6426 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of 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.

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Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.

Q: Hey Mark, fellow Willamette Valley-ite here again

So here's where I'm at. I recently built a brand new hobbyweight. Spent months on the design, meticulously selected components, machined every part, and promptly went 0-2 in my first competition.

The flaw lay in a very poor hardware choice. The output shafts for my drive gearboxes have a 3mm hole. I designed wheel hubs to slip over the shaft and used an M3 alloy bolt to lock the shafts to the hubs. Bad idea. About a minute into each fight, both bolts sheared and turned my bot into a very expensive paper weight. I didn't take any major hits, and I believe that the bolts sheared solely from the torque of the motors turning the wheels back and forth.

I am now wondering if a 3mm hardened pin will be sufficient to lock the hubs to the shafts. The ones I have in mind have a 1600lb shear rating. I emailed McMaster-Carr to try and get a shear rating for the bolts so I could compare, but they said that they don't test shear on their bolts. I am wondering if the pins would hold up, or if I should completely redesign the hub?

I'm running 12 volt Johnson RS-550 motors (40:1 reduction). Also attached is my CAD of the hub and gearbox.

As always, I appreciate your input very much! [Albany, Oregon]

A: [Mark J.] That's what's so great about combat robotics; you learn all sorts of unexpected things that you wouldn't run across in a normal lifespan. Sometimes it's fun, sometimes it's painful.

Bolts are designed for optimal tensile strength at the expense of shear resistance. Make that 3mm bolt out of alloy instead of steel, add in the leverage advantage of a (perhaps) 50mm radius wheel on the outer surface of a 2.5mm radius shaft, and then try to abruptly reverse the momentum of a 12-pound robot moving along at 9 MPH, and presto -- shear failure.

  • The Good News Replacing the alloy bolt with a 3mm hardened steel pin will very likely eliminate the shearing problem as the weak link in your driveline.
  •  The Bad News  Shearing the alloy bolt was probably all that prevented the failure of the small gearbox output shaft with the large cross-drilled hole.
Based on your CAD, the gearbox shaft is very small for a hobbyweight, and that 3mm hole removes a great deal of the shaft strength at a critical location. A 3mm cross-drill thru a 5mm shaft removes more than 70% of the shaft material at the minimum cross-section. The hardened pin will transfer the torque load directly into that weak area of that shaft -- which will be your new failure point.

If those shafts are as small as they appear you don't just need new hubs, you need new gearboxes. For a hobbyweight, I'd look for minimum 8mm diameter steel shafts with no cross-drilled holes.

Q: So it sounds like a hardened pin isn't going to be my simple solution. I don't know of any other offset gearboxes that I could switch to and fit inside the robot in the same way. I'm trying to avoid redesigning the whole bot and have a couple ideas. I could try to modify the gearboxes and install a larger shaft, but I'm sure that is easier said than done. My other idea seems a little more appealing (for the moment).

I was thinking I could press fit a square hub (red in the picture) made out of a somewhat flexible plastic onto the existing shaft. This would take the cross-drilled hole out of the equation and add some flexibility. Is this a viable solution, or recipe for disaster?

A: I'm concerned that the gearbox weaknesses run deeper than the shaft -- why would there be combat-quality components up to but not including the output shaft? I suspect that you're going to chase a series of weak points thru the entire structure of those boxes, ruining your combat success for as long as you keep them.

I realize that you've invested effort and heart into your current design, but there are reasons your competition does not use those gearboxes. Cut your losses and start fresh.

Q: Hi. I am building a 15 lb combat robot, and I have a 1700Kv Brushless Inrunner motor by Dr. Mad Thrust. I am running a 25v system with a reduction of 1:1.5 so the weapon will be spinning at around 28,000 rpm. Do you think this is too fast for the weapon? I have seen many beater bar robots spinning insanely fast, and the downfall of my last robot I believe was the weapon speed. I have attached my weapon so you can base your answer off of that as well. Thanks :) [Dublin, Ohio]

A: [Mark J.] Your drawing/CAD failed to 'attach' so I don't have the details of your weapon design. I'll have to be general in my comments.

