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4836 Questions and Answers about Combat Robotics
from Team Run Amok
Ten Years of Ask Aaron

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) requested 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   

Welcome to my website Recent Questions
Not all the questions we receive at Ask Aaron are serious. Some are odd, some misdirected, and a few are incomprehensible. Aaron enjoyed dealing with these questions, and I've collected some of his best responses in a in a special page:
Aaron's Greatest Hits
Aaron's Greatest Hits

Q: hey mark .. can this esc [link to very nasty $5 'H.K.' ESC removed] run the pmdc in reverse too ? if yes ... how? [India]

A: [Mark J.] No.

The very first comment at the top of the discussion section just below the product listing answers your question. This is a model aircraft ESC, and airplanes don't back up. General rule: if the product description for an ESC does not specifically say that it features reverse, it does not.

Note: I don't recommend ANY products from the 'H.K.' hobby warehouse for use in combat robots.

Some builders use a few of their products (batteries, radio gear) with some success, but H.K. is known for very low prices -- NOT for high quality, customer support, readable documentation, or bulletproof reliability. In the case of ESCs, their stated amperage ratings are entirely fictional and a fair number of them fail immediately on power-up. IMHO their products are marginally suitable for backyard hobby use, but not for competitive robotics.

Q: The power of robot rotary weapons is measured in joules, but joules doesn't mean much to me in real world terms. Can you give me an example of a 1000 joule impact with everyday objects? [West of the Pecos]

A: [Mark J.] Sure. A baseball bat hitting a major league home run is an impact of just about 1000 joules. Bear that in mind when you're designing and building your robot -- can you hit it as hard as you can with a baseball bat and have it bounce back for more?

Baseball batter

Q: HI mark . There is a motor I found . 250 watts , 24 v , 3000 rpm reduced to 330 rpm ( looks like a pankake motor ) and no idea about torque .
I measured the pole resistance , its giving 2.3 ohms , will two of these motors will work for a 60 kg wedge bot ( 4 wheels attached with two motors by chain sprocket) ... the dealer told me its a bicycle motor ... [India]

A: [Mark J.] As a general guide, a combat robot with an active weapon needs a bare minimum of 4 watts of drive motor output per pound of total weight. Effective rambots, wedges and spinner killers need about three times that much. By that fomula, your 132 pound robot should have at least 524 watts total drive power -- pretty close to what a pair of these motors claim to deliver.

However: I have some doubt that these motors actually deliver 250 watts of output power. If the pole resistance is 2.3 ohms, that works out to about 10.5 amps stall current at 24 volts (24 volts / 2.3 ohms = 10.43 amps). You can get a rough estimate of output power with the following formula:

Peak Output (approximate watts) = Voltage * Stall Amperage * 0.25

That works out to less than 65 watts output power (24 volts * 10.5 amps * 0.25 = 63 watts). If your pole resistance reading is accurate these motors will not deliver enough power for your 60 kilo combat robot.

Note: a 250 watt peak output PMDC motor with 3000 RPM unloaded speed would produce about 440 oz-in torque at stall and draw about 45 amp stall current.

Q: hi mark .. the pancake motor guy here ... I think you are right ... my multimeter is having some error for the resistance , probably the wire leads have some of its own resistance

I did one thing ... I have run the motor on 1 volt from the DC power supply and stalled the motor ,it takes 2.2 amps .... from this can I calculate how much power it is rated on 24 v there a formula?

A: The relationship between stall amperage and voltage is linear: 1 / 2.2 = 24 / X, solve for X. That gives a stall amperage at 24 volts of 52.8 amps. Plug that into the formula I gave above for approximate power output and you get: 24 volts * 52.8 amps * 0.25 = 317 watts output power. That is a rough approximation, but it's close enough to the rated output of 250 watts to give me some confidence in that rating.

A pair of these motors geared down 9:1 (330 RPM output) with 4" wheels for a 60 kilo robot would provide reasonable acceleration and good pushing power, but a low top speed for a wedgebot -- less than 4 MPH. You might want to consider four motors and larger wheels.

Save a few dollars in your budget to buy a new multimeter.


I would first like to thank you for this wonderful website that you and Aaron were able to create. It has been an invaluable resource to builders all across the world. I am sorry to hear about Aaron's passing and I will keep your family in thoughts and prayers. [Maryland]

[Mark J.] Thank you for your kind compliments and compassion. Aaron would be pleased to hear that our work here is appreciated.

Q: Hi Mark! I'm planning on building a 30kg bot & this'll be my first combat robot. I was thinking of making a spinner killer just like 'Breaker Box' because there are many spinners in the Indian robotic scene. When watching videos of it fight, one thing I noticed that being fast & controllable is a necessity for this type of a design. 'Breaker Box' combat robot

For my robot i'm planning on using 2 motors driving 4 wheels with timing belts. These motors are originally designed for use on hydraulic power packs. The specs of the motor are 24v, 500W, 2800 rpm, Duty cycle: S2- 5min (I don't exactly know what this means but it was written on its spec sheet & I think it might be the 5min working duration), 2.6kg weight, 3.75Nm stall torque, 80A stall current, 4A no load current.

I'm thinking of applying a 4:1 reduction with 6 inch wheels (or would 5:1 be better?)

The plow would be made of tempered 4340 steel including the arms. The rest of the robot would be 6061-T6 aluminum. I'm thinking of using wiper motors for the arm articulation.

I'm using a custom made motor driver with dual channel & PWM so that it controls both the motors with 80A current carrying capacity (is it enough?). The robot would be powered by NiCd cells & controlled with a 2.4ghz 6ch controller.

