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Privacy Policy 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.
It Isn't Only Driving Skill
Q: What's the biggest mistake you see new builders making? Is it an error with weapon design? A problem with the drive train? What needs the most improvement?
A: [Mark J.] An effective combat robot must have balance between all of its systems.
If you're interested in using the full capability of your computerized transmitter or need suggestions on which radio system to buy in the first place, Team Run Amok has several guides on the subject.
Multiple Considerations
Q: How many wheels are optimal on a combat robot? Two or four? I've also encountered bots with six wheels in competitions. Any advantages to that?
A: [Mark J.] "Optimal" is hard to define. Some weapon types work best with a specific number of wheels, and some wheel lay-outs work better in some weight ranges. Some considerations:
Static and/or Dynamic
Q: I have a Hellraiser robot kit with the competition upgrade add-on. When testing it out it's being very bouncy as soon as the drum spins up. Any advice as to what's causing this and how to make it more stable? A: [Mark J.] That drum is a multi-piece UHMW plastic drum with internal counterweights in the form of steel washers to balance the single-side impact teeth. The weights are critical. You didn't happen to have any parts left over? Q: I gotta plenty of those washers left over, but I put in as many as the video instructions said. Seventeen I believe. They also shorted me several of the square nuts. A: Every screw, nut, and washer is important to the rotational balance of that drum. With the 3S battery the drum will spin north of 10,000 RPM and even a gram out of place will make for a jittering and uncontrollable 'bot. Check to make sure everything that's supposed to be in there is actually in there. Q: I have the 2S battery as well as the 3S battery. Should we try to configure with the 2S battery you think? Would we still need the BEC regulator if we switch batteries?
A: The imbalance problem will be less @ 2S but will still be an issue. Drums can be difficult to balance as their length may produce both static (bounces up and down) and dynamic (rocks side to side) imbalance. Suggest contacting the manufacturer to see if they have suggestions on restoring balance to the drum.
The tinyESCs that come with the competition upgrade will provide enough current to the receiver if running the 2S battery -- you will not need the seperate BEC.
Response: Thank you so much for taking the time to respond! Depends on the Transmitter
Q: Is it possible to program a switch on my radio control so that depending on its position it inverts the mix? I say this thinking about a robot that can run inverted, so that by doing this it can achieve better control when it is inverted. A: [Mark J.] Some transmitters can do this, some transmitters cannot. What transmitter do you have? Q: At the moment I'm using Flysky FS-i6 but I'm going to switch to Taranis X-Lite.
A: Bad News - The FlySky is a great inexpensive radio, but it can't assign custom functions to a switch. You're out of luck there.
Good News - The Taranis X-Lite uses OpenTX firmware and an invert switch is very simple to implement. See: OpenTX: Simple Invert Switch.
Low / Hard / Both / Neither
Q: Hi & thanks for sharing all these questions and answers!
I'm a new builder & I've been having fun trying to create a design for an two wheel antweight undercutter. (I am planning to make a wedge bot first when it comes to constructing something). I'm having a really hard time understanding a few of the details of how to mount the weapon and the weapon shaft. Some more important context: I've been thinking of making almost everything out of laser cut metal with some sheet metal bending & tying it together with bolts. Also, I'm aiming to use a pulley to separate the weapon shaft from the weapon motor. Here I have two questions. First for the weapon shaft, I've seen the horizontal spinner diagrams you've posted but these seem to have a flanged bearing / bearing housing with bolt holes to attach to the frame, to handle the axial loads (eg from being hit by a vertical spinner). (Those diagrams were really helpful, BTW, in helping me understand that it's important to think about the inner ring and outer ring of the bearing separately!) Most bearing housings I have found are very large but it does look like some small ones can be purchased (although they seem expensive to me and might be heavy). Perhaps I am searching for them in the wrong way. Anyway, would you think such a bearing housing is reasonable in antweight robots or would it be better to use some sort of washer with holes in it to hold a flanged bearing in place? I keep thinking that there must be something simple here that I am missing.
Second I have it in my head that it's better for an undercutter to be as low to the ground as possible (without hitting the floor), partly due to reading some of the answers here talking about how drum spinners are more vulnerable on the bottom of their rotation. But having a whole hub and bolts (and sliding foot, for a 2 wheel design) below the weapon seems to leave a lot of space. I can imagine putting the hub above, but then there are still the bolts. They could be countersunk / counterbore, but that presents some issues as well: I'm not expecting the weapon to be very thick anyway; I don't have a machine shop; and if I have my parts made at a laser cutting service, it doesn't look like I can get countersunk / counterbore holes unless I use a softer material (e.g. chromoly instead of ar500). I have a feeling that one or both of these goals (hardest weapon material or lowest ground clearance) is not so important for antweight. Can you help me understand to what extent these are reasonable things to chase after for an antweight? Or do you know of a more one-sided mounting method that I could try? Thanks!
