4983 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) 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
'Ask Aaron' gets a lot of questions about designing spinning weapons for combat robots. I've gone thru the archives and edited together a selection of Q&A that should answer the most common spinner questions.

Ask Aaron Spinner Weapon FAQ Ask Aaron Spinner Weapon FAQ

Q: Hi Mark, Sage here. Hope you're doing well.
I have bought the iMAX Quattro B6 charger. Although the site states 60W per port, instead it's 50W per port.
Soon I'm gonna buy 6S 5000mah LiPos. (Although I'd realistically need about 2000mah worth of battery capacity in a match, I'll just double it to be on the safe side. I read it in the Riobotz Combots Tutorial. Plus 25% reserve for unexpected situations.)
What bothers me is that my charger might take too long to charge my batteries. The manual states: "For 50W charge power, current is regulated accordingly. Eg. 11.1v battery, charge current approx. 4.5A. Eg. 22.2v battery, charge current approx. 2.2A."
The current is user selectable but I kinda have a bad feeling that it won't allow me to increase it past 2.2A. That's gonna take >2hrs to fully charge my battery!
Is the charger too puny for my application or will I be fine with it?
If the charger is puny, I thought of some alternate ways to make it work.
First, I can use 2 small battery packs instead of a big one. Like a pair of 3S 5000mah batteries in series or 6S 2500mah batteries in parallel. I'll also buy another charger. In this way I have 8 charging ports and 10 batteries in total, 2 in my bot and 8 on charge. And so I'm effectively doubling the power pumped into the batteries from 50 to 100 watts as I'm utilizing 2 ports at once, which halves the charging time to just above an hour for one set (2 batteries).
Second, instead of five 6S 5000mah batteries, I can buy 7 or 8 and just hope I don't run out of charged packs and stick with just a single charger. What do u think Mark? Which way should I go? [India]

A: [Mark J.] I know you're asking about your battery charger, but there are a couple of assumptions hidden in your question that I think I'd better address before I get to the charger...

  • Mounting LiPoly batteries properly is tricky -- they're a little 'squishy'. If you just strap the pack to the chassis with a couple narrow zip-ties, a hard impact will cause a lot of localized pressure that can crush the battery enough to cause an internal short. Shorted LiPolys burn! Ideally, the battery pack should be padded and securely enclosed within a rigid container.

  • Good LiPoly mounting does not make for quick and easy battery swaps. If you're planning on swapping packs between matches you'd better place a lot of thought into making those swaps quick and foolproof. If your robot took a lot of damage and you need to scramble to get it ready for the next match you don't need any extra trouble when swapping out the battery.
Now, back to the charger:

You're worrying way too much about battery power. Your calculations are telling you that you only need 2000 mAh for a full match, you added 25% to that for good measure, and then doubled the whole thing! If you only pull down the charge on a battery pack by 2000 mAh in a match, your charger can top that off in an hour. If you get in a rush, you can easilly run two matches back-to-back on a fully charged 5000 mAh pack. You've got enough battery power to run 10 full matches without any charging at all!

Your current charger is fine, and I think five battery packs is overkill. Given that there's a lot of time between matches in the early rounds of a tournament, your current charger with just three 5000 mAh packs should get you thru a tournament just fine.

Q: Hello,for 1hp 900rpm DC motor to push 60 kg of weight, how many 12v 7.5Ah sealed batteries are require? [India]

A: [Mark J.] Depends on how far and how fast you want to push it, and how hard it's pushing back. I can't summarize a full kinematics course here, but I can simplify your question and provide what may be a useful real-world answer.

The Tentacle Drivetrain Calculator can estimate the pushing force and current drain for a combat robot under heavy pushing conditions based on:

  • Operating voltage
  • Stall amperage
  • Motor voltage constant
  • Motor torque constant
  • Robot weight
  • Number of motors
  • Gear ratio
  • Wheel diameter
  • Traction conditions
  • Length of match

Since you haven't given me most of that information and since you probably don't have good motor specifications, I'm going to have to make a series of blind guesses that may or may not yield a figure of any use to you.