Are the drive motors which i chose good enough for this design? AmpFlow motors aren't a choice because they would completely blow my budget if I import them. One more thing that concerns me is that they're designed for hydraulic power packs. Can I still use them in the drive system or will it have any adverse effects?

And would the gear reduction suffice or should I consider a 5:1 reduction for Indian arenas?

Any comments which could make the robot better would be greatly appreciated!

Thank you!

P.S.- I tried using the Team Tentacle calculator but couldn't make out much maybe because I'm not that technically sound with motors. Sorry!

Thank you again! [India]

A: [Mark J.] The hydraulic power pack motors should be fine. An 'S2- 5 min' duty cycle indicates that the motors are designed for five minutes of full-power operation followed by a good cool-down period. That's pretty much what robot drive motors go thru, so you should be OK.

You'll need more than a 4:1 or 5:1 reduction with 6" wheels. Acceleration would be poor in a small arena with those ratios. For a small arena (3 or 4 meters across) you'll need 8:1 or 10:1 reduction to give the responsive control you're going to need.

Wiper motors are marginal on power for your plow lifting arms. A pair of very powerful wiper motors ( would provide about 50 pounds of lift at the end of a one-foot pivot arm, but the stress on the gearbox would be considerable. More importantly, if you attach the plow arms directly to the wiper motor shafts you're going to transmit a very great amount of impact to those fairly small shafts, bearings, and gearbox housing. That's a gauranteed failure point. Consider a fixed (unpowered) spinner-killer plow for your first robot.

I'm worried about your 'custom made motor driver'. If the '80 amp current carrying capacity' is based on the simple power rating of the power chips, you're going to be in trouble. Those ratings are for instantaneous power surges at the rating temperature. As soon as the power chips start flowing power, their temperature rises and their power rating starts to drop. Read Chuck McManis' article on real-world power ratings for robot motor drivers for a full explanation.

For what it's worth, the Tentacle Calculator estimates that your motors will pull no more than 27 amps each with an 8:1 reduction ratio, or 54 amps each with a 4:1 reduction ratio.

Q: Thank you so much for your help regarding the motors!

Just one more question. I need just general advice whether I should stick with 6 inch wheels or should I swap them for 4 or 5 inch wheels. You know all the technical details much better than i do, so what would you choose if you were in my place? 6 inch wheels with a higher reduction ratio or 4(or 5) inch wheels with a lesser reduction ratio? And thanks! - Sage

A: [Mark J.] Wheel diameter is likely not a critical factor in your design. A 'Breaker Box' style robot would need to be invertible, so the wheels would need to be large enough to extend above and below the chassis/armor. In general, I recommend the smallest wheel that will meet the design needs of the robot. Smaller wheels are lighter, which allows for more weight to be used elsewhere in the design.

Q: Hi mark... planning to buy Sabretooth 2*60A dual motor drive to drive my ampflow motors. The ESC is rated 60amps per channel, so when I power that board with 100amps from my SLA battery will it harm the sabretooth? [India]

A: [Mark J.] Short answer -- no, the Sabertooth controller should be fine.

Longer answer - a motor controler's current rating refers to the maximum current that can safely flow from the battery, thru the controller, and then thru the motor.

  • If the motor is not placed under load conditions where it can flow more than a continuous 60 amps at the available voltage -- you're fine.

  • If the battery cannot supply more than 60 amps to the motor regardless of the motor load -- you're fine.

  • If the motor controller has effective overcurrent protection that will limit current flow regardless of motor load or battery current capacity -- you're fine.
Since the Sabertooth 2x60 controller does have overcurrent protection it should not be harmed when used with a battery that can provide more than 60 amps to a single channel even if the AmpFlow motor controlled by that channel is loaded so heavilly that it could attempt to draw more than 60 amps continuous current.

Note: most AmpFlow motors will attempt to draw well more than 60 amps under heavy loading. If such amperage is not available, they will not be able to produce their full rated torque and motor performance will suffer. You didn't mention which AmpFlow motors you are using, their gear reduction, the wheel diameter, or the weight of your robot -- so I cannot calculate the current the motors might require under a heavy pushing load. A 60 amp controller may not be sufficient for the motors to perform well in your specific application.

Q: Hello Mark,
I am comparatively new to the heavyweight robotics. I wanted some guidance from you.

I am making a war bot first time and I am very interested in the wedge design. Can you please help me make an ultimate wedge bot - what are the essentials for a wedge design? I have searched online but one thing confuses me: if the bot has zero clearance it gets difficult for it to drive because of some back force it gets from wedge (scraping of the wedge to the ground) and if we keep some distance we cannot get under the opponent. How do I deal with this?

Also, if the wedge has zero clearance some weight is supported by the ground (because the wedge is touching the ground with a force) so we loose some grip too! I was just thinking about all this but couldn't get anywhere. Can you please help me out? [India]

A: [Mark J.] Yes, a zero-clearance wedge does cause a bit of drag and it does take some weight off the drive wheels, but neither of those things is a serious problem:

  • For a two-wheeled wedgebot, the components (batteries, speed controllers, etc.) can be concentrated close to or behind the axle line to shift the majority of the weight (80% or more) on the drive wheels to maintain acceptable grip.

  • A four-wheeled wedgebot should have the wedge hinged to drop down with only its own weight, not taking any of the 'body' weight off the drive wheels.

