5439 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 the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

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.
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   

 Recent Questions

Q: Hi! Can u suggest a good light weight race bot design.And what motor rpm will be suitable for this bot ? [Tamil Nadu, India]

A: Team Run Amok answers questions about combat robots. We have never entered 'roborace' events like those that are popular in India; no such competitions are held in the US.

  • We do not provide full robot designs. See Frequently Asked Questions #4 for info on the types of design questions we do answer.
  • Indian robot builders often ask about selecting a motor by its RPM, but you cannot select an optimum motor for your purpose based only on motor speed. You couldn't pick an automobile engine based only on the engine RPM, and the same applies to robot motors. Robot speed depends on motor speed and wheel diameter, and robot acceleration depends on motor torque. Suitable speed and torque for the robot depend on the specifics of the roborace course, the weight of the robot, and the driving perferences of the operator.
I suggest that you find successful competitors for race courses similar to the one you will be running and see what motors and designs they use as a starting point for your own design.
Q: what do i need to look at to chose the best motor to drive my lightweight robot? or do u have any suggestion ? Thank you :) [Quebec, Canada]

A: [Mark J.] See Frequently Asked Questions #21 for guidance on motors. Assuming that this is for the lightweight flipper 'bot in the question below, you might be interested in the DeWut?! Gearmotors utilizing the 18 volt DeWalt drillmotors overvolted to 24 volts.

Q: we can use drill motor in lightweight? I knew u can in featherweight but i thought they were not strong enough for heavier class (lightweight, middleweight, heavyweight)

A: [Mark J.] There are drill motors and there are drill motors, and the DeWalt 18 volt is no ordinary drill motor. Safely overvolted to 24 volts it pumps out a reliable 1.2 horsepower at a very useable output RPM. That's a whole lot better than the common inexpensive hacked cordless drills used in lighter 'bots.

The DeWut Gearmotor Kit places the DeWalt motor and gearbox in a strong and easy to mount cradle with a well-supported 1/2" steel output shaft. That makes a great package for a new builder of larger 'bots.

Want crazy power? Use four of these motors.

Q: HI i plan on doing a lightweight 4 bar flipper who will be shape like storm 2. The chassis will be made of aluminium (1cm thick) and i would know what dimension are usually the light weight class... thank you for your answer [Quebec, Canada]

A: [Mark J.] A couple tips:

  1. Don't start with a material thickness. The appropriate material thickness will depend on the chassis design, size, the loads that are placed on the chassis, and the shape and placement of the armor. Starting with an arbitrary thickness forces the rest of the design to conform to that decision.

  2. Don't start with an arbitrary size. Sketch a compact layout for your components (motors, gearboxes, batteries, speed controllers, wheels, weapon actuator, radio...) and add up their weight. Calculate the sizes of the chassis and armor pieces needed to wrap around the components. From that you can 'back into' the thickness of your chosen material that can be used and still get in under the weight limit. Leave an allowance for nuts, bolts, wire and such.

There are certainly other ways to design your robot, but this method is simple and easy for a beginning builder to understand and follow.

Q: Mark I am doing an argumentative research paper in my English class, and the teacher said we could do any topic we wanted. I was wondering if you had some good robot combat topics would be good to "argue" about. This seems childish but I think it will be very enjoyable. By the way, I love the work you have done for the robotics community.
-Sincerely, teen from [Huntsville] Alabama.

P.S not a "fanboy", just a person who enjoys discussing robots.

A: [Mark J.] You're gonna get me in trouble. The topics in robot combat that I believe are worth a good arguement are topics that the robot community doesn't like to discuss. Here are a few:

  1. Robot combat is growing increasingly popular in India, but the tournaments there are often held in terribly unsafe arenas where spectators can be (and have been) badly injured. Should the robot community support the growth of combat robotics in India and leave it to the locals to sort out the safety issues, or should we impose our ideas of proper safety upon their culture?

  2. The original BattleBots series on Comedy Central was largely responsible for the explosive popularity of combat robotics in the US largely because it was an open tournament that anyone was welcome to enter. Hundreds of people built robots and entered each of the tournaments. The new version of BattleBots on ABC is no longer an open tournament; a limited number of contestants are pre-selected to make for 'better television'. Is this in the best interests of the sport, or does it make it less appealing to new competitors?

  3. Combat robots are violent and destructive, and the level of violence continues to increase. Is this violence 'OK' because it's 'just machines', or does the violence feed into and fuel an unacceptable level of violence in American society?

Q: Hi mark. I am building a 4lb wedge bot and am indecisive on a final fact. I am indecisive on using 2 wheels with drive motors or 4. With 2 drive motors my dimensions are 6" wide X 7" long. With 4, 6" wide X 10" long. The area my bot takes up is a concern as the arena is only 5' X 5'. I am wondering if the approximate 3.6 lb of pushing force of the 4 motors is worth the extra 3 inches when compared to 2.25 of the 2 motors. Pushing force and maneuverability are key but would the shorter length and less surface area be better given it has less pushing force but still breaks traction (4 motors break at 30% 2 motors break at 50%)? Thanks. [Warrington, Pennsylvania]

A: [Mark J.] Fairyweights (150 grams), antweights (1 pound), kilobots (2.2 pounds), beetleweights (3 pounds), mantisweights (6 pounds); aren't there enough insect weight classes? Who decided we needed a 4 pound class???

You haven't given me much about your design or what rules the event will run, but in general I like a large footprint on insect 'bots. Team Run Amok's successful six-wheel drive beetleweight lifter 'Zpatula' is 12" wide by almost 16" long!

Conventional design theory says that a smaller robot is better because the armor can be thicker with less area to cover. This is true, but the larger footprint allows for much greater turning torque, plus the larger 'bot will react less to weapon impacts. Judges also generally respond better to a larger robot; the size differential works in your favor.

