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4872 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 .. fork tooth guy again .... sorry I am troubling a lot .... We have a lot of drum bots in India , so my primary concern would be getting under a drum bot .... now these things have the skates which support the heavy load of the drum .. so with a wide wedge it would be difficult to get under head on ... ( getting the wedge form the side would be tough manoeuvring ) .. can you suggest something to easily get under a drum if you go head on ... OR what should be the strategy against a drum bot? Can I have some other attachments against horizontal spinners using the same bot ? [India]

A: [Mark J.] Aha! Thank you for the explanation. I better understand what you're trying to accomplish. The 'spinner killers' in the US all use wide scoops or wedges, and these work well against drums too. Here's why:

  • If the low wedge runs into the drum before it hits the front skids, the low contact angle hit will 'pop' the drum weapon upward and lift the skids to allow the wedge to penetrate beneath them.

  • If the wedge hits the skids before receiving impact from the drum, your four-wheel drive should give better traction than the two-wheel drive on the drumbot and you can shove them wherever you like.
You're in control either way. If you're having trouble getting under the skids with a full-frontal attack, try easing the edge of the wedge in toward the middle of the drum at an angle a little off head-on.

Q:something about tyres ...... they say friction does not actually depend upon surface area ... assuming that coefficient of friction remains same throughout the arena.... how would it actually make a difference using [wide] and [narrow] tyres . theoretically it should not ... but what happens in real? And how [wide a tire] should I use for my 4WD wedge bot ? one thing I noticed is that the skid drive is difficult in [wider] tyres

A: You are correct. Wider tires - in theory - do not generate greater traction than narrow tires bearing the same load.

Race cars use wide tires to better control heat build-up under racing loads, not directly for increased traction. Race tires also get physically 'sticky' when hot and can generate coefficients of friction greater than 1.0 at the expense of greatly increased rolling resistance. Neither of these situations apply to combat robots. Some early combat robots used go-cart race tires with very poor results because the robots just don't generate the high-energy loads needed to bring the race rubber 'up to temperature'.

So, why do some robots have wide tires? There is that little caveat you mentioned in your question, "...assuming the coefficient of friction remains the same throughout the arena". Combat arenas are dirty, dusty, oily, greasy, messy places. A wider tire covers a larger 'contact patch' and will be less effected by small slick spots.

I can't supply you with a formula to calculate the optimum width for your tires. It would depend on robot weight, power, rubber compound, and arena conditions that you really aren't going to know in advance. Your observation that wider tires do make skid-steer robots less maneuverable is a good one. It would be nice to be able to arrive at the arena with an assortment of tire widths and compounds to try before the event, but that isn't terribly practical.

My advice is to stick to moderate tire widths and to clean them with a solvent before each match. I use 'lighter fluid' (caution: flammable), but alcohol (caution: flammable) works well too.

Q: Hi aaron, excuse me I have a question... Is this battery enough for 2 motors banebots Rs550 for build a sumo robot? This robot weigh 3kg without force magnet. [link to cheap Hong Kong 5000 mAh 4S Lipo battery removed] [Equador]

A: [Mark J.] Scroll down the page about nine posts for a previous answer to battery capacity required for a 3kg sumo robot powered by RS-550 motors. The 5000mah battery you found has at least five times the capacity you need for a sumo match. To give a more precise answer I would need to know the motor gearing, wheel diameter, and the amount of magnetic downforce you're using.

Advice: since you don't need a battery this large, spend some of the money you're saving to buy a better quality Lipo -- something from a different distributor.

'Newton' beetleweight combat robot Q: hi mark
Newton - Weapon test - Beetleweight combat robot from TeamExpat
impressed in seeing their weapon. i want to know the construction of that weapon and they have used 1200kv brushless motor for their weapon. can brushless motor be used for running weapons in combact robotics [India]

A: [Mark J.] The full build log for 'Newton' is in a thread on the FRA website: Team Expat Beetleweight Adventures.

Nearly all current insect-class robots use brushless motors for their spinning weapons - see FAQ #23.

