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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.
Spin or No Spin
Q: I have a FlySky FS-i6 transmitter for my beetle with the weapon controlled by channel 3. I recently saw another builder with an FS-i6 who used one of the transmitter switches as a safety that kept the weapon from spinning until it was flipped down. Can you tell me how to do that? [The Heartland]
A: [Mark J.] It's easy to set this up on a FS-i6 by activating 'Throttle Hold' in the function menu:
More setup tricks are in my FlySky FS-i6 Combat Guide.
Push it Through
Q: I'm looking for a way to quantify material strength and I'm hoping for some guidance. I'm playing around with the idea of a crusher robot and I'm trying to figure out how much force I need to pierce through different metals. I can look up a materials tensile strength and it usually gives me a value in PSI, but does that take into account the materials thickness? Obviously a smaller tip on the crusher means a higher force over a smaller area, but how would I calculate it? Sorry if this is a little out of the scope of this site; any help would be appreciated thanks. [Paso Robles, California]
A: [Mark J.] There are a number of different standard measures of the strength of a material exposed to different types of stress and strain. Tensile strength is a measure of resistance to a force attempting to pull the the material apart along its length. That's very different from resisting a piercing force. As you go thru the list of different measures of strength you will notice that none of them directly measure force required to pierce. This is because piercing resistance is as dependent on design elements like thickness, support, and ability of the structure to flex as it is to the material itself. A simple calculation is not going to give you a useful number.
My best suggestion here is to build a mock-up of a 'typical' robot structure that you might encounter with your crusher and subject it to destructive testing with a model of your piercing tool and a hydraulic press. Sometimes you just gotta go out and break stuff.
Dead or Live
Q: Hi, I was wondering how to make sure an axle on a vertical spinner does not come out the sides but rotates, well, vertically? [Seattle, Washington]
A: [Mark J.] There are two broad types of weapon shaft design -- see: dead shafts and live shafts for a drawing and explanation. Insect class robots will typically use a 'dead' shaft that is a non-rotating part of the chassis that strengthens and stiffens the entire structure. The spinner weapon and any drive sprocket or rotor are on a hub that rides on shaft bearings. If you want a 'live' shaft that spins with the weapon you may use shaft collars on the outside ends of the shaft to hold it in place, but as a very wise builder once said, "Set screws suck." A Kinda Lumpy Disk
Hello, Mark! I’ve been working on and off with my ant as of late and have two design-related questions: [Naperville, Illinois]
Q1: How would I calculate the kinetic energy for this design using your calculator?
A: [Mark J.] The kinetic energy storage capacity of a given spinner weapon design depends on the speed of rotation and the Mass Moment of Inertia (MOI) of the rotor. The online Team Run Amok Spinner Weapon Kinetic Energy Calculator can determine the MOI of a rotor made up of simple geometric shapes: disks, bars, and tubes. Calculating the precise MOI of more complex shapes like your weapon is more difficult. Options:
Pumping those numbers into the kinetic energy calculator with an assumed speed of 8,000 RPM gives kinetic energy storage of...
A: An interference press fit requires a high precision drilled hole -- much tighter tolerances than you might get from a waterjet cut hole. If you have a correctly sized hole a it's not all that tough to tap or vise-press a needle roller bearing into place.
I know of builders that fix bearings into slide-fit holes with Loctite 640 retaining compound. This is not the same stuff as the common blue Loctite threadlocker. The Loctite is effective -- but don't get it in the bearing!
You might consider flanged oilite bushings instead of needle roller bearings. The flange keeps the bushing in place and the bronze bushing can absorb enourmous impact loading. Correctly oiled, the frictional difference is negligible.
The Guide Says...
Q: Hi, Back again, but this time building a beetle! What brushless motor would you recommend for vertical or drum spinners of the 1.5KG weight class? Thanks! [York, England] A: [Mark J.] The Ask Aaron Combat Robot Brushless Motor Selection Guide will give you general parameters for weapon and drive motors for a given robot weight. For reasons provided in the guide, a brushless motor massing about 90 grams with a power output around 450 watts will do nicely for a typically sized beetle weapon. Examples: Selection of motor diameter/length will depend on available space in your design, and selection of motor Kv rating will depend on the weapon design and drive method. Since all you've told me is "beetle vert or drum" I can't narrow it down more than that for you.  20 Years in Combat Robotics 
2021 marked Team Run Amok's 20th anniversary in combat robotics. We dove straight into the deep end of the competition pool, starting our career with a win at the nationally televised 'Robotica' tournament. In the years that followed we won more competitions, travelled to compete in England twice, and finished in the top three at more than half the events we entered.