Go bigger and dominate the small arena.

Indian combat arenas are not safe! Q: i am completly freshers about making this combat robot...can u give me a model which have high power and some opposing or attacking performance ... [Orissa, India]

A: [Mark J.] Click here.

The mega Q&A session about the design of the Texas Bar Spinner grew so large that I moved it to its own page: Ask Aaron: Texas Bar Spinner. I think the discussion is over -- for now. See how it came out!
Q: I am designing a two wheeled robot (plus front caster) with horizontal bar spinning weapon. I know, I know, it's been done and while a few spinners have done well, most do below average.

I used the torque, acceleration and battery calculator to which you refer. Inputs were 250 pound robot using two T74 motors with AmpFlow dual ESC (160 amps per channel) turning 10 inch wheels. I am planning on using five in series Turnigy 7.5 aH 90C 7.4V hard case LiPoly batteries to run the motors at 37V... (read more)

Tex going crazy over too much data.

Q: I'm the beetle spinner builder again [from Oregon]. My design is coming along alright, right now I just need some assistance on some of the most critical aspects of the design: the weapon shaft, attachment method, and reduction method.

I was originally going to use a Fingertech Blade hub to mount my weapon to the weapon shaft, but I had two major problems with that:

1: The hubs are notorious for loosing up and coming off
2: The maximum drill out size for the bore is 1/4"

The first problem I can work with, as Fingertech are redoing the design entirely, but the second part is a big problem. My original weapon of 6"x2"x.125" 4130 steel has become 10"x2"x.25" 4130 steel, running off of a 2:1 ratio of my motor(Maximum RPM of the motor is 21460rpm). While max speed of the weapon will be theoretically about 10100rpm(A whopping 2000 joules of energy storage in a 3lb robot), I really don't expect more than the mid end of 8000rpm(The less insane but still very high energy storage of 1200 joules) due to air resistance. With that much energy being thrown around, I don't expect a 1/4" shaft of kind to survive the sheer force. I know to make it as thick as I can while still being in weight, but I'm wondering if a .375" grade 8 bolt is enough handle the forces, or should I go up to .5" shaft for the weapon? I know bigger is better, but I'm on a very tight weight budget with so much of the bot dedicated to the weapon, and anywhere that I can save as much weight as possible, the better (Then again, the weapon shaft may be the last place I want to steal weight from).

The second issue is getting the weapon to stay on. Because my shaft needs to be bigger to handle the forces required, I was looking at Servocity hubs(The .770" bolt pattern ones to be specific)and bolt on the weapon, but I'm pretty sure if the screws didn't shear themselves first, the hub would snap like a cracker. I was wondering if I could take some washers(Like NORD-LOCK washers) and clamp tightly down on the blade with a pair of shaft collars like Hazard did. If I have to get the weapon mount custom machined for me, what would be the absolute best way for it to stay on the hub, and the hub stay on the shaft, no matter what? I feel like a weapon that under performs is still usable, but a weapon that flies off is worse than useless.

A: [Mark J.] I've never been able to figure out how 'Hazard' managed to get enough clamping force from the shaft collars to adequately hold the weapon blade in place. Tony B. got it to work, but I can't tell you how he did it. I also can't recommend a keyed shaft for a bar spinner -- too much localized stress around the key. I think the optimum solution is a keyless lock bushing that will position and clamp your weapon bar to the shaft. Shop around a bit to find one that meets your design needs.

I do not recommend bolts - grade 8 or otherwise - for weapon shafts. Bolts are designed with the metalurgy to survive high tension loadings, not the shear loading you would expose them to as a weapon shaft. Bolts should always be protected from shear loads, either thru the design of the elements they are holding or by insertion of hardened pins that will take the loading before it is transferred to the bolts. Use a shaft suited to the type of load it will encounter.

You didn't ask about the wisdom of spinning a bar weapon that accounts for 45% of the robot's weight at 8500 RPM, but I think I should warn you of a few things.

  • You didn't mention what specific weapon motor you plan to use. I'll assume it's brushless. Brushless motors put out enormous power for their weight, but they do not like to be bogged down spinning up a heavy weapon connected via a small reduction ratio. Also, pushing against aerodynamic drag that drops the free RPM of the motor by a very conservative 15% will likely place a continuous load and amp draw on the motor outside its design parameters. Combined result - you're gonna melt your weapon motor, and quickly.

  • Impact angles for large and small diameter weaponsThere is a design issue about weapon 'bite' that I didn't put in the Ask Aaron Spinner Weapon FAQ because it's a trigonometry issue that's difficult to explain, but I'll give it a shot. Let's call it 'bite angle':

    The larger the radius of the weapon, the shallower its impact angle will be for a given bite depth.

    You can see in the diagram that for any given amount of bite depth a larger diameter weapon will impact at a much shallower angle than a smaller diameter weapon -- causing a more 'glancing' blow and decreasing energy transfer into the target. A larger diameter weapon typically more than makes up for this by spinning at a slower speed while carrying the same energy as the smaller weapon that spins faster, giving a greater bite depth.

    Bottom line: high RPM hurts the effectiveness of a large diameter weapon more than it would hurt a smaller diameter weapon. The large weapon is good, but slow it down and you'll get a much better hit.

Q: Last thing is getting the energy from the motor to the weapon. I was thinking of the Fingertech timing belts, but I feel like the forces the weapon would create would snap the belt(I've seen so many Last Rites clones on Youtube fail because the belt/chain failed). I also feel like O-rings would also snap on start up, so they're out. Should I move up to the wider 1/4" MXL belts, go with XL belts, or should I go for chain and sprocket, like the ones found on Servocity(Assuming the above problems aren't a problem for them, which I have a good feeling that they might be)? Obviously, this risks the weapon motor and its associated electronics, but I want to make sure that if weapon motor/ESC does die, its after delivering a huge hit that KO's my opponent.