Q: hi mark ... I have decided the general strategy for playing in 60 kg category . As I have a descent control on manoeuvring the bot so as to put the wedge under the opponent from the side . I have decided to make a 4WD wedge design with only a single long teeth like solid fork coming out. this will be non adjustable and only the tip of the wedge will be touching ,scraping the ground .there will be a similar wedge on the back side having inclination upwards so as to play even if the bot is inverted ( the lateral view will look somewhat like a parallelogram ) . the bot is vulnerable to undercutter types ... but i plan to ram them before they catch speed .... because the arena is very small ... can you please tell if there is something i can improve in the design . the body will be box type .... so how can i avoid spinners because they take the corners . also can you help me out in choosing the correct material is the wedge is solid ... long teeth like protruding .... has this kind of design implemented before [India]

'Mini Maxbot' combat robot A: [Mark J.] Relying on a strategy of attacking a vulnerable exposed side is not terribly workable. It's a rather long path to maneuver around to the side of your opponent, and all they have to do to counter is pivot. You might get lucky against your first opponent, but your second opponent will be waiting for that move.

I've seen a number of combat robots employing a wide range of fork-like wedge teeth. The design has been fairly common when used with an integrated lifter weapon ('Pack Raptors', 'Tazbot', 'Pipe Wench', 'Vlad the Impaler', 'Wendingo', 'Takatakatak'...). Individual long wedge fingers are not as strong as a solid wedge might be and are susceptible to impact damage. Solid plate wedges have proven more durable and more useful against a wide range of offensive weapons -- particularly spinners.

Dual-ended parallelogram-profile robots were also fairly popular back in the BattleBots era ('Bad Attitude') but are no longer seen in robot combat. The added weight and awkward design is not worth the protection against being inverted. A single hinged wedge that can drop down into a new position if inverted is more effective if being inverted is a real concern. Do you really expect your low wedgebot to be flipped very often in a small arena?

If you're worried about sharp corners on your robot attracting spinner attack, round the corners off! Even a little work with a hand file can round off a sharp edge enough to be useful, but a gently curved surface is ideal. Very hard surfaces are also useful in preventing a spinner from digging into the surface and getting 'bite'. Best option is probably keeping your wedge between you and the spinner.

There is a lot of discussion about wedge material choice in the Materials & Components archive -- suggest you look there for ideas. A great deal depends on what materials are available to you in India. A tough titanium alloy is ideal, but resilient heat-treated steel alloys can be a good solution as well.

'Origninal Sin' and 'Sewer Snake' combat robots Q: hey mark ... the fork tooth guy ..... Now , if I make a hinged wedge which is invertible .... and I bend the edges of the wedge downwards ( for preventing side attacks ) like woloop or original sin.... then when the bot gets upside down .. then the portion bent down will be upwards ... will that be a good wedge?

A: I really wonder why you're so worried about being invertible. Small arena + low wedge robot = low chance of being flipped. The wedges on 'Wallop' and 'Original Sin' would be next to useless if flipped over. I'd suggest worrying more about building an effective robot and less about being flipped.

Q: Or else how about forks like skewer snake? are they invertible? are they vulnerable to spinners?

A: 'Sewer Snake' can self-right if inverted, so it doesn't worry about inverted operation. It also has an assortment of front attachments. When fighting horizontal spinners it has been known to replace the forks with a smooth conventional wedge -- a much better choice.

Q: if we want to give maximum impact of drum teeth to the oposition,then what should be the position of the skid,it should be closer to the drum or should be away. [India]

A: [Mark J.] Suggest you read section 6.6 of the RioBotz Combat Tutorial for a full treatment of weapon support and maximizing impact. Read the rest of the RioBotz tutorial while you're at it.

In general, your skid should be placed as close to the drum axle as possible and be as stiff as possible.

Q: hi mark.
i got two ampflow geared motors e30-150
its rated rpm is 700rpm i need to reduce it to 300rpm
what are the possible ways to reduce the rpm of the motors [India]

A: [Mark J.] I could give you the full list of ways to reduce the output RPM, but I'd rather give you a single method that makes sense for your application. Offhand, I can't think of a reason why you'd want a specific 300 RPM output for a combat robot. You paid good money for the power and speed of the AmpFlow motors. My question to you: why do think you 'need' to reduce the RPM?

Q: reply to mark: in india arena is so small so 700 rpm is difficult to control im not using speed controller (out of my fund) in such cases i would like to run those ampflow motors at 300rpm.