A: [Mark J.]
You did a fine job of pouring thru the Ask Aaron archives in search of examples and diagrams, Silver Spring. The drawings you found are somewhat generic and include elements necessary for larger robots than you are planning. I think you may have missed one post down at the very bottom of the Ask Aaron Robot Weapons archive that is more relevant to an insect-class robot:
A: [Mark J.] The mounting is pretty much like an FBS turned upside-down. The weapon shaft is rounded on the ends and usually sticks out both below the blade and above the chassis to allow the 'bot to invert.
There is such a thing as too low for an undercutter blade. You want enough clearance to keep the blade clear of arena floor imperfections and to avoid floor contact if the 'bot gets 'jostled' from a hit. Having a weapon that is more dangerous to your 'bot than to your opponent is embarassing.
Take a look at the design of the FingerTech Blade Hub.
Small Things Make a Big Difference
Q: Why is it that very similar weapons have vastly different results? Uppercut and Deep Six both have huge, aysmetrical, and fast bars, but Uppercut is so much more destructive. Kraken boasts more crushing power but is so much less dangerous than Quantum. [Vestal, New York]
A: [Mark J.] Up above the questions and answers in the Ask Aaron Robot Weapons Archive is a framed bit of advice labeled 'Aaron's Wisdom'. I placed it in this prominent position because it makes a very important obervation about the importance of weapon design relative to other factors:
Aaron's Wisdom I've said this often but builders don't want to believe me:
Specifically why 'Robot A' looks like 'Robot B' but is less successful requires a detailed engineering study of the robot's fuctional design and components. If this was an easy process the builders of 'Robot A' would have already made the changes to improve their robot's performance.
The weapon may be the least important system on a combat robot.
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.
If you're not winning matches it isn't because you have a poor weapon. Stick to the Mission
Ask Aaron is receiving an uncomfortable number of questions on topics that fall outside our mission:
I've been lax in enforcing the Mission Statement lately and things have gotten out-of-hand. There are plenty of on-line forums for fans who want to post memes, create drama, discuss trivia, and speculate on the outcome of this week's televised robot matches. Ask Aaron is not such a site. I'm willing to answer questions on topics not listed in the mission, but I don't want to fill the pages of Ask Aaron with posts that make it difficult for combat robot builders and historians to find the information important to their needs.
If you'd like to ask a question that falls outside our mission, please include an email address where I can respond directly to you.
The Store is Closed
Q: How long is delivery for the hat and cd? [Vestal, New York]
A: [Mark J.] Good grief - is my webstore still on-line? I was sure I took that down six years ago when I moved to my new web server. It's 2023 -- they don't even put CD drives in computers anymore. I had a few extra Team Run Amok hats for sale and I was tossing in a video CD from the 'Oregon Clandestine Street Fight' as a bonus. Here's an excerpt:
No TV coverage - No spectators - No prize money - No trophies - No scale - No clock - No rules
Comment: Thank you so much for the hat. Im a big fan of the site if you couldnt tell. Reply: You're very welcome, Vestal. What Is That Thing?
Q: If you remove Run Amok's front wedge what is that thing that helps push the box in the maze and go under Killer B in the fight to the finish, is it another wedge or something and speaking of Run Amok's wedge what are these things on the sides of it i am guessing these are the iron weights.[Erskine, Scotland ☆]
A: [Mark J.] You can't just hang a sheet of metal off the front of your robot and call it a wedge -- a wedge requires structural support. With the wedge itself removed the tubular steel 'bumper' and support members are exposed. The top supports angle upward and are welded to the strong front chassis beam that also carries the steering bushings. The lower supports extend all the way back to the steel battery box. The widely separated anchorage locations spread impact forces across the chassis.
Early testing revealed too much flexing along the sides of the wedge. Before shipping out to Robotica we welded 1/4" thick aluminum angle stock along both edges to reinforce this potential weakness. Q: what would you say is the biggest upset from the battlebots reboot? I think its ripperoni vs end game. [Vestal, New York]
A: [Mark J.] This falls outside the range of question types we answer here. Ask Aaron is not a fan site. See FAQ #14.
