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![Fifteen Years of Ask Aaron [Click Me]](pic2/Ten_2.gif)
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.
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.
Click to browse thousands of previously answered questions by category, or search for specific topics. Includes  
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.
Something's going on in the UK and I think 'Ask Aaron' is being abused. I'm getting multiple versions of very specific robot design questions about the same layout from the same region of southern England. This sounds to me like a school project assignment, and I'm not interested in doing homework about vaporbots for all of Blighty. See for yourself:
Q: Hi, i have 2 questions to ask
A: [Mark J.] What's going on? Scan down the page and read the last few questions. I'm seeing a theme here...
The short answers to your questions are:
Q: I need to find a motor that will generate 0.2Nm of torque per wheel for a fightbot. The back 2 wheels will be direct driven from 2 motors, and the front 2 will be belt driven. I have a £50 budget so i know gearing will be required, could you help me choose a good ratio and also how I can find a suitable motor? what is the best type of wheels for a small robot and which material is best? [Brighton, England]
Once you have determined your actual torque, speed, and voltage requirements you can look thru gearmotors at on-line robot suppliers in your country -- like 'Robotshop'. A quick search there found a 12V 970RPM Econ Metal Gearmotor that may meet your requirements and budget.
About wheels and tires: There is no single 'best' wheel/tire type. How 'small' is your robot? What type of surface does the arena floor have? How important is traction versus durability? Will the wheels be exposed to direct weapon impacts or are the wheels protected by armor?
There are dozens of posts on wheel and tire selection in the Ask Aaron Materials and Components archive, and for very small robots there are additional posts in the Ants, Beetles, and Fairies archive. Search there for guidance on wheel/tire selection.
Q: i have to produce a combat fighting robot and my part is weapons electrical, and therefore what calculations do i make as my robot consists of two axes on the sides of the robot and a spinner blade in the middle [Edgware, England]
A: [Mark J.] I can't teach you Mechanical Engineering in a few paragraphs, Edgware -- but I do have a collection of on-line tools and Excel spreadsheets here at runamok.tech that can help with your design calculations:
What weapons are most effective in robot combat? The answer may surprize you: What Weapons Win?
Q: Hi, if i am building a robot that is 3kg, is there a way of choosing the right wheel radius using calculations or do i just make an assumption. [London, England] A: [Mark J.] There is no single 'right' wheel radius for a robot of a given weight. For a specific motor and weight, the correct drive train will be a function of wheel radius and gear ratio: larger wheels require greater gear reduction, smaller wheels require less gear reduction. See the post immediately below for links to equations and tools to select the correct combination of motor, gearing, and wheel radius. Q: Hi I am doing a project where I am building a fighting robot and was wondering what the best way is to choose a motor with calculations. The only thing I have to work with is that the mass of the robot is 3kg. What equations do I need to consider [Brighton, England]
A: [Mark J.] I've got a whole webpage on that topic, Brighton: Optimizing Combat Robot Drivetrains.
Once you have the theory down you can automate the selection of drive motors and gear ratios with the on-line Tentacle Drivetrain Calculator.
If you have trouble with the Tentacle Calculator, I have a step-by-step Example Drivetrain Analysis.
Q: Hi! My team is building a 15-lb bot. We are going with a kiwi drive holonomic setup and a drum spinner, all powered by brushless motors. 
We have calculated that we can use Propdrive 2836 2200kv outrunners with 20:1 gearboxes for our drivetrain, and the same motor with a 2:1 belt drive for the spinner. Since all 4 motors are the same, could we run them off one of the 4 in 1 quadcopter ESCs? (With the requisite firmware flashing, control board, etc.) Something like this. These ESCs are designed for quadcopter use where there is plenty of airflow, but we figure we can manage heat by mounting it to our aluminum chassis with a thermal pad.
Thanks for your help. [Cambridge Massachusetts]
A: [Mark J.] Massive wheels... Tiny drum... But you only asked about the ESC, so let's talk about that.
