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Questions and Answers about Combat Robotics from Team Run Amok.

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Fifteen Years of Ask Aaron [Click Me] Ask Aaron banner image
6817 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email about designing and building 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.

New Archive Feature!

You can now take a tour of posts in the Ask Aaron Archives that have been referenced to answer new Ask Aaron questions -- let's call them Less Frequently Asked Questions. I'm testing it in the 'Radio and Electrical' archive, but if it proves popular I'll convert the other archives as well.

Click the green button and give it a try:


Q: Brushless drive motors have me really confused. My beetle has brushed motors but I'd like to build a feather and it seems that everybody is using brushless for their feathers. I don't know where to start. Is there a beginner guide someplace? [Pittsburgh, Pennsylvania]

A: [Mark J.] My usual advice for beginners in any aspect of combat robotics is to look at what successful builders are doing and follow their example. That doesn't work very well with brushless drive. where the difference between poor and excellent performance relies so much on the unseen ESC firmware and user-modified settings. Unfortunately, both firmware and currently favored settings change quite frequently.

I've pulled together a collection of posts from the Ask Aaron archives that may be of some use in selecting suitable brushless motors for both weapon and drive use. It also contains a complete 'off-the-shelf' solution for featherwight brusless drive that requires no modification:


Q: I've been watching videos about battle hardening brushless outrunner weapon motors. Doesn't globbing a lot of epoxy on the outside of the stator increase heat build-up? How much epoxy do I really need? [Ames, Iowa]

A: [Mark J.] I don't know where the practice of building a thick epoxy 'shell' around stator windings became popular. It does reduce stator cooling, and it does not prevent the lower layers of wire from shifting under impact load. A better method:

  • Drop your epoxy dispenser (long-cure, not 5-minute epoxy) into a glass of hot water to get warm and 'runny'.
  • Pop the stator into a 150-degree oven and let it warm up all the way thru.
  • Mix up a batch of the warm epoxy and brush/pour/drip it onto the stator windings.
The warm epoxy will 'wick' into the windings. Keep adding epoxy as it continues to soak in. Rotate the stator to better distribute the epoxy and check for drips. When satisfied, pop the stator back into the warm oven -- the heat speeds curing.

The epoxy that soaked in between the windings now aids cooling via improved conduction, and the wires are bonded together to prevent shifting.


Q: Seth Schaffer gave you a shout out on [social media site] for solving the weapon instability problem he had with his antweight "Mini Mulcher". What did you tell him? [Tacoma, Washington]

A: [Mark J.] Seth is just showing his appreciation for a very small pointer. Both Justin Marple and I remarked that the long, narrow, and heavy weapon blade on antweight 'Mini Mulcher' was a prime candidate for a bad case of 'polhode instability' that would explain Mini Mulcher's wild gyrations. Seth did the hard work of chasing down the problem with an extensive testing program and created a new 'axe head' weapon design that eliminated the polhode problem. A word to the wise was sufficient.

Q: I don't understand what caused Mini Mulcher's instability. The weapon was spinning on it's "maximal inertial axis" which should be stable. What went wrong?

A: Yes, according to the intermediate axis theorem it was spinning on a stable axis and everything was fine... until there was a change in the stored energy level. A change in stored energy will induce a small polhode motion wobble.

If the energy storage capacity of the intermediate axis is significantly different from that of the axis of maximum inertia, the wobble will self-damp and pass without notice. Unfortunately the weapon blade's intermdiate axis momentum was quite close to that of the maximum inertia axis and the wobble was able to flip the rotation axis 90 degrees -- throwing the entire robot perpendicular to the arena floor. The weapon striking the floor then sent the 'bot skyward.
The new weapon blade has the inertia of the intermediate axis well separated from that of the major principal axis and the polhode wobble damps out. Problem solved.
Flashback Post from 2018: Gyro Effect Calculator
Q: Is there a calculator to determine if a bot will flip over based on the MOI of the weapon, the distance from the wheels to the weapon shaft, and the diameter of the wheels? Can you calculate how fast you would be able to turn based off the information from the spinner spreadsheet? Does overall width matter in stability of the weapon and in the turning ability of the bot - are wider bots more stable?

Many Thanks! [New Castle, Pennsylvania]

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

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

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

Follow the link above to our design tools page and scroll most of the way down the page to find the link to the 'Total Insanity Gyroscopic Effect Calculator'. I'd give you the direct link here, but you'll benefit from looking over the other tools on that page.