A: Overall, it would have been better to select gearmotors more closely suited to your requirements -- but you do have some options:

  • Running the motors at 12 volts instead of their rated 24 volts will cut the RPM in half. It will also cut torque in half, which will reduce acceleration. If your primary concern is controllability, this may be an effective and simple option.

  • A chain drive running from the gearmotor output to a new wheel carrier stub axle with sprockets in a 3:7 ratio (like 12 teeth on the motor, 28 teeth on the stub axle) with give you 300 RPM. This is heavy, adds new failure points, and wastes the very well constructed output shaft on the MagMotor gearbox. This reduces the RPM but increases torque, which will increase the responsiveness of the robot and may add to your controllability issues.
You didn't mention the construction details of your robot, like weight and wheel size:
  • If your main concern is a reduced top speed, you can reduce speed without changing RPM by reducing the wheel diameter. The MagMotor gearbox can be used with wheels as small as 4" diameter. This has the same torque-increasing issue as the chain and sprocket solution but is simpler, more reliable, and actually reduces weight.

  • If your main concern is controllable acceleration, you can reduce acceleration by increasing wheel diameter. A larger wheel will make the robot more controllable with simple on/off/reverse switches. Top speed will be limited by the slowed acceleration in the small arena. If you chose this option, make sure the larger wheel diameter does not permit stalling of the motors under heavy pushing loads. A 60 kg robot can effectively use up to an 8" diameter wheel with the MagMotor E30-150 gearbox.

UnMakerBot - lightweight combat robot Q: Hi..Mark..what is the dimensions,motors and details informarion of UnMaker bot.. [Maharashtra,India]

A: [Mark J.] I have no details on the 2014 Stem Tech Olympiad lightweight champion. Best I can do is the photo at left of the 'bot with its cover removed. The motors look like Castle Creations products, but I can't say which ones. If you're trying to duplicate the 'bot you'd better have serious machine shop skills and deep pockets.

Weapon shaft clamping collar Q: Hello, I am designing a 15lb horizontal spinner robot, with an uncanny resemblance to Hazard. I was trying to figure out a way to securely mount my 5.16lb, 24in bar across the top. I have visited Team Delta's website, and they seem to be using shaft collars both on the top and bottom. I am currently running a dual bearing support system, and a v-belt pulley driven by a beefy brushless motor. Any help would be appreciated! Thanks -- Erik [Winchester, Massachusetts]

A: [Mark J.] I wrote one answer to this question, briefly posted it, thought a little more, took the original post down, and started again. Let's see if this makes sense.

Hazard's 22 pound spinning blade is not directly fixed to the 1.5" diameter steel weapon shaft. It is squashed firmly between two shaft collars (see picture) that are tightened onto the shaft by machine screws that reduce the diameter of the collars for a clamping fit -- not held in place by set screws! Custom phenolic washers on either side of the blade form a mechanical clutch that allows the blade to slip on impact and limit the shock transmission back to the drivetrain (Hazard Build Report).

I'm not very keen on squishing a weapon blade between two collars and relying on friction to spin it up, but it worked very well for 'Hazard'. If you do decide to use shaft collars, I would strongly recommend clamping collars over set-screw collars, but I think I'd axially fix the blade by broaching the shaft and blade and fitting a key to lock rotation -- let your v-belt act as the slippy-clutch.

Q: I've found an inexpensive ESC [link removed] that looks like it will work for my robot, but it doesn't have any type of overcurrent protection. Is it practical to go into combat without some current protection? [Deep 13]

A: [Mark J.] You've found a very simple ESC with no reverse current protection, no current limiting, no thermal protection, no LiPo voltage shut-down, no mixing, no midpoint throttle sensing, no DIP switches, no jumpers, no cooling fan, no mounting bracket - no frills at all. I rather like that. There's no complex manual to read thru and no way to get the set-up wrong for your combat application. Just make sure you wire it correctly or you'll fry the ESC.

