny ones which makes them a workable option for both drive and weapon power. The Turnigy motor you referenced has about the right amount of power for a hobbyweight weapon but spins at better than 38K RPM, which could be difficult for a belt drive to handle -- simpler to use an outrunner. For drive use, a pair of the Turnigy inrunners would have enough power for a featherweight 'bot, and this is probably the lightest class where I would consider using inrunner drive motors.

By the time you get up to heavyweight 'bots, the outrunner motor styles have disappeared and everyone uses inrunners if they're using brushless.

Q: Can I use my Quantum pistol-grip radio for an antweight robot? [The Aether]

A: [Mark J.] Yes, with a little added hardware. Your Quantum radio, like most pistol-grip R/C systems does not have the 'mixing' capability to to control a robot. Your steering 'wheel' (radio channel 1) controls your steering servo servo to point your front wheels, and your throttle 'trigger' (radio channel 2) controls the motor speed controller for forward/reverse motion. To control a 'tank-steer' robot the control channels must be 'mixed' to each control the motor(s) on each side of the robot, but in different ways:

  • The 'trigger' must instruct both motor controllers to respond together to move the robot forward/reverse; and
  • The 'wheel' must instruct the motor controllers to respond in opposite directions to turn the robot.
Fortunately, you can add a small an inexpensive bit of electronics called a 'channel mixer' to your robot to sort this all out. These are available from multiple sources. Many builders use the FingerTech tinyMixer for its robot-friendly design and functions.
Q: what flavor of Machine screws are best for a battle bot? (GR 8/alloy steel, Titanium, stainless) specifically what is needed for holding armor on?
Is harder always better? [Hagerstown, Maryland]

A: [Mark J.] Machine screws / bolts are designed and rated to resist a 'pull' force that is tested by a machine that applies a gradually increasing force. That type of strength is important in many applications, but it is not the type of load fasteners will face when holding combat robot armor. Forces encountered in combat are typically sudden 'shock' loads that may have a high 'shear loading' factor. Ideally, mounting should be designed to prevent shear load on bolts, but combat impact vectors are unpredictable -- you need to design for all possible loads.

The desired material and temper of the bolts depends a great deal on the armor mounting style, but there are some general considerations. Grade 8 bolts are strong, but their added hardness results in a lack of 'toughness' needed to survive shock loads. Stainless steel fasteners have much greater toughness, but are not nearly as strong overall. Titanium falls in between, but the added expense and bother far outweigh any advantage. What you most commonly see in use are standard grade 5 steel bolts: greater strength than stainless and greater toughness than grade 8. The loss of strength can be made up by adding a few extra bolts. If your bolt heads are exposed to possible spinner attack, you'll want to countersink the heads to prevent the spinner from grabbing exposed 'spinner bait' and tossing your 'bot and/or shearing the bolt heads.

Need more info on machine screws? Curious Inventor: All about screws.

You might also be interested in searching the Ask Aaron Materials and Components archive for 'shock mount'.

Q: Hey, Mark! I've been goofin' around with the Tentacle Calculator, and I've come up with a couple questions about it:

1) Is there a way to determine the gearing ratio on a robot with a chain or belt-driven drive with front and rear wheels of different sizes? For instance, the rear wheels are 3", and the front wheels are 4". The rear has, say, a 1.75" sprocket, the motor itself has a 2" sprocket, and the front has a 3" sprocket. These aren't real numbers, they're just random numbers to be used as an example.

2) Is there a way to determine the speed of a shufflebot? I ask because the calculator does calculations for wheels, and IDK if there's a way to calculate for shufflebots.

Thanks again! [Jacksonville, Illinois]

A: [Mark J.] By the numbers...

  1. If you're driving front/rear wheels of different diameters, the front/rear sprockets/pulleys should be in the same proportional diameters as the wheels to avoid wasteful tire 'scrub'. In your example:
    • The front wheels are 4" diameter with 3" sprockets, making the sprockets 75% of the wheel diameter.
    • The rear wheels are 3" diameter, so their sprockets should also be 75% of their diameter: 3" * 75% = 2.25".
    If the proportions of the wheels and sprockets are correct on front and rear, entering the gear reduction for either set of sprockets and tire diameters into the Tentacle Calculator will give the correct result.
  2. I'll tell you what the speed of a shufflebot is - it's too damned slow. The faster you try to go the bouncier it gets. At speed you spend as much time with the shufflepods in the air as on the ground and a lot of the drive energy goes into making the whole robot shake. Think 'robot with hexagonal wheels'.