Yes, you can run all the motors from a single 4-in-1 quadcopter ESC, and the motors don't even need to all be alike. I know of several small 'bots running two drive motors and a dissimilar weapon motor from a single compact quad ESC.
NOTE Quadcopter ESCs in general do not use the common 'PWM' receiver output protocol with one three-wire connector per radio channel. The specific ESC you are considering uses the 'DShot' serial protocol, so you will need a receiver with that type of output. Check the requirements of any quad ESC before you proceed.
Update - I asked some buiders familiar with Quad ESCs about your choice. It seems the APD f-series ESCs may not be programmable for reverse operation. Another builder suggested the Racerstar ReachUP 100A, but I don't have confirmation on useability. Stay tuned.
Q: I give up. How does it work? [Multiple Requests]
A: It's called a Killough Platform -- similar to an omni-wheel in action, but different in structure. The two wheels in each of three cradles are connencted by gears to each other and to a drive motor which can rotate the wheels while they remain oriented at 90 degrees to each other. One of the pair of wheels is always in contact with the floor as they rotate and 'walk' the platform along. The wheels remain free to spin on their own axles and roll sideways to comply with motion imparted by the other two cradles. You can see the action clearly in this video.
Q: I had a question regarding belts drives for weapons: does belt width matter? For context, I have a 30lb weapon system consisting of 2 15-lb spinners for a 250lb robot and I'm debating (for each spinner) a single belt of either 1/2" or 3/4" in width. Although it seems reasonable to go "the bigger the better", would you find there to be any problems with the 1/2"? Let's just say the smaller size may allows for more compact spacing and such. [U.C. Irvine]
A: [Mark J.] You're not giving me much to work with, Irvine. Before I make a belt recommendation I consider four primary factors:
I'm also puzzled by the information you did include about your weapon system. Twin 15-pound spinners are very small for a 250-pound combat robot. A typical combat spinner robot might devote 30% of total mass to the weapon system, including the motor. I'm not sure what weapon design you might have in mind that would compensate for that light weight, but an unusual design could alter my belt recommendation.
Best advice: Find a successful robot with a similar weapon design and emulate their belt system. Weapon belt drives are an all-too-common failure point, so base your design on a proven concept.
Q: I had a proposal for multi-layer armor: 3/16" titanium on top with 3/16" kevlar underneath. What do you think?
A: It's tempting to think that layering dissimilar materials might make a composite that has the strengths of each of the materials. What you generally end up with instead is a composite that accentuates the weaknesses of each of the materials, particularly when the materials used do not share similar stiffness.
Search the Ask Aaron Materials and Components Archive for "composite armor" to find multiple discussions on different types of layered armor. My general advice is to avoid the practice.
Q: I saw a youtube video of a fight with a beetle named "Droopy" that has two horizontal disks but no wheels. It kinda skates forward on skids, but I dont think its a bristlebot. Can you explain how it moves? [Atlanta, Georgia] 
A: [Mark J.] Builder Tommy Wong calls 'Droopy' a gyro walker, but it doesn't work on the same principle as vertical gyro walkers like 'Wrecks'. It's more accurate to call it a torque reaction walker that gets help from a small gyro effect.
When you apply power to a horizontal spinner weapon, the weapon motor apples torque to the spin the weapon in one direction, but it also applies torque in the opposite direction that attempts to spin the robot chassis in the other direction. By increasing and decreasing weapon motor torque you can get the robot to rotate back and forth, but it won't move forward.
Droopy's name gives you a clue about how it turns that torque rotation into forward motion. A perfectly horizontal spinner does not lift the leading wheel when turning like a vertical spinner does, but Droopy's counter-rotating weapon disks each angle down just a bit toward the outer edges of the 'bot. That little bit of 'droop' creates enough gyroscopic force when the 'bot turns to transfer weight off the leading skid and force the 'bot to pivot around the skid more distant from the advancing side.