I made several updates to the website to observe our anniversary year. Check the Team Run Amok webpage for a full list. Flashback: an archived post from 2019
Q: How well do clamping-style shaft collars resist axial shock? Cheers.
Q: If I were to machine my own clamping-style shaft collar and shaft, would there be any benefit to including a Morse taper on the mating surfaces?
A: Adding a matching shaft and collar taper will greatly improve axial shock resistance in one direction -- and entirely screw up resistance in the other direction. Unless the shock loading direction is completely predictable (and nothing in combat robotics is completely predictable) I would strongly avoid any taper.
I'm assuming that you're asking about shaft collars because your application requires axial adjustment along the shaft. If not, a simple machined groove and snap-in circlip are the standard and preferred solution.
The Miracle is...
Q: Hi again, this time having a problem with Lipos…
I bought a 2s LiPo charger from the BBB shop and when I attempt to charge any of my Turnigy 300mah LiPo batteries, the light is red for about for seconds, then flashes briefly and repeats this. I have plugged these batteries into my robot, but it’s not turning on? Thanks for your help! [Eton, England]
A: [Mark J.] This same charger is sold on Amazon.com as the Blomiky H102. Several reviews of the Blomiky report problems similar to what you describe. The miracle is that most of these little £3 ($5) chargers actually do kinda work. Your's doesn't. Send it back to the Bristol Bot Builders shop and ask for a new one.
Note: The Blomiky advert on Amazon estimates that the H102 will take about 5 hours to charge a 1000 mAh battery -- that's about 90 minutes to charge your 300 mAh pack. You might want to consider spending a bit more for a faster charger.
A: You have options:
Flashback: an archived post from 2019
I had a conversation with a builder today that convinced me it's time to repost this explanation of brushless torque:
Q: I have a choice of two versions of the same brushless motor to power my weapon: a 750 Kv and an 1100 Kv. The 750 Kv version will give me more torque, right?
A: [Mark J.] There are a lot of 'half-truths' about brushless motors floating around out on the 'net that are confusing robot builders and leading to poor design choices. You can find this one all over the 'net:
There is a kernal of truth in that statement, but it is incomplete and misleading.
The true part -- the motor speed constant (Kv) and torque constant (Kt) are inversely proportional -- if one goes down the other goes up. Compared to a higher Kv version, a lower Kv motor will produce more torque per amp of current.
Here's a real-world example comparing two versions of the AXI 2208 motor:
The incomplete part -- lowering the motor Kv increases the electrical resistance (Ri), which reduces the current the motor will draw. The misleading part -- if allowed to pull unrestricted current, the lower Kv motor version will produce both less torque and less power.
AXI 2208/34 Gold Line V2 motor
AXI 2208/20 Gold Line V2 motor
The 1820 Kv version of this motor generates greater torque than the 1100 Kv version throughout the RPM range. When bogged down to the '60 second' current maximum it produces 8% more torque, and it does so while spinning 85% faster than the lower Kv motor -- a power increase of:
There are a number of reasons why you might choose a lower Kv version of a motor for a specific application, but Technical K.O.
Q: Why would my combat robot be browning out when it gets hit? [Bellevue, Washington]
Electron Maze
Q: Hi, I’m still working on my antweight spinner and space doesn’t seem to be an issue anymore which is great! I am still struggling with the wiring though so could you create a wiring diagram utilising 2 red bbb ESCs, the BBB brushless motor and Brushless speed controller for a basic antweight spinner bot?
Thank you very much! [Oxford, England]
A: [Mark J.] This previous post in the Ask Aaron archive [link fixed] has a wiring diagram and notes for the Bristol Bot Builders Red ESCs with N-20 motors. Adding the wiring for the brushless weapon ESC and motor looks like this:
Note that the BBB Red ESCs are being phased out in favor of the BBB Edition Antweight Dual ESC. The wiring diagram for the Dual ESC is the standard combat robot wiring diagram as seen in FAQ #19.