  • A steel/aluminum/titanium wedge with a nice flat, smooth, and wide bottom edge will produce little drag sliding along an equally smooth arena surface. Problems only arise if the arena has exposed edges where the individual floor sections come together that can 'catch' a wedge.

  • Wedge bevels and glidesWedge drivers should have the ability to raise the front edge of the wedge a bit if the the arena requires it. A little grinding to bevel the leading edge will usually do the trick. Alternately, a few Teflon 'glides' can be attached to the sliding bottom edge to rise it just a bit.

  • Once you get the wedge under your opponent it will lift them and transfer some of their weight and grip off of their wheels and add that weight/grip to your drive wheels. Thet's the whole reason you use a wedge.
Plenty more wedge ideas and design concepts in the Design & Construction archive.

Q: I see builders asking a lot of questions here about weapons and motors and once in a while a question about wheels. Are there questions that builders should be asking but aren't? [Western US]

A: [Mark J.] Weapons give the opportunity to deal damage if you can apply them to your opponent, and your motorized drivetrain gives the ability to to move toward your opponent -- but the it's the interface between the driver and the machine that allows the robot to be responsive and well controlled. We get very few questions about tweeking the control interface to make the robot driveable.

I see plenty of examples of combat robots with poorly set-up transmitters. Machines that wander about like lost sheep -- unable to reliably point their nose at their opponent, incapable of driving across the arena in a straight line, and spinning around uselessly when attempting a simple turn. Many builders don't even know what functions are avaiable on their transmitters that might be useful to them.

If you're interested in using the full capability of your computerized transmitter or want to know which radio system to buy in the first place, Team Run Amok has a few guides on the subject:

Q: Hey Mark, I'm designing an antweight spinner, I've been wondering what are some good cannon fodder objects for ant spinners to chew on, something with enough chunk to give a reasonable approximation of another ant weight, soda and soup cans can only go so far :) [Oregon]

A: [Mark J.] I made up a few targets by filling tuna cans with plaster of paris to give them proper mass. Suitably sized chunks of 2x4 lumber make satisfying targets as well. Stay safe -- keep adequate shielding between yourself and a spinner of any size!

Indian combat arena Q: Hi guys this is Aravind from India, im building a wedge robot for my battle events
weight ; 25 kg
motors ; 2 ampflow E30-150G
suggest me wheel dia for the wooden arena
i need to push the opponent out of arena
so shall i go for 2 wheel drive or 4wheel drive-driven by chain mechanism?
will it gives that much stall torque when i give 24v 70 amps for two motors [India]

A: [Mark J.] That's a VERY small and cramped arena! About 4 meters across with two one-meter holes doesn't give much maneuvering space. I'm not sure how you're expected to get your opponent between you and one of the holes. I'd consider a 'dustpan' style robot to scoop up the opponent and sweep them toward a hole.

Your chosen AmpFlow E30-150G gearmotors are powerful enough to push a 55-pound 'bot with 4" wheels to about 6 MPH in just 2 feet, and you'll have more than enough torque to maximize your pushing power. A chain 4-wheel drive can put the entire weight of the robot on the drive wheels, which maximizes pushing power. However, four-wheel robots are not as maneuverable as two-wheel 'bots. In an arena this small maneuverability may be critical. I think I'd go with a 2-wheel robot.

Dustpan style robot
You don't need to worry about stall torque -- the motor torque will be limited by the available traction. The AmpFlow E30-150G motors can each draw up to 125 amps at 24 volts if stalled, but even with very grippy 4" tires the motors have enough torque to exceed the grip available and spin the wheels well before they can stall. The light weight of your robot will allow the wheels to break traction and spin at about 15% of stall torque, reducing the maximum amperage draw to 15% of the max: about 17 amps per motor. A 24 volt 70 amp supply will be more than enough for the motors with 4" wheels in a 55 pound robot.

Q: Is the acceleration of your robot more important than the estimated top speed of your robot? In 1997 Robot Wars US BioHazard's top speed was slower than Vlad the Impaler but its better acceleration made sure that it could keep up with its opponents' pace and win the match. [Chinese Forum]

A: [Mark J.] Blistering top speed does no good if you can't accelerate to that speed within the confines of the arena. Watch videos of recent robot fights and you'll see that the contenders are rarely more than a few feet away from each other. There simply isn't time or opportunity to back across the arena for a high-speed ramming run -- your opponent will follow and you'll lose aggression points for not constantly moving toward them.

Rule of thumb: gear for enough acceleration to reach top speed in about half the width of the arena.

Q: hi mark what will happen if I connect 24 volt 8 ah SLA in series with 7.4 volt 5ah lipo to obtain 31.4 volt and also its effect on my E30-400 motors to run them on such a overvolting [Chandigarh, India]

A: [Mark J.] There are two general rules about combining cells in series, and you're violating both of them:

  • DO NOT mix cells of different chemistry; and
  • DO NOT mix cells of different capacity.
In this case, your LiPoly cells would deplete first, and the Sealed Lead Acid battery would continue to power the circuit and reverse charge the LiPoly. The result is a destroyed LiPoly and almost certainly a nasty fire.

A 30% overvolting of a motor will result in a 30% increase in speed, a 30% increase in torque, and a 30% increase in current. Since power is the product of speed and torque, the output of the motor would increase by 69% (1.3 * 1.3 = 1.69).

How well an AmpFlow E30-400 might cope with this overvolting would depend on how heavilly it is loaded. As you have told me nothing about the load you plan to place on the motors, I cannot comment on the wisdom of a 30% overvolting.