  • Two motors similar to the 1 horsepower AmpFlow E30-150 - for which I have specifications
  • Operating at 24 volts
  • Geared down 6.3:1 to give 900 RPM output
  • 60 Kg robot weight
  • Four-inch diameter drive wheels
  • Standard traction conditions -- smooth, hard, clean surface and grippy rubber compound
  • Five minute long match

Assuming that the robot is pushing at maximum capacity 70% of the match, the total current draw is around 6.175 Amp-Hours, so you'd need two of the batteries in series to provide 7.5 Amp-Hours of current at 24 volts.

If your motors are rated 1 horsepower at 12 volts, current drain doubles to 12.35 Amp-Hours (half the voltage requires double the current to provide the same wattage output), so you'd still need two batteries, but wired in parallel to provide 15 Amp-Hours of current at 12 volts.

If ANY of my guesses are wrong, the entire calculation is flawed and you're on your own.

Note: the amp-hour ratings for lead-acid batteries are based on a slow drain period (10 hours). They must be de-rated for rapid discharge, but the degree of de-rating depends on the specific battery type and manufacturer. Given that it's unlikely that your robot will be pushing at full-throttle for 70% of the match length, I think you'll be OK with two of the batteries.

Q: Hi Mark Sir......can you please tell me which motor is good to rotate a 13kg drum and 22kg single tooth spinner ,ampflo A28-400 or A28-150? [Mumbai, India]

A: [Mark J.] I can't recommend a motor based on just the mass of the weapon. I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in spinner weapon design and motor selection.

Search the Archives!' Q: Just wanted to know the Mechanism SKF Warrior by Team Whyachi !!!
what kind of mechanism is that ?
They say it stores energy when it spins, and then uses it to flip the opponent. How is this energy stored, spring ?
Or is it something else ? [Mumbai, India]

A: [Mark J.] Previously answered.

Search the Weapons Archive for multiple posts on 'Warrior SKF' and its Spin Kinetic Force (SKF) weapon.

Hi Mark, hope you are doing well. Today, I visited the website after quite a long long time! Reading so much questions from my fellow Indians put a smile on my face. I must admit, the number of questions from Indians nowadays are quite large compared to back in those days (I assume, we were the only ones asking from India back then). Although we are not in the combat robotics scene anymore, someday we hope to start again from where we stopped.

Anyways, I just wanted to let you know Mark, that your website is one of the most valuable resources available to Indian Combat Roboteers. I highly respect and admire your decision to continue this website even though you were devastated because of Aaron's demise. I'm sure that's what Aaron wanted, and he's smiling from the Heavens to see "Ask Aaron" growing bigger and popular day by day. Regards, Las Vegas. [Niedersachsen, Germany]

[Mark J.] Thank you, Las Vegas. There were people kind enough to answer some of our questions when we got our start in combat robotics, and we put 'Ask Aaron' together to pay that favor forward. I'm very pleased that you and others have found our information and advice to be helpful.

Note: 'Las Vegas' earned his nickname here because even though he was in India his internet service went thru a server in Nevada. For quite a while we labeled his location as 'Las Vegas' until he cleared up the situation for us. Now his emails are being routed thru Germany. He claims to actually be in Niedersachsen, but I've given up on trying to figure out where he really is.

A few reflections on 'Ask Aaron' as we approach our 5000th question.

  • The first archived copy of 'Ask Aaron' from 2003 was a very simple page with only seven questions plus Aaron's first robot haiku. Today the site consists of twenty interlinked 'Ask Aaron' pages with a custom keyword search and more than 450 graphics.

  • In the last week 'Ask Aaron' has hosted visitors from Argentina, Australia, Austria, Bangladesh, Brazil, Bulgaria, Canada, Chile, China, Denmark, France, Germany, Greece, Hungary, India, Indonesia, Iran, Ireland, Italy, Kazakhstan, Korea, Malaysia, Mexico, the Netherlands, New Zealand, Pakistan, the Phillippines, Romania, Saudi Arabia, Singapore, Slovakia, Slovenia, South Africa, Spain, Sri Lanka, Sudan, Syria, Thailand, the United Arab Emirates, the United Kingdom, Vietnam, and at least 37 of the United States (Map).