    If you want an estimate from the Tentacle calculator of how fast it would be going if it wasn't hopping, double the offset distance for a lobe on the shufflepod cam and enter that value as the 'wheel' diameter. But seriously...

Q: How much can the performance of a brushless ESC be improved by swapping capacitors?

I saw on Charles Guan's website when he was learning brushless drive setups that he swaps the capacitors out on all his ESCs for high-performance capacitors. I know this is supposed to help shield the ESCs and prevent burnouts from current surges when the motors are pushing at low RPMs, but how much does it help, and how much should I upgrade the capacitance of my ESC?

To be more specific, I have a 30 amp brushless ESC with a pair of 25v 550 uf capacitors. Would it make sense to swap those for a pair of 35v 2200 uf capacitors? They only weigh 6 grams and the ones I'm losing are probably 3, so if I can get any increased survivability for my ESC that kind of weight would be worth it.

The ESC will be running a .102 Ohm 4S 580kv brushless motor with a 1.8:1 pulley reduction to power a large horizontal spinner (about .0025 Kg M2).

Also, I read on some hobby forums that ESC capacitors should generally be rated at twice the voltage of the system, is that true? My system is 15v, and the current caps on the ESC are 25v so is it worth it upgrading for that alone?

Thanks again for all your help! The bot is coming together nicely and I've registered for the Robothon in Seattle in October. [Mark from Vancouver]

A: [Mark J.] How much extra salt should you shake onto your hamburger? Some need a little, some need a lot, some have too much already.

Charles Guan is an MIT engineer who designs and manufactures ESCs in his spare time 'cause he thinks it's fun. He absolutely knows what he's doing. If you're not a electrical engineering graduate of a respected school you're likely gonna to do as much harm as good by tinkering with the components of a complex digital electronic device.

That said, the little brushless ESCs robot builders 'borrow' from the hobby R/C aircraft industry are designed for very different performance targets than we expect from them in our applications. Bumping up the voltage rating of the filter capacitors will do no harm and may provide some level of extra reliability.

Blindly increasing the capacitance is another matter. I'm pretty sure that Charles Guan had his drive system fully instrumented and was watching voltage spikes on a oscilloscope as he changed the capacitor values. Simply pouring in more capacitance is shooting in the dark.

A drive motor ESC sees a lot of high-load/low-speed action that can benefit from added capacitance, but a spinner weapon ESC operates much closer to the propeller-spinning job for which the unit was designed. I'd suggest keeping the factory capacitance value as-is unless you discover a specific reliability problem as too much capacitance can adversely effect high-speed operation. If you really want to tinker, keep the increase to no more than double the original value. I suspect that most spinner ESCs fail for reasons that more capacitance isn't going to overcome.

P.S. - I'm happy to hear that your build is going well and that you're registered for Robothon - that's a great event. Say 'hi' to the Seattle crowd for me.

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 crystaline 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 metalurgical 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: Hi Mark,

I'm designing a 'Breaker Box' style rammer bot for 15# competition. I was curious about different types of drive motors. There are several variations that will allow the full pushing force, but I imagine a higher torque motor would win in a pushing match. In my area, Leopard and Castle Creation 540 size motors attached to Banebots gearboxes are most common. I was curious if you can use an outrunner with the same or similar bolt pattern, hooked up to a Banebots gearbox, as a drive motor. I know AmpFlow motors are all the rage in the higher weight classes, would one be suitable here?

I was also wondering how to maximize grip on the area floor. I have cut treads in tire and clean my wheels religiously with mixed results. Do lower shore hardness wheels offer better grip, ex green banebots vs black banebots or Colsons? Has anyone had success with adding "grip" to the tire? I was thinking studs like Riobots did, low grit sandpaper, two sided tape or something that can be sprayed on that adds grip.