Q: Which weapon type is good for beginners? I know wedge is the best but because my local event has active weapon rule, I can't build wedge. So could you tell me which one is recommended and which one is not? [Seoul, Korea]

A: [Mark J.] A combat robot is a complex assembly of components and systems that all have to work together to be successful. Forcing new builders to add an active weapon (lifter, flipper, spinner, crusher...) is a disservice to both new builders and the combat robot community. It creates a barrier to participation that practically guarantees frustration and failure for novice builders.

If your first robot must have an active weapon, concentrate on the basics and build a robot that does not rely on its active weapon to be effective. Start with a strong, capable chassis with a wedge and enough weight allowance left over to add a small active weapon system. The weapon can be whatever you're comfortable bulding. A simple electric lifter would be a good choice, but don't allow the active weapon to compromise your strong drive train and wedge. Your active system is secondary and entirely expendable.

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

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

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


Q: Somebody made a comment about the "hamburger is bad" rule on [a combat robot social media site]. Can you tell me what that rule is? [Henderson, Nevada]

A: [Mark J.] You came to the right place.

'Ask Aaron' has a long history of receiving requests for very specific recommendations on materials or components that provide far too little information for us to give a useful answer. We needed a name for this type of question and a way to explain our problem in answering. In 2010 we published a webpage titled "The Hamburger is Bad" that uses a fictional exchange of questions and answers about whether a 1/4 pound hamburger is good to depict the problem. The name caught on in the combat robot community.

So, "the hamburger is bad" is not so much a rule as a shorthand request to provide adequate information when asking a question.


Q: I have a 3S LiPo and I want to run the drive on the full 11.1 volts and tap the balance plug to get 7.4 volts for the receiver and servo. I'm worried about screwing something up and getting a short thru a 'ground loop'. How do I do this safely? [Social Media]

A: [Mark J.] You're wise to worry. If you tap the balance plug incorrectly you will have the receiver 'ground' at a different voltage potential than the drive 'ground' and bad things will happen: wires melting, LiPo cells exploding, the works. Tapping a balance port When tapping the balance plug for a voltage less than the full battery voltage, always use the full-pack ground (pin 1) on the plug. You could, for example, get 7.4 volts from the balance plug by tapping pin 2 and pin 4, but failing to include pin 1 creates the dreaded ground loop that shorts out one or more cells thru the receiver ground bus.

Make sure everything uses the same ground and you'll be fine.


Q: Maybe it's just my imagination, but Hypno-Disc was always interesting to me in that it when it turned, it would turn about a point that was closer to the front of the robot rather than the centre and sling the back out. How did it manage this? Was it just weight distribution that caused this effect or was it something about the wheels? [Ashford, England]

A: [Mark J.] It's not your imagination. I won't comment on 'Hypno-Disc' directly (see FAQ #37) but I will direct you to the Hypno-Disc website where you may notice some differences between their front and rear tires starting in Series 5.

You may also find this post in the AskAaron Archives interesting.


Based on questons I'm seeing on social media, I think it's time for me to repost this block of questions about brushless motor sizing:
Flashback Post from 2018: Sizing Brushless Motors
Q: Is 2x Castle 1406 1900kv @ 20:1 3in wheels drivetrain underpowered for 30-pounder? [Ann Arbor, Michigan]

A: [Mark J.] There's a rule of thumb for brushless motors circulating out on the builder forums that appears to work well for drive motor selection. The rule states that a set of brushless drive motors that together weigh between 2% and 3% of the total weight of the robot will have more than enough power and adequate 'thermal mass' to survive. Exactly why this works is a topic for a future discussion.

A pair of Castle 1406 1900kv motors weigh in at 2.9% of a 30 pound robot. That puts you on the high end of the golden range. On 4 cells with 3" wheels and 20:1 reduction you'll have about 12 MPH speed. Sounds healthy to me.

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

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

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

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

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


Q: "Mounting plates" sold by Fingertech have served me well on ant & beetle motors, is there an equivalently easy solution for fairyweight motors? [Woburn, Massachusetts]

A: [Mark J.] The popular 'N20' style motors used in 150 gram 'bots are constructed differently than commonly used ant and beetle motors. They require greater support for their open gearboxes than 'face mount' style plates like the FingerTech Flat Motor Mount provide.