Ready for Anything
Q: When I went to Norwalk Havoc (as a spectator) I noticed a sawbot called 'Whittle by Whittle'. I happened to notice that this robot - as well as 'SawBlaze' on the rare occasion they use the saw - use a certain type of circular serration saw blade that differs from the common very-serrated design. The blade seems to be much flatter and stable, however it also seems to not be able to cut as quickly. Are there any reasons why different bots use these different saw design?
As always, Go Run Amok! [Worcester, Massachusetts]
If your're planning to attach a saw blade to a combat robot you're going to face some unusual challenges:
Risk and Reward
Q: Do you ever think Motorama will return once Covid settles down? NHRL is cool and all but they don't run anything below 3lbers? A: [Mark J.] Your wish is granted. According to the Motorama Robot Conflict webpage, registration for the February 19-20, 2022 event will open November 14th, 2021. Fingers crossed -- check there for updates. Q: Additionally, Fish or Carne Asada in tacos?
A: How high is your risk tollerance?
A: [Mark J.] See this previous post in the Ask Aaron Archives about spinner impactor shape for guidance.
A: [Mark J.] When you buy the 22mm gearmotors from ServoCity or BotKits the gearboxes are 95% of the expense; the brushed motors only add about $1.50. You'll need the pinion gears from the stock motors for the conversion, so just buy the whole package so you can back-convert if you don't like those fancy Nerf motors.
A: [Mark J.] Note for unseasoned American builders: antweights in the United Kingdom are restricted in both weight (150 grams) and size (must fit in a 4" cube). Some general tips:
Q: Hi, about my recent question involving a quest for space, you mentioned batteries - I am currently running a turnigy nanotech 2S 300mah battery and I know it is extremely overkill. Where could I find a (much) smaller battery and what mAh rating would be nescessary? I think the BBB site recommends a 180mah battery but browsed the entirety of google and could not find any of those at all. Thanks!
A: The battery capacity requirement depends on details of your drive train and weapon. The BBB battery recommendation of 180mAh is for their drivetrain kit without weapon: adding a spinner weapon may put you back close to 300mAh. Run some simulated matches with your 300mAh battery - strapped to the outside of the 'bot if necessary - to see how long it provides power and downsize as appropriate. If your charger tracks amp-hours needed to recharge your battery after a standard length match you can use that as a guide.
Small capacity LiPo batteries have become scarce. A search for "E-Flite 180mAh 2S" turns up a good number of responses. They are not cheap, but they are half the size of your 300mAh Turnigy.
A: [Mark J.] It would help to know exactly what you're doing but in general, yes. You can drive two unsynched ESC/motors from a single receiver output port signal. Do not try to drive a single brushless motor with two ESCs.
A: [Mark J.] There are very good reasons why builders use UHMW-Polyethylene for those Saifu-style wheel guards: it's light, slick, bends easily, flexes to absorb a lot of energy, and stretches rather than breaks. Acetal Copolymer (POM-C) has very different material properties:
Particularly with lighter weight classes, a good weapon hit can send a 'bot spinning so violently that a 'heading hold' gyro can lose its directional lock and behave erratically. Many builders have switched to 'rate' gyros that do not try to keep a heading lock because they are less susceptible to this type of confusion. You lose some of the 'on rails' feel a heading hold gyro provides, but your 'bot won't go crazy if it takes a good hit.
What you're looking for is a single servo gyro with a remote gain control. These used to be fairly common, but with R/C aircraft moving toward integrated 'flight controllers' they are now hard to find.
A: [Mark J.] Repair time can be a limiting factor at a tournament. Highest priority for spares are components that:
If I were going to BattleBots I'd have two complete spare robots ready to pull out of their crates and toss into the arena, plus enough parts to build one more 'cause that how TV works these days. Your spares list should probably fall in-between these two extremes.
Thanks for your time, man. [Philadelphia, Pennsylvania]
A: [Mark J.] Yes, spinners are MUCH more effective now than they were fifteen years ago when the armor guide was written, but I think you should re-read it. The guide states that mild steel and 6061-T6 aluminum are cheap and easy to obtain, but that other alloys are stronger. There's nothing about 'solid choices' or holding up to spinners. It also says that you should do your homework and:
If I go back to the start of Ask Aaron and update one old question every day it will take me nineteen years to get back to this question -- I'll never catch up! The archives all have date markers every year or so to help readers judge the age of the question they're reading, but I can see the merit in posting a specific warning notice in the armor guide. I've added a note.