Your insight and experience is much appreciated. [Pittsburgh, Pennsylvania]

A: [Mark J.] You're asking about very popular design topics. A search of the Ask Aaron Combat Robot Design archive yields 127 hits for 'traction' and 128 hits for 'pushing'. The Ask Aaron Robot Motors and Controllers archive has dozens of posts about using brushless motors for robot drive systems -- and there are several on this page.

Here are some quick topic highlights -- search thru the archives for details:

  • Brushless motors mated to assorted planetary gearboxes are VERY popular in sublight robot classes. There are plenty of examples and build logs to provide guidance, as well as many posts in the Ask Aaron archives.
  • Using brushless drive motors is much more complex than using brushed drive motors. Hobby grade brushless motors and controllers are borrowed from the R/C model aircraft industry and are made to spin lightweight propellers -- not push heavy robots. Getting the motor controllers repurposed to do this work is complex and often frustrating.
  • Torque is only useful to the point where the tires 'break traction' and start to slip. Torque past that point does not significantly add to pushing power. The Tentacle Drivetrain Calculator is invaluable in designing a robot drivetrain for pushing power -- if you're using brushed motors. Click its 'Help' button for details.
  • The high performance AmpFlow motors are needlessly heavy for a 15-pound robot. You can get all the torque you need from much smaller/lighter motors. Want more torque? Increase the gearbox reduction ratio and/or go with smaller diameter tires to trade speed for torque.
  • For maximum pushing power get as much of your robot weight as possible onto your powered wheels. A 4-wheel drive robot with all the robot weight supported by driven wheels is a great start. Two-wheeled robot? The RioBots combat tutorial has a section on calculating center of gravity placement for 2-wheel robots to get optimum traction.
  • Want to win a pushing match? Get a wedge or lifter under your opponent to take weight off their drive wheels and put that weight onto your drive wheels.
  • Soft 'sticky' tires have better traction than hard tires when clean, but 'sticky' tires collect a lot of grit and dirt from the arena surface which eliminates their traction advantage. Cutting a tread helps tires shed grit and oil, but if your tires are super sticky you're fighting a losing battle.
  • Foam tires are soft but aren' great for traction. They can be coated with liquid latex or silicone sealant to improve their grip.
  • You can GREATLY improve traction with magnetic downforce -- if the arena surface is magnetic, and if the event allows such designs. You can also boost traction with vacuum fans. Search the design archive for 'vacuum'. While you're there, search for 'sumo'.

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 π * (5/16)2 = 0.307 in2 cross-section area and your proposed 3/4" cylinder has a π * (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: I'd like to try brushless drive motors for a new robot, but I can't figure out how to input brushless motor data into the Tentacle drive train calculator. How do I calculate the stall torque and torque constant for a brushless motor? [West of the Pecos]

A: [Mark J.] The Team Tentacle Torque/Amp-Hour calculator was designed for brushed motors and assumes an inverse linear relationship between motor speed and torque. Common hobby brushless motors do not have that speed/torque relationship, and the entire concept of brushless 'stall torque' doesn't really make sense. To further complicate the issue, the torque curve of a brushless motor is highly dependent on motor controller firmware and on the specific user-adjustable firmware settings. The Tentacle calculator is nearly useless for designing a brushless drive train.

See what successful bots in your weight class are using for brushless drive motors, gearboxes, ESCs, and (importantly) controller firmware/settings. In general you'll find motors with high Kv constants, high reduction ratio gearboxes, and controllers flashed with SimonK or maybe BLHeli firmware. Brushless is a whole lot more complex than brushed drive, and I'd strongly recommend that you start with a proven combination. Experiment after you have a working drive train.

Q: Great site! I'm getting a lot of good info as my son and I are building a beetleweight.

He wants to use a pistol-grip style radio, we're looking at the Spektrum DX5C with the SRS6000 receiver due to its built-in gyro. My concern is that it only lists a failsafe on the throttle channel. I see that some of the Spektrum receivers have the (sometimes undocumented) ability to switch to a mode where all of the channels will fail in the bind position if you bind it the right way, but I have not found any info on this receiver yet.