A search for "N20 mounts" will find many clamp-style mounts, like the Pololu Micro Metal Gearmotor Bracket shown. That's your fairyweight equivalent easy solution.


Q: How do I charge a 36v Li-ion battery like those found in hoverboards? [Roseville, California]

A: [Mark J.] Call me crazy, but have you considered a 36v lithium charger? Lithium batteries need 'smart' chargers that carefully monitor the charging process. Hoverboards and scooters have on-board charger electronics and just need a power supply, but you'll need a complete charger. Do not use any charger not intended to charge the specific type of lithium battery you're using. Improper charging can very easily result in a fiery and explosive failure of the battery.

Note The battery you reference does not have the high discharge rate commonly needed in a combat robot. A typical combat 4400 mAh LiPo battery would have a max continuous discharge rate of more than 250 amps, while the battery you reference has a discharge rate of only 30 amps. If you can live with that low discharge rate, shop around a bit; I've seen that same battery from other sources for half that price.

If you're interested in the technical details of lithium ion battery charging, check this article at Battery University.


Q: I'm using a 20A Afro esc flashed with blheli to power a Turnigy 2209-980 brushless outrunner. I'm also using a Turnigy 850mAh 3 cell battery to power all the electronics. 30c-60c discharge. At low and high rpms, the weapon, when it gets a good hit on something, shuts off and re calibrates. I have installed a large capacitor between the battery leads that helps with this problem quite a bit. Anything above 50% power on the throttle shuts down the weapon when it gets a good hit. Rapidly throttling the power all the way up to 100% also achieves this.

While watching the electronics during the incident, they all seem to shut down for a quarter of a second before coming back on and re calibrating. On my previous robot with a drum spinner, I also had this issue with a 750kv motor. During the competition I was able to wait for the electronics to come back online and drive away. If you need any other information about the electronics or the robot, just tell me. [Arlington, Virginia]

A: [Mark J.] The clue to the cause of the problem is that your installation of a 'large capacitor' across the battery leads helped. You're getting a serious voltage drop when the weapon motor is pulling a lot of power and your electronics are 'browning out'. Two possible solutions -- I'd do both:

  • Since ALL your electronics are shutting down I'm suspicious that it's the receiver that's causing the system reset when the voltage dips. Remove the 'large capacitor' from the battery leads and connect it across the power and ground pins on an unused receiver port. This will sustain voltage on the 5-volt receiver bus.
  • How old is your battery? The math says that it should provide (60 * 0.85 =) 51 amps of peak surge power, but it sounds like it isn't doing it. The current batch of Turnigy 850 mAh 3S LiPos are rated 60c-120c and should have no trouble keeping up with your weapon motor.

Q:'UnMakerBot' builder Zach Goff says his lightweight champion has a reprogrammed quadcopter brain that keeps it from flipping over. Is this real? How does it work? [Bay Area, California]

A: [Mark J.] Many combat robots use a single-axis 'heading hold' gyro to improve straight-line tracking and provide steady turn response. The solid-state gyro accomplishes this by adjusting drive motor speeds to make the actual turn rate (yaw) matches the turn commands coming from the R/C transmitter.

Zach has expanded on this by repurposing a multirotor flight controller with three solid-state gyros aligned at right angles to each other to detect rotation in any of the three axis of motion: pitch, roll, and yaw. Unlike an aircraft, a combat robot has no direct control over motion in the pitch and roll axes -- but information about movement on those axes can be used to modify robot motion in a way that may keep it out of trouble:

  • A sudden up-pitch to the front or rear of the robot may indicate a weapon strike that threatens to flip the 'bot over. If you still have wheels on the ground, a command by the pitch gyro for hard accelleration away from the detected strike may keep the robot upright and on its wheels.
  • A 'gyro dance' caused by too high a turn rate on a robot with a vertical spinner weapon can be detected by the roll gyro, which can then command a reduction in the turn rate or weapon speed to keep the 'bot on its wheels and avoid a vulnerable position.
The hardware found in a stock flight controller can perform these functions, but the firmware must be modified to direct robot-correct responses. This isn't a project for the timid, but you can gain some advanced help in keeping your 'bot on the ground.
Flashback Post from 2018: What's an OPTO?
Q: I see several different brands of brushless ESCs labeled 'OPTO'. What does that mean? When would I want 'OPTO'? Would it ever be a bad thing? [Pomona, California]

A: [Mark J.] A complete answer requires a bit of history.