Q: Hello Mark, in an interview with Carlo Bertocchini in 1999 he said BioHazard was built with defensive ability on first and said if your bot can't be destroyed,you will probably go all the way,was that right? [Chinese Forum]

A: [Mark J.] I can't recall Carlo ever saying anything that wasn't right -- at least at the time he said it. In 1999 combat robots were fairly unreliable contraptions. It was very common for wheels to come off, chain drives to fail, batteries to break loose from their mountings, and motor controllers to melt. None of those things ever happened to BioHazard because it was very well designed and constructed. Carlo's championships and win/loss record speak for themselves.

In current robot combat it isn't enough to just survive. The judging criteria have changed to place all the emphasis on aggression and damage, while more powerful weapons have made survivability more problematic.

Q: A robot weighing 25 kilograms is expected to move with a top speed of 3 meters/second. To produce such speed with a 10 cm diameter wheel the motors should rotate with around 600 RPM. Suggest at least two motors that can produce the required RPM and torque. Also suggest batteries to power the robot for at-least 30 minutes and driver circuits (motor driver IC) to drive the motor. [Ghaziabad, India -- just east of Delhi]

Q: A robot weighing 25 kilograms is expected to move with a top speed of 3 meters/second. To produce such speed with a 10 cm diameter wheel the motors should rotate with around 600 RPM. Suggest at least two motors that can produce the required RPM and torque. Also suggest batteries to power the robot for at-least 30 minutes and driver circuits (motor driver IC) to drive the motor. [Delhi, India -- just west of Ghaziabad]

A: [Mark J.] Two absolutely identical questions submitted to Ask Aaron within a few hours of each other -- from two locations very close to a technical university. Questions that read very much like class assignments.

Why do you pay for an education and try to avoid learning? What do you expect to achieve by asking others to do your assignments for you? If you believe that I am so stupid that I would not recognize this as a class assignment, why would you trust my answers?

The worst part? All the tools you need to perform the calculations are available here on the Ask Aaron site and are discussed in multiple posts just a bit further down this page. If you had spent a few minutes reading you could have learned how to perform the required analysis and been done with your assignment.

You are embarassments to your school and to your families.

Team Fatcats' antweight robot 'Triton' Q: how did andy sauro's "triton" work? [Havertown, Pennsylvania]

A: [Mark J.] Team Fatcats' three-wheel antweight triangle spinner was an implementation of the 'melty brain' spinner technology -- also known as 'cyclone drive' or 'translational drift'. The tech was originally developed to give two-wheeled 'thwackbots' controlled motion while spinning, but three-wheel and even single-wheel variations have been implemented.

While the entire robot spins, on-board sensors read rotational speed and a microprocessor briefly cuts power to each drive motor at a critical point on each rotation to cause the robot to 'drift' in a specific direction in response to radio control input.

A full explanation of the technology along with demonstration videos and access to schematics, code, and hardware lists can be found at the Nothing Labs Open Melt page. There are also several posts about melty brain 'bots in the Ask Aaron Design & Construction archive.

UPDATE: Oops, I was wrong about 'Triton'. The antweight hyper-spinner was NOT an implementation of 'melty brain' technology. It turns out that Andy's three-motor spinner didn't actually have any real directional control at all -- it just spun, drifted randomly, and bounced off the arena walls. It did have a servo-controlled arm that could be lowered to drag on the floor and throw the whole robot off in some random direction if it wasn't being active enough -- but that hardly qualifies as 'controlled motion'. I don't think many event organizers would allow such a system to compete. Not recommended.

Q: What happens when we remove the gear assembly from a wiper motor and use only it with the main axle which is connected to the armature? [India]

A: [Mark J.] The power of a motor is the product of its torque and RPM. Gear reduction reduces the output RPM and increases the output torque, but the product of the two remains unchanged -- save for the mechanical losses caused by gear friction.

Automotive windshield wiper motors have a high ratio worm gear reduction -- around 60:1. Removing the gear assembly would increase the output RPM by a factor of 60 and reduce the available torque by the same factor of 60. You'd be left with a rather heavy and bulky motor that provides around 6000 unloaded RPM with perhaps 60 oz-in stall torque at 12 volts -- about 1/10th horsepower. That's not nearly enough torque to direct-drive a wheel for a robot drivetrain, and not close to enough total power to provide adequate robot performance if it was properly geared.

The smaller, lighter, and less expensive BaneBots RS-550 motor produces similar stall torque and three times the RPM at the same 12 volts, giving three times as much total power. Even if overvolted, wiper motors are inadequate for combat robot propulsion.

Q: Greetings, I wish to know what kind of batteries to support rs 550 motor, and what time it lasted??? [Equador]

A: [Mark J.] The current requirement for an electric motor depends on the load placed on that motor. An automobile will consume much more gasoline running at high speed, uphill, and pulling a heavy load than it will idling along a level road with no load -- and so to will an electric motor consume more current when asked to produce large amounts of power than when is is spinning along under a light load. To calculate the battery requirement for a motor you must first estimate the loading that you will place on that motor.

There are many posts in the Motors & Controllers archive that discuss estimating the battery requirements for motors used for robot drivetrains, and many posts in the Robot Weapons archive that discuss estimating the battery requirements for motors used in spinning weapons. Search those archives for 'battery capacity'.


  • Lightly loaded -- a 12 pound robot powered by two BaneBots RS-550 motors at 12 volts geared down 30:1 with 3" diameter wheels might consume less than 0.4 amp-hours of current in a 3 minute match with a peak current draw less than 11 amps.