  • I've been running 'Ask Aaron' by myself for almost a year and a half now. It's not as much fun as it was when Aaron and I worked thru the questions together. Sometimes it's comforting; sometimes it's frustrating; sometimes it hurts. I'd considered calling it quits at 5000, but Aaron wouldn't approve of throwing in the towel. I've made a few cosmetic changes to the site to prepare for the start of our next thousand questions. Onward!

Q: hi mark ... this might be a dumb question , but i would still ask . Is there any substitute to ampflow motors ? like motors having same characteristics weight and power bt cheaper than ampflows ? If not same weight , the same power for a little bit of more weight . but cheaper ? expected power is about 2 hp at 4000-6000 rpm . [India]

A: [Mark J.] Carlo Bertocchini went to considerable effort to design, and produce a series of PMDC motors ideally suited to the demands of combat robots. AmpFlow motors are so widely used in combat robotics because there is no cheap substitute.

You can't get a documented 2 hp in that RPM range with anything close to the reliability, efficiency, and ease of use of the AmpFlow in a 'bargain' motor. If you could, you wouldn't need to ask me what motor that was 'cause a great many robot builders would be using them.

Q: I am going to make an combot robot...weight 30 kg.......I want to make drum will be hollw and of 4 to 5 kg.....I only want to know which motor will be best in our budget bcoz we can not afford we want other cheap substitutes....please help us.....I want to rotate drum at 2000 rpm [Haryana, India]

A: [Mark J.] I can't tell you what motor will be best in your budget because:

  1. You didn't tell me what your budget is;

  2. I don't know what motors are available to you; and

  3. Many of the weapon motors used in India (automotive starter motors, e-bike pancake motors...) have no reliable performance figures to use in calculating their acceptability for a specific weapon application.

'Cheap substitutes' are cheap for a reason. Without performance specs you're just guessing, and you're likely to throw your money away on an entirely unsuitable motor.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in weapon design and motor selection.

Q: Hey Mark, something is off with one of my tinyESC V2s. The problem is that once power and signal are applied, the motor starts spinning at max speed with no control. No matter what I do on my radio or what channel it's on, the esc won't respond, unless I shut it off and then the failsafe kicks in. It's not a problem with the radio, my receiver, my battery, or my motors. The esc responds the same way every time. The red LED is also stuck on, with the green one not even turning on, if that helps. Any clue whats wrong with it, and if its even possible to fix? [Aurora, Oregon]

A: [Mark J.] It doesn't sound good -- but if you're lucky the ESC may have just lost its calibration settings. Try resetting to the default: tinyESC calibration jumper

  1. Plug your tinyESC into the unpowered radio receiver.
  2. Jumper the two tinyESC 'calibrate' pins with a piece of wire.
  3. Power up the receiver - leave the transmitter off.
  4. Remove the jumper.

If that doesn't help, email Kurt at FingerTech Robotics:

Q: hi aaron! after a long time whether roboclaw 60A speed controller(from orinion robotics) will be good to drive a pair of e30-150 motors and any one them have specially used that, comparing with sabertooth 60A driver can u speicify the positives and negatives of both.because sabretooth resets on its impact and results in low voltage of rx [Tamil Nadu, India]

A: [Mark J.] I'm very sorry, but Aaron is not available to answer your question. I'll do the best I can.

I can't recommend a speed controller based solely on the motors that a robot will use -- see Frequently Asked Questions #21. The speed controller amperage capacity requirement for your motors will depend on design factors such as:

  • robot weight;
  • wheel diameter;
  • drivetrain gear reduction;
  • battery voltage and current capacity; and
  • percentage of total weight supported by the drive axle(s).

The merits and drawbacks of the Roboclaw and Sabertooth controlers have been discussed in previous posts on Ask Aaron. I suggest that you search this page and the Motors and Controllers Archive for those posts.

Very briefly:

    Search the Archives!'
  • There is evidence that the Sabertooth controllers have a serious problem with impact resistance. I cannot recommend their use in combat robots at this time.

  • The Roboclaw controllers are designed for general robotics applications -- not specifically for combat robots. They are difficult to set-up and lack key features useful in robot combat.