I'm sure this question would be welcomed on the Battlebots subreddit.
A Rat Has Resurfaced
Q: I found on your archive that Andrew Lindsey was the creator of the Robotica Rats. Did he and someone else control them? And do you think he has them in storage, or are they scrapped?
A: [Mark J.] Robotica host Dan Danknick says the rats were controlled by "an experienced Hollywood SFX team" which makes sense. You wouldn't want any of the competitors to feel that the rats were anything but impartial.
Audiences Want Action
Q: How come no one uses a cord ball or dispenser to entangle spinner weapons. The have everything else. [Generic Verizon Server]
A: [Mark J.] Most all combat robot rule sets prohibit entangling devices -- they make for dull matches. Example from the BattleBots Design rules Section 8-j:
Prohibited Weapons
The following weapon types are not allowed under any circumstances:
Definitive Answers are Rare
Q: Hello Aaron, I love all the work you've done on the multitude of topics for combat robots. I, however am not an engineer and I'm struggling to come up with a definitive answer for my current build. Could you help out with the spinning weapon calculator for a 1lb Fingertech beater bar? My google attempts have only come up with "up to 10k rpm". I'm trying to figure out what an ideal weapon RPM should be to maximize bite with a ground speed of approx 6 mph. Any help you could provide is greatly appreciated!
Thanks, Eric
A: [Mark J.] Take a look at the Rotational Speed section section of the 'Ask Aaron: Spining Weapon FAQ'.
The Good News The section provides complete and comprehensive answers to:
The Bad News You'd like a definitive number, but there is no single definitive number. The optimum weapon speed is situational and varies with the design of your opponent, the arena in which you're fighting, and how good a 'run' you have at your opponent. Most spinner weapon builders design their systems with more speed than is useful in most situations and enter a match at reduced weapon throttle.
More Distortion Than You Think
Q: Why arent full body spinners just giant brushless outrunner motors with wheels on the bottom? [Vestal, New York]
A: [Mark J.] Several reasons come to mind:
Weight - Take a look at the photo of the Lumenier LU8 LITE 150kv outrunner and count up all those copper-wound steel-core electromagnets so tightly packed around the circumference of the motor stator. Larger diameter means more electromagnets. Copper is heavy, steel is heavy, and all those neodymium rare-earth magnets in the rotor are heavy.
Speed - Everything else being equal, the larger the diameter of the motor the more slowly it spins and the more torque it produces. The pictured motor is about 3.5 inches in diameter and spins about 3300 RPM @ 22.2 volts. Make it larger and it spins too slowly to be an effective weapon.
Fragility - Even a thick FBS shell distorts a surprising amount on impact, and there isn't much clearance between magnets and stator on an outrunner motor. You shouldn't batter down doors with your face and you shouldn't slam your precision motor directly into a combat robot.
Time Out to Pump!
Q: I have thought about converting a bb gun that you pump to store up pressure with into a pneumatic flipper is this possible? I'm guessing that you'll give me a cheerleader button. [Vestal, New York]
A: [Mark J.] No, Vestal - the Cheerleader doesn't get this one. You haven't told me much about your conversion plan, so I'll have to guess:
A: Combat robot weapons typically measure their power in joules of kinetic energy, and there isn't much energy output from a single pump on an air pump gun. The kinetic energy of a moving object is equal to half of the object's mass (1/2*m) multiplied by the velocity squared. A 'BB' pellet weighs about 0.35 gram and a small pump air gun might propel a 'BB' at 200 feet-per-second from a single pump. This works out to 0.65 joule of kinetic energy.
That level of kinetic energy does not compare well to other weaponry available: the 5-inch plastic bar spinner weapon you've previously written in about stores about 56 joules. Given the weight, complexity, and construction challenges of a converted air gun flipper I think it is a poor choice.
If you are interested in a more practical electric flippers take a look at Combat Robots: Four Spring Flipper Designs.
Q: has anyone built a full body flipper? Where the whole shell lifts up?
A: [Mark J.] Not a successful one.
I originally had short video clips of the fights in the journal, but one day I was stepping thru the full video frame-by-frame and realized how much more detail and drama that type of replay gave. I grabbed four critical frames from each of the videos, individually enhanced them for sharpness and color, and spliced the results together into a pair of slow-motion gifs.