  • Heavily loaded -- a 60-pound robot powered by two BaneBots RS-550 motors at 18 volts geared down 16:1 with 6" diameter wheels might consume 7.0 amp-hours of current in a 3 minute match with a peak current draw of 200 amps.

Q: we are making a rammer bot and we have a pair of drill motors with the following specs: 24 Nm torque, 1200 rpm and 14.4v. We dont know the stall current of the drill motor. I assumed a stall current of 100 A (is this an appropriate assumption?) and used the tentacle calculator; it told me that a pair of these motors would be sufficient to move my bot at around 9 mph (6 inch diameter wheels are used). The drill that we are buying (einhill rtcd 14 4 li) has two speeds that it works on (one high and one low, decided by a switch on the top), which components do we need to keep so that our motor only works on the high setting? I could not find any videos or guides on the subject.

Also, what is the best way to attach drill motors to our wheels? The wheels that we bought have a 1 inch hole in the center where we could attach the motor axle but this hole is too large. Would welding some metal into this hole to reduce the diameter be effective? or is there a better way? [India]

A: [Mark J.] Several problems:

  • You are misinterpreting the specs for that Einhill drillmotor;
  • You have made multiple errors in your use of the Tentacle calculator;
  • Two-speed drills are very awkward to use in robot drivetrains - you bought the wrong drills;
  • You will need to machine custom wheel hubs to adapt your existing wheels to your drillmotors -- you bought the wrong wheels; and
  • You didn't mention the weight of your robot -- I'll guess at 30 kg for my calculations.
The torque spec I have for the Einhill RT-CD 14.4 drill locked in 'low' speed (~300 RPM) is 36 N-m. With the gearbox in 'high' speed (~1300 RPM) the stall torque will drop to about 9 N-m. You can have the high speed or the high torque, but not both. I don't have an exact stall current figure, but 100 amps is pretty close to being correct based on comparable drills.

With the gearboxes locked in 'high', the 1300 RPM output with 6" wheels gives a calculated top speed of about 23 MPH but with very poor acceleration. In this configuration the motors do not deliver enough torque to prevent stalling the motors under heavy pushing loads, and because of the poor acceleration true top speed in a 12 foot arena is only about 5 MPH.

Locking the gearbox in 'low' with 6" wheels will give much better pushing power, acceleration, and a useable top speed around 7 MPH. This is still short of good rammer performance, but much improved over locking the gearbox in 'high'. Maximum current draw under heavy pushing will be about 27 amps per motor in this configuration.

It's a poor idea to buy drills to convert to a robot drivetrain without checking to see if other builders have had success using those drills. You will have several problems converting this particular Einhill drillmotor to robot use. The drill housing is all that holds the motor to the gearbox -- there don't appear to be any screws attaching the two components. The housing also holds the components that keep the gearbox in high/low speed mode. It may be best to retain the drill housing and trim away the excess material you don't need. If you're unwilling to retain the drill housing, you'll need to fabricate something like the Dewut?! Gearmotor Kit to securely hold the components in place and lock the gearbox in your preferred speed setting. It's a lot of work, but some bodged solution will likely come apart in combat and leave you without propulsion. A single-speed drill with the motor firmly attached to the gearbox is a MUCH simpler solution.

The junction between the gearbox output and the wheel -- the 'hub' -- is a critical area in combat robot construction. The hub must:

  • Match the diameter of the gearbox output to the wheel opening;
  • Prevent the wheel from falling off the end of the shaft; and
  • Lock the wheel onto the shaft to allow RELIABLE power transfer from motor to wheel.
Given that your gearbox output screw is round and your wheel has a round hole, that last part is going to be tricky. A simple friction clamping of the wheel to the shaft is NOT adequate for combat -- you aren't going to be able to just reduce the wheel hole size, stick a screw thru it, and tighten it down. A web search for 'robot drillmotor hub' may give you some ideas for functional hubs.

Q: We are making a combat robot for a 30 kg weight class event and have most of the components we need except the motors. The bots final weight is around 25 kg without the motors. We are planning to make a rammer type robot with wheels of around 12 cm diameter. We looked everywhere but cant find any decent motors to use (it would take too long for ampflow motors to arrive if we had them imported) so were thinking of either drill motors or wiper motors for our wheels (the bot uses two wheels).

Can you tell us which motor is better to use? The wiper motors seem to be extremely slow so i guess we would have to do some kind of gearing to increase the speed. They also look hard to attach wheels to.

We found some drill motors that had ratings of 15N-m and 500 rpm. Would two of these be enough to effectively drive a 25kg rambot? [India]

A: [Mark J.] I'm weary of saying this -- the Tentacle Torque/Amp Calculator can answer this type of motor selection questions for you. The calculator will provide speed, acceleration, current consumption, and even battery requirement for a robot of a given weight, specified motors, number of motors, wheel size, and battery voltage. Learn to use the calculator!

In general, automotive windshield wiper motors are slow and limited in total power. If you were to gear it up for more speed you would lose too much torque to maintain pushing power. I don't recommend wiper motors for any combat robotics drive.

A pair of drill motors with the rating you give would be much better than wiper motors, but still poor for a 30 kg ramming robot. Acceleration and top speed in a small arena are both less than adequate ramming 'bot performance, although pushing power would be adequate.

Seriously, learn to use the Tentacle calculator.

