5845 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.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
In Memorium: 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   
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

New! Happy New Year! I spent some of the last few hours of 2016 adding a new feature to 'Ask Aaron': you can now directly attach a photo, drawing, or other file to your question. Type in your question and choose your file. Click 'Send' and you're on your way.

Q: I'm building either a fairy weight or an ant weight bot similar to 'bad attitude'. What would be the best way to defend against most spinners? The bot will be metal (aluminum most likely a few mm thick) and I am thinking about using would blocks on the side to have the spinner (probably horizontal) damage itself. [Smithtown, New York]

A: [Mark J.] Spinner defense is best accomplished with perfectly smooth, hard, gently curved surfaces on the outside of the robot. No sharp edges to give spinners a place to 'bite', no soft materials for sharp spinner teeth to dig into -- so absolutely no wood blocks!

You'll want to make your wedges out of harder material than aluminum. A hard, curved scoop is the preferred shape for spinner defense. Keep it between you and your opponent, and watch that they don't get to your wheels. Practice your driving until you can do this without even thinking about it.

Q: After some thought I think I want to make an undercutter type spinner either 12 or 30 pounds. I came up with the attached sketch, nothing is to scale, which is my question.

First does this make sense? I was thinking that making the part as close to the blade as strong as possible was the way to go, and tapered bearings are stronger than regular ones as long as the bolt is tightened right.

Second, how do you figure out the proper size of everything? I have no idea how big of a shaft I should be using, how big of bearings to use, how close I am to the blade cracking because I put too much stuff through it, ect. [Baton Rouge, Louisiana]

Questionable Undercutter design...

A: [Mark J.] Multiple suggestions:

  • Scroll down the page about four posts to find the undercutter layout question with an attached diagram. The two weapon bearings must be well separated to eliminate destructive 'moment loading'.
  • Tapered bearings are not commonly used for spinner weapons. The primary bearing loading when the weapon 'hits' is radial -- the lesser axial loading is accomodated by the freedom of the robot to move along that axis in response to the impact force. Standard ball or roller bearings will do nicely.
  • Find some 'build logs' for robots of your proposed style and weight. Like it says in the FAQ: "Look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger."
  • Grade 8 bolts are very strong in 'tension' -- stress applied along the length of the bolt -- but bolts in general are not designed to withstand lateral 'shear' forces of the type your design places on the weapon shaft. An undercutter in the sub-light weight classes should use a 'tougher' steel shaft that can absorb shock loading without breaking.
  • The hub and pulley must be attached to the shaft in a manner that prevents rotation. Set screws suck. Ideally the pulley, hub, and shaft are all broached for a keyway. Keyed shafts @ Robot Marketplace. Examples of keyways and other attachment options are available in the Ask Aaron archives.
  • The fewer holes you put on your weapon blade the better, and minimal localized stress on any hole that does exist is critical. The preferred blade hub design is a 'clamping' style that holds the blade in place without adding bolt holes thru the blade itself. There are examples of clamping hubs in the Ask Aaron archives.
Don't try to re-invent the wheel. Find examples of successful weapon mounting designs, study them, and base your design on what you have learned.
Q: I have desgined (not yet built) a beetle weight robot with a drum that lights up (with LEDs for entertainment purposes) but what if I spin my drum like so fast that it wont cause damage? I'm sure I'm going to get 'The Burger Is Bad' or 'Check the Archives'. [TCRM, Malaysia]

A: [Mark J.] The hamburger is good and you don't need to check the archives. Read thru the Ask Aaron Spinner FAQ. The 'Rotational Speed' section of that FAQ discusses the factors involved in getting the weapon to 'bite' into your opponent rather than just 'skitter' across them without doing any damage.

Read the whole Spinner FAQ while you're there -- it'll save you a lot of time and trouble.

Q: Hi, Mark. Could we turn a Viper Kit spinner into an overhead spinner? We have the V2 because V1s are discontinued as you know. Well anyway, my viper spinner keeps pulling a 'Don't Ask' (moving itself impossible directions like sideways ad by sideways I mean my bot is pointing forward and it is going sideways). Please do not make this go to 'Hamburger is Bad'. Ok, well here is the main question: (drumroll) will it work? [Herndon, Virginia]

A: [Mark J.] Well, you could just turn it upside-down and...

Q: P.S by turning it into an overhead spinner I do not mean flipping it upside down. I mean making it an overhead spinner.

A: You mean like...

Q: P.S.S *cough* Hazard, Brutality, and Justice *cough*

A: So a longer bar-blade centered on the top of the 'bot high enough to clear the wheels? I see two major problems with that:

  • You don't have enough weight allowance on the Viper for an effective spinner bar that long; and
  • Using the direct drive method of the Viper for a blade with that great a moment of inertia would both strain the motor and give a very slow spin-up.
I'd suggest finding out why your undercutter is behaving so strangely -- I've seen many Viper spinners and never one that acts as you describe. Your weapon motor may not be aligned correctly or the weapon blade may be imbalanced. Work on getting your current weapon working correctly before moving to something new.

Q: In reply to the Viper kit owner, I've had similar but less pronounced issues with 'Original Robot Do Not Steel', where the robot tends to want to drive in a slightly diagonal line when spinning up. It's probably something to do with the large contact surface between the ground and the blade hub. It's probably something to do with the large contact surface between the ground and the blade hub.

