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Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
How do antweight big-wheeled vertical spinners like "Oathbringer" constrain the weapon shaft between their chassis halves? I would guess that the weapon shaft is hollow with wires threaded through (like on "SLAM PLAN") due to the chassis being too skinny to contain independent electrical systems, but I am uncertain as to how everything else is held together. [Zanesville, Ohio]
A: Mark J. Given that the weapon dead shaft on a HUGE-style big-wheel vertical spinner takes a great deal of stress and holds the entire 'bot together, the mounting of the shaft to the chassis halves is critical -- regardless of weight class. The preferred method is to clamp a good length of shaft in machined alloy blocks pulled tight by heavy machine screws. The blocks are in turn secured to the chassis at multiple points with screws. An example of such mounting in hobbyweight 'SLAM PLAN' and a typical clamp mount can be seen below:
Q: As seen from a question you recently answered, India continues to receive negativity from the combat robot community for their still-abysmal safety practices. With the World Cup underway, I couldn't resist asking this. Do you think establishing an international governing body for robot combat with universal rules and standards (similar to FIFA in soccer) could help improve safety in the region? What do you think?
A: Mark J. FIFA is a major sports organizaition with ample financial influence to reward compliance with their rules and regulations. Their current tournament has a prize pool of a billion dollars. Huge numbers of fans spend time and considerable money to travel across the globe to their events, and countries vie to attract their favor.
Combat robotics is many orders of magnitude too small to support an international governing body. What "carrot" could it offer for compliance? What "stick" could there be to chastise violations? Why would a technology fair in India pay any attention to rules they could not afford to follow?
Q: I am making a chassis for a beetleweight robot out of hdpe [High Density Polyethylene] and I want it to be a unibody so assembly would be a bit easier. But I don't have a milling machine, so I thought that molding would be a better solution. I was wondering if making it this way would be less durable. Thank you.
A: Mark J. When you warm up HDPE to molding temperature it does not become a pourable liquid like parafin wax -- it becomes a viscous semi-solid glob that must be pressed into the mold while still very hot. Getting this gummy mass to fill the corners and edges of a mold as small and complex as a beetleweight robot chassis would very likely leave you with voids and poorly adhered folds that would compromise both the strength and functionality of the finished product.
Plastic Smithing: How to Make Your Own HDPE Plastic Anything
Plastic insect-class robot chassis are exactly what 3D printing technology is good at doing, which is why you see so many printed chassis and few if any molded beetle chassis. Cannot recommend.
- Which common low-speed robot control problem can be corrected by appropriate use of the 'Subtrim' transmitter function?
- If you use the 'Mode' function to switch your transmitter from the US standard 'Mode 2' to the European standard 'Mode 1' stick assignments, what will happen to your EleVon mix rotation (steering) stick assignment?
- Which specific transmitter features are needed to program a switch that will correct stick commands when your robot is flipped over? Can the FlySky FS-i6 do this?
- What is the effect of adding positive exponential response to the rotation (steering) channel on your transmitter?
- The servo powering your antweight lifter has a little less range of motion than you planned for when you designed the mechanism. Which transmitter functions can help with this problem?
The idiom "jumping the shark", or "shark jumping", or to "jump the shark"; means that a creative work or entity has evolved and reached a point in which it has exhausted its core intent and is introducing new ideas that are discordant with or an extreme exaggeration (caricature) of its original theme or purpose.
A: Mark J. You are well advised to be worried. NASA-quality machined cycloidal drives have very high efficiency, but miniature 3D printed versions do not. You would be lucky to get 70% efficiency per stage, which means that in a two-stage drive about half the input power would turn into heat.
Cycloidal drives are also not well suited to high input speeds, and you're looking to feed one better than 50K RPM. I foresee a very short lifespan for your proposed drive. If you want to use a high Kv motor, stick with conventional gears.
Shock loading and heat are the things that kill stators. If you're spinning a big beater bar the aerodynamic loading may be too great for a 2836 -- consider a larger motor. Also think about epoxy coating the stator windings (photo) to improve cooling and prevent wire shifting.
A: Mark J. It's common to use an electrolytic capacitor soldered between the power input leads on the weapon ESC if running greater than 4S voltage to protect the ESC - not the motor. The capacitor (around 1000 uF rated for voltage at least 150% of your battery) smooths out voltage spikes that can fry vulnerable ESC components but the motor is MUCH less sensitive to voltage spikes.