Q: would 4 30kg-cm motors with a 7.5 stall current and 10 cm diameter wheels be enough to effectively drive a 25 kg rammer bot? I used the tentacle calculator but im not sure what weight to enter since the rammer bot would need to overcome its own weight plus the opponents

A: [Mark J.] ALWAYS use ONLY the weight of YOUR robot as input to the Tentacle calculator. Only the weight pressing down on your drive wheels counts toward pushing power and drivetrain performance. Unless your opponent is on top of your 'bot, its weight does not enter into the drivetrain calculations -- and if it is on top of your robot the only impact it will have on a level surface is to slow your acceleration.

You haven't given me enough information about your proposed motors for me to calculate their performance with the Tentacle calculator. I need the speed (RPM) of the motors and voltage.

The best I can do is to take a guess at voltage and estimate the total output power of the motors to see if that is in the appropriate range for a 55 pound robot. Assuming 12 volts, the peak output power of each motor is about 20 watts (see the Motors & Controllers archive for the formula to estimate output power from stall amperage and voltage). Total for four motors = 80 watts. That's a bit less than 1.5 watts per pound of robot. A ramming 'bot needs more than ten times that much power to have effective speed and acceleration in a small arena. Even if these are 36 volt motors, you'd still be far underpowered. These motors won't do for your purpose.

iPhone 6 as designed by robot builders. Q: If a Robotic Combat builder designed an iPhone, I wonder what it would be like? [Facebook]

A: [Mark J.] If it was designed by an 'average' robot combat builder:

  • it would be held together with duct tape and hose clamps,
  • the critical parts would come from a cut-rate warehouse in Hong Kong, and
  • the interface would be awful because they finished programming it in the car on the way to the release party.

Q: m using ampflow e-150 motors for my drive. do i need to provide any relays or solenoids or can i directly connect it to my is a wired robot [India]

A: [Mark J.] If the cables and switches for your wired remote are capable of handling the large amperage required by the AmpFlow E30-150 motors, you can control the motors directly from your remote. However, the E30-150 motors can pull as much as 125 amps @ 24 volts if stalled; high current requires heavy cables and switches. It would be more effective to use solenoids on the robot and run only a small controlling current thru the cables and switches in your remote.

You will also find it very challenging to properly control your robot with simple forward/off/reverse control of the drive motors. Read thru our Solenoid Control FAQ for more information.

Badminton racquet and bird Q: i want to build a robot which can play badminton .How to choose dc motor for base platform having four wheel mecanum wheel(100mm)of robot ,if robot having wieght of 20 kg? [India]

A: [Mark J.] See answer to previous post on motor selection five questions down this page. In order to select drive motors you must first define your performance requirements -- in this case acceleration and top speed. Once you have those requirements, the Tentacle Drivetrain Calculator can assist in matching motors to your robot.

Note #1: Mecanum wheels add complexity to the calculations. A robot with Mecanum wheels will have both speed and torque reduced by up to 30% along diagonal axes (download table). To correct for this, increase minimum acceptable acceleration and speed performance by 41%.

Note #2: Mecanum wheels typically have poor traction, so high acceleration rates may be very difficult to achieve with Mechanum wheels regardless of the motors selected. This may severely limit the performance of an omnidirectional badminton-playing robot.

Periodic table entry for aluminum Q: Hi Mark. Hope you are well.

I found a local material place that stocks decent aluminum for robot purposes, mainly stuff like 6061. I noticed they also offer 2011 grade for the round bar. Price is very good, about the same/slightly cheaper than 6061.

Do you know if it is any good for robot uses? I would be using it to make gearbox housings, motor mounts and mounting standoffs in a 30lb full combat machine.

Thank you for your time and continuing the website. [Surrey, England]

A: [Mark J.] You're very welcome, and thank you for your kind wishes.

A source of 2011 grade aluminum at a price similar to 6061 alloy is a good find. 2011 aluminum typically runs about twice the price of 6061.

Known for its excellent machineability and bright finish, 2011 aluminum also has a bit better strength and hardness than 6061. It is well suited to your intended uses.

Note that 2011 aluminum has poor welding characteristics. For welded structures I would switch to the 6061 alloy.

Q: On the subject of sumo robot. What kind of motor can you recommend to achieve great speeds and great impact? [Guayaquil, Equador]

A: [Mark J.] See previous answer six posts down this page.

Team Run Amok does not compete in sumo so I cannot personally recommend motors for the sumo classes. I do know that some successful competitors use the 12 volt Maxon RE40 motors in the 3kg class, but they are VERY expensive.

I suggest that you find builders who compete in sumo to discuss drivetrains.

Q: I've been working hard on the drum weapon design for my bot. Is there anything else I want to concentrate on other than the weapon? [India]

A: [Mark J.] Your weapon is pretty much the LAST thing you should worry about. The really important things are the basics.

  • If your speed controllers fail, you lose.
  • If your drive motors fail, you lose.
  • If your wheel hubs fail, you lose.
  • If your gearboxes fail, you lose.
  • If your battery fails, you lose.
  • If your radio fails, you lose.
But if your weapon fails, you still have a chance to win. Make sure you get the basics right before you decide to complicate things by adding an active weapon.

Q: hello sir,i am building a thirty kg drum bot i am deciding to use radio system for drive the four locomtion motor and a weapon motor.But still now i have no idea about the radio system ,as i am a eee student i have good knowledge on now tell me where exactly i have to start and learn and do it? [India]

A: [Mark J.] Read thru the first six or eight posts in the Ask Aaron Radio and Electrical archive. In those posts you will find links to:

  • our R/C Radio Reception guide;
  • an article on How R/C Radios Work;
  • our Combat Radio Function Guide;
  • an R/C robot circuit diagram in our FAQ;
  • our Radio Transmitter Programming Guide. and
  • our Combat Robot R/C Gyro Guide.
It's not like I'm trying to hide this stuff.