  • There are other controllers you should consider that may better meet your needs, such as the Talon SRX, Ragebridge, and BotBitz 85 amp. These have each been discussed previously here at Ask Aaron.

Do your homework and find out how much amperage your motors are likely to use, then you will be better equipped to select the proper controller for your purpose.

Q: Who was the composer for Robotica? [United States]

A: [Mark J.] The end credits for TLC's 'Robotica' series list veteran Hollywood composers Danny Lux and John Carta for music.

Q: thinking of building a bot with horizontal sppiner fr 25kg catg. compitations.. whic ampflow shud i use considering the cost factor also [India]

A: [Mark J.] It's generally not a great plan to start by picking a motor and then 'back into' a weapon design to suit that motor. You'll have greater success if you start with a weapon design (type, dimensions, material) that is capable of storing the energy you require at a reasonable speed -- then select a motor that can spin up the weapon to the desired energy level quickly enough to meet your needs.

I used the Run Amok Spinner Excel Spreadsheet to generate this simple example:

  • A steel bar 400 mm long, 90 mm wide, and 25mm thick weighs 7 kilos. That's a reasonable weight for a 25 Kg class spinner weapon.

  • Spinning like a lawnmower blade, the bar will store about 1000 joules of energy at 1400 RPM. That's an acceptable amount of energy for a 25 Kg class weapon and a reasonable speed.

Now that we have a practical weapon design, we can look at weapon performance when powered by a specific motor.

  • An economical AmpFlow E30-150 motor running on 24 volts with a 4:1 reduction belt drive to the bar weapon will spin the bar to about 1400 RPM in less than 2.5 seconds. That should be adequate performance in even a small arena.

That's only one possible weapon that may or may not suit your overall design. It does show that it's possible to design a 25 Kg class weapon powered by an Ampflow E30-150 motor. A more powerful motor could spin a weapon with greater destructive potential -- the choice is yours.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for additional guidance in weapon design and motor selection.

Q: hey mark, can you tell me that how much energy (J)/kg would be sufficient for a drum weapon to attack a bot of 15 kg?- sam [New Delhi, India]

A: [Mark J.] According to Stroker McGurk's Law:

"If some is good, more is better, and too much is just enough."

I suggest that you read the Ask Aaron Spinner Weapon FAQ for a more precise answer to your question, Sam. The first three questions there are:

  1. How much energy should my spinning weapon store?

  2. How do I calculate the kinetic energy storage capacity of a spinning weapon?

  3. How quickly should my weapon spin-up to speed?

Read the whole thing while you're there -- it might save you from asking a few more questions in the future.

Q: Can a Gyro sensor could be used as a substitute by attaching it to the robot's wheel so that the sensor spins around as the wheel turns? [Houston, Texas]

A: [Mark J.] A subtitute for what, exactly?

There is some general confusion about the operation and uses of R/C peizo gyros. Our Gyro Guide answers many of the questions about how and why to use a peizo gyro in a combat robot.

The gyro is mounted on the robot chassis and is connected to the R/C receiver. It monitors the turning motion of the robot and compares it to the turning signal output from the receiver. If the turning motion differs from the turning signal from the receiver, the gyro adjusts the turning signal to correct for the difference before passing it on to the motor controllers.

Mounting the gyro to a wheel would only let it monitor the rate at which the wheel was rotating, not if the robot was turning at the rate the radio was commanding. It would also make it hard to wire the gyro to the receiver and motor controllers!

Q: which type of dc motor suits to rotate 7kg horizontal drum (outer diameter 160mm, thick 40mm , made by MS) ?
it is advisable to use E-bike brushless dc motor having capacity of 200w , 12v , 20-18 A , 2000rmp (torque = p*30/3.14*2000)=0.9Nm) ?
or any other? [Gujarat, India]

A: [Mark J.] First, you've misapplied the formula used to calculate the torque constant (Kt) of a motor in an attempt to calculate the actual torque your motor supplies, and you've incorrectly entered the actual RPM of the motor into that formula instead of the motor's speed constant (Kv). The correct formula for estimating the stall torque of a brushless motor is given in the instructions page of the Run Amok Excel Spinner Spreadsheet. While you have the spreadsheet open, spend some time learning how to use it and you can do your own evalution of motors for powering up a spinning weapon of any size, shape, and material.

