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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.
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: 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:
Q: So I recently got a FrSky QX7 Transmitter to replace my dying HobbyKing entry transmitter, but frankly, I'm a little overwhelmed - seems a lot of the OpenTX and Taranis tutorials are geared towards planes, so filtering out and applying the information I want is proving tricky. I've successfully bound the receiver to the tx, but I need to do the following:
A: [Mark J.] I'm not so sure that all of those functions are simple, ore even possible on your old HobbyKing transmitter - but they are certainly functions that the Q-X7 can handle. There is very little that an OpenTX transmitter cannot be set-up to do, but all that capability comes with a large and complex system of menus and adjustments. Figuring out how to implement even simple functions can be a challenge. But don't worry, I've got you covered:
I've answered a wide range of individual questions about using the Taranis Q-X7 transmitter for combat robots. The OpenTX firmware used in the Q-X7 offers features previously unheard of in a transmitter priced below $100, but the user interface is radically different and intimidating for even experienced radio users.
I decided to author a combat robot guide for the Q-X7 based on the format of my FlySky FS-i6 transmitter combat guide, but exploring the OpenTX firmware with step-by-step examples setting up functions commonly found in combat robots. Work through these examples and you'll have a good working knowledge of the OpenTX firmware.
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]
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.
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.
Flashback Post from 2018:
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.
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.
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. 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]
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]
'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.
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.
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 a36v 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 (link removed) 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:
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?
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:
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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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.
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:
Seth did the hard work of chasing down the problem with an extensive testing program and created a new 'axe head' weapon design that 
A: [Mark J.] You came to the right place.
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 
A: [Mark J.] Many combat robots use a single-axis 