  1. It's unlikely that your actual weapon speed will approach 28,000 RPM even with a very powerful weapon motor. The Kv 'speed constant' value is accurate for an unloaded motor. As you add load to the motor the speed drops, and the aerodynamic load on a spinnng weapon increases with the square of speed. Spinning a weapon at 28K RPM requires four times the power needed to spin the same weapon at 14K RPM -- sixteen times the power needed at 7K RPM. Your weapon will fall farther off the calculated speed as aero drag increases, and will pull greater current in direct proportion to the increasing load.
  2. Read thru the Ask Aaron Spinner Weapon FAQ for considerations in evaluating spinner speed. Note in particular the section about drums/beaters going 'weapon-to-weapon' against similar designs.

Q: What type of screws have the best hold in UHMW polyethylene? Can I tap threads and use machine screws? [Hershey, Pennsylvania]

A: [Mark J.] Depending on your specific needs, you have multiple options:

  • The popular solution for structural joints is wood 'deck screws'. The coarse threads hold well in soft plastic. Drill a pilot hole and run 'em in.
  • Specialty 'Plastite' screws are great in firm plastics, but are harder to find and may not be worthwhile for soft UHMW.
  • Polyethylene is poor at holding fine tapped threads. If you want the strongest possible hold with machine screws you can cross-drill and insert cross-dowel barrel nuts. This is way too much effort for a small 'bot, but might be handy for larger builds.
  • There are threaded inserts to drive into soft plastic that then take machine screws. They're useful for screws that must be removed frequently.
  • Heat-set threaded inserts are not recommeneded for UHMW combat uses.

Q: hi mark. i was watching season 1 of "robotica" recently. is it pure coincidence that the ram force and whyachi teams have the same uniforms? [Philadelphia, Pennsylvania]

A: [Mark J.] Yes, kinda. Team Force made their TV appearance on Robotica a few weeks before Team Whyachi appeared on BattleBots, but neither team had seen the other when the shirts were selected.

When you order embroidered hats and shirts you typically pick from a selection of 'stock' clothing on which to put your logo. The red/white/black 'Pit Crew' shirts were common stock items at the time, and both teams independently picked them out of the catalogue.The Team Whyachi and Team Force hats, however, were quite different -- I have one of the 'Team Force' hats in my collection.

Q: Are Auto reset breakers a good idea for Amp spikes In a combat robot? [Hagerstown, Maryland]

A: [Mark J.] A better idea is to design your robot with components spec'd to handle the current loads -- with some to spare. You really don't want to be a sitting duck for 15 seconds while your breaker resets.

Additional Problem Auto reset breakers are not designed for the physical shock loads combat robots experience. A sharp impact will trip the breaker well below its current rating, which makes them worse than useless.

General Rule Unless tournament rules require, don't put ANYTHING into combat that can shut down the robot without your intervention. This includes battery voltage cutoffs and any type of fuse. Audible warnings or telemetry alerts are fine 'cause they leave the critical decision to stop or continue with the driver.
Put in all the safegards you like for testing, but I'd much rather get a few more seconds out of a component and risk it failing than save the component and lose the match. If your budget is so low that you can't aford to burn a few components for a win you're in the wrong hobby.
Q: So I was thinking to start building an antweight for Massdestruction. I was thinking a overhead grappler (not a crusher) and a wedge. This is gonna be my first combat robot and I'm not even sure she I'm gonna build it. Is this a good one for beginners? Thanks! [Turks and Caicos Islands]

A: [Mark J.] I really ought to put this in the Frequently Asked Questions...