The bottom hole of the blade hub is threaded, so you might want to put a machine screw in there. [Black Mountain, North Carolina]

A: FingerTech's blade hub webpage recommends installing a screw in that threaded hole:

"The hub has a 6-32 threaded hole so that your horizontal spinner robot can rest on a screw head and not be pulled in different directions when the blade spins."

I assumed this was standard practice. I'd recommend a round-head nylon screw if you have one.

All horizontal spinners will face some newtonian 'action/reaction' deflection force as they spin-up, but once up to speed there should be little interference with mobility unless there is poor weapon alignment or poor balance; the blade should be perfectly parallel to the arena floor.

FingerTech blade hub with screw head

Q: I am the viper kit guy and I forgot to tell you that we are still working on an arena. And in the meantime we use a cardboard blade to practice. Plus we don't plan to fight the bot until spring fling 2017 so while we practice we use tape instead of the screw.

A: It seems the hamburger was bad.

  • You failed to mention your cardboard blade that will deform at speed and create odd aerodynamic effects.
  • You didn't think it was important to tell me about the the piece of tape that moves the blade hub contact point away from the center of the spin axis.
No wonder the 'bot behaves oddly! Get the 'bot in full fighting trim before you complain of poor performance.
Q: I am using two E30-150 ampflow motor for my mobility and Kavitsu KA-02 planetary gearbox for reduction. The problem is that the reduction provided by the gearbox is too much and so my mobility speed is a bit slow (Gearbox is attached to motor by gear meshing which further reduces rpm by half). The gearbox is also quite heavy which is causing problem in weight management. So i was thinking of making a custom gearbox for my E30-150 with decent speed and torque. Can you suggest me what type and what material etc. should I use in making my gearbox?

Another question is can you tell at what rpm I should rotate my drum and what diameter and material I should go for my drum? Can you suggest some motor alternative to A28-400 ampflow motor for rotating the drum as it's too costly.

P.S My current drum is about 13 kg with 18cm diameter and 4800 rpm rotated by E30-400 ampflow motor. I am going for 120 lbs category [Tamil Nadu, India]

A: Although I very much wish to support the technical aspects of robot construction in the energetic and expanding Indian subcontinent, I am also greatly worried that I may be contributing to an extremely dangerous situation for both builders and spectators. This has brought me to a painful decision:

The 'Ask Aaron' website is closed to questions from builders competing in India

Aaron certainly wouldn't approve of continuing to support reckless endangerment of life and limb from the operation of high-energy weaponry without adequate arena containment. Until there is a significant and universal change in Indian arena safety and/or rules to control dangerous weapons, I cannot in good conscience accept new questions from the region.

I will point you to the Ask Aaron Spinner Weapon FAQ for guidance in the design of drum weapons and weapon motor selection. I will not, however, make specific recommendations.

I can also warn you that the construction of a reliable custom gearbox is well beyond the ability of the average robot builder. You may find some useful information on multi-stage chain and sprocket drivetrains in the Ask Aaron Design and Contruction archive.

Q: Hi Mark,
I am the Indian guy who asked the question. I think you are still under the wrong impression that arena used in our competitions are unsafe. Now most of the places in India use polycarbonate sheet walls for the arena. The good teams don't participate in the competitions whose arena are not safe for fighting due to obvious safety reasons. The sheets in the image are 10mm thick.

A: I follow Indian robot combat closely. I'm one of the moderators for the 'Combat Robotics India' Facebook group and I review the arena construction details of all the events that post there. I also scour YouTube for video from recent Indian combat events.

I can agree that Indian robot combat arenas have improved in the last few years, but there are still MANY events run with completely inadequate containment. The current Indian standard for a 'safe' arena for 120 pound robots would not be viewed as adequate for 12 pound robots in the US or Europe. Your current drum weapon - as described in your post - stores more than 10,000 joules of energy. On a 120 pound robot that's more than enough to breach 10mm polycarbonate walls, let alone the more common Indian arena containment.

Modern Indian robot combat arena
VIT Chennai arena

I receive many requests from Indian builders who are interested in building more and more powerful weaponry for their robots even though the arenas barely contain the current weaponry. I have no way of knowing if the question comes from a 'good' team that will compete in a 'safe' arena, or what their workshop and testing safety practices might be. I don't want to be involved in a death or maiming resulting from design assistance I might provide.

I believe I understand Indian combat safety quite well. If you want information on making events safer I'll be glad to assist. If you want information on more powerful robots for your current arenas the answer is 'no'.

Q: do you have a drawing on how an undercutter blade is fixed to a bot? i know u did for a fbs and a horizontal spinner. [Quebec, Canada]

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.

Undercutter blade mounting

Q: Hi, its me again. I wanted to know if using 22:1 fingertech silverspark motor at 7.4v with 1 5/8 wheels is ok for an antweight? i put all the spec in the geartrain calculator and the number didnt means anything to me.. could you help me with that? thanks a lot your work is appreciated. [Quebec, Canada]

A: [Mark J.] The Tentacle Drivetrain Calculator you referenced is a valuable tool in the selection of drivetrain components and robot design. Your understanding of the calulator's output might be improved by reading the Example Drivetrain Analysis using the Tentacle Torque Calculator that I published a few months ago.

For the gearmotor, voltage, and wheel diameter you propose: the calculator shows a too-slow top speed around 2.5 MPH with a maximum current draw near 33% of stall. Acceleration is OK with top speed is reached in 2.5 feet from a standing start. A wedge in a small arena needs better performance than that.

Most builders run the 22:1 Silver Spark gearmotors with 3-cell LiPoly batteries (11.1 volts) and wheels around your size range. At that voltage the performance of your drivetrain improves greatly: 3.8 MPH top speed, maximum current draw at 22% of stall, and top speed still achieved in just 2.5 feet. Those a good numbers for an antweight wedge in a small arena. Bump up to a 3-cell battery.

Q: hi, my antweight "wedge" is design so i can swap the attachment i put on it, fot now, i have these 2 fork as the main one, but i want to have a solid wedge to fight against spinner. But, cant it be just a flat piece of steel at an angle of about 35 degree, or do i need to add the little wedgelet thingie on the side like algos and many other bots have? is it really necessary? i joined a pic of the design as it is for now [Quebec, Canada]

A: [Mark J.] Top-ranked combat robots like 'Algos' do not have design features present just to look cool and waste weight allowance. In this case, the turned-down wedge end caps serve two important purposes:

  1. Spinning weapons get excellent 'bite' on any exposed sharp edges they can find. Leaving sharp edges on the ends of your wedge where spinners can easily get to them is suicide. Builders call exposed edges like those "spinner bait". The gently curved-down ends on the 'Algos' wedge move the edges back and to the side where a spinner weapon will have a much more difficult time taking advantage of them.
  2. Extending the wedge around the side of the robot helps deflect spinners and other weapons away from your vulnerable exposed wheels. A wedge that ends right at the side of the robot gives a clear path for an opponent to 'feel its way' right down the side into the wheel. Save your wheels -- offer them a little protection!
An 'Algos' style wedge is very effective. I'd suggest imitating it closely.
Quebec's antweight
Quebec's spinner bait design


Q: i recently bought my first transmitter, a spektrum dx6i, and i found out the antenna on the receiver is really long.. what should i do with it to make it more compact? [Quebec, Canada]

A: [Mark J.] Several different receivers can be used with the DX6i transmitter and you didn't mention which receiver model you have. Perhaps it's the Spectrum AR610 with two antennas: one short and one a few inches long?

Do not cut the antenna wire. Try coiling the part of the longer antenna between the receiver and the shrink-wrapped 'amplifier' around a short piece of plastic soda straw. A piece of tape will hold the coil in place.

You may want to read our guide on Radio Reception Problems in Combat Robots for more antenna tips.

[Mark J.] I generally don't accept 'guest commentary' from builders, but I received a note from Rob at ARC Robotics UK offering support in response to the recent question from 'Sheffield' three posts down the page. Since I'm 4900 miles away, I thought it might be useful to port some advice from a more local source:

Sheffield - firstly, I know your pain. My friends and I are building our first heavyweight, and it's running and driving. It can be very hard convincing anyone (especially if they are the money holder) to become involved in any project, certainly one with potential for severe injury. My advice would be the following:
  • Do more than just drawings. Cost out the entire project, and know what stages you want to build your bot in. (I am assuming it's for Robot Wars, and I'm also assuming you're of school age here). Make sure you have a cost on every item, and a place to get it from. Google is your friend here.

  • Make sure you actually have a design that you've fully thought out. If you're in education, you can get a free version of AutoDesk Inventor, but if your design is nothing more than generalised drawings on a piece of paper, you'll be woefully unprepared for just how complex a build can be. You can save yourself a lot of work by downloading some CAD models from

  • Listen to Mark when he says that gusset plates are an acceptable way of bolting together a robot. I've done several builds of cars that were held together with gusset plates, and they worked exceptionally well. If you do go down the gusset plate route, remember that tight tolerances are your friend, and that standardised components will be readily available from places such as Screwfix or B&Q. If you're using 30mm Box section, I would recommend M12 bolts. If you go down the Nyloc route (which I 100% say you should), remember that Nylocs have to have 2.5 threads past the end of the nut to work to their specified standard, so don't underspec on your bolts in order to save money.

  • Alternatively, waterjet cutting services are readily available, and fairly cheap. A small bot's worth of plates (& spares) can be got for 500 or less.
Remember that in the UK, the Fighting Robot Association is there to help. You'll get a lot of comments saying that you shouldn't build a heavyweight for your first robot (and they're probably right), but don't let that put you off. Approach the community with well thought out ideas and, even though they're all a bit sarcastic, it's a great resource and community. My username on there is 'emisnug' should you wish to PM me.

In terms of approaching your mother, think through it. How much is it going to cost? What tools will you need? Do you have any help/teammates? What could you do to negate some of the cost? Are there local resources that you could tap into (for example, a school's DT lab)?

-Rob [North Walsham, England]

Date marker: January 2017
Q: Hello, Mark. I have recently received a drone and I was wondering if it might be suitable for combat in the Battlebox. I know of the nessesary requirements in joining another team and whatnot but, I mostly just need to know how to attach a proper weapon to the drone. Thanks-Luke from Alabama

P.S I love the new file attachment system, it makes asking questions so much simpler.

A: [Mark J.] Nice little drone, Luke -- but I don't believe it's suitable for use in robot combat. The only current event allowing weaponized drones is the re-booted BattleBots, and that presents three problems for you:

  1. The support drones used at BattleBots are much larger, heavier, and complex than the hobby drone you have received;
  2. An effective drone weapon can take hundreds of hours and thousands of dollars to develop -- see "Buzz" Fire Drone for Team Caustic Creations; and
  3. BattleBots has apparently not been renewed for a third season on ABC.
See also my earlier comments about drones in the Ask Aaron Events archive. Have fun with your drone, and don't let a robot swat you out of the air with a garden rake.
Luke's drone
Luke's drone

Q: Hi Mark,

I'm a big fan of direct drive spinning weapons in insect weight classes. The concept, to me, seems absolutely genius...if done right. In my experience I've had no reliability issues with direct drive drums and the like and prefer that method to belt driven weapons. But at what weight class does the direct drive concept start to fall apart?

Thanks, David R. [Livermore, CA]

A: [Mark J.] I don't share your enthusiasm for direct-drive weapons, David. As much as I admire simplicity in design, direct driven weapons are not as successful as belt-drive weapons anywhere above the 150 gram fairyweight class.

Of the six top-ranked antweights at SPARC/BotRank on 12/20/16 there are:
  • two belt-drive disk spinners;
  • two belt-drive bar spinners;
  • one wedge; and
  • one direct-drive mini-drum spinner ('Algos' - see text below).
Of the six top-ranked beetleweights at SPARC/BotRank on 12/20/16 there are:
  • two belt-drive mini-beaters;
  • one belt-drive disk;
  • two wedges; and
  • one direct-drive disk ('Margin of Safety' - custom hubmotor).

Direct driving a spinner weapon has two drawbacks:

  1. Direct drive places large impact loads on the motor bearings, which are poorly designed for that purpose; and
  2. The motor must be oversized to provide enough torque for a decent spin-up time with high Moment of Inertia (MOI) weapons.
Note that Mike Jeffries made extensive modifications to the weapon motor and bearings used in top-ranked antweight 'Algos' to harden it for direct-drive use. You can read the details of the modification in his guest-post in the Ask Aaron Weapons archive.

In summary, weapons with high MOI are poorly suited to direct-drive. Direct-drive 150 gram fairies work well, but at antweight size your direct-drive weapon is best restricted to a low MOI design such as a small drum or mini-beater.

Q: Well, in that case...

One of the big turnoffs with belt-driven weapons for me is "The Belt Jump" after a particularly violent hit, and the belt disconnects from the weapon drive system. Do you have any tips on how to help prevent this?

David R. [Livermore, CA]

A: Two tips:

  1. Belts jump off pulleys when the chassis and mountings 'flex' under load from an impact and shift the pulley alignment. The chassis between the weapon and motor needs to be structurally stiff, and the motor mount must be placed as close to the motor pulley as possible to avoid flexing. No overhanging 'firewall mount' for outrunner-style brushless motors!
  2. A larger diameter 'pulley flange' can replace or be added to the existing flange on one or both sides for greater security.
I think it goes without saying that great care should be taken with pulley alignment during construction. Make certain that the motor and weapon shafts are perfectly parallel and coplanar, and that the pulleys have no axial offset.
Outrunner weapon motor mounting

Q: Hello Mark, the guy from Sheffield, England again.

So I have my ideas pretty much all drawn out. But it's just that, just ideas. The problem I have is convincing my parental figure (my mother, my father (who funnily enough introduced me to Robot Wars) passed away 13 years and 2 days ago on writing this message) into doing this, as she seems a little pessimistic with it all.

She has two concerns that kind of stand out, understandably;

  • One is the cost of it all. I know about the solution of that, which can be eBay, and a few other local places. Not only that, but research goes a long way as well, so that part isn't really a problem so to speak, but it is quite annoying to try and convince her for me to get decent parts such as the electrical side of things (ESC, Batteries, R/C equipment, chargers, but especially the ESC.). Now I know I had asked about the ESC before, but what would be a decent ESC for a heavyweight robot? Not sure if that'd fall under the Q&A section because... well, I can be a bit of a dingbat haha.

  • Two is how I'd put everything together, especially the metalwork. I plan on using 30 mil box steel tubing for a chassis for the robot (With, if need be, weight reduction). Only problem is the only decent way I see of putting the chassis together is MIG welding (as I feel screws and bolts would be a little bit weak, especially when it comes to taking impacts), which is a HUGE problem right now, as my mother, understandably, is a bit concerned about the safety side of things. I already know about the precautions you need to weld, but it's convincing her which is the main problem. How does one convince a parental figure on doing MIG welding? Especially as it's a safety concern.

One more question: Would a decent scooter motor work? Say, a 750 watt motor ran at 24/36 volts? Or would that lead me to "The Burger is Bad"?

If you've answered these questions before, then I deeply apologise. But I thank you for the time you take for reading this question in the first place.

Oh, and Merry Christmas and a Happy New Year when it comes around! [Sheffield, England]

A: [Mark J.] Hello, Sheffield. Thank you for your kind holiday wishes.

Your component selection questions are all very reasonable and address problems encountered by all combat robot builders. They are also difficult to answer, particularly with the limited information you've provided about your design.

Take a look at Frequently Asked Questions #21 for advice on drive motor and ESC selection. A great many heavyweight robots have been powered by the motors from salvaged electric wheelchairs ('invalid chair' is the Britishism, I think). These are easy to mount, have an intigrated gearbox, and often come with suitable wheels.

Many successful combat robots have bolted steel tube chassis. If the chassis members follow good structural design guidelines and use gusset plates to reinforce the joints, the chassis strength can equal that of a welded chassis. Bolted construction also makes on-site repairs a good deal simpler.

Also, arc welding is something of an art -- there are lots of ways to do it wrong and only one way to get it right. It takes many hours of practice and guidance to become good enough at it to reliably make welds that will hold up to combat stresses. I would much prefer to have a novice bolted joint than a novice welded joint holding my robot together.

Gusseted square tube joint

Q: Hi Mark, are the VEX Robotics pneumatics kits suitable for beetle robots? If so is there any documentation on their performance in terms of speed of actuation? Also if suitable, is there any suggestions you'd make for mounting? And the most loaded question: single-acting or double-acting - which one is better?

Thanks. [Monroeville, Pennsylvania]

A: [Mark J.] Suitable? Depends on what you want to do with it. The VEX pneumatics systems are powerful enough for a lifter, but as supplied they are not quick enough to be effective flippers. The system is also quite bulky to pack into a beetle.

  • The maximum actuator force at the rated pressure is about 12 pounds. That's comparable to the discontinued Inertia Labs antweight pneumatics system, but larger and much more expensive.
  • Weight of the system is about a pound, with most of that weight in the pressure tank.
  • A Google video search for 'Vex Robotics pnueumatics' finds many videos showing these components explained and used in various robotic applications. You can judge the size of the components and the system speed.
  • A actuator speed decreases as the load increases, and decreases as the tank pressure drops.
  • The actuators come with clevis mounting brackets that allow the actuator to pivot.
  • The single-acting actuators are equipped with a spring that returns the piston to the 'rest' position when the gas pressure is removed. That spring force must be overcome by the piston when actuated, so the net available force from the actuator is reduced. If you want the full 12 pounds of force you'll need to remove the spring or switch to the double-acting system.
  • The valves supplied with the double-acting actuators keep one side of the actuator pressurized at all times. The pressurized side only de-pressurizes when the valve starts to pressurize the other side. This greatly slows the actuator. For greater speed you could use two of the single-actuator valves and two radio channels -- allowing you to independently pressurize and de-pressurize either end of the actuator.
Given the price, bulk, and weight of the system and its limited power output -- I think I'd pass on trying to use this system in a beetle.
Q: Hello, I am trying to build a Featherweight robot in my garage, but our budget is semi-tight, and we live on Cape Cod, I want to know where I can get some parts for cheap, like a surplus house or something.

Thanks and Sincerely- Mike K. [Hyannis, Massachusetts]

A: [Mark J.] See Frequently Asked Questions #16 -- and read the rest of the FAQ while you're there.

Read the FAQ!

Q: Why does RioBotz choose to have 13 radii and 18 sections [in the design of their integrated single-tooth 'snail drum' weapon]? [Dublin, Ohio]

A: [Mark J.] The selection of 18 initial sections was arbitrary -- it came from dividing the 360 degree polar coordinate plot into twenty-degree sections. That seemed to be a reasonable number of facets to machine into the drum in the final design. Two of the sections (40 degrees) were combined for the impact tooth at full radius and one section is the curved 'tooth notch' designed to reduce stress concentration that has no single radius. Then:

"After observing the nearly flat shape of the optimal solution in the regions between 220o and 320o... the algorithm [was] re-evaluated considering only 2 flat sides in such region. This new optimal solution is very similar to the previous one, but it is easier to machine due to the reduced number of facets." -- Drum Shape Design and Optimization Using Genetic Algorithms

The creation of just two facets ('k' and 'l') in the region from 220o and 320o reduced the number of radii to thirteen.

Q: Hi. I'm a complete novice in building combat robots and the one I am building is just for a family tournament between me and my 2 brothers (dont worry - we have secure arena sorted for us). I do have engineering experience but not in this field. I have a couple of questions:
  1. How do you stop damage to a motor powering a bar spinner when it impacts another robot? I was going to use a belt/chain that has the ability to slip when it hits a robot and only using the momentum store in the bar.
  2. What motors would you suggest for driving a light weight 27.5kg robot (2 wheels)? Looking to move the robot at reasonable speeds but still have enough power to push a similar robot.
Thank you for this site as many articles have helped me greatly. George S [Mablethorpe, England]

A: [Mark J.] Family robot fight -- awesome!

A slipping belt drive is the standard weapon drive in larger robots like yours. A standard 'V-belt' set a little loose works nicely. Include some tension adjustment in your design to fine-tune the amount of slippage and you'll be fine.

A pair of 'hacked' generic 18 volt cordless drill motors are adequate to give your 'bot enough speed and push for a family fight. There are many guides on hacking drill motors on-line. Keep your wheel size small -- no more than 10 cm diameter -- to keep your motor loading reasonable.

A two-wheel drive 'bot won't have a huge pushing potential, but keep your weapon spinning and you'll earn some respect.

Q: Hi Aaron, could you tell me, for a spinning drum that stores 10,000J [30 Lbs class] what is the difference between a 10mm bite and a 20mm bite? Thanks. [Valle del Cauca, Colombia]

A: [Mark J.] Bite (what's bite?) is calculated as a maximum depth of opponent insertion into the arc of a spinning weapon at a given weapon RPM and forward velocity. You'll get that maximum bite rarely, just like 13 black only comes around rarely on a roulette wheel. Sometimes your luck will be very poor and you'll hit your opponent just as an impactor is facing them and get no bite at all! On average, you'll get half the max bite -- less as your attack speed drops.

  • Consider an opponent who was wise enough to avoid exposed sharp edges in their robot design. Given a nice sharp angle to attack you don't need a lot of bite, but if you're forced to attack a flat or gently curved surface you need all the bite you can get.
  • Greater bite also is a bonus when your attack velocity drops. A weapon with a lot of bite can still be effective in close quarters when you don't have a chance to back away and take a high-speed run. Watch some combat videos and pay attention to the speed at which most weapon hits are made.
What's the diffference between 10mm and 20mm bite? A few more match wins. Bite is good -- more bite is better.
I've been writing quite a bit of JavaScript lately, so what's a little more? Here's a quick 'Bite Calculator' -- fill in the blue input cells with your values and click 'Calculate':

Rotary Weapon Bite Calculator
Weapon: rpm Insert Time: sec
Impactors: # Max Bite: mm
Attack: mph

I've added this calculator to the Spinner Weapon FAQ.

Q: When did the insect class combat robots first appear? Where was the first insect class tournament? [a question I asked myself, since nobody else had]

A: [Mark J.] The first antweights were something of an accident. At the time of the earliest combat robot events, R/C transmitter / receiver sets came with a full set of servos, servo mounting hardware, servo 'horns', and a receiver battery pack. The builders each had a drawer full of servos and no particular use for them.

Someone figured out how to modify servos for continuous rotation which turns them into tiny high-torque gearmotors with built-in ESCs! Attach a jar lid to a one of those servo horns, screw it onto the servo output shaft, plug two of them into a receiver along with one of those unused receiver batteries and tape the whole mess onto a stiff piece of cardboard -- instant mini robot!

The first 'Antweight World Series' tournament for 150 gram 'bots was held in September of 1999 in Reading, England (video). The event appears to have been held on a scrap piece of wallboard propped up in someone's living room. I'm not sure it should count.

The first organized U.S. insect tournament I can document was the SOZBOTS antweight event in Sun Valley, California on February 23, 2002 (video).

SOZBOTS welcome

Q: Rank these designs from a to f .
  1. A complete control type of bot with a vertical saw blade
  2. A clamp bot with a drill on it's clamp
  3. Number 1 with the saw replaced with a horizontal bar [Herndon, Virginia -- suspiciously close to Ashburn and Reston]

A: [Mark J.] If you're asking someone for a favor, the correct form is: Please rank these designs from a to f.

You didn't mention what weight class you're considering, but there are some serious problems with these designs:

  • An effective clamp and lift system like 'Complete Control' takes up a lot of weight, which leaves VERY little for a workable saw, drill, or spinner weapon;
  • You have very little control over what part of your opponent happens to be beneath your drill or saw, which makes the chances of something vulnerable being exposed to your weapon very slim;
  • Lifting a clamped opponent up into a spinning weapon offers very little advantage over simply running the weapon into them, plus there's now a very good chance that the impact of your spinner will rip your clamp apart.
If you're fond of this type of weaponry, think about a simpler design. Consider a 'dustpan' with a saw on a light moving arm like 'Gloomy'.
Q: i've read lots about overvolting motors, but haven't found anything in the archives about overvolting the ESC. if my ESC is rated at 50 volts and i'm using two 8S lipo batteries in series to produce a nominal voltage of 59 volts, will i instantly fry my ESC or will it just get very hot until it dies? [Utah]


  • Flowing too much CURRENT thru an ESC will cause heat to build up faster than it can dissipate and lead to component failure, either slow or fast depending on the degree of overload.
  • Putting too much VOLTAGE into an ESC will exceed the dielectric properties of the components. What's that smell? Instant failure.
Manufacturers do allow for a bit of extra voltage. A hobby ESC rated 12 volts will be fine with a 12 volt 'nominal' battery that shows a 13+ volt 'resting' voltage, but you don't want to push it farther than that. There are a few exceptions -- usually toy-level controllers that carry an artificially low voltage cap for safety (like the Vex Motor Controller 29. However, bumping up the nominal voltage nearly 20% on a seriously rated 50 volt ESC is a quick ticket to ozone city.
Q: Why doesn't witch doctor have gyroscopic forces acting on it, one side doesn't lift [Reston, Virginia]

A: [Mark J.] See this post on gyroscopic forces that just moved to the Ask Aaron Weapons Achive.

Q: Hey, Aaron! I was thinking about building a beetleweight drum spinner for a competition in Chicago. The design I want to build will have a 1-pound drum. There's only two problems:
  1. What type of design should I use that would support a heavy drum like that?
  2. How do I cut out a drum from a piece of steel?
If you could answer one (or both) of my questions, that would be excellent! [Kollier W. , Illinois]

A: [Mark J.] I can offer some help:

  1. Take a look at the photos of the Weta 1 and Weta 2 beetle drum spinners for some general guidance on drum support. They use strong, thick-section extensions of the main frame rails to support their drums.
  2. I can't teach you metal shop fabrication in a short answer. I can tell you that most builders do not start with a solid block of metal! Typically a builder will find a suitable length of thick-wall aluminum tube and will fabricate end caps and add impactors.
There are many posts on drum design and fabrication in the Ask Aaron Weapon Archive. Search there for 'drum'.
Q: what types of particles are represented by the crosses (x) [Ovenden, England]

A: [Mark J.] You might have included a link to the diagram. It took me a couple minutes to find it.

The section heading is "simple covalent bonding" and a covalent bond is defined as "a chemical bond that involves the sharing of electron pairs between atoms". It's not a great leap to infer that the crosses are electrons. I don't think you have a future in chemistry -- why don't you go build a robot?

Q: Hey Mark,

How does the flipper on Lock jaw in Battlebots Season 2 harness the power of the springs? From what I could tell it was winched back but how was it able to fire then reset again? Additionally, could this method of flipping be utilized in all other weight classes as an alternative to pneumatics? Thanks in advance! [Straight Outta Facebook]

A: [Mark J.] Donald H. doesn't divulge much about his robot designs, and I can't see enough detail in the photos of 'Lock-Jaw' to understand the clutch mechanism. Fortunately there are builders who do share their spring-powered flipper designs:

  • Dale Hetherington has built pretty much every exotic robot design there is. Take a look at his very detailed build log for 'Dead Air'.
  • I'm personally very fond of the snail-cam spring loader for it's simplicity and adaptability to small weight classes.
That will get you a start.
Q: Does the RPM of motor matters while building battlebot? [Kathmandu, Nepal]

A: [Mark J.] Build a lot of BattleBots in Nepal, do they? The power of the motor matters, and power is the product of torque and RPM:

Power [watts] = Torque [Nm] * RPM / 9.549

American builders aim for about 100 watts of power per pound of robot weight, although sucessful robots can be built with as little as 10 watts per pound. When you have enough power you can adjust gear reduction between the motor and the wheels to get the required speed and acceleration for your design.

Team Run Amok offers tools to help with robot design and motor selection: here is an example drive motor analysis using the Tentacle Drivetrain Calculator.

Kitbots beetle spinner kit Q: I have a saifu v2 kit and I want to make it like Klazo from near chaos Robotics with the wedgelets. The wedgelets need to be used to get under DDT. Ok here is the question. Will the wedgelets work? [Ashburn, Virginia]

A: [Mark J.] The titanium wedgelets or 'horns' used by 'Klazo' and recently adopted for the Kitbots 'Weta 2' beetleweight kit (photo at left) are not added to assist in 'getting under' a specific opponent type -- the UHMW wedgelets formed in the side rails of the 'Saifu 2' are entirely adequate for that. The problem is that the thin slicing blades on horizontal spinners like 'DDT' cut thru the UHMW plastic like butter. The titanium holds up much better to that type of attack.

If you're going to add a design feature, make sure you know what it's supposed to do.

Q: Hello,
Do you know a site where I can buy pinions? I am trying to mount a motor with a 5mm shaft to a 72.3 gearbox. (I'm the dude with a ton of grammar mistakes, I'm from Ohio.) (This will be my third robot.) Thanks! [IP address: Orlando, Florida]

A: [Mark J.] You'd think that something small and apparently simple would be easy to find in the 'net - wouldn't you? Unfortunately, pinion gears are a problem.

Advice: don't try to order an internet pinion to match up with that gearbox. All you know is the motor shaft size you need -- the PDF doesn't give the required gear pitch, tooth count, or length. The number of 'wrong' gears you might find, buy, and throw away is mind boggling.

I suggest that you, the gearbox, and the motor visit your local R/C hobby shop. They likely have a box of assorted pinions under the counter that you can go thru to find something that works or that can be modified to work.

It may be easier (and possibly less expensive) to search eBay for complete gearmotors with a 5mm motor input shaft. Pull the motor, switch the pinion to the motor you want to use. and bolt the new motor to the gearbox. This is common practice when adapting a specific motor to a combat robot application. It saves a lot of trouble.

Q: Hello Mark,

I've heard it said that Wrecks' vertical disk (30-35 lbs) has a much higher Moment of Inertia than Electric Boogaloo's vertical spinning weapon (~70 lbs). With the assumption that Wrecks is using a similar motor (big assumption), how is this possible?

-David R. [Livermore, CA]

A: [Mark J.] A little clarification:

The Moment of Inertia (MOI) is NOT a direct measure of how much energy a spinning weapon stores; the motors or speed of rotation have no bearing on the moment. MOI is a measure of the energy needed to change the rate at which the weapon is spinning. Its value depends on the mass of the weapon and (most importantly) on how that mass is distributed relative to the rotational axis.


  • A small chunk of matter is rotating around an axis at a given RPM at a distance of six inches. With each rotation the chunk of matter travels a distance of ( 2 * 6" * pi = ) approximately 37.7".
  • That same small chunk of matter is now rotating around an axis at the same RPM at a distance of 12 inches. With each rotation the chunk of matter travels a distance of ( 2 * 12" * pi = ) approximately 75.4".
  • The chunk of matter 12" from the axis must be travelling twice as fast as it did when only 6" from the axis in order to complete a revolution in the same length of time.
  • The kinetic energy of a moving object increases with the square of its velocity (K = mv2), the mass 12" away from the axis has four times as much kinetic energy as an equal mass 6" from the axis when rotating at the same RPM.
  • Doubling the distance of the mass from the rotational axis doubled the speed of the mass, which raised the energy stored by the spinner at any given speed by a factor of four (velocity2). This also increases the MOI of the spinner by a factor of four, even though the mass of the spinner has not increased.
Now, let's take a look at the spinner weapons on 'Electric Boogaloo" and "Wrecks":

'Electric Boogaloo' has a bar spinner weapon. A bar has a lot of its mass close to the rotation axis and relatively little mass far from the axis. The formula for the MOI of a rectangular bar spinner is:

MOI Rectangular Bar = (mass/12) * (length2 + width2)

'Wrecks' has a large diameter spinner with most of the mass concentrated in a ring at the outer edge -- far away from the axis of rotation. The formula for a the MOI of a thick ring (discounting the supporting spokes) is:

MOI Thick Ring = (mass/2) * (inner radius2 + outer radius2)

Let's use the new Run Amok JavaScript Spinner Weapon Calculator to compare the MOI of the two designs based on rough estimates of their sizes:

  • A steel bar similar to the spinner weapon used by 'Electric Boogaloo': 24 inches long by 4 inches wide by 2.5 inches thick weighs 68.6 pounds and has an MOI of 0.99 kgm2.
  • A steel ring similar to the spinner weapon used by 'Wrecks': 15 inch outer radius, 12 inch inner radius, and 0.5 inch thick weighs 36.2 pounds and has an MOI of 1.95 kgm2.
The ring weapon is a bit more than half the weight of the bar weapon, yet has nearly twice the Mass Moment of Inertia. That's a much more efficient use of weapon mass for energy storage.

Q: hello, so I'm building my first battle bot, i was going for ant weight but my college club is doing a beetle weight so i am looking to do that, my motors are brushless 4900kv, and they are cogging on the low end when i try to use them, also I'm having issues finding a proper wheel that gets enough traction due to my lack of torque, my question is i am trying to gear down the motors to help with the cogging, i have all the parts but i have no idea how to attach an axle to a battle bot. I just get confused on how to add a shaft with the ability to rotate. [South Brooksville, Florida]

A: [Mark J.] It sounds like you jumped into the deep end of the pool before you even watched someone else swim.

  • 'First combat robot' and 'brushless drive motors' are a very poor combination.
  • You've attempted to simply attach a wheel directly to the shaft of a 4900kv brushless motor? You're lucky it's cogging and didn't just melt.
  • You've told me nothing about your motors except the 'kv', but as a guess you're going to need to gear them down about 40:1 to get adequate torque and controllable speed. That's not something you can do with a simple pinion and spur gear. The most common approach is to attach a brushless motor to a multi-stage planetary gearbox -- but I'm getting ahead of myself.
Advice: use the brushless motors for another project. The 'kv' is too large for a combat robot drivetrain, and you shouldn't be using a brushless drivetrain in your first 'bot anyhow -- they're a real pain to get working well.

More advice: get out on the 'net and find some build logs for beetleweights to get an idea of what works and get some design ideas before you go any farther. Here's one to get you started: Naked Singularity beetleweight.

Still more advice: browse the shop at to get an idea of the type of components used in competitive beetleweight robots.

Q: i am the new builder who recently asked about brushless motors, im on a tight budget main supplies were bought by my school club, so i cant go and buy brushed. our club is building 4 bots for a competition, but in my team none of us have built, and teams cant help each other. any tips for gearing down my brushless motors, they are 4900 kv 19 turn [South Brooksville, Florida]

A: [Mark J.] So... tight budget, no experience, unspecified components, the other teams can't help, you're 3000 miles away, and you want to build a combat robot.

I cannot even guess at the power available from your motors from the information you have provided -- '4900 kv' motors come in all sizes and power outputs. The Hamburger is Bad. Useful information would include the brand / model of the motor and how many volts your battery provides.

As I said in the earlier post, if you want to use conventionally sized wheels for your beetleweight robot (say about 2" diameter) you're going to need a gear reduction of around 40:1. From a practical standpoint that will require a multi-stage reduction that would be complex, fragile, and (I'm guessing) beyond your capacity to produce. It's time to get creative!

Here are two high-reduction combat robot drivetrains that require no gears, pulleys, or sprockets:

  • Friction drive The shaft of the motor presses directly against the surface of the tire and relys on friction to transmit power to the wheel. You will get some slippage, but if your motor has a 2mm to 3mm shaft the reduction ratio works out about right for a fast spinning motor like yours.
  • Tiny wheels If direct-drive for a normal sized wheel isn't practical, how about a really tiny wheel? Mount a very small wheel to the motor shaft and mount the motor at an angle to put the wheel in contact with the floor. Unconventional, but a successful antweight robot used this method as a weight-saving measure.
In your situation, I'd go for the tiny wheels. The 'wheels' could be something as simple as a small plumbing washer, a short length of small diameter rubber tubing, or even a few layers of electrical tape wound around the shaft. They won't last long, but they're easy to replace.
Q: If you attach 2 motors to one spinner blade will the blade go 2 times as fast as one motor? P.S don't say anything about weight. [Ashburn, Virginia]

A: [Mark J.] If you tie two people together, are they twice as fast? Power is additive, speed is not:

  • two motors = same speed
  • two motors = twice the power
The weapon will be no faster but it will spin up to that same speed in half the time -- it will be twice as quick.

You can gear up to get greater speed, but you will get a proportional reduction in torque. Ain't no such thing as a free lunch.

Q: How do electric hammers not burn out [...just down the road from Ashburn, Virginia]

A: [Mark J.] If you aren't careful, they do burn out.

The motor for an electrically powered hammer weapon needs to be powered off at either end of the weapon swing to avoid an extended 'stall' condition where the motor would consume damaging current levels. This can be done a couple of ways:

  • Most builders control the hammer with one of the spring-centered joysicks on the transmitter and release the stick at either end of the weapon swing. Just don't forget to release the stick!
  • Some R/C relay boards and a few ESCs (like the Talon SRX) have provisions for 'limit switches' to sense when the controlled device has reached the end of its travel and automatically shut off power in that direction. This allows an electric hammer to be actuated by a simple single-throw switch on the transmitter -- typically channel 5.

Q: Show me a picture of killerhurtz and deadpan side by side [Ashburn, Virginia]

A: [Mark J.] You've confused 'Ask Aaron' with Siri. Show me a little respect.

Remembering Aaron... 

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.
Killer Robot drawing by Garrett Shikuma

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