Most ESC 'protection' circuits fuction very simply: when they sense a problem they shut down the robot. 3 malfunctioning robots

  • Getting too hot? Shut down.
  • Too much current? Shut down.
  • LiPoly battery voltage low? Shut down.
This can be handy in set-up and testing sessions, but THE LAST THING you want in actual combat is for the robot to decide on its own to shut down. I'd much rather squeeze the last few seconds of performance out of an ESC and be mobile at a critical point in the match than have the controller decide it's had enough and quit. An extra few seconds of power can be the difference between winning the match and being ripped to shreds.

A combat robot should turn off ALL 'protection' in combat: no fuses, no LiPo voltage, no thermal, no overcurrent - NOTHING that lets the machine decide it should shut down on it's own except radio fail-safe. It's fine to have sensors trigger audio or visual warnings about overloads, but let the driver decide if it's a good time to ease up or shut down.

Q: hey mark ..... what is the best wedge for a 3 kg sumo match ? my sumo bot is 4WD ...and I have placed a wedge at 45 degrees. It did work out for some matches but not very successful . also can you please tell some traction increasing tips for this weight category? the arena for sumo in India is just a round with 2 meter diameter . [India]

A: [Mark J.] Please check at least the FAQ and the recent questions before submiting a new question.

  • Team Run Amok does not compete in sumo. Any advice that we give is gathered from other sources. You are best advised to get your sumo info elsewhere.

  • Four posts down this page are links to everything we know about sumo traction.

  • Check this old post in the archives for general advice on sumo wedges.

Q: Hello,

I am using a duratrax 550 motor to drive a 3 kg robot 6WD. The motor will be geared 21:1 and the wheel size is 36mm in diameter. I am having trouble with the selection of the correct battery (lipo?) for it. Would appreciate it if i am given some advice on this matter. Many thanks [Singapore]

A: [Mark J.] A single Duratrax 550 motor? I'm going to assume two motors, as one motor would be an odd design for a 6 wheel drive robot. I'm also going to assume this robot will compete in the 3kg sumo division.

Two RS-550 motors is a whole lot of power for a 3kg robot. Acceleration performance will be excellent, and the motors will be very lightly loaded so they won't consume much power. The Tentacle Torque/Amp Calculator says the motors will consume only a bit over 5 amps total under heavy pushing before the wheels break traction and limit greater current flow. Total power consumption for a three minute long match (very rare in sumo) would be about 300 mAh. Any small three-cell LiPoly pack with 400 or more mAh capacity should be fine for your purpose.

Q: hi mark, hows u? ... lucky here again I need ur help instead using sabretooth 2*60 motor driver can I use sabretooth 2*32 or sabretooth 2*25 for my bot ....
actually fact is about cost 2*60 is much expensive
actually i thought with proper care I could use 2*32 for my two ampflow e-series I mean e30-150, what u say ? [India]

A: [Mark J.] I'm still sad. Lucky. Still very sad.

We've discussed your drum weapon extensively, but I don't think I have the details of your robot drivetrain. An AmpFlow E30-150 motor can pull as much as 125 amps at 24 volts under heavy loading, so it seems like wishful thinking to hope that a 32 amp motor controller might be enough -- but there is a simple way to check...

The Tentacle Torque/Amp Calculator can quickly answer the question of the maximum sustained amperage that your motors might be expected to pull given the weight of the robot, wheel diameter, gear reduction ratio, and battery voltage.

Suggest you do a little analysis and see what your actual current consumption might be before selecting a motor controller. You really don't want to be worrying about 'proper care' of your ESC in the middle of a combat match!

Q: lucky again aacording team tentacle amperage calculator motor will need 20 amps continous and 40 amps peak . and [the Sabertooth] 2*32 [motor controller] is able to supply 64 amps at peak and 32 amps regular then sabretooth 2*32 will sufficient I think so?
cause my sla could not provide more than 70 amps

A: You're mis-reading the results from the Tentacle calculator, Lucky. If you click on the friendly 'Help' button on the right side of the middle section of the Tentacle calculator display, you will find detailed descriptions of the input and output fields.

  • The output labeled 'Amps (per motor) to spin wheels:' is the greatest continuous current consumption expected under heavy pushing by each single motor in the drivetrain. If you have one motor on each channel of your motor controller, then this is the absolute minimum current your controller should be able to continuously provide.