Your first combat robot should NOT have an active weapon; no spinner, no lifter, no flamethrower, no crusher. Keep it simple -- build a wedge. This is standard advice from multiple sources, and it is both for your benefit and the best interests of the sport. See this post in the Ask Aaron archives for a summary of the reasons.
Update I've tucked this in as part of FAQ #8.
Q: Is there a smaller and lighter ESC solution than a pair of tinyESCs for an insect class bot? I'm having trouble with weight and space on my design. [Saskatchewan, Canada]

A: [Mark J.] If you're using a DSM2 protocol transmitter you can replace your receiver and both drive ESCs with the Endbots Lemon-RX / Dual ESC combo. The 6 gram package eliminates a lot of wire clutter and weight. The disadvantage being that it's one piece rather than three chunks that you can wedge in. See if the (about) 3cm x 2cm x 1cm footprint works for you.

Q: So I have a question regarding battery eliminator circuits.
  • Last year I ran an antweight wedge with 4 motors and 2 drive fingertech tiny ESC's, each with a BEC and I did not clip any wires, just plugged it in and it worked fine.
  • This year I am running 2 tiny ESC's on top of a 12 amp brushless BLHeli esc for my weapon motor.
I am a bit worried about the BEC's stacking and overpowering the receiver, so I checked your archives and sure enough you recommend clipping all but one of the BEC's. This year I'm likely going to clip the BEC on the Tiny ESC's and leave the one on the brushless ESC, is this correct?

Also out of curiosity, I had no problems last year with both controllers providing power to the receiver, any idea why that might be? [Ballwin, Missouri]

A: [Mark J.] Versions 2.4 and later of the tinyESC have components to prevent multiple BECs from effecting the tinys. As noted on the FingerTech tinyESC page:

Version 2.4 ...There is also now protection on the 5V receiver line so that other BECs will not affect the tinyESC - no need to disconnect the red wire anymore!
I'd suggest leaving the red wires on the tinys alone since they're safe to operate together. If your brushless weapon ESC has a BEC (not all do - check the documentation) I'd clip its power line to make sure the tinys don't mess with it.

Note BECs don't 'stack' to overpower the receiver or other electronics -- they're all wired in parallel. The problem is that multiple unprotected BECs may confuse each other's voltage regulators and cause an unstable fluctuation that effects receiver performance. When in doubt, use only one.

Q: Have you ever heard of a robot named 'Mr. Quacker"? I can't find a record of it in BotRank. [Columbus, Ohio]

A: [Mark J.] You won't find an official record, but there is a video: Robot Fight Club: Mr. Quacker vs. The Incinerator.

Q: So I heard from somewhere that 'Tombstone' had carbon armour but from somewhere else that he had metal armour. Which one is true? Thanks! [Turks and Caicos Islands]

A: [Mark J.] Builder Ray Billings doesn't much believe in armor. I'm not sure you should even call the thin panels that stretch over Tombstone's old-school tubular chassis 'armor'.

Non-BattleBots versions of his big spinners use thin aluminum or Lexan panels, but out of concern for arena hammers and saws the 250-pound version has titanium panels top and bottom. The black side panels are painted -- not carbon.

Q: So I heard that 'Warrior Clan' or now called 'Warrior Dragon' used kinetic energy from their spinner. Is this true? If so how? By the way I'm a big fan of your website! [Turks and Caicos Islands]

A: [Mark J.] Team Whyachi's kinetic spinner/flipper has a long history, having first fought in 2009 at the non-televised 'BattleBots Professional Championship' as 'Warrior SKF'. There are multiple posts about Warrior SKF in the Ask Aaron Robot Weapons archive: start here.

Note Some sources incorrectly claim that 'Warrior SKF' is a rebuild of Team Whayachi's earlier rambot 'Warrior'. The name was recycled, but the two very different robots are structurally unrelated.

Q: I'd like to try brushless drive motors in my next 'bot, but I'm really confused about the motor controllers. How do I tell if an ESC can be flashed with simonk firmware, and how do I actually flash an ESC? [Central Florida]

A: [Mark J.] Builder Robert Cowan has a very nice video on his YouTube channel - SimonK Firmware Flashing Tutorial (Brushless Drive):

This video covers how to flash reversible SimonK firmware on to any compatible ESC. In the video, I'm using a HobbyKing F-60A, but any ESC with an ATmega-8 processor should work. This should allow you to use a brushless motor for a forward/reverse drive system for a robot.
If the process is a bit intimidating to you, there are small brushless ESCs that come pre-flashed with SimonK. Search your favorite hobby source for 'SimonK'.