Q: I'm having an issue with my scorpion esc recently to where it will power a motor off of one terminal, but not the other. We are running it off a 3s lipo with the kitbots motors, and in the past it has worked fine. But recently it has had issues running a motor from one of the terminals. The lights on the scorpion mini esc board will generally light up red or green based on which direction the motor is spinning. For the terminal that's working properly, everything is fine. However, when you connect a motor to the terminal that isn't working, then there are some issues. If you don't connect anything to the problematic terminal and push the sticks back and forth, the lights will turn green and red (as expected, this is normal). However, the second you connect your motor to the problematic terminal, the red and green indicator lights become extremely dim and there's no output at all. We know it isn't a motor issue because we swapped motors and terminals, and the issue is localized to the one terminal on the esc regardless of which motor is used. You can connect the motor to the terminal, push forward on the sticks, and then see the light very dimly come on with no response from the motor. And then the second you disconnect the motor, still pushing forward on the sticks, the light shines at its nominal brightness again. It seems that it's an issue with the ESC. But is this something that can be fixed, or will I have to write off the ESC? [Louisiana State University]
If you can't find anything from your visual examination I'd suggest starting over with a fresh ESC.
P.S. -- I hope you aren't trying to build an 12-pound 'bot for the LSU robot competition around the Scorpion Mini and a pair of KitBots motors.
Reply: Haha, no! This is actually a beetleweight build for the LSU combat robotics club here. But I'm surprised you got word of that competition. We actually are designing a robot for that, too... we have every intention of defeating the senior design students. Maybe you can check it out when the time comes. I'm sure it'll be streamed somewhere. Response: I've heard quite a bit from competitors in the LSU 12-pound competition -- both here at 'Ask Aaron' and on other forums. Judging from the questions being asked, the competition is being taken quite seriously. Best luck to you. 
Q: Hello. I need your help. I have a flysky i6x Transmitter and I have accessed the factory menu where I set on the self-centering option and the wherever is the throtle the connection between tx is made with rx. How can I undo that because I can't access the factory menu. Thank you. [Bucharest, Romania]
A: [Mark J.] The Factory Menu is the only method I know for toggling the 'safe start' parameters to allow startup with the throttle centered/down. If you cannot now access the factory menu, reset all of your gimbal trims to zero and try again.
If you are still unable to access the factory menu you can do a Factory reset from the System menu. This will wipe all changes and model settings and return the transmitter to the 'new from box' settings.
Q: I have tried looking through the archive, But could not find the specifics.
I am designing a 12 lb hobby weight battle bot with 2 wheel drive. The arena is 8X8 ft and I am trying to find torque calculations for the wheel to spin. Please help. [Louisiana State University]
A: [Mark J.] My visitor log shows that you found appropriate pages here at RunAmok.tech about 30 seconds after you sent in your question -- but you missed the page that gives you the theory to write up for your class project:
Q: Is it possible to make a hub motor with 3D printed NylonX? I'm afraid it's too weak to survive on full-combat fights. [Taegu, Korea]
A: [Mark J.] As you didn't provide details on the weight class or the design of your proposed weapon, I can't directly comment on the suitability of your fabrication process. The hamburger is bad.
I will say that I see combat robot builders using 3-D printers to produce components that would be much better fabricated by other means. There is a well-known saying often credited to Abraham Maslow that I will paraphrase here:
"If your favorite tool is a hammer, all of your problems resemble nails."
I'll add that I wouldn't personally consider printing stressed full-combat weapon components for anything above the 150-gram class.
A: [Mark J.] Chassis magnets can be more trouble than they are worth, getting you stuck to kickplates, uprights, and even your opponent. Magnet wheels can drive you right out of awkward situations like those, but the neodymium ring magnets themselves are fragile, particularly in larger sizes.