  • The output labeled 'Total Peak Amps:' is the sum of the continuous current consumption of all motors in the drivetrain. Since your 'bot has two motors, it is simply two times the 'Amps (per motor) to spin wheels:' field. This value is used for battery calculations and does not directly relate to motor controler 'peak' capacity.
The Tentacle calculator assumes that the drive wheels are loaded only by the effort required to push the robot. If additional drag or loading caused by mechanical interference or malfunction is added to the drivetrain, the current consumption of the motors will increase. Examples: bent bodywork rubbing on a tire, malfunctioning gearbox, debris wedged between wheel and chassis.

Now that the output of the Tentacle calculator is clear - yes, I agree that the capacity of the Sabertooth 2x32 ESC is sufficient for the normal combat operations of your robot.

I do wonder why you believe your SLA battery cannot provide more than 70 amps. I've never seen a SLA battery supply data on absolute peak current. The 'Cold Cranking Amps' (CCA) often cited in SLA specs refers to the maximum current a battery can deliver at very low temperature (0F/-18C) for a continuous 30 seconds. Peak burst amperage at normal ambient temperature can be much greater, and certainly enough to fry a small motor controller. Make sure you know what the numbers mean or they'll turn around and bite you.

Q: Hi Mark, Sage here. Thanks for the previous answer [scroll a couple questions down the page]. You are right, I guess I shouldn't skimp on an ESC. Originally I thought that the Sabertooth 2X32 Dual Motor Speed Controller or Roboclaw 2X30A Dual Motor Controller would suffice. But I kinda have a bad feeling about them and my gut tells me that I should get the Sabertooth 2X60 Dual Motor Speed controller or Roboclaw 2X60 Dual Motor Controller. As I have to import them the 60A ESCs would end up costing about $150 more than the 30A ones. Should I go for the 60A ones or save them for my next bot? And which one do you think would be a better choice Sabertooth or Roboclaw?

Thanks again and sorry for bugging you with a hell lot of questions! [India]

A: [Mark J.] No need for an apology, Sage. If I didn't want to answer robot questions, why would I run a robot question site?

I would not hesitate to use a 30/32 amp ESC for your current project. But if there is going to be a 'next bot' that will have greater amperage demands, why spend money now for an ESC that you will soon outgrow and replace? It may make better economic sense to buy a larger ESC now.

It's difficult for me to offer a recommendation between the Sabertooth and Roboclaw controllers. Each has drawbacks:

  • The Sabertooth ESCs have known quirks in the R/C interface that give some builders trouble - particularly the 'autocalibrate' feature. I know these are commonly used in India.

  • The Roboclaw controllers lack acceleration/decelleration 'ramping' to protect the ESC from overloading in combat conditions that require rapid reversing of motor direction. Not commonly used in robot combat.

  • A third option: you might consider a pair of the 85 amp BotBitz ESCs. I've never had my hands on these, but they're popular in Austrailia. These are simple and compact single channel ESCs (buy two) that seem to hold up well to robot combat.

Q: hey mark ... what is wheel traction compound exactly ... I read about it online , how does it work ? does it remove a small layer of rubber or just cleans it ? is it suitable fol all kinds of rubbers? I was planning to use it for my 3 kg sumo wrestling bot . I have used cricket bat grip wrapped around tyres for grip ... can we do anything more to increase traction ? [India]

A: [Mark J.] Many different products are sold as 'traction compounds':

  • Some are chemicals that break down rubber molecules into shorter lengths, softening the rubber surface and making it more 'grippy';

  • Some are resins that bond to the rubber and provide a thin high-traction layer;

  • Some are just sticky goo that forms a weak adhesive bond between tire and surface.
FingerTech sumo wheels Different types of traction compound are applicable to different types of rubber and to differing conditions. First, check your event rules about tires and traction compounds. Many events forbid applying anything 'sticky' to the tire, and most would not be pleased to have you use a compound that leaves a residue on the arena surface.

Serious sumo tires made from soft polyurethane resin or silicone rubber do not require chemical help to deliver optimum grip on smooth, clean surfaces.

The Ask Aaron archives have many posts on traction tricks, the best 'rubber' for sumo tires, and links to instructions on how to mold your own super-sticky urethane treads. Search the Design & Construction and Materials & Components archives for 'sumo'. You'll also find reference to some more exotic methods of improving traction, including magnetic and vacuum downforce. Again, check your event rules carefully.