Comment: Regarding your last answer. There's a table of ESCs and their relative SimonK & BLHeli compatibility in this Google docs spreadsheet.

It may not be 100% perfect, but it's the best I know of. [Mark from Vancouver]

Reply: That resource and several others appear in Robert Cowan's comments directly under the YouTube video linked above (click on 'show more'). I'll reprint those links here for those who missed them:

Q: I have a question on DX6i Spektrum transmitter. On the main board there is only 1 BLUE capacitor. I need the part # for this or the rating. With this part I will have a working radio, so help me out please? Thanks in advance. [Spur, Texas]

A: [Mark J.] I didn't have a DX6i in my workshop, but builder Travis Schmidt found the answer for me: it's a 220uF 16v electrolytic capacitor. Thanks, Travis!

Q: My horizontal spinner has a big reaction on weapon startup that rotates the robot opposite to the weapon direction. I'd like to activate a high-traction 'foot' that presses down against the arena floor to brace the robot against rotation when the weapon is under high spinup load. Is there a sensor that will detect high current flow to the brushless weapon motor that I can use to activate the foot? [The Forums]

A: [Mark J.] There is a simpler and more workable solution. A heading hold gyro of the type used in R/C helicopters will detect an unwanted rotation of your 'bot and signal the drive motors to compensate for the rotation. The gyro is lightweight, inexpensive, uses your existing drive train, is simple to implement, and has the advantage of operating when the robot is moving rather than locking you in place 'til the weapon is up to speed.

Q: I'm building my first robot - a hobbyweight with drill motors. Are there any receivers that will operate on 18 volts, or do I need a separate receiver battery? I'm using a Spektrum Dx6i transmitter. I asked about this on one of the forums but didn't understand the answers. [North America]

A: [Mark J.] Builders often fall into jargon to answer new builder questions without realizing how confusing that can be.

Most electronic speed controllers (ESCs) used for robot drive motors have a 'Battery Eliminator Circuit' (BEC) that supplies ~5 volt power to the receiver via the three-wire cable that connects the ESC to the receiver. If any of your ESCs has a BEC (check their docs) you will not need any additional power for the receiver.

If your ESCs lack BECs you can purchase a small and inexpensive external 'universal' battery eliminator circuit (UBEC) that connects to the main battery and plugs into any available receiver port. A 'Switch Mode' type UBEC is preferable for battery voltages greater than 12 volts.

More info on BECs: Dimension Engineering BEC FAQ.

Q: Hey, Mark! Apologies for sending you a question right after my last bunch, but it wasn't until this morning that I actually remembered it. Okay, so I noticed 'Wrecks' uses a vertical flywheel that is much thinner than conventional flywheels.

What material did they use (I'd assume S7 steel, but I didn't know)? Other than less weight, is their any other advantage to having a thinner flywheel? Are there any notable disadvantages? [Champaign, Illinois]

A: [Mark J.] 'Wrecks' relies on gyroscopic precession generated by its vertical spinner (made of AR400 steel) for it's 'walking' motion. A larger diameter weapon has a greater mass moment of inertia (MoI) and generates greater precesssion force at the same RPM. Cutting the flywheel width in half allows for a 40% increase in diameter at the same mass, and doubles the MoI.


  • Thinner allows you to make the spinner larger in diameter, which makes it better at generating gyroscopic precession, which makes 'Wrecks' a better walker.
  • More precession force is good for a precessional walker, but it causes problems for a wheeled robot. See this post in the archives on 'gyrodancing' for details.
  • A thinner vertical spinning weapon is more vulnerable to damage from horizontal spinner strikes.