In the gif above you can see Killer Bee's rising wedge clip Run Amok's top armor and lift her front wheels off the arena surface as the Bee tips over the edge of the platform -- a detail that went unnoticed in the video.
A: [Mark J.] Hobby servos do not come with a rated 'lifting power'. The specs they come with are:
Two additional notes:
A: [Mark J.] Robotica Seasons 2 and 3 did have challenge stages redesigned to allow greater opportunity for robot vs. robot conflict. Most competitors chose to focus on the points-gathering tasks, although in a few matches the close quarters of the starting turntable in "The Labyrinth" saw weaponry deployed.
I think it would make sense to have active weaponry for a Robotica-style event comply with "Sportsman Class" rules as defined in section 4.2 of the SPARC Ruleset. That should allow for ample capacity to interfere without destroying the opponent or the set.
A: [Mark J.] The symptoms fit a dirty or faulty potentiometer on the transmitter throttle stick. Plug one of your drive motor ESCs into the receiver weapon port and see it the drive motor behaves the same way as weapon. If it does you can try to clean the pot [Cleaning a DX6i Potentiometer] or for about $15 you can buy a whole new gimbal and pop it right in.
A: [Mark J.] Magnet downforce adds virtual weight to the drive wheels but does not add corresponding mass, so there are multiple inputs involved in simulating the effect on performance. I have a spreadsheet that I use to modify calculator input values to simulate downforce before entering them into the calculator, but it's a little clunky:
The in-calculator 'Help button' has been updated with this description:
Magnet downforce will improve pushing power and acceleration, but will cause greater current consumption by the drive motors and require greater ESC and battery capacity. The calculator will show the changes in performance and capacity.
Team Tentacle Drivetrain Calculator: Brushless
Looking for help with magnet selection? See the Magnetic Downforce Calculator.
JavaScript debugging is not my long suit, so I called in Roger the Web Gerbil and he found it in a flash. Roger is refusing to divulge the details, but from the wry smile on his face I'm assuming I simply failed to properly define a passed variable. Roger got his cracker and we have a working tool -- case closed.
A: [Mark J.] A few months ago I got tired of answering 'what's wrong with my transmitter mix' questions and decided to answer all possible mix problems at once. I wrote an 'expert system' program [a primative and specialized A.I.] that asks three questions about what your bot is doing and spits out a precise and direct solution. I call it the Run Amok Mixer Fixer.
From your description, the answers to the three questions the Mixer Fixer chooses to ask are:
March, 2023 - We celebrated our 20th anniversary by giving away a stack of complete termite-weight robot kits in our 'Secret Identity Contest'. The contest is now closed, but you can still try to solve the puzzle. Think you know your combat robots? Give our anniversary puzzle a try:
A: [Mark J.] We won't upload copyrighted video to the internet services -- see FAQ #13. However, I've recently learned that the Internet Archive has a complete set of Robotica episodes from all three seasons available to view, cast, or download:
Consider clicking on the Internet Archive's "donate" button and chipping in a couple bucks to support their work.
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
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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It was a hot July afternoon and the finest heavyweight and super-heavyweight combat robots
in the Northwest gathered at an undisclosed location to test their mettle.
According to 
A: [Mark J.] You're referencing the pair of animated gifs in the 
Q: how do you convert a servos lifting power to a servos crushing power? [Vestal, New York]
Q: My favorite part of Robotica was the challenges that were nominally point-based, but the robots could interfere with each others' progress. Given the massive advancements in weapon effectiveness & destructiveness, if such a contest came back in 2023, would there be a fair way to allow some degree of moving weapons for glassbreaking, self-righting, and a reasonable amount of interference, without the dominant strategy becoming "kill the other robot, then scoop all the points unimpeded"? [Newton, Massachusetts]
OK! Both the brushed and brushless versions of the Tentacle Drivetrain Calculator now have a 'Magnet Downforce' field highlighted with yellow text right under the 
The previously reported bug in passing magnet downforce data to the Acceleration Calculator has been squashed. The calculator is now correctly showing the improved acceleration from reduced wheelspin with magnet downforce. 
Q: Feel dumb because I’ve screwed up my transmitter and can’t figure it out. I have right and left on my transmitter doing forward and reverse and vice versa, what do I have screwed up here? [Boulder, Colorado]
Q: I'm a big fan of the TLC Robotica series, but I'm having trouble finding some of the season 2 and 3 episodes. Would you consider uploading these seasons from your library to one of the video services? [Tempe, Arizona]
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
We welcome combat robot questions. Please search the 