You haven't given me enough info about your E-bike motor to model its performance in spinning up your weapon, but I can tell you that in general a 200 watt motor isn't going to be enough for the weapon you describe. The AmpFlow E30-150 motor provides 750 watts of output power, and would be a marginal motor for your purpose. If I correctly understand your description of the weapon, the E30-150 is capable of spinning it to about 1300 RPM and 1500 joules of energy in about 4.5 seconds (3:1 belt reduction). That would make an adequate weapon for a 40 kg robot.

Suggest you read the Ask Aaron Spinner Weapon FAQ for additional information in designing your weapon and selecting a weapon motor.

Q: Hi Mark... bang bang guy here. I have read the transmitter requirements in the archives, and I have become more confused as to which remote to buy. I have seen many transmitters on the net but I am terribly confused! Can you please suggest some which have been used in combat and are trusted and are also not too expensive?

I would like to use the left vertical axis for controlling both the motors together (like the bot going straight and reverse) and the right stick for steering. I will definitely need a inverting function 'coz my bot will be invertible. I also have a weapon to control.

There is also one more confusion. As in bang control, we can make two types of turns:

  • one is 360 degree turn - the left and right drive wheels running a same speed but opposite direction;
  • the other is a half turn, in which only one side drive moves and other is stationary.

I understand how we can achieve the 360 turn by just operating the steering throttle, and smooth turns by using combination of both the throttle and steering sticks, but how will I be able to make a half turn?

And also, can we get ready-made transmitters with the left vertical control axis already positioned to self-center? If not which transmitter should I buy so that I can get parts easily for that modification? Please help me out as I am buying a transmitter for the first time and I dont want to make mistakes.

Also can you explain the modes, the control which I want comes in which kind of mode? Will I require a mode switching?

Thank you SOOOO much! [India]

A: [Mark J.] Lots of questions -- but that's why I'm here.

Your control preferences are entirely normal for combat robots and can be met by just about any full-function 6 channel radio system.

  • Throttle on the left vertical stick axis and steering on the right horizontal stick axis is my preference as well. It requires 'Elevon Mixing' (sometimes called 'Delta Wing') which is a very common function, and a 'Mode 1' transmitter. Your motor controller inputs will plug into channels 1 and 2 on the receiver.

  • A 'Mode 1' transmitter has the 'Throttle' (channel 3) assigned to the vertical axis on the right stick and the 'Elevator' (channel 2) assigned to the vertical axis on the left stick. A 'Mode 2' transmitter has these reversed. A true Mode 1 transmitter will have the vertical axis on the left stick spring-centered - you won't need to worry about changing it.

  • If you purchase a transmitter with 'Mode Switching' capability either the right or left stick may have the vertical axis spring-centered. It's fairly simple to open the transmitter case and swap the spring centering mechanism from one stick to the other as needed.

  • Avionic RCB6i transmitterI know that the Avionic RCB6i radio is available to Indian builders at a good price. Features include:

    • mode switching;
    • Delta Wing mixing;
    • position-settable failsafes on all channels;
    • 3-position switch on channel 5 to operate a reverseable weapon controller forward/off/reverse;
    • 2-position switch on channel 6 to operate the invert function on the 'RageBridge' controller;
    • all of the most needed combat control functions (servo reverse, dual-rates, digital trims, model memory); and
    • a user manual that makes sense.

  • Another possible radio would be the Orange T-Six transmitter and matching Orange R620 receiver. Inexpensive and widely used in combat robots, the Orange radio has more features than the Avionic but lacks the position-settable failsafe controls and useful 3-way switch on channel 5 that the Avionic offers. For your purpose, I'd buy the Avionic.

Once you get used to proportional control of your robot you won't need to worry about the type of maneuvers you had to resort to with your 'Bang Bang' system. If you really need a 'half turn' it can be accomplished by combining equal amounts of throttle and turn input. The turn input will command one motor forward and one motor backward, and the throttle input will balance out the backward motor command with an equal forward command leaving that motor motionless.