Team Dark Forces has an interesting solution for larger 'bots. Their lightweight spinner 'Dark Ripper' has 2.5" wheels constructed by making a triple-layer sandwich of three ring magnets between four steel washers. Claimed downforce is 50 pounds per wheel. The slightly larger diameter washers protect the brittle magnets and focus the magnetic flux for greater attractive force. The recesses also provide a place for any magnetic swarf the wheels pick up to slide out of the way. I'd suggest bonding the entire 'sandwich' together with epoxy or Goop to keep it in one piece if a magnet does break.
Q: Hi there. Kelpie's builder David Small here. Your comments in the "Unregulated Poppet Valves with Leaky Rams" section in Tips and Tricks for Robot Pneumatic Weapon Systems are a little misguided. Kelpie's ram was hydrotested to 150% operating pressure just in case something happens and it needs to handle the full pressure. It's 100% safe and legal at any tournament. It will absolutely not "turn into a grenade". It's not just an insect thing either, this "awful" design is used in robots like Rocket the lightweight too just at a larger scale. I'd appreciate it if you update your article accordingly. [Phoenix, Arizona]
A: [Mark J.] I'm always happy to receive and post comments from builders, David. I'll add your comments to the article, but I do have a question for you: how do you hydrotest a leaky ram? A loose clearance ram with no seals and a vent doesn't seem to be a candidate for pressure testing. Q: Kelpie's ram actually has no holes and only leaks because I don't wrap the pressure fitting with Teflon tape. There's also a groove in the piston for an O-ring. During the hydrotest I add the tape and install the O-ring, but in its normal operation in the robot the O-ring and tape are removed. I also considered drilling and taping a hole in the bottom that could be plugged to make a seal, but just not using tape was easier. A: I don't consider your system to be a true 'leaky ram'. It could operate safely with a 3-way solenoid valve, but you've choosen to run with a poppet and an intentional fitting leak. It's the type of system described and classed 'safe and legal' in the last paragraph of the article:
Why even mention this system here? There is a safe and legal use for a 'leaky' system with a pneumatic ram and a poppet valve. If your flipper design allows for a gravity-powered retraction, you can replace the solenoid valve in a conventional system with a high-flow poppet valve to get an improved flipper action. You shut the poppet valve off as soon as the flip is complete, and a small 'leak' anywhere downstream from the valve will allow the ram to 'leak down' for a slow retract.
I've added your comments to the article and re-written the description to better descibe the type of system I find unacceptable. Thank you for pointing out the need to improve the article.
Q: I've learned a lot about my Taranis Q-X7 transmitter from reading your combat guide. Thanks for writing that up!
I've got a problem trying to do something not covered in your guide that I thought you might help with. In addition to my combat robots (two ants and a beetle) I'd like to use the transmitter for an 1/12th scale R/C race car I've had for years. I have a very simple radio set-up to control the single-motor ESC (on channel 3) and the steering servo (channel 2). When I power everything on the steering servo is correctly centered and responsive, but the drive motor immediately powers up and runs at medium speed. I have to pull the throttle down about half way below center to get it to stop. Is there something wrong with the ESC or is there some adjustment needed to the transmitter? [Pensacola, Florida]
A: [Mark J.] Glad to hear that the Taranis Q-X7 Combat Guide helped with your 'bots, Pensacola.
Robot ESCs and R/C car ESCs have different expectations in the 'center position' signals they get from the receiver.
Combat robot drivers who wish to use a pistol grip transmitter with their robot ESCs have a similar problem, but with a different solution. Team Run Amok has a page with details and instructions:
Q: I am designing a 3lb generic beater bar spinner (think: Kinetic) except the weapon support horns are made of titanium like 'Weta, God of Ugly Things'. I like the design of these robots, but they seem to always lose to big horizontal spinners, and I think the reason is the exposed edges of the supports are vulnerable and let spinners "bite" into them. How can I design my robot to block these big spinner hits? [Bellflower, California]
A: [Mark J.] Don't block -- deflect. Have you seen any of the Team Dark Forces 'bots fight? Builder Russ Barrow is fond of protecting his vert spinner weapons with wide, curved plows that will deflect attacking horizontal weapons up and over the weapon supports. Granted going 'full Russ' requires some radical design commitment, but adding a couple of wide armor 'ramps' up the sides of your support horns will perform a similar function.