A: You haven't told me about your motor torque. Drive belts are rated by torque and RPM and you've only given me the last half of that info. All I can suggest is that you look at the drives used by weapons in heavier weight classes that transmit a comparable level of torque at comparable RPM.

Q: Also, one last thing: How do you tell when a weapon bar is on its last legs and should be replaced before it snaps in half?

A: 'Taint easy. Generally a bar looks just fine right up to when it snaps. You may get lucky and detect a small change in the sound it makes when you strike it and let it 'ring', but that isn't a reliable test. The 'real' method to detect early signs of trouble is a magnetic dye penetration test. It takes some time, it isn't cheap, and it's not a perfect indicator. My recommendation is to carry a spare blade and hope for the best.

Q: Beetle spinner guy again, thanks for the help. Those keyless bushings you linked to seem like they would be perfect method to mount my weapon. They're a bit expensive, but for the weapon hub I think I should spend the extra bucks.

I completely forgot that motor torque is kinda of a big deal in transmission selection. The motor is question is Turnigy D3536/5 1450KV motor at 14.8 volts, a fair bit bigger than what most beetles run. I'm also thinking that I should bump up the gear ratio of the weapon from 2:1 to 4:1. Even if its not Tombstone ratios of energy storage, 500 or so joules of storage should more than sufficient for a beetle. Plus, if I want more energy, I can always change my weapons shape.

A: I think those are fine decisions.

The keyless bushing is an under-appreciated hub option. Shop around a bit -- there are different manufacturers and different designs that might click with your ideas, and might save you a few bucks a well.

Yea, that motor's a 'fair bit bigger' than the standard beetle weapon motor alright. The little bugger pumps out close to a full horsepower! Start-up torque is always a question mark with sensorless brushless motors -- a lot depends on the controller software. I think we can safely guess at around 300 oz-in of torque. Yes, I think 500 joules at around 5000 RPM will be VERY impressive in a beetle. You'll have a quick spin-up, good bite, and moderate motor loading.

In commercial usage, the XL timing belts are used for up to about 160 oz-in of torque and 4000 RPM. Combat robots overstress everything, so I wouldn't hesitate to use an XL belt in this application. I'd suggest the 3/8" width and the largest pulleys that work for your design and selected ratio -- larger pulleys reduce the effective torque transmitted by the belt. Be extra careful with pulley allignment; at the speed you're running alignment is critical.

Q: I hot glued some wires to insulate them, and after a particularly intense match, I found that the glue had melted. Other than that, the insides were fine. Is this concerning? It's a Beetleweight robot with a 1000 mAh battery, if that makes any difference. [Andover, Massachusetts]

A: [Mark J.] It's concerning that you'd use hot melt glue on something that has the potential to get hot!

It would help if you'd mentioned what the wires are connecting and where you applied the 'insulation'. Common low-temperature hot melt glue starts to soften around 140 degrees farenheit. Some parts of your wiring -- motor connections, for example -- can routinely get MUCH warmer than that.

If you need to apply a coating to insulate an electrical connection that gets hot, pick up a small tube of Silicone sealant at your local hardware or auto store. It's an excellent insulator, easy to apply, and when cured it withstands at least 400 degrees farenheit without complaint.

It's going on ten years since I wrote the Team Run Amok Spinner Weapon Excel Spreadsheet. There have been several upgrades over the years (battery capacity guidance, english/metric conversion calculator, brushless motor torque estimator...) but it needed an update in the recommendations for energy capacity and spin-up time, as well as new default specs for the 'example' weapon.

Version 1.6 is now available for download: Team Run Amok Spinner Weapon Excel Spreadsheet

Indian combat arenas are not safe! Q: What should be the min. Angle of slope of wedge bot?? [Maharashtra, India]

A: [Mark J.] Click here.

Q: Which material should i use for best armour in flipper bot? [Gujarat, India]

A: [Mark J.] Click here.

Q: Hi, Mark. Team WhoopAss's two flippers, HexaDecimator and Hexy jr, seemed quite different from some orthodox US flippers, such as Intertia Labs' flippers in many aspects. Do you think the two flippers should be classified as orthodox US flippers? What do you think is the edge of the two flippers? [Jiangsu, China]

A: [Mark J.] The only unusual element in the design of the Team WhoopAss flippers is the forward placement of the flipper hinge. A conventional flipper design has the hinge point as far to the rear of the chassis and as high as possible in order to maintain a constant mechanical advantage for the pneumatic actuator throughout its range of motion. Placing the hinge so far forward reduces the effective power of the weapon, but does have the advantage of allowing the actuator to be placed low and flat in the chassis -- lowering the height of the robot.

As is so often the case, the weapon is not what made the Team WhoopAss robots successful. Although not nearly as powerful as the flippers from Inertia Labs, the realtively modest weapons could be employed effectively because the rest of the robot was well designed and well driven.