Back in the day...

The small, high-power brushed motors powering early electric model aircraft created a huge amount of high-frequency electrical 'noise' and voltage spikes from the arcing of the motor brushes at high speed. This 'noise' traveled along the power wires and found its way to the sensitive and vulnerable electronics in the R/C receiver, wreaking havoc with reception.

To protect the receiver from this electrical noise, 'OPTO' speed controllers included a device called an optical isolator in the line between the receiver and the power circuits. This 'opto-isolator' converted the receiver output signal to a modulated light beam, then back to an electrical signal for the ESC. Electrical noise couldn't jump back across the optical gap to get to the receiver. Genius!

But there's a catch...

If you use a battery eliminator circuit (BEC) to power the receiver from the main battery pack the electrical noise can use it as a path to the receiver, nullifying the benefit of the opto-isolator. Fully isolating the receiver from motor 'noise' requires that the receiver have its own battery, so OPTO ESCs had no BEC. Inconvenient, but it worked.

Jump to present day

Brushless motors have replaced the 'noisy' brushed motors in model aircraft and the need for opto-isolation no longer exists in most applications. Few brushless 'hobby' ESCs have an optical barrier to electrical noise, yet many still carry the OPTO label. In current usage it's simply an indication that the ESC has no battery eliminator circuit. Eliminating the BEC shaves off a couple grams of weight and eliminates the need to 'clip' the red power line in the receiver cable when multiple BEC equipped ESCs are on the 'bot.

You need something in your power system to provide power to your receiver, and a convenient way to do that is to make sure that one of the ESCS for the weapon or drive motors has an BEC -- but you only need one.


Q: Hi there. I’m looking for information on competitors who competed at San Fransisco Robot Wars event in 1997 and I’m also looking for pictures of them. Thank you. [Apparently from New Zealand, via two proxy servers in the UK]

A: [Mark J.] Info from the 1997 San Francisco Robot Wars is more difficult to obtain because the entire US Robot Wars organization was shut down immediately following the event. They didn't even have time to update their website with photos and results. There are a few good sources of info and photos:


Q: What are the materials that are used to make weapons in combat robots? [Digital Ocean]

A: [Mark J.] Your question comes from a web server associated with web proxy services that mask user location. Please remove your VPN service and resubmit. If you have nothing to hide, don't try to hide it. No Proxies! I'm very serious about safety in combat robotics. I no longer answer 'Ask Aaron' questions from geographic regions that ask only about building more powerful robot combat weapons with little to no regard for building safer robot combat arenas. Builders from these regions often attempt to sneak questions thru by disguising their true locations with VPN and proxy services. Questions that arrive via known proxy servers are generally ignored because of the time it takes for me to determine the true origin of the question.


Q: Hi Mark. Thank you for sharing your snail cam design. I work in a different field and am interested in using it to generate impact. We have a tight space, the minimal cam radius is 5mm and the maximal cam radius is 8mm. We want to use disk spring to generate 1000N impact force. Do you see if it is possible? Any comments will be appreciated. Thank you! [Weston, Florida]

A: [Mark J.] You're not giving me much to go on, Florida. The snail cam is a reasonable mechanism to compress and release spring force, and it is fairly simple to calculate the energy stored in the compressed spring, but translating stored energy into 'impact force' is complex, requires many assumptions, and varies with the characteristics of the object being impacted.

Here's a link to a good article at wired.com: How Do You Estimate Impact Force?


Q: Given that μ = 52, σ = 35 and n = 35, what is P(X ≤ 73) and P(X ≥ 73)? [Singapore]

A: [Mark J.] Not a robot question, but it's a slow Saturday morning and I do have a minor in math...

Calculate the Z-score:

Z = (x - μ) / σ = (73 - 52) / 35 = 0.6

Consult a Z-score table:

P(x ≤ 73) = P(z = 0.6) = 0.7257

P(x ≥ 73) = P(z = -0.6) = 0.2743


Q: hi ive been enjoying reading you site for a few years finally decided to ask a questions hopefully this doesn't qualify for a bad hamburger or cheerleader question

anyway im currently designing/building a 30lb ring spinner (not a good idea but i like a challenge) and im looking at ways to power the ring so currently my plan is to use two brushless motors on a 3 to 1 ratio on the inside of the ring via a large hdpe ring gear. my main concern with this is that the impacts would damage the weapon motors or break teeth off the inside of the ring so my plan is to use a ridged tpu [Thermoplastic Polyurethane] my thinking is that the tpu teeth would flex and "skip" on a big hit protecting the motor would this work or am i just going to rip apart the rubber if this wouldn't work what would you suggest to protect the motors. i unfortunately cannot use belts due to a lack of internal space (mostly rubbish design on my part) i could run the motors with wheels on to drive the ring but my fear was that i wouldn't be able to get the torque required to spin the weapon

heres some info that might be useful

  • the ring weighs just short of 5kg

  • the brushless motors are N5065 320KV 1820W from banggood because i could afford to buy lots of spares due to the cost admittedly this will probably bite me in the butt

  • the module of the gears is 4.2 with 100 on the ring gear and 30 on the motors

again apologies if this is a rubbish questions and for the shocking grammar [Abingdon, England]

A: [Mark J.] Well, you already know that a ring spinner is a bad idea -- that saves me some time.

The combination of "flex" and "gears" is a bad one. The teeth are large, but I doubt you'd have enough clearance for them to flex enough to 'skip'. I worry that the flexi TPU teeth would distort under drive torque loading and jam at the gear interface, causing huge drag. That would bog down the motors and create enough heat to soften the plastic, creating more trouble.

I'm not a fan of friction drive for just the reason you mention. Stick with HDPE and make the ring and pinions as tall as possible for strength. I assume the pinion gears are mounted on the motor cans? Support both ends of the weapon motors: extend or reverse the motor shafts and add bearing blocks to the far end. If the ring support is strong enough you should be OK.


Q: Hello Mark
I come back to you for our robot flipper machine. We followed your advice and installed a RC Polulu 2802 switch, but as regards the solenoid valve the best is a 5/2 model. Should the pilot be internal or external, and naturally open or closed?
Thank you in advance for your advice. [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] Hello, Jean Pierre...

For your purposes you'll want a solenoid operated valve with an internal pilot. Valves with an external pilot are for systems that must be able to actuate even when there is no pressure for them to control -- never the case in a robot flipper.

All 5/2 (five port, two position) pneumatic valves have two output ports. In the non-energized state:

  • One output port is connected to exhaust: normally closed (N.C.)
  • The other port is connected to pressure: normally open (N.O.)
You will connect the N.C port to the extension side of your actuator, and the N.O. port to the retract side. In that configuration the actuator will not move from its retracted state when the system is pressurized with the valve non-energized.
Q: I stumbled onto some crazy butane jet lighters that are intended for cigars but seem pretty dang impressive. Would it be practical to put one of these in an ant or beetle? [The Aether]

A: [Mark J.] I fought against BattleBots champion Donald Hutson's jet lighter weaponed ant 'Incinerator' in a melee at 'Bot Bash' in 2002. Somebody ejected it thru a pushout quickly, but it continued to wander around the 'moat' attempting to fry the other 'bots that fell in. Donald's like that.

All flame bots are just for showing off -- they do not win tournaments. You need a method to turn on the gas and light the flame remotely because no organizer will let you 'flame on' before the arena door is locked and the match has started. Don't write back and ask me how to make that ignitor; if you don't already know how you're not ready to build a flame-bot. The event also has to allow flame weapons. Most organizers aren't too keen on 'bots that could melt holes in their expensive Lexan.


Q: I'm building a robot for a competition which was cancelled due to the COVID-19 obvious. Before everything was shut down, I was able to record the actual free-running RPM of the output shaft on one of the gearmotors. The RPM translates to about 14 feet per second. I'd like to have at least 8 feet per second in actual performance. I was curious what kind of decrease in speed I might expect when the robot is completely assembled versus this theoretical speed. Each motor would be driving two sets of wheels and I figured we would be close, but are there any methods I can use to calculate this that isn’t crazy complicated? Thanks! [Reddit/battlebots]

A: [Mark J.] Brushed Permanent Magnet Direct Current (PMDC) motors produce their maximum torque at stall, with torque falling away linearly with increasing RPM. At no-load free-running RPM the torque usable torque has dropped to zero -- if there was any available torque the RPM would continue to climb. Brushed DC motor performance curves From a standing start your robot will accelerate more and more slowly as the speed climbs and torque diminishes. With greater speed comes greater friction and aero drag. When the total drag climbs to the point that it matches the available torque from your drivetrain, acceleration stops: you're at top speed.