A: You're wise to worry about shearing fastners. Machine screws are made of material and temper to optimize tensile strength -- not resistance to shear forces. Take a look at this video from Robert Cowan for details. You can use Robert's hardened pin technique, or you can 'slot and tab' (see illustration) the panels to take some of the shear load off the screws. Q: Does UHMW plastic have the same "wedginess" capability as a metal like steel or titanium? I want to use UHMW for my insect bot's wedgelets as titanium/steel is a pain to machine. I only have access to small hand tools.
A: If UHMW was a good wedge material you'd see it being widely used in this type of application -- but it isn't. UHMW has many useful attributes but it is very soft. A hard (and possibly sharp) metal weapon impactor will dig in and 'grab' the soft surface rather than slide up the wedge and deflect. Not what you want.
Now, you could screw mount a nice wide UHMW wedge to either side of your weapon support structure and 'clad' the impact surface with a strip of hardened steel or titanium. The UHMW would be light, energy absorbing, and easy to mount -- while the hard metal strip would be efficient at delecting weapon impacts. 
Q: How long do I have to wait for blue Loctite to dry before it gets good locking strength? When I tighten down a nut and then put a drop on the exposed threads it stays wet for a long time. [Salt Lake City, Utah]
A: [Mark J.] The correct usage of tread locking compounds like Loctite is to apply a small drop to the clean screw/bolt threads before you spin on the nut and tighten (video). Anerobic threadlockers like Loctite cure when confined in the absence of air between close fitting metal surfaces. Applying a drop to an assembled screw and nut does not provide these conditions. Properly applied, blue Loctite produces a fair bond in 20 minutes and a full cure in about a day. Q: Has something about the Tentacle Torque Calculator changed? I made some calculations for an antweight drive train a few weeks ago and now the same setup gives me different performance figures. I didn't make any changes. [Hollywood, Florida]
A: [Mark J.] Something has changed, but it only effects the Fingertech 'Silver Spark' and 'Gold Spark' gearmotors. These gearmotors use the Mabuchi FK-050SH motors, and the Calculator had been using the performance figures from the BaneBots version of that motor. A builder on one of the on-line discussions noticed performance figures for the FK-050SH given on the Fingertech site indicate a different variant of the motor.
I changed the Calculator data numbers for Fingertech gearmotors to mirror the numbers on the Fingertech website. I also adjusted the default voltage up from 6 volts to the commonly used 7.4 volts. The new power figures are about 30% lower than the older BaneBots numbers.
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
A: [Mark J.] My robots don't care. My robots don't spread, suffer from, or die from Covid-19 -- but you can. Don't be selfish. Follow the science. Stay safe. Q: In your Taranis Q-X7 Combat Guide you say that it's possible to program a Mecanum wheel omni-drive mix in OpenTX firmware. What would that look like? [Buena Vista, California] 
Assuming that you want a standard single-stick drive on the right stick with the added side-strafe control on the left 'rudder' axis, your channel layout and MIXER page are shown in the pictures above. Real omni-drive jockeys may prefer putting strafe on the right stick and rotation on the left. Just swap the 'Ail'eron and 'Rud'der assignments in the MIXER screen to give that a try. 