Hexadecimator weapon drawing

Q: Mark,
I'm having trouble with my drive ESCs. I'm using a pair of Tiny ESCs coupled with two 1000rpm motors. When I go full forward the wheels both go full forward and then suddenly one of the wheels will reverse or stop completely. I've tried recalibrating the ESCs and adding capacitors across the terminals and neither worked. The only thing left that I can think of is the battery. It's a 450mAh 3S 45C LiPo. Is this not enough? Or is it something else that I'm missing? [East Cleveland, Ohio]

A: [Mark J.] There is a LOT of variability in the "1000 RPM Motors" from KitBots, but your battery (assuming it's properly charged) should be ample to cover that variation, and a voltage drop shouldn't cause an ESC to reverse. The problem is likely in the ESC / receiver / transmitter chain. You haven't given me enough info to fully troubleshoot the problem, so let's try a little diagnostic tree:

  1. Are your Tiny ESCs the current version (2.4)? Multiple Battery Eliminator Circuits (BECs) fighting each other to supply power to the receiver can cause odd problems like you report. The newest 'Tinys' have a diode in the red power line that runs to the receiver to prevent this problem, but earlier versions don't have this protection.  The problem can be made worse if there is a weapon ESC that you aren't telling me about. 
    • Yes, I've got the current Tinys: goto 2.
    • No, they're older Tinys: pick one ESC with a battery eliminator circuit to power the receiver and clip the red receiver wires on any other ESCs that have a battery eliminator circuit. If this doesn't solve the problem, goto 2.

  2. Is it always the same wheel that stops or reverses? If it's always the same wheel, it might be a bad ESC.
    • No, either wheel can act up: goto 3.
    • Yes, it's always the same wheel: swap the left and right side ESCs.
      • If the problem is still on the same side after the ESC swap, goto 3.
      • If the problem also swaps sides it's likely a bad ESC. Send it back.

  3. It's looking like a radio problem. Check your transmitter battery for charge, wipe any programming you have and re-enter, and just for good measure re-bind the transmitter. If it's still acting up the radio is suspect.

It's possible that you've failed to mention something important, like a huge weapon motor that cuts in just before the problem crops up. I'll assume that you wouldn't do that to me.

Q: Mark,
Guy with the TinyESC 1000 rpm motor problem. I'll try to give you some more information.

  • I'm using the new TinyESC v2 so I don't think that is the problem. But I tried removing the power lines from them anyway and still had the problem.

  • I tried swapping the motors and that made it worse. I discovered that one of the motors had seized which led to part of the problem, but it didn't fix it. Now both motors work, but every so often they reverse and the robot doesn't move as expected.

  • According to your past recommendation this would be a Tx/Rx problem. I'm using the Spektrum Dx5e and the Spektrum AR6255 receiver. I know the transmitter isn't the issue because I use it with other bots and it works fine.
Do you know if there's anything I can do besides buying a completely new receiver?

P.S. - I forgot to mention that I'm using the built in mixing on the Spektrum DX5e. Do you think getting a vtail mixer might fix this problem?

A: OK, both motors can reverse unexpectedly and control is inconstistent. You've got caps across the motor terminals, all the batteries are charged, and clipping a red power lead from an ESC to the receiver makes no difference. The transmitter is used with other 'bots and performs just fine. Got it!

The transmitter-side mixing isn't the problem. I'm suspicious of two things:

  1. You didn't mention if you re-bound the transmitter to the receiver as I suggested. I suspect that the fail-safe points for the drive motor channels are mis-set. Carefully follow the binding procedure in the Spektrum AR6255 User Guide paying close attention to the receiver fail-safe points which are set when you bind the receiver.

  2. The AR6255 has dual antennas. Read up on their correct positioning in the user guide and try re-orienting them in the 'bot. Pull them out and let them dangle free to see if that corrects the problem. I really hope you didn't cut the antennas short to fit them in your 'bot! Try stretching them out and away from electronics / motors / metal surfaces to see if that does the trick.
Note that the AR6255 has a feature that tells you about radio signal drop-outs caused by poor reception. From the User Guide:

The AR6255 has a red LED (labeled with H) that indicates the number of holds that have occurred since the receiver was last powered on. The LED will flash the number of holds then pause (e.g., flash, flash, flash, pause, flash, flash, flash, pause indicates three holds occurred since the receiver was last turned on). Note that holds are reset to zero when the receiver is turned off. During the first flights of a new airplane, itís recommended to check the red LED hold indicator. If itís flashing, itís important to optimize the installation (move or reposition antennas) until no hold occurs. On later flights, the LED Hold Indicator can be used to confirm RF link performance.

Check for that flashing LED the next time the 'bot acts up -- it's an indicator of reception trouble. Get those antennas out where they can do some receiving!

Q: Mark,
I wanted to update you on where I was at and how I fixed my problem in case anyone else has this issue in the future. I ended up swapping out both the receiver and the ESCs but still had the problem. What I think the issue was is that the brushless ESC I was running for my weapon motor had some fault with its BEC. Since I was using the new Tiny ESCs that have the diode that prevents over powering the reciever, it was running off of the Brushless ESC's BEC. That was somehow causing issues with the TinyESCs. I removed the power wire from the Brushless and re- attached the ones on the drive ESCs and that 95% solved the issue. It still occasionally flutters, but I can live with it.

A: Thank you for the follow-up!

I'm glad you were able to track down the problem, but you never told me that the 'bot had a weapon ESC --  even after I prompted you in the first step of the diagnostic tree.  It's really tough to correctly diagnose a problem with a component that a builder won't admit exists.

Q: I am currently building my first combat robot. It's a featherweight bar spinner. I just have a few questions about motor and gear selection after using the spinning weapon spreadsheet.

My favorite option so far is using a geared AmpFlow E30-150. This geared motor has a no-load RPM of 670 and a stall-torque of 360 in-lbs. I plan on using a single stage pulley around 2.5:1 to bring the RPM up to the 1600-1700 range. My weapon is going to be approximately 16 inches long and weigh about a quarter of the robot's weight. Using the spreadsheet, the weapon will spin up in about 2 seconds with a tip speed of around 80 mph and store 880 joules of energy.