Update: I decided to put a single reference to all of the Run Amok radio guides in the FAQ. I believe it's FAQ #20.

Q: Hello will you please help me out by suggesting the torque and rpm required to move a robo car which is having a weight of 60kg and need to push a weight of about 40kg of a DC electric motor if possible mention in brief what should be specification i am planning for a four wheel drive. [India]

A: [Mark J.] The answer depends on how fast you want the 'bot to push that weight, how fast you want it to accelerate, and the diameter of the driven wheels. A motor with very little torque and speed can be geared down to push a 60kg 'bot and a 40kg weight slowly, but if you want more speed you'll need more power. Examples:

  • A 60kg 'bot with 3" diameter wheels powered by four gearmotors each delivering 3 kg-cm of torque and 150 unloaded RPM could push a sliding 40kg weight at about 0.5 MPH.

  • The same 'bot with 6" wheels and gearmotors delivering 130 kg-cm torque with 1000 RPM could push a sliding 40kg at about 12 MPH.

You can't calculate the power you need until you figure out your performance requirements. There are MANY posts discussing motor selection in the Motors & Controllers archive.

Q: The point (6,a) and (b,8) lie in the line y=3x-7, find the value of a and b [Glasgow, United Kingdom]

A: [Mark J.] Algebra homework? No robots? Sigh... just for fun:

If the two points are on the line described by the equation, they must both satisfy that equation. We'll substitute the given x and y and solve the equation to get a and b:

  • For the point (6,a): x=6, and we plug that into the equation to get

    y=3(6)-7 = 18-7 = 11, a = 11.

  • For the point (b,8): y=8, and we plug that into the equation to get


    ...add 7 to each side of the equation to get


    ...and divide each side of the equation by 3 to get

    5=x, b = 5.

You're welcome.

Q: hi mark ,i was planning to build a 30 kg bot with vertical spinning bar of dimensions 20*15*5 cm (l*w*th) of mass around 7 kgs (mild steel) and i want to run it around 3000 rpm ,i was using an ampflow e30-400 to power my weapon and i want know the reduction ratio and energy storage of my weapon(i was unable to get through the spinner spread sheet)....and can i use ms for teeth ? [India]

A: [Mark J.] Lets start with the reduction ratio. From the AmpFlow website: the no-load speed of the E30-400 motor at 24 volts is 5700 RPM. Allowing for mechanical and aerodynamic drag on a spinning weapon will reduce that speed by about 15%, so let's call it 5000 RPM. You want a 3000 RPM weapon speed, so the formula is:

Motor Speed / Weapon Speed = 5000 / 3000 = 1.67 to 1 reduction.

A chunk of steel 20 cm by 15 cm by 5 cm isn't as much a bar as it is a square, and it will weigh closer to 12 kg than 7 kg. If you are removing material from that almost-a-square to reduce it's mass to 7 kg, I'd need details of the true shape of the rotating mass to calculate the energy storage.

IF the weapon is a simple 12 kg steel block of the dimensions you give, it will store about 3000 joules of energy at 3000 RPM. Spin-up time will be about 3 seconds.

Tooth material should be harder than the material you'll be hitting with the teeth. Mild steel isn't very hard. If your particular steel alloy can be hardened, I would consider at least surface hardening the teeth.

Q: hi mark ,iam the spinning bar guy again sorry i have given wrong info about it..... actually its dimensions are 20*12*4 cm (l*w*t) around 7.4 kg ,what will be the energy storage of my weapon ?and was that weapon is enough for my 30 kg bot ? OR i should chage my weapon dimensions ? thank you for your info about reduction ratio......

A: [Mark J.] That's a big difference. I'm not sure how you confused the dimensions that badly and sent me down the rabbit hole with the other numbers. The 'Ask Aaron' service is free, but that doesn't mean my time has no value. Please double check your numbers in the future.

A steel bar 20*12*4 cm spinning at 3000 RPM will store about 1700 joules of energy with a spinup time of about 2 seconds. That's better than 25 joules per pound of robot weight, which isn't bad for a bar spinner.

If your design allows, a longer and narrower bar of the same weight would store more energy. Examples: a 24*10*4 cm bar spinning at 3000 RPM will store about 2100 joules (2.5 second spinup), and a 28*8.5*4 cm bar will store more than 2600 joules (3 second spinup).

I'd strongly suggest that you learn to use the Run Amok Excel Spinner preadsheet so that you can examine many weapon designs and find the one that best fits your needs and your design parameters.

Q: Greetings, my question: are two Banebots RS-550 motors with 16:1 P60 gearboxes enough for a 3kg sumo robot for metal dohyo? [Quito, Ecuador]

A: [Mark J.] We don't compete in sumo, and my initial reaction was that a pair of RS-550 motors can provide WAY more than enough power for a 3 kilo robot, even with magnetic downforce. Then I went out and found some video of recent 3 kilo sumo matches. YIKES! Take a look:

If you're competing at this level, I think you need to find someone who is a current competitor in 3 kg sumo to discuss drivetrains. The RS-550 motors may be adequate, but the BaneBots P60 gearboxes are probably too heavy for your purpose -- they are built to take heavy impacts that you aren't going to have in sumo. I suspect you'll want more than a 16:1 gear reduction to get that lightning acceleration in a very small arena -- maybe 26:1? Sorry I can't be of more help.