Q: sir which weapon is more effective from a Chucker or a lifter in 60kg bots ??and plz suggest me a best mechanism for it! [Low Earth Orbit over India]

A: [Mark J.] The answer depends on your competition rules and the design of the combat arena.

  • 'Flipper' weapons are quite effective in arenas where there is enough room between the arena boundry and the protective screening to toss your opponent out of the arena for a win. Flippers require an effective pneumatic system to provide the speed and power needed to toss an opponent end-over-end thru the air, and I know that pneumatics are not allowed in many Indian competitions. Direct use of electric power simply won't do.

  • 'Lifter' weapons are effective in breaking your opponent's traction and possibly toppling a top-heavy opponent. Few Indian robots can function when inverted or simply tipped on their side. Either pneumatic or electric power can be used for an effective lifter, although an electric lifter will require great gear reduction in order to provide the torque needed to lift a heavy opponent.

The 'best' design for a lifter or flipper will depend on the tournament rules, your ability as a builder, the materials and components available to you, and integration with the rest of your robot design. Suggest you search thru the Weapon Archive for many previous posts about lifters and flippers to see what design might be best for you.

Q: hi mark . I am used to the " bang bang" style control (with contactors) in the 60 kg weight category . It is easy for me to make manurers for me with that style . But the arenas are growing big so I have to shift to high Rpm motors and bang control would cause the slippage of tires on the spot . I want to shift to speed control . would it be difficult to adapt? [India]

A: [Mark J.] Not terribly difficult -- but maybe a bit of an expense. Since you ask about radio gear farther down, I'm guessing that your current robots use wired controls to actuate the contactors and that you are interested in going wireless. Frequently Asked Questions #19 features a wiring diagram of the components for a typical combat robot. Compare that to your current robot and you should have a good idea of the modifications you will need.

Q: Also can you suggest me a good 2 channel speed controller giving out 30 - 40 amps per chanell continuous current at 48V or 36 V . I am using two motors rated at 36V 750 watt for drive overvolted to 48 V . I want to buy a speed controller or higher rating so that I can use it with other motors in the future .

A: It's a good plan to look to future uses of the controller and invest in something that you can use for a long time. Your list of requirements is difficult to meet, largely because of the need to control up to 48 volts. The only speed controller I can recommend is the RageBridge -- which is a very fine ESC that can handle up to 50 volts at up to 40 amps continuous current. Note that 50 volts is an absolute maximum for this controller, and that four freshly charged '12 volt' batteries in series can exceed 50 volts. Be cautious.

Q: Do you have any idea about roboclaw speed controllers ? the 30 and 60 amp version .

A: According to the RoboClaw manual:

"The main power input can be from 6VDC to 34VDC on a standard RoboClaw and 13VDC to 48VDC for the HV (High Voltage) RoboClaw."

The RoboClaw HV 2x60 Dual Motor Controller is more expensive than the RageBridge and much more difficult to set up. It has a great many features that are not used in combat robots, and lacks the true current limiting feature of the RageBridge. I can't recommend a RoboClaw for your purpose.

Q: Also which transmitter would u suggest to buy for first timers ? regular aviation style or the gun style ( piston style) used for rc cars. also can you tell me how is the control on the piston style controller, my bot runs on skid drive. I am more into wedgebots. I also run one weapon bot .

A: Unless you have a VERY strong personal preference for the pistol-grip style transmitter, there is no reason to consider their use for a combat robot. Pistol grip transmitters in general do not have the Transmitter Functions needed to properly and safely control a combat robot, and the few that do are quite expensive. Read thru our Radio Guides to learn about radio selection and programming, and purchase a full-featured twin-stick style radio system.

If absolutely necessary, a pistol grip transmitter can be used to control a skid-steer robot by using the channel mixing function of the speed controller. Channel mixing takes the output from the transmitter throttle channel and splits it between the two motor control channels of the controller to direct them both forward/reverse as needed. The transmitter steering output is likewise split between the two motor control channels to provide differential power to the motors on either side of the 'bot for turning. But seriously, you don't want to use a pistol-grip transmitter for your combat robot.