Actual top speed should be 85% or more of calculated speed. Getting to only 57% of calculated top speed would indicate that you either have a great deal of drag in your drivetrain or you have insufficient gear reduction and are 'bogging' the motors. Check for free-running in your drivetrain before adjusting gear ratio.

I've written a guide to help determine optimum gear ratios for combat robots. It goes thru the math and then points you to my version of the Tentacle Torque-Amp calculator to let you compare the effects of gearing changes on acceleration and top speed within an arena of given size: Combat Robot Drive Train - Optimum Grear Ratio Selection.

I've also written up a walk-thru of a drivetrain optimization for a combat robot: Example Drivetrain Analysis using the Tentacle Torque Calculator.


Q: Hey there, i am purchased a leopard 58110 motor for my 30kg bot for weapon and I am using a ZTW 200amps ESC but I don't know why my motor isn't able to pull the weapon. I am unable to figure out that did I made a mistake in selection of each or i am putting some wrong programming values while programming my ESC.. please help me (nepal)

A: [Mark J.] You claim to be from Nepal, but your IP trace says your message came from Mumbai. Ask Aaron no longer answers questions from robot builders competing in India (Why not?).

Assuming you are from Nepal, you have provided far too little information for me to even take a guess at a solution to your problem. All you've identified are the weapon motor, weapon ESC, and robot weight. So many things that could be wrong -- so little information provided. About all I can tell you is that the motor and ESC are compatible and that the default ESC programming should be an adequate starting point. Something else is wrong. The hamburger is bad.

Q: Hey Mark, thanks for answering my previous question. It seems that you haven't visited india in past 2 years.. the arena quality is much improved since 2018 as the Indian robotics community boycott all those events having unsafe arena...

May be you should start answering all indian roboteers from today. Thank-you. [Mumbai, India]

A: Maybe you should develop a sense of honor and stop lying about where you're from.

Maybe you should stop kidding yourself about robot combat safety on the Indian subcontinent.

Here are a few video highlights from regional 2019 tournaments with horrible safety conditions:


Q: Hi there, I dunno if you'll know the identities of [these three] UK entries during the 1995 UK Robot Wars pilot. I know they were all built by Derek Foxwell and I've checked the archived website for images and names but I'm struggling to discover who they were. Any ideas? [Bodmin, England]

A: [Mark J.] The best info I have is that the three robots in the photos were small machines weighing perhaps 5 Kg that were built by Derek Foxwell for 'proof of concept' trials staged months prior to the Robot Wars pilot event that came to be known as the 1995 UK Open Competition. Short film clips from a compilation of the UK Open were released years later as part of the 'Robot Wars - The first Great Wars' video. That compilation shows the three small robots in what may have been a 'Robot Soccer' event.

The featured fights at the UK Open had early versions of U.K. robots 'Eubank the Mouse' and a multibot version of 'Grunt' called 'Grunts' versus well-known US Robot Wars competitors 'The Master', 'La Machine', and 'Thor'. Both Eubank and Grunt appeared in later seasons of Robot Wars, but the names and full story of the three 'bots you ask about are undocumented.


Q: My son is making a 3lb battle bot. He wants to run the drive motors off of a 2s 300 mAh battery and run the weapon on a 3s 1100 mAh battery. I sent a sketch up of how he wants to wire it up. Somebody on [social media] said we would have a "ground loop" and I'd blow something up. What's a "ground loop", and do I really have a problem with this circuit? [Orlando, Florida]

A: [Mark J.] I've re-drawn your sketch for clarity.

  1. A ground loop may occur when you have two or more batteries wired in series to obtain a higher voltage and some component taps into the wire connecting the two batteries in order to operate at reduced voltage. This can create a 'floating ground' at a higher voltage potential than the ground used by other components that will 'dead short' thru the receiver ground leads and melt receiver leads and/or destroy one of the batteries. You can see an animated diagram and a more complete discussion of ground loops in this archived Ask Aaron post.
  2. Your circuit has no ground loop. It does use two batteries, but they independently power two separate components that each share a common ground. No floating ground -- no ground loop. It will work, but there is a simpler circuit that will accomplish the same power distribution.
The simpler solution is to wire all ESCs to a single 11.1 volt battery and restrict the output of the transmitter sticks controlling the drive motors by transmitter programming. The function used to do this has different names on different makes of radio; check your transmitter manual for 'Adjustable Travel Volume' (ATV), 'Travel Adjust', or 'End Points' for instructions. Setting the ATV to 66% on forward, reverse, left, and right stick commands will reduce the voltage the drive motors 'see' from 11.1 volts to 7.4 volts. One battery, one power switch, and simpler pit procedure.
Q: How exactly does the 'Greedy Snake' lifter mechanism work? I’ve seen it used on bots like 'SlamMow', 'Claw Viper', and 'Barróg Doom' and kinda have an idea as to how the system works, but I feel like I don’t get it entirely despite being a seemingly simple setup. Are there any diagrams, pictures, or videos that best describe how it works?