Is this a good design for what I hope to be a hard-hitting featherweight? What would be an ideal RPM for this weapon/weight class? Are there any other geared motors that you know of which would be better for this application? Any advice is appreciated! [Albany, Oregon]

A: [Mark J.] Several suggestions:

  • The AmpFlow E30-150 is a fine weapon motor, but consider how you plan to use it. You're attaching it to an expensive, heavy, and power-wasting gearbox that takes the RPM down too low, then you're running it thru a belt drive to speed it back up. Spinner weapons typically use a single belt drive reduction to drop the motor speed and increase the torque to the weapon. Cheaper, lighter, more efficient, and less to go wrong.

  • The AmpFlow is a brushed motor: durable, easy to control, high in torque, and difficult to screw up. Brushless motors are currently very popular for spinner drives -- they have more power per pound than brushed motors, but they are also less durable, tricky to control, low in torque, and very easy to screw up. As this is your first robot, I think the brushed AmpFlow is a fine choice for a weapon motor.

  • Your energy storage is too low for a modern feather bar spinner. Trash the gearbox and run a 2:1 reduction belt drive to your bar. That gives you 2800 RPM and 1750 joules in 4 seconds -- 770 joules in the first 1.3 seconds. Much better.

  • Your remaining problem is stuffing that E30-150 down low enough to get an effective spinner height -- or is this a vertical spinner?
Have some fun with it, tear up a few opponents, and learn a whole bunch. Remember: a combat robot is more than just a weapon. Don't put all your effort into the weapon at the expense of the rest of the 'bot.

Q: Hey Mark, thanks a lot for the advice. I'm happy to get to trash the gear box. I guess my only concern was the weapon storing too much energy for the size of the robot (based on the 19 J/lb. mentioned in the spreadsheet).

I plan on matching the motor with a Talon SRX ESC and angling my bar so that it hits low while the motor can be mounted up higher (if this configuration raises any red flags, please let me know).

Other than that, my main concern is this: I have a half inch cold-drawn steel (C1018) shaft with compatible mounted bearings. My ever-changing CAD model has the shaft length around 5-6 inches. Would this shaft be sufficient for my weapon setup? Or should I spend a little more on going up to 3/4" or 1"?

A: The Weapon Spreadsheet was written about ten years ago -- back when weapons were much 'kinder and gentler' than the current standard. Around 60 joules per pound is entirely in line, particularly given your relatively low weapon speed.

Angled bar spinners were once fairly common -- I recall Chris Hannold's 'Six Million Dollar Mouse' at Robot Wars Extreem Warriors. The design has no serious flaws and solves a number of design problems. I haven't seen one built in quite some time, and it may come as a surprise to your opponents.

The Talon SRX has tested very well in similar applications. It should be fine -- just don't try a high-speed reverse of the weapon when it's spinning!

Your 1/2" shaft is likely too small for a weapon with this much power and that great a distance between supports. I don't have the details of your weapon construction, but I'd spend some of that money you saved by scrapping the gearbox on a 3/4" shaft and bearings.

Q: My robot is an 10kg combat robot. It is a four wheel drive bot. I have used 2 1000rpm motor for the back wheels and 750rpm and 700 rpm motors for the front wheels. Will this misbalance the bot??can it cause any major problem; if yes then can u give me any solution?? [Leeds, England (or possibly Amsterdam, Netherlands)]

A: [Mark J.] I think this is a record -- three different drive motors in one 'bot. May I assume that all the wheels are the same size?

You haven't told me enough about the motors to do a full analysis, but in spite of what some builders might tell you this shouldn't cause any major problems.

As your robot accelerates the torque available from the front motors will taper off more quickly than the available torque from the rear motors. At some point the front motors will no longer be able to 'keep up' and the rear motors will take over all the load. There will be a little drag on the front wheels at speed, but it shouldn't be a cause for concern. Many all-wheel drive automobiles deliberately design a front/rear torque imbalance into the drivetrain to improve handling -- they drive just fine.

The small difference in speed between the front motors might cause a small 'pull' to one side, but again I believe this will be barely noticeable.

If you're really worried about this you could fit 40% larger diameter wheels to the front of the 'bot (or 30% smaller to the rear) or you could provide differing voltages to the front and rear to level out the motor RPM. I wouldn't bother unless a real handling problem develops. Give it a go; I think you'll be fine.

Q: Hey Mark, question regarding 'coil' and 'contact' voltage for a solenoid. I've got a solenoid here that has a 24V 'coil' but the contacts can handle up to 48 V. I'm planning on using an 8S lipo for the weapon battery so continuous voltage would be somewhere in the realm of 30V. So I guess I can't power the coil directly with the weapon battery? What am I supposed to do... use a separate battery pack just for my solenoid coil? Seems a little counter-intuitive. [Charlottesville, Virginia]

A: [Mark J.] You bought the wrong solenoid. Manufacturers typically offer a variety of coil voltages for a given solenoid to allow flexibility in the control circuits. There is some tollerance for over/under voltage on the coil, but it's typically +10%, -25%. Check the specification sheet for your specific solenoid.

What you're 'supposed to do' is return the solenoid and get one with the coil rating you want.

If you don't like that option, it is possible to add a power resistor in series with your coil to allow it to operate at voltages higher than its rating. The formula to calculate the resistance needed is:

Resistor Ohms = ((Operating Voltage / Rated Voltage) - 1) * Resistance of Coil

Example: to safely operate a 24 volt coil with a resistance of 60 ohms at 30 volts requires an additional resistance of:

((30 volts / 24 volts) -1) * 60 ohms = 0.25 * 60 ohms = 15 ohms

The power resistor will require a power rating equal to the power consumption of the coil at its rated voltage -- see your contactor documentation.

Q: Mark, I see what you mean, looking at the specs for a White-Rodgers solenoid. My solenoid in question is the Whyachi C1 contactor. The data sheet mentions only a 24 VDC coil and current ratings for 48 VDC for the contacts... not as clear. Any advice on this? Thanks.