Q: Dear Mark,
Beetleweight eggbeater 'Wave of Mutilation' I am looking into building a beetleweight Eggbeater robot. I am trying to find a place to manufacture the actual eggbeater part of the robot. Its 2.5"x3.5"x.625" made out of steel (preferably tool steel). I looked at e-Machine shop to try to manufacture it and their price that they gave me was around $600! That seemed outrageous to me. That's over twice what I am spending on parts for the rest of the robot. I know that robot combat is an expensive hobby, I've built two already, and that the weapon needs to be the sturdiest part of the robot but my question is does this price seem reasonable to you? [location withheld]

A: [Mark J.] You didn't include a link to a drawing of your beater design, so I can't comment directly. It's possible that some element of your design may unintentionally call for very time-consuming work. If the design is a conventional rectangle with simple holes for the shaft, the price is not reasonable.

I'd suggest that you post your design to the Facebook 'Robotics Community' for comment and a referal to a shop to water cut your beater bar at a reasonable price.

Q: VestedMadScientist here again. I recently noticed that there is a robot in the lightweight class known as 'GLaDOS'. Do you know if that team affiliated with VALVe Software, or are they just using the name? [North Carolina]

A: [Mark J.] Lightweight 'GLaDOS' is the most recent robot from the very successful Team Adrenaline -- a group of students from Olathe Northwest High School in Olathe, Kansas. They have no connection to VALVe.

Q: Hello sir. I am new to this site. I would like to build a 30 kilo war robot with a drum weapon. Would a hollow drum or a solid drum be better? What kind of a material should I use? And how should I use this? [Maharashtra, India]

A: [Mark J.] Welcome to 'Ask Aaron'. I do ask that you do me the courtesy of reading thru the Frequently Asked Questions and the 'Recent Questions' page, and then performing a keyword search thru the 'Ask Aaron Archives' to see if we've already answered your questions.

  • Your solid/hollow drum question is answered just a few questions below on this page, and there is a great deal of additional information on drum construction in the Robot Weapons archive.

  • Your question on drum material is answered two questions below the answer to the hollow drum question, and again is discussed in the Robot Weapons archive.

  • Your question about how to build the weapon is -- you guessed it -- answered below on this page and discussed in detail in the Robot Weapons archive.
Read thru the earlier questions and answers. If you have NEW questions, write back.

Q: I bought the wrong BaneBots gearboxes! I have the 256:1 gearboxes which are far too slow for my purpose. Is there something I can do to change the gear ratio and speed up the gearbox? [India]

A: [Mark J.] BaneBots P60 gearboxes are modular. Your gearboxes have four 4:1 reduction stages housed inside a '4-stage' cylindrical ring gear. BanBots sells ring gears in 1, 2, 3, and 4-stage lengths. You can disassemble your gearboxes and reassemble them into one of the shorter housings while leaving out 1, 2, or 3 of the reduction stages. You'll end up with a 3-stage 64:1 gearbox, a 2-stage 16:1 gearbox, or a single-stage 4:1 gearbox. One of those ratios will likely be close to what you need.

Chain and sprocket drive Q: As I told you, I bought four BaneBots 256:1 gearmotors for locomotion in my 30 kilo robot. Since it is very slow I am planning to make an 18" wheel and make a dead wheel unit in that (see drawing). If I do this will I increase the speed? Is it reliable? [India]

A: Every gear reduction stage you put between the motor and the wheel steals power from the motor. You've already got 4 gear stages in your gearmotor, and you want to add another one? The chain reduction will sap still more power, add extra weight, and create a new mechanical system that can fail.

A simpler, stronger, lighter, and more reliable solution is to remove one gear reduction stage from the gearmotor. If you are unable to obtain the correct shorter housing, you may reduce the length of the current housing on a lathe. That will give you a 64:1 reduction, which will work well with about 7' wheels in a small arena.

Q: As a newbie I have been trying desperately to find a template list of basic components for a hobbyweight robot but so far nothing. Its not that I am lazy or want a short-cut and in fact I have already read loads of articles on robot building and have browsed endless parts catalogues.

What I am after is a set of basic components (motor/hub/wheel/controller/battery) to use as a base reference (a sort of "minimum specs" with which the robot would at least function).

The thing is some of these motors are really expensive! I certainly don't want to buy a motor that is overkill but neither do I want my robot to fall apart due to plastic gears or weak shafts or some silly thing like that! So its not just a trivial decision. [Tarxien, Malta]

A: [Mark J.] I can certainly understand your desire for guidance on components, but combat robotics really isn't a 'template' type of endevour. A fighting robot is often greater than the sum of its parts, and a good portion of the challenge is to create a design that compliments the strengths and minimizes the weaknesses of available components.

  • All the components of a 'bot must function well together, so choice of 'motor/hub/wheel/controller/battery' will vary with the design you have in mind for the rest of the robot. For example, a wedge or rambot will call for more speed and power from the drivtrain than would be appropriate or needed for a drivetrain that will push around a big spinner weapon.

  • The hobbyweight class is currently lacking in off-the-shelf components that are well suited to the purpose. In particular, robust gearmotors with reasonable output and simple mounting options are just not available. The traditional source of hobbyweight motors has been hacked cordless drills, but they require some work to adapt to the purpose and are awkward to mount.

  • If you ask a dozen different sources for recommendations on hobbyweight 'motor/hub/wheel/controller/battery' you'll get no concensus on components. Everyone has had a good or bad experience with specific items.
Bottom line: the hobbyweight class is currently a poor entry point for new builders. If you have the option to build for a different weight class I'd recommend you give that some consideration.

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.

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