A: Mark J. This can be fixed in the transmitter or by a hardware adjustment. Since you have not mentioned what transmitter you're using I'll give you the hardware fix:
Reply: It's not that the motors are running in reverse, it's that CH1 and CH2 are swapped and I can only change it via the transmitter.
Response: That's incorrect. If your 'bot spins in place when you try to go forward or reverse one of the motors is responding in reverse -- and that problem CAN be resolved by swapping motor leads as described above.
If you prefer a transmitter settings solution, you can use the "reverse" function to invert the output from the Catalyst port controlling the motor that is responding incorrectly. I believe the Catalyst assigns the left and right side motors to Ch1 and Ch2 respectively.
You are still not telling me what transmitter you have. Why is it a secret? If you are using the popular FS-i6 transmitter my FlySky FS-i6 Combat Guide gives full instructions on use of the reverse function.
A: Mark J. A few, but I can't vouch for any of them...
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Olives Cheese Oranges Salt Egg Salad Beer Dirt Blood |
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2k RPM4k RPM 6k RPM 8k RPM 10k RPM 12k RPM 14k RPM Let it spin down some more. |
Q: Am I really supposed to read through the 7928 questions and answers on Ask Aaron to learn how to build a combat robot? The RioBots Combat Guide is seventeen years old, 367 pages long, and badly needs another update. Isn't there a concise instructional guide that's been written in this decade that can tell me how to build a beetleweight robot?
A: Mark J. Ask Aaron was never intended as a course in combat robotics. It's a source to turn to if you're stumped on some specific problem, seek clarification on design theory, or just browse for your own entertainment. The RioBots guide is a great engineering resource even given its age, but it also is not a beginner's course. If you want a recent guide that covers building a beetle from the ground up I've got just what you're looking for.
Leo Ping, captain of the University of Toronto Robotics Association Combat Robotics team, has published a free download "Beetleweights for Dummies -- The Ultimate Guide to Building your Own 3lb Death Machine" -- 135 pages, written in the Fall of 2025. It's been widely distributed on the NHRL discord. Like the RioBots guide, I don't agree with everything presented therein (e.g. the section on weapon 'bite' is misleading) but in general the material is quite good and covers all the bases.
A: Mark J. Malenkis have some quirks that can trip up the unwary. One simple rule will avoid most of the trouble:
"Super well done, that's exactly right. It does have a config mode as well as a bind mode. Config mode is super easy to exit, just drive the robot (forward or reverse) and it immediately goes into drive mode."
There has been both a push for safer events and more robotiers have gone away from street wars, flimsy cages, and unregulated combat. IIT Bombay Robowars has created a much better space with a fully enclosed arena and better safety regulations so much so that robots from other countries have started competing at their events.
The fact most Indian teams refuse to compete at unsafe events is also a preeminent sign of how supportive of safety they are now. Do you think you could back away from your negative comments on Indian robot combat safety?
A: Mark J. Tournaments in India are looking better, but appearances can be deceiving. Here is a video clip from the I.Fest tournament in Gajarat, India on the 15th of November, 2025.
Arenas that LOOK safe are not always arenas that ARE safe. That 10mm thick "polycarbonate" (was it?) panel shattered, a robot with an active spinner weapon plunged into the audience, and multiple people were injured. My policy on robots competing in India remains in place.
Also, thank you wholeheartedly for maintaining this website. It's a wonderful community resource and obvious labor of love. It reminds me of the more personal, and expressive "old internet" that I don't see enough anymore.
A: Mark J.
I don't keep an updated list, but Aaron gave these five bot names as his favorites in an archived post from 2009:
Im redesign a robot I posted on here in 2024, mainly the weapon system. Im wondering how bad the gyroscopic effects would be between these two designs
Both have the same diameter of 127mm, with the disk being around 78.5 grams and the bar being around 36.7 grams
A: Mark J. The amount of gyro reaction your robot will get when turning is dependent on drive train width, robot speed, robot mass, weapon speed, and the weapon mass moment of inertia (MoI). If your CAD program can provide the weapon MoI, I just happen to have a calulator that will give you a full answer to your question:
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.
A: Mark J. Things would need to go 'just right' for your planned weapon to do more damage to your opponent than to itself:
A more practical approach is to secure your robot against upward impact reaction and only then apply a downward impact to your opponent. Sliding forks or a 'dustpan' style tray under your opponent as you approach will allow a downward vert to impart full destructive force without vertical separation. Remember your Newtonian physics -- your weapon and mounts must be strong enough to absorb the "equal and opposite" force from such an impact.