Q: Is Stainless Steel 304 a good armor for middleweight 60kg category against spinning drums? If not which are other materials which are better? [India]

A: [Mark J.] Right next to the blue button you pressed to ask your question it says:

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.

Then, right below the question box is a super-easy method you can use to search the archives and recent questions:

Thousands more robot combat questions and answers in the searchable Ask Aaron archives.

I don't think that's too much to ask.

A pair of questions remarkably similar to yours are answered in a single post farther down this page. Entering '304' in the search box would have told you that, and would have found another '304' question in the archives. Next time do that first and you won't have to wait a day for an answer that was already here.

Q: Hi sir,i want to know which bane gearbox motor is most suitable for a 40 kg robot?? [India]

A: [Mark J.] I'd need much more information about your 'bot than just it's weight before I could recommend a specific gearmotor.

  • What diameter wheels will you use?

  • How many motors will your robot have?

  • How large is the arena in which it will complete?

  • Will your robot have an active weapon, or will it rely on pushing power?

  • Is there a restriction on the operating voltage or amperage for the motors?
See Frequently Asked Questions #21 for more information on motor selection.

Q: hey m using e-150 motors to drive my 30kg robot.
i want a substitute for curtis albright dc66 contractors [India]

A: [Mark J.] The correct substitute depends on why you want a substitute.

  • Is the DC66 not available to you?
  • Do you want something less expensive?
  • Has the DC66 failed and you need something with greater capacity?
In any event, a web search for "DC motor reversing contactor" will reveal any number of possible substitutes.

Q: Hi mark recently i had purchased rocker switch with 10A 277 VAC 20A 125 VAC so how much amps at 12V it can deliver? [India]

A: [Mark J.] Estimating ratings for switches outside their listed voltage range is a tricky business.

  • First, Direct Current (DC) is harder for a switch to handle than Alternating Current (AC). Direct current is a continuous flow, but alternating current flow is in the form of a sine wave that passes thru zero current flow many times a second. This periodic stoppage of current allows electrical arcing to extinguish and saves the contacts from burning or welding shut when switched at high current loads. Switches must be de-rated for use in DC circuits -- typically by a factor of about five.

  • Second, switch ratings are typically for 'resistive' loads and I'm assuming you'll be using this to control a motor, which is an 'inductive' load. Inductive loads prolong arcing when the switch is opened and require additional de-rating of the switch current capacity by about 25%

  • Finally, in addition to being an inductive load, a DC motor also produces 'back EMF' when power is removed and the motor coasts to a stop. This is separate from the extra load caused by the inductive load and further de-rates the current capacity of the switch by perhaps 50%.
There are other factors involving the physical design and materials used to construct the switch. I can't give an exact current rating for your switch when used for 12 volt DC motor operation, but in general it would be unwise to use a switch rated for 20 amps resistive load at 125 VAC for more than 20 amps inductive motor load at 12 VDC.

If the switch does fail, it will likely weld the switch contacts 'on' and be impossible to turn off, which could be disasterous in a robot. I'd strongly recommend that you find an automotive switch properly rated for DC useage at the current you need for your motor control.

Q: can you please give the specifications of the dc motor we need to use to rotate a 3kg cicular disc at 1000 rpm ? [India]

A: [Mark J.] The power needed to spin a 3 KG disc to 1000 RPM depends on:

  • How quickly you want the disc to reach 1000 RPM; and
  • The diameter of the disc.
The power to sustain the disc at 1000 RPM is negligable, so it all comes down to how quickly you want your disc to reach the required speed. A larger diameter disc will take longer to spin up to speed than a smaller disc of the same mass, but it will store more energy for a greater impact.


  • An aluminum disc 300 mm in diameter and 15 mm thick weighs 3 kg. It will store about 170 joules of energy at 1000 RPM. A 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 0.8 second -- very quick, but very little stored energy.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. The same 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 3.5 seconds -- much slower, but much better energy storage.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. An Ampflow E30-150 motor with 5.0 N-m stall torque and a no-load 5700 RPM speed geared down 5.7:1 will spin this disc up to 1000 RPM in about 1.7 seconds -- a good balance of speed and energy storage.
I will mention that most builders choose to spin their weapons