A: You have a few options:

  • I make the coil resistance about 50 ohms -- add a 10 ohm 15 watt power resistor in series with the coil.
  • Ask Team Whyachi ( what the actual voltage tollerance is.
  • Run it and hope it holds up.

Q: Hi Aaron. I want to know what Ziggy`s pneumatic pressure use, the Store pressure and the Regulator pressure? Ziggy builder's website can not open [Yunnan, China]

A: [Mark J.] There are descriptions of Ziggy's weapon system in the Ask Aaron Robot Weapons Archive

Quick summary:

  • Gas of choice: high pressure air or nitrogen
  • Storage pressure: 3000 psi (~200 bar)
  • Regulated pressure: unregulated [correction -- see below]
Testing and operating such a weapon system is  EXTREMELY DANGEROUS  -- Do Not Attempt to construct a similar weapon without extensive experience in pneumatics and mechanical design.

Q: Hi Mark, I noticed it in the recent post that the Ziggy use a unregulated system which means the pressure in actuator is 3000psi, same as the pressure in the tank. However, I believe the Robogames rules limit the pressure at 250psi, Ziggy seems violated the rules. Is there some reason or just I made a wrong judgment about the pressure in the actuator is same as it in the tank? [Guangdong, China]

A: [Mark J.] Hmmm...

I had assumed that 'Ziggy' received an 'event organizer exception' for a high-pressure system, as allowed under the rules. The standard photos of 'Ziggy don't show a pressure regulator, but just to make sure I went thru my photo archives for a picture from a different angle. Guess what I found - a pressure regulator! According to my sources, that is a PR-59 GO Regulator.

So, I stand corrected. It appears that 'Ziggy' ran a 250 psi regulated pneumatic system. This does not mesh with the team's claim that their weapon put out 14,000 pounds of flipping force. You just can't get that much out of their actuator on 250 psi. My warning about the danger of a powerful pneumatic weapon like this still holds.

Combat Robot 'Ziggy' - armor removed
Combat Robot 'Ziggy' showing pneumatic regulator position

Q: Do LiPo's come charged fresh out of the box? The other day I was fitting some connectors to an unused, uncharged battery, when I produced a spark and a tiny wisp of smoke. [Andover, Massachusetts]

A: [Mark J.] Lithium polymer batteries are severely damaged if discharged below about 3 volts per cell, so they are stored and shipped with a partial charge -- typically about half of their rated capacity.

Watch that battery carefully for early signs of damage - like 'puffy' swelling of the wrapping. Discard it if you see problems.

Robot Wars logo - 1999 Q: Dear Mark, what's your opinion on BBC deciding to reboot Robot Wars? Does that mean that audiences are interested in robot combat again? [Chinese Forum]

A: [Mark J.] It means that someone at the BBC believes that audiences are again interested. They've signed on for six episodes with no start date announced. If television programming boffins could actually predict what audiences wanted to see there wouldn't be a 65% cancellation rate for new television series'.

Q: Hi mark. I am an upperclassman in highschool and often give guidance to the freshmen teams (usually groups of 3 or 4) trying to design, construct, and compete with beetle weight robots. I credit you and your amazing website for most of my knowledge. Thank you.

Because of your website and teachings and watching many competitions and participating i have a fair amount of knowledge at my disposal. I could complete any of the designs the teams come up with within 4 months start to finish and feel bad because there is so much the underclassmen do not know. Their lack of understanding and ability to focus on the important parts of the robot (proper belt tension, actually driving before the competition, etc) and inability to complete a design in a reasonable time saddens me as many good robots have never been built simply because of time management. You and google and myself, and my tech teacher can always offer knowledge but ability to properly budget time for the design process is needed. I don't know how to convey this.

First designs actually begin in October and competition is in April. Many groups get lost in the virtual engineering and never get around to ordering parts and actually building the robot. Do you have a suggestion how to fix this problem or can you offer a suggested time table given the club is strictly afterschool from 2 until 4? Your help is appreciated as I am just trying to help others. [Pennsylvania]

A: [Mark J.] It's relatively simple to tell someone how to do something, but infusing someone with lasting enthusiasm is a more difficult challenge. Although the best teachers can do this it seems to be more a talent than a skill. While I don't claim any expertise in this area there are some well-known general guidelines for keeping groups moving forward:

  • Competing with a combat robot is a large undertaking that can overwhelm. Clearly break the path to the goal into small, manageable tasks. Each task should have a well defined achievement and end point.
  • Make a big chart of these tasks and track each team as they complete each task. Display this prominently at club meetings to drive competitiveness between the teams. Maybe something like:

    Selection of General Robot TypeOctober 20
    Draft of Robot LayoutNovember 12
    Full Parts List with Prices and SourcesNovember 22
    Order and Receive PartsJanuary 4 Ordered
    Test Electrical ComponentsJanuary 17 ......
    And so on...............
  • Keep their eyes on the goal. Maybe a video of a particularly good robot fight at the start of each club meeting?
  • When a team is falling behind, step in early to help them with the obstacle that has them bogged down.
  • Hand out praise for good work accomplished.
I don't think I can help with a suggested timeline or even specific tasks. I know how I build a robot, but everybody does it in their own way and has their own motivators. Be ready to offer the support each team really needs, but remember that these are their robots and give them room to build them their way. A robot that fails can be at least as good a learning experience as a robot that wins.

Q: Hi Mark. Thanks for all the advice. You have some really good ideas that I hadn't even thought about and I consider myself to be an out of the box thinker. I totally agree, competition is helpful but I would also include overall teamwork as a club so groups help each other. This was absent in my freshman year and I have been striving to get that specific point across.