A: [Mark J.] I'm happy to hear that you enjoyed my judging guidelines quiz. The RFL guidelines attempted to be very specific but caused some unexpected outcomes if they were closely followed. The purpose of the quiz was to point out those odd decisions in hopes that the guidelines could be adjusted to eliminate nonsensical results.
Jump forward a decade. The judging guidance for the BattleBots reboot is less specific and has corrected many of the problems I had with the RFL guidelines, but a number of controversial judging results have appeared each season. The guidelines are constantly updated in response to builder input, but there is still a lot of heated discussion on fan sites that official BattleBots explanations have done little to resolve.
I appreciate your suggestion, but I don't believe that throwing myself into that particular briar patch would help the situation. I'll pass.
A: [Mark J.] I have known builders who took their heavyweight robots up onto the roofs of their homes and threw them off onto concrete driveways -- repeatedly and at all angles. While this may or may not be effective at uncovering weakness, it is not a common practice as most builders like to show up at an event with a pretty robot. Anything with enough impact energy to properly test is gonna leave a mark, and by the time you hit your 'bot from all possible angles you're not gonna have much of a robot left.
Get your robot out and fight. Creating a great robot is an iterative process: build, fight, evaluate, improve, repeat. Take a look at Jamison Go's blog post on the evolution of his Hall of Fame beetle 'Silent Spring' for some of his tips on testing and improvement.
I'll leave you with a quote from Team Juggerbot:
A: [Mark J.] Yes, kinda...
"Cut and Fold" paper models of combat robots were a thing at the time. I know of three printable paper models from the Robotica series:
A: [Mark J.] I get a fair number of questions about fixing R/C mixes. In the past I referred such questions to a multi-step process to diagnose and correct the mix. It worked, but it took a while and the diagnosis sometimes had you changing things and then changing them back. Inefficient.
I recently finished an online javascript 'expert system' to handle this type of question. The Run Amok Mixer Fixer asks just three questions about your mix and then tells you exactly how to fix it -- no trail and error! Give it a try.
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.
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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Q2: How would I go about fitting the bearings into the weapon? I don’t have access to any fancy or expensive tools and figured it wouldn’t be very easy to just press-fit two metal parts together.
A: [Mark J.] Many designs qualify as a 'clamping' collar, and some are better than others for both torque transmission and axial shock. Holding 'well enough' depends on the specific application, information you have elected to withhold. I can comment that clamping collars are generally preferable to set screw collars.
Q: About my recent question concerning chargers, I do have a fingertech, proper lipo charger, which came with the fingertech viper kit, but I don’t know how to use it, and also my £3 BBB charged batteries much faster than 90 minutes when it worked. If my fingertech charger isn’t suitable either, could you link me with a decent, working one with correct accessories and a video on how to use it?
Many thanks again!
A: [Mark J.] It would help a great deal to know a bit about your robot and more details as to how it behaves when hit. You're asking the equivalent of, "Why won't my car start on a cold morning?" 
You'll need to add a switch in the battery line, as covered in the 
A: [Mark J.] If you know specifically what type of material you'll be cutting with a circular saw you can match the saw blade design to that material and get an optimum cutting rate. The selected saw blade might have twenty teeth or a hundred and fifty teeth, and those teeth might be steel, carbide, or something more exotic. For some materials you might choose a blade made entirely from an abrasive ceramic.
Q: Do you know if I can buy 22mm gearboxes (like the one's used in D2 kits) without the motors? I want to mess around with some high power 130 size motors for my next build and I'm just going to toss the stock ones anyway. [Roseville, California]
Q: Hi, Still working on this second antweight and after will definitely move up to a beetle, but still after 4 design revamps am struggling to fit all of my electronics inside my robot - I have looked over multiple designs on the internet but am still having trouble trying to fit everything into a 4’ cube - bearing in mind this is a spinner. I was planning to make something not too dissimilar from the pictured robot but was wondering how all of these electronics were fitted in. Probably not a very answerable question but any tips you can give me would be much appreciated.
Thanks! [Oxford, England]
Hey, I just realized the sheet of UHMW I thought I bought is actually a sheet of acetal copolymer. I’m not too familiar with plastic properties, and I’m looking to make saifu kit-style rounded wheelguards. Is acetal copolymer a bad choice for something like that? [Philadelphia, Pennsylvania]
A: [Mark J.] It's been a few years since I wrote the 
Q: Hey Mark, just stumbled across your "
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