Q: According to the Kinetic Energy calculator, my beetleweight weapon is spinning [up to]
1000 joules by 2 seconds. The Spinning Weapon FAQ said that a weapon should have the weight class of the bot in kilos x 60 x 2. That adds up to about only 163 joules. The motor in the calcs is the Sunny Sky Outrunner in the Vector Kit and the weapon is a 10 x 2 inch steel bar with a thickness of about 1/10 of an inch. Does my weapon really have that KE? [Potomac Mills, Virginia]
A: [Mark J.] No, your weapon does not spin up to 1000 joules of kinetic energy in two seconds. It would have helped if you had included the numbers you entered into the Run Amok Spinner Weapon Kinetic Energy Calculator so I could see where you went wrong. Here are my numbers...
According to the EndBots product website your weapon bar has the following specs:
Q: I am making a beetleweight 'Tombclone' horizontal spinner. About how much of the total robot weight should be allocated to the spinner? [Winchester, Virginia]
A: [Mark J.] Grant Imahara's "Kickin' Bot: An Illustrated Guide to Building Combat Robots" was the first to promote the well-acepted '30-30-25-15 rule' for robot weight allocation:
Q: I am the Tombclone person and I am not experienced with beetle spinners, only antweight ones, so I am asking for a brushless motor recommendation for the beetleweight HS.
A: I recommend that you search the Ask Aaron Archives before asking a new question. With more than 6800 questions and answers, there is a good chance that your answer is waiting there for you. Here's a Q&A from the Ants, Beetles, and Fairies Archive:
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: I saw the graph of the number of robot fights and tournaments for each year on the Team Run Amok webpage. You say there that the data for the graph comes from SPARC Botrank. Is that data available to everyone? [Sacramento, California] 
A: [Mark J.] It is! You can go to their events list to find the events listed by date and the number of individual fights for that event. Clicking on the event name will take you to a list of all the fights at the event listed by weight class.
You might also be interested in this graph of similar data from the combat robot tournament registration site BuildersDB for comparison. Mouse-over the graph above, or click this link: Builders Database Events Chart. The BuildersDB chart shows 'Registered Bots' instead of 'Fights' because that's what the site tracks. Not all events report results to BotRank, and not all events use BuildersDB for registration -- but both charts show similar trends. 
Q: I'm having trouble setting up my new FS-i6 transmitter with my endbots desc. I set up the FS-i6 with elevon mixing according to the instructions in your FlySky FS-i6 Transmitter for Combat Robotics guide but I can’t get the channels to behave. Moving the stick up with neither channel reversed, the right side moves forward and the left backwards. Reversing channel one has no effect, and having channel two or both reversed results in the left moving forward and the right backwards.
What am I missing? The wiring is the same as I had working on my last transmitter and I’m pretty sure I had just reversed one of the channels. Thanks! [Boulder, Colorado]
A: [Mark J.] The odd responses to stick input that you describe are expected if you have both transmitter 'elevon' mixing and Endbots DESC on-board mixing turned on. The transmitter mixing is much more configurable, so leave it on and turn off the Endbots mixing.
The Endbots DESC has an absurd an unusual method of selecting on-board mixing: it toggles on/off each time you calibrate the DESC. Recalibrating your DESC will turn off the on-board mixing and should fix your problem.
Reply: Desc mixing did the trick, thanks!
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:
Type your question in the box, attach files if needed, then click: 'Send'
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A: [Mark J.] I agree that it's an ESC issue, but your problem is not a standard failure mode for the Scorpion. From your description I'll guess that you have a broken PC board trace and/or a failed solder joint. Break out a magnifier and spend ten minutes examining the copper traces and solder junctions. Keep a lookout for stray scraps of wire that might be shorting something. Pay close attention to the motor connector block in question.
Q: I know what you're talking about in the answer with the defensive wedges on the support horns, but do you have any advice on how to attach them to the robot? I could just use 1 NutStrip and screw the wedge onto the bot, but I am skeptical that only a few screws won't fall off or shear.
The performance numbers are likely a bit optimistic. Trying to spin a big bar that fast will generate enough aerodynamic drag to limit top speed and lengthen spin-up time. I'd guess that you meet the 'typical' spin-up speed and energy storage formula, and not much more.