As always, thanks! [Social Media]

A: [Mark J.] The mechanism in question is often referred to as a single motor clamp lifter. Powered rotation of a spur gear (shown with a red dot) drives a clamping arm attached to a second spur gear downward until on object stops it's motion and prevents further rotation of its gear. Continued rotation of the powered spur gear will then rotate the entire gearbox and attached lifting platform around the axis of the powered gear. Reversing the rotation of the powered gear will lower the platform and release the clamp. In practice, there may be additional gears involved to provide additional gear reduction in order to gain the substantial torque needed for the lift. The animation has been reduced to only the critical components.

Note The clamping force is created by weight on the lifting platform. As the lift angle increases there is less weight pressing down on the platform: clamping force becomes essentially zero when the lifter is vertical.


Q: Hello Mark,
First of all I want to thank you for your previous answer [five posts down the page].

For our flipper type robot we are considering a Burkett 5404 2 port 24v type valve associated with a Wasp motor controller. What do you think of this choice? And what connection for a double acting cylinder?

Thank you for your reply. Best regards, Jean Pierre [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] The Burkert 5404 is the solenoid valve of choice for pneumatic flipper systems because of its ability to flow a large volume of gas very quickly, but I don't think it's suited to your use. As I recall, you are building a 20 KG robot. A single Burket 5404 weighs more than a kilo and you'll need at least two valves of this type for your flipper.

The 5404 is a simple 2-port open/close valve. It can open to allow gas to flow into your actuator but when it closes there is no place for the gas in the actuator to go -- you need a second 2-port valve to open and exhaust the gas from the actuator to allow it to retract -- see the diagram. If controlling both valves from a single controller you will want the pressure valve to be the 'normally closed' type and the exhaust valve to be 'normally open' so that an actuation signal will open the pressure valve and close the exhaust.

Two 2-port valves will suffice for a flipper if you can live with a gravity or spring-powered retract. If you want a powered retract you'd need more valves to operate the retract side of the actuator. By this point a quarter of the weight of your 'bot could be valves and hoses!

Take a look at our Tips and Tricks for Pneumatic Weapon Systems page for some alternative designs.


Q: My FS2A 4 channel receiver has the failsafe only on the third set of pins. What small receiver can I use with my FSi6x transmitter that will failsafe correctly? [Social Media]

A: [Mark J.] The FS2A receiver will failsafe correctly with your FSi6x on all channels, but the set-up process is not well described in the receiver documentation. Chinese radio gear is like that. Here's what the docs say:

Failsafe settings:
The receiver can normally receive the transmitter signal, push the rocker to the the failsafe set, long press the receiver bind button, the blue light flashes quickly several times and then always bright , failsafe settings is successful.
Allow me to translate Banggood English to [Actual English]:
  1. "The receiver can normally receive the transmitter signal,"

     [Turn on the transmitter and receiver]

  2. "push the rocker to the the failsafe set,"

     [Move the transmitter sticks and switches to the failsafe positions and hold them there for the next step]

  3. "long press the receiver bind button, the blue light flashes quickly several times and then always bright,"

     [Hold down the receiver bind button 'til the LED flashes, then stays steady]

  4. "failsafe settings is successful."

     [You're done]

This is a separate process from binding the receiver, and is done after binding is completed.

Q: What kind of robot would be most effective/useful in this era of COVID-19? (Preferably in the form of a haiku) [Pittsburgh, Pennsylvania]

A: [Mark J.] Who wants dibs on the name "Antisocial Distancing"?

Social distancing:
Ship bot, but no transmitter.
Control via Skype.


Two photos of Aaron Joerger 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   
Killer Robot drawing by Garrett Shikuma

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.

Aaron's Greatest Hits! More of Aaron's Poems Aaron's Minecraft High Dive Video Aaron's World of Warcraft Player Guide


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