Q: Hey! I'm back again with some more questions! This is another collection of random questions, and as always, I appreciate you taking the time out of your day to answer them! [Champaign, Illinois]

1) 'Double Dutch' is quite an unusual machine, both in shape and its idea of using two counter rotating blades as weaponry. My first question about Double Dutch is simply this: does using two counter rotating blades generate more force than, say, using two blades rotating in the same direction? I'd assume that the answer would be no, but hey: that's why I came to ask you!

A: [Mark J.] A major problem with a horizontal spinner is that stiking your opponent sends them off in one direction and sends your 'bot off in the other direction, wasting about half of the impact force (see Newton's Third Law). If you can manage to hit both sides of an opponent more-or-less at the same time you can transfer more of the impact energy to your opponent and less into sending both 'bots sliding across the arena... in theory. In practice it may well just 'spit' the opponent out like a watermelon seed and shove itself backwards.

A second 'benefit' is that the gyroscopic forces from the counter-rotating blades cancel each other out. There's no danger of odd 'gyro dance' effects if the 'bot is kicked up off horizontal. This is less of a problem with horizontal spinners than with verticals, but it's still a bonus.

2) I recall builder Kevin Lung mentioning in a reddit post that 'Double Dutch' was capable of omni-directional drive. Is this true, and if it is, how given that it uses normal, non-omni wheels?

A: Take a close look at the photo of 'Double Dutch'. Notice how the wheels aren't all pointing in the same direction? If you drive those wheels independently in the same way you drive omniwheels you can get a sideways 'strafing' motion as well as conventional motion. It requires lots of power to spin/slip the wheels in this manner, but it can kinda work.

3) I noticed that 'Sawblaze' uses a flamethrower alongside its powerful saw. Does this fire actually help the saw cut through the opponent, or is it more or less for special effects?

A: Fire pretty.

4) What exact purpose do the curved bits of 'Bronco' serve? I'd assume they're built to keep vertical drums at bay, but if they are, aren't there simpler ways to deflect vertical drums?

A: Yes they are; no there aren't.

5) I saw a video of a robot named 'Bad Vibrations', alongside the robot 'Motored Brush'. How exactly do these robots use vibration to drive? I know 'Clean Sweeper' and 'Vibrator' both use the forces of their spinners, but these two don't wield two spinners to use the spinner as the main source of power...

A: Exactly how bristlebots work is far more complex than their simple construction would lead you to believe. Simplified explanation: an imbalanced rotating mass jiggles the robot up/down fore/aft and if you get the speed right the loading and unloading of the brush bristles contacting the floor produces a net vector in a direction perpendicular to the axis of motor rotation. Motion is slow and with very little force. Place an independently controlled motor/brush on each side of the 'bot and you can get it to turn. Reverse the motor rotation and maybe it will back up -- or maybe not. Like I said, its a complex and inefficient system.

  • The 'Bad Vibrations' video demonstrates how puny the drive is compared to a conventional drive. You don't need big off-balance spinners to get the drive to work, just small off-balance motor-weight systems will do.
  • 'Motored Brush' is not a bristlebot. It's literally a brush, but a conventional wheeled robot drive train is hidden under the bristles.

6) Why aren't Horizontal Crushers, like 'Kan-Opener' and 'Tough as Nails', as popular as Vertical Crushers, like 'Mohawk', 'Spectre', and 'Petunia'?

A: More moving parts are required for a horizontal crusher. The real question, given their generally poor performance, is why builders consider any type of crusher viable.

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

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

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

A: [Mark J.] I was careful to specify insect weight classes. If the guideline (not a rule) extended beyond insects, I would'a said so.

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

Extrapolating to larger robots is non-linear. The Square-Cube Law requires larger robots to devote a greater percentage of their mass to structural elements, leaving less for other components. As robot weight increases you find proportionally smaller spinner motors and those motors are pumping out less power per unit weight because they are also subject to square-cube issues for structure and heat dissipation. A heavyweight spinner motor might typically produce 50 watts per pound of robot at about 3% of the robot mass. Fitting this to a log curve gives the chart shown -- but it's only a guideline.

    Remembering Aaron Joerger, 1991 - 2013
The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary
- Mark Joerger   
Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.

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