A: Mark J. From the wording of your request I assume that you wrote this for some AI service but did not like the answer you received. I dropped it into ChatGPT and got a jumbled mix of useful information and contradictory mush. A quick search of the Ask Aaron Archives for build a safe robot combat arena would have saved both of us time and frustration.
See the Ask Aaron Frequently Asked Questions #38 for resources, plans, and guidance on building safe robot combat arenas for various weight classes. A proper arena of the size and capacity you specified will be quite expensive.
A: Mark J. The wedge and forks are both good, but in different situations:
It's best to have both and swap in the appropriate counter to your opponent for each match. If you only get one, the wedge is the better choice.
Q: A builder on [social media] recently mentioned using a 2205 2300kv brushless weapon motor in their combat antweight. Was that a typo? Does anyone make an outrunner only 5mm thick? [Akron, Ohio]
A: Mark J. I understand your confusion. There are two ways to measure a motor for classification:
A '2205' quad motor like the pictured 'Emax RS2205-2300' would be a '2817' if classified by its can dimensions.
A: Mark J. It's largely a manual process:
A: Mark J. You've told me nothing about your robot, but let me guess: two wheels set WAY at the back of the chassis and something heavy up in the front. That layout gives very little weight on the drive wheels, and little weight means little traction. When backing, any irregularity in the arena surface will shift what little traction you have to one or the other wheel and cause an unexpected turning motion. Lateral weight transfer caused by the turn locks you in and spins you around. Your problem is entirely mechanical - no transmitter adjustment will help. A two-wheel drive 'bot should have about 2/3rds of the total weight on the wheels for best pushing power, agile response, and good tracking.
Short of redesigning your bot, you can examine the parts in the front of the 'bot that slide along the floor. Polish those contact points smooth and tweak the chassis as needed to assure left and right sides touch with equal weight. If you have the option to move the drive motors forward or shift heavy components rearward you may get some improvement -- but this design error is something best handled when first laying out your component placement.
Take a look at this post from the design archive in early 2025:
A: Mark J. For best traction you want as much weight on the drive wheels as possible, but leaving too little weight on the front of your 'bot will allow it lift up off the arena floor under acceleration and make it vulnerable to attack. Designing for 65% of the robot weight on the drive wheels gives good traction and leaves enough front weight in most cases.
Note 2 - Magnet downforce may be used to correct traction and lift problems in a steel-floored arena. Combined gravity and magnetic weight of 65% on the drive wheels will still be a good starting point.
Im trying to choose the right brushless motors for the drivetrain of my 30kg combat robot (Its a drum spinner). Im wondering if the "6354 190kv Belt Motor BLDC Motor for On-road and Off-road Electric Skateboard and Robot" would be a good fit for my two wheel drive? our wheels diameter is 25cm (9.843 inches i believe) and the voltage input is 22.2v (Will be using a 6s Lipo batter)
if i need to provide more information then please tell me! Hope this isnt a case of a bad hamburger
Thanks
A: Mark J. From the Ask Aaron Brushless Motor Selection Guide:
Something more like the HobbyStar 4120 (5055) 710 Kv Brushless Outrunner would be close to the low end of the suggested mass range and would produce full power on 6s voltage. Gearing depends on arena size, but a 45:1 reduction (with those 25cm wheels -- why so big?) would give good performance in an 8 meter wide arena.
I'm unable to find an exact websearch match for a specific "6354 190kv Belt Motor BLDC Motor for On-road and Off-road Electric Skateboard and Robot", but a pair of generic 6354 motors like the "Flipsky 6354 BLDC Offroad Electric Skateboard Brushless Outrunner" weigh in at better than 1100 grams and require 10s voltage to perform up to their specifications. As power is proportional to the square of voltage, on 6s these motors would have only 0.6 × 0.6 = 36% of the 10s power. Too heavy and too little power.
Q: Hey, person with the 30kg drum spinner again.
Thanks for the hobbystar motor recommendation. Unforunately, i think these are hard to find in my country.
I have a few questions though.
A: It is unusual and limiting to decide on battery voltage before picking drive motors -- I'm glad to hear you're willing to consider alternatives. Let's take a look at the proposed drive motors with the Team Tentacle Drivetrain Calculator: Brushless Version:
A: Mark J. The short answer is "no". PETG is heavier for equivalent strength, more difficult to print, and no tougher than a good quality PLA+. Avoid.
Be sure to check tournament rules for the specific filament types they allow.
A: Mark J. I've never actually tried, but it shouldn't be very hard. You can only go where the rails go, so just give it a little throttle and toot the whistle once in a while.
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
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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
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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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That's obviously A question from your homework. Do your own research. |
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