General tip: wipe your tires clean with a solvent before each match to remove any oily residue and dirt -- a tire covered with dust is not a grippy tire.

Q: Hey Mark! It's Sage again, the hydraulic power pack motor guy. I experimented with the Team Tentacle calculator and I noticed that my motors won't pull more than 27A each at 8:1 reduction ratio with 6 inch wheels before they lose traction. I also found an ESC with 30A continuous current with 50A surge current for 10s. It's link is: Phoenix Series 24V/30A
I was thinking of attaching a fan & heatsink to it myself.
Can i use this ESC for driving my lightweight without toasting it? [India]

A: [Mark J.] What you've found is NOT an Electronic Speed Controller (ESC). The Phoenix Series motor drivers are somewhat deceptively advertised relay boards for simple forward/off/reverse operation of two motors. They provide no speed control.

Manufacturers are interested in selling their products and will describe them in the best possible terms. I suggest using products that are in wide use by robot builders with a well established record of performance. Don't be the first guy in line to discover the problems with a new product. Talk with other builders and use products that are proven to stand up to combat robot use.

Note: it's rarely possible to add current capacity to a commercial ESC with additional heat sinks and fans. The power chips must be designed and mounted in a manner that will allow the added cooling to efficiently draw heat away from the crucial junction where the heat is being produced. If additional heat sinking would add significantly to the capacity of the ESC, the manufacturer would already be doing it - or would at least provide information on additional heat sinking in the product documentation.

Q: Hi Mark, it's Sage. Thanks for pointing out that the Phoenix series was a relay board, I wouldn't haven't even known that! I completely understand that an ESC is an essential part but I just don't have the money left to buy a high end one. I know that using the above relay board has demerits as compared to an ESC out of which lack of speed control is probably the biggest one. Can the Phoenix series still be used in my robot WITHOUT TOASTING IT?? (the motors draw 27A before the wheels lose traction at 8:1 reduction and 6 inch wheels) i'm pretty curious about it's suitability I just wanna know will it handle the load or just burn. I know that it only offers bang-bang controlling so it's either full forward, off or full reverse... But I'll work my way around it.
I used the phoenix board previously on an all terrain robot and I observed that it handled 40A surges easily I measured it myself.. So I think the numbers on the site probably hold good.

A: As I recall from your earlier questions, you're building a 'Breaker Box' style robot -- about which you correctly observed that "being fast & controllable is a necessity." Now you've got motors that will give you the 'fast' part, and you're trying to get the 'controllable' part from a relay board? Rethink.

If the specs on the Phoenix board are reliable it should be able to control your drivetrain without toasting, but you won't like it.

Q: hey mark .. if you stand behind a two wheeled bot , and consider the bot is making a left turn( differential drive) ie . right wheel moving forward and left wheel stopped . there will be some force acting which will try the right wheel to come out of its shaft .... I mean to say ... if a bot is taking such a turn and the wheels are not fixed properly , the right wheel will then tend to come out of the shaft , that means there is some force acting along the axis of the drive shaft of the motor , how can I calculate this force ? ... a little deviated from the combat topics , but I want to know . I tried to draw the free body diagram but couldn't figure it out ... [India]

A: [Mark J.] Your question reeks of a homework problem. Whether it is or is not, it isn't directly related to combat robots.

I'll put you on the right track: your free body diagram isn't working because the axial force is caused by the turning motion dragging the wheel sideways thru the turn. It's a sticky problem with a lot of unspecified variables (turn radius, weight on wheel, speed, coefficient of friction...) Figure out how far you're dragging the wheel sideways per unit time and you'll have a good start on the solution.

Q: Hi , we have already designed 15 kg robot like last rites with horizon spinner and defense robot with wedge in 15 kg bot. Now we are trying to design K2 design, so kindly mention the merits and demerits of K2 bot. Clearly tell to design weapon of K2 for weight of 15 kg(weapon alone 15kg) and also the material selection for weapon including shafts, bearing and insert materials. [India]

A: [Mark J.] 'Ask Aaron' is not a free engineering service, and we aren't going to design your robot for you. See FAQ #4, and read the rest of the FAQ while you're there.

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.

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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