Looking through the radio archives, you have a lot of suggestions for radios/receivers with good (or at least passable) failsafe capabilities. However, when I search for them they seem to all be coming up as "discontinued" items. Do you have any suggestions for current receivers, or know of a good list somewhere? [Fredericksburg, Virginia]

A: [Mark J.] Well, now you've opened up the jumbo can-o-worms. Where to start...

I'm assuming that you've found our Robot Combat Radio Guide, but in case you haven't I'm gonna suggest that you take the time to give that a read. And since you mentioned gyros, our Guide to Combat Robot Gyros will be handy as well.
Combat robots 'borrow' most of their R/C gear from model aircraft because their control needs are the closest to those of our machines. Better than 90% of the transmitters you see at tournaments are twin-stick aircraft units. A fair number of novice builders decide that pistol-grip or gamepad-style transmitters would suit them better because they are familiar with their use in R/C cars or gaming. The great majority of those builders soon realize that combat robots have much different control requirements than cars or games and quickly switch to twin-stick. I strongly suggest that you skip over the "this doesn't work as well as I thought it would" stage and buy a twin-stick radio to start, if only for the much greater selection and the much larger support base from other builders.

Radio gear has been changing at a ridiculous rate since Chinese manufacturers jumped into the game headfirst a few years back. New transmission protocols, open-source firmware, onboard serial networks... which is why all the receivers you've found in the radio archives are outdated and no longer in production. Our robots don't really need all this new fancy stuff that's aimed mostly at the drone market, but we make up such a tiny portion of R/C gear sales that we have to go along with whatever happens in the larger industry.

The upside of the Chinese R/C takeover is a stupendous drop in prices. The downside... well there are several downsides:

  • Unreadable Documentation The quality and completeness of the manuals for Chinese R/C systems is truly awful. Here's an actual quote from one:
    The time-recorder is used calculating comparable bo stipulated time unexpectedly, or the possible time of flight under the state that the fuel fill it up with, it is very convenient. The pattern of the timer-recorder is the count-down. Pour time-recorder from set for time is it is it count to change, show surplus time at interface to begin.
    I recommend downloading the manual for any radio you consider purchasing to see if you can make any sense of it -- before you buy. I also recommend going with a system that is in use by a large number of combat robot builders, so that you have a knowledge base to tap when you realize the manual doesn't cover what you're trying to do.
  • Factory Support There isn't any. That's why you need your friendly knowledge base.
  • Initial Quality When you're paying $50 for a radio system with features that would have cost you $350 fifteen years ago, something's gotta give. The feel is cheap, the switches and gimbals wear out, and sometimes it doesn't work right out of the box. At these prices, you just go buy another one. Some are better than others.
  • Unreadable Documentation I know I already mentioned this, but it's so bad I wanted to put it in twice.

Alright, so what do I recommend? I personally prefer Futaba radio gear such as the '6J' for high quality and excellent manuals -- but they are not widely used in robot combat so a new user would not have the knowledge base of established users they might need. Consider one of these:
Flysky FS-i6 Probably the most common system in use in robot combat, and certainly a good choice for a first system. Comes with a very useable receiver that fully failsafes on all channels, with a wide variety of specialized receivers available should the need arise.

Taranis Q X7 A very sexy system widely used in robot combat. Better than usual quality, looks great, but it's a complex radio that will intimidate novice builders. Does not come with a receiver, but multiple full-featured receivers are available.

A quick word about 'failsafe' As fully discussed in the Robot Combat Radio Guide, there are at least three different things a receiver can do to on signal loss that qualify as a 'failsafe'. Some of these are suitable for robot combat, and some are not. Make sure you understand the differences.

One last thing I don't recommend receivers with built-in gyros. A gyro has to be specifically oriented within the robot relative to 'up' and 'forward'. This can make fitting your receiver-gyro unit into a small robot awkward. Robots also have special requirements for gyro shut-off when the robot happens to be inverted that a receiver-gyro combo is unlikely to provide. I you want a gyro, get a stand-alone unit.

Sorry for the long and rambling answer, but I did warn you that this is a big can of worms. If you'd like to hear other opinions on this subject there is an active 'Combat Robotics' group on Facebook that would love to give you a full spectrum of opinion -- and then some.

    Remembering 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   
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:

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Do your own research.

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