There is just one more specific question I have. The groups as previously mentioned always get caught up in the virtual design, not on ordering parts and problem solving in 3D. We utilize the software Inventor. The advice everyone gives the groups is to finalize their design then order parts. This is done in minute detail and then parts never get ordered and the got ultimately just becomes a box.

I am the only one that suggests to only make rough dimensions of your parts, do a rough layout in person, and readjust size and material accordingly. Not to shape the weapon at all, only as a square or circle maybe with a few holes, definitely not to make an assembly file, and to just utilize the software to give a rough weight estimate. Am I wrong in doing so?

Thanks again. I know your website is more directed to robot engineering and building not teaching others but I appreciate your feedback.

A: It sounds to me like your CAD software is more of a hinderance than a help. It isn't 'AutoCAD Club', so if the software is getting in the way of project completion you need to clamp down on it. You genuinely do not need software to build a successful beetleweight combat robot, and you certainly don't need a refined engineering drawing before you order parts. Give 'em ten days to come up with their plans anyway they want (pencil sketch, cardboard cutouts, CAD) and pull the plug on the design phase.

Q: What is your opinion on the use of nutplates (similar to for attaching armor to structural members? Assume attachment is to a thin flange off the side of the structural member rather than drilling and tapping a hole directly in the structure. [Charlotte, North Carolina]

A: [Mark J.] Too finicky. Builders commonly use a stronger solution called Nutstrip. It's strong, comes in assorted sizes, reinforces the flange, and is considerably less expensive than the nutplates you've found.

Q: Nutplate guy here... Thanks for the info on nutstrip, that does look like a good standardized product. A couple points about nutplates though:

  • They are pretty cheap if bought in bulk
  • Many different configurations available - even 'floating' styles which correct for misalignments
  • They work really well in tight spaces or irregular arrangements which don't match the linear nutstrips
  • Because they are riveted to the flange (or other structural member) you can fasten to them without needing access to the backside of the workpiece
I've been around aerospace for years, where these are very common for small assemblies, so I wondered if any bot builders had used them.

Thank you for keeping this site running, this is a wonderful service for the community.

A: It's good to keep a mental file of unusual hardware and fastener solutions in case a unique situation arises.

  • Some builders will no doubt see the beauty of a nutplate and find places to use it.
  • Other builders will tack weld a conventional nut to the back side of a flange and take pride in their solution.
  • A couple guys will wrap duct tape around the whole thing and call it good.
Takes all types.

Q: To continue the 'how to put threads in thin materials' discussion... Rivnuts are pretty cool. This guy does a better explanation than I can. [West Chester, Pennsylvania]

A: [Mark J.] Another specialty fastener for unusual circumstances. Requires a special installation tool, can loosten and rotate in the hole, and can pull thru more easilly than the other options. Not really 'combat rated', but can be installed 'blind' in hollow tubes.

Q: Hey Mark -- A user from China just recently asked a question very similar to mine but I want to expand on it just a bit. This is the first year I am planning on using LiPo batteries in my robot for their energy density. My wheelmotor ESCs have Lipo protection so I'm not too worried about pushing their limits there. I'm more worried about my weapon.

The motor I'm planning to use on my spinning weapon has a stall torque of 285 A, and some calculations on the spinner spreadsheet says my required battery capacity should be in the realm of 2 A-h. The Lipo I have (temporarily) selected has a continuous discharge rate of 275 A and burst discharge of 550A. My main concern is that I am activating this weapon with a relay, not an ESC.

As far as I know, main Lipo failure/fire modes are accidental over-discharge leading to low voltage, being damaged or punctured, and being inappropriately charged. Assuming I'm choosing a battery with plenty of extra capacity for safety factor, and it is thoroughly padded from impacts, would you expect a problem? I'm mostly worried about the millisecond motor stalls experienced during relay activation. The weapon is powered with a belt drive so impacts should not completely stall the motor either.

Thanks for the help! [Sayville, New York]

A: [Mark J.] I think you have a good understanding of the issues and have made a good battery selection for your purpose. Watch the LiPo for any signs of outgassing (a 'puffy' expansion of the battery case caused by gas release) and dispose of the battery if you see such an indication of impending battery failure.

About LiPo protection mode on ESCs: unless strictly required by the event rules, turn the LiPo protection 'OFF' in actual combat. You do NOT want your ESC shutting your robot down when you're winning with 30 seconds left in the match! You can get LiPo sensors that emit a loud warning buzzer when the battery voltage hits the danger zone, giving the driver the option of shutting down to save the battery or continuing to stay in the match. Burn the battery if you need to, just win the fight!

Q: This video on combat robot construction says that an LED should be plugged into the receiver so when the machine is on it will light. Is that right? If the Receiver lost connect the LED will not light but the power is still on. [Tokyo, Japan]

A: [Mark J.] The video is correct. The receiver is powered with five volts from the Battery Eliminator Circuit (BEC) in the motor controller. The receiver output has three leads: power + (red), ground (black or brown), and signal (white or orange). The signal lead carries a pulse coded digital signal that is typically shut off when the receiver loses contact with the transmitter, but the power and ground leads are a simple pass-thru from the five volt power source and are 'on' whenever the motor controllers have power from the battery.

A power indicator light can be fitted anywhere on the far side of the the main power switch from the battery, but it is convenient to connect it to the power and ground outputs on an unused receiver port as that provides a constant low voltage suitable for an LED with a small resistor.

Q: Hello Aaron
I want to ask if I can use the Polymer Lithium-Ion Battery when I use the Motenergy ME0708 Motor as weapon motor ? If not Why? Thanks a lot. [Hebei, China]

A: [Mark J.] In theory, you can power any motor with any type of battery you like. The problem here is that the ME0708 motor can draw an enormous amount of current -- more than 4000 amps at 48 volt stall. LiPoly batteries are subject to damage if they are asked to supply more current than they can safely deliver, so you must either find a LiPoly battery with very great current capacity, or you must use some sort of 'soft start' system to reduce the current draw for the first part of your weapon start-up. Search the Ask Aaron Robot Weapons Archive for 'servo slower' to find a previous post on this topic.

Many builders will choose to equip other battery types (sealed lead acid, nickle metal hydride...) that are not prone to damage from attempting to meet the large current demands of very powerful motors rather than risk damage and possible explosive failure of a large LiPoly battery.

Q: Hello Aaron
The Interia Lab`s webside can not open, I want to know the structure about Toro , T-minus and Bronco , Can you show some pictures about them? thank you [Yunnan, China]

A: [Mark J.] See Frequently Asked Questions #30. Here's a link to the archived inertia-labs site:

Q: Hello,Aaron.
Why the CO2 cannot be used in Balltebots' competition ? [Hebei, China]

A: [Mark J.] When CO2 undergoes a phase transition from its compressed liquid state to its gaseous state it absorbs a great deal of heat energy, cooling itself and the materials it contacts. The cold surfaces condense water from the air. There is a small chance that condensed water can freeze into an ice plug and block a gas port used to depressurize the pneumatic system. This can lead the robot's operator to believe that the system is fully depressurized when it is not -- a dangerous situation!

Compressed air or nitrogen does not cool nearly as much as CO2 when powering a pneumatic system and does not risk building up ice blockages. Battlebots chooses to avoid this possible safety risk in their current rule set -- although they are alone in this action. I suspect it's an insurance issue.

Correctly designed snail cam
Q: Mark,
I came across this video of a spring powered flipper and it inspired me. I have been wanting to build a flipper for a long time but my budget wouldn't allow anything bigger than a beetle and ant/beetle weight pneumatics aren't quite up to combat specs. My question is how would you determine the torque needed to turn the cam in order to load the flipper? Thanks [Cincinnati, Ohio]

A: [Mark J.] Oh my! The concept is sound, but the components and layout of the mechanism in the video are horribly designed. The motor torque requirement is constantly varying as the oddly shaped lifting cam rotates. The motor visibly slows at one torque peak. In order to minimize the motor torque needed to load the flipper:

  • The spiral 'snail cam' driven by the motor must have a continuous and gradual lift.
  • The spring should be given a straight pull -- none of that 'bend in the middle' crud that requires calculus to figure out the non-linear change in spring tension.
  • Assuming a straight spring pull, the profile of the spiral cam must be a non-euclidian parabolic spiral to adjust the pull rate to the increasing load imposed by the spring as it is extended (see Hooke's law).
I've given you just a start on how the calculation might be done, but it's almost midnight and I have to get some sleep. Let me think about this for a day to see if I can sort out the equations.

Q: Mark,
Spring powered flipper guy again. Would this be a better layout for the components?

A: Yes! Much better -- a proper snail cam and a nice straight spring extension. I'm not certain that the cam profile is entirely correct to provide an even loading on the motor, but the general layout is very nice.

Chart showing correct snail cam profile for constant torque when extending a springIdeal parabolic rise snail cam

Q: Also would using conservation of energy be a good approximation (i.e. rotational work converted into spring energy)?

A: Your question beat my update to the prior post. Calculating the rotational work needed to arm the spring and backing into the required torque is the path I decided would be best. Assuming that you get a straight pull on the spring and get the spiral cam correctly profiled to even out the torque requirement to a constant level, we can derive the required torque from the rotational work formula:

Work = Torque * Angle Thru Which the Axle Rotates [in radians]

In our case the work is the extension of the spring, which takes place over the course of one revolution of the axle. The spring extension force is a linear function (Hooke's law), so we can calculate the average force required to extend the spring as follows:

Average Spring Force = (Spring Force at Start + Spring Force at End) / 2

That plugs into the basic linear work equation [Work = Force * Distance] like this:

Work = Average Spring Force * Distance Extended

Getting close now. One full rotation is 2𝜋 radians, so with a little algebra the rotational work formula transposes to our needed torque equation:

Torque = (Average Spring Force * Distance Extended) / 2𝜋

Example: if you're extending a spring with a rate of 200 ounces per inch from its rest state (zero force) to one inch of extension with a parabolic snail cam over one full rotation, the torque required will be:

Torque = (((0 oz + 200 oz) / 2) * 1 inch) / 2𝜋 = 100 oz-in / 6.283 = 15.9 oz-in

There are some losses due to angular inefficiencies and rubbing friction with the cam, and you will want the motor to deliver that level of torque at some reasonable motor speed to extend the spring quickly. I'd use a gearmotor with a stall torque about three times the formula result to avoid bogging and let the motor operate up near its horsepower peak.

Addendum: There is some discussion out in the forums about the amount of force required for an effective sping flipper. There are too many variables (force, stroke, geometry, angle, expectations, ect.) to give a good calculated result to that question. Further, the dynamics of a spring flipper are different than those for a pneumatic flipper and cannot be directly compared. I'd suggest mocking up the weapon geometry and trying differing spring rates on a proxy opponent to find a result you like.

Q: How come we don't see more robots using two wheels on each hub like 'Sewer Snake?' Looks like it works pretty well for them. [Andover, Massachusetts]

A: A better question might be "Why did Team Plumb Crazy stop running two wheels per hub on their heavyweight robots?" The answer is that it didn't work all that well for them. They only did it in the first place because they found a source of really inexpensive narrow wheel-tires. In the end the extra weight and complexity involved simply wasn't a net gain. Their 2015 BattleBots entry 'Stinger' reverted to a conventional one wheel-per-hub.

Combat Robot 'Sewer Snake'

'Sewer Snake' during its
12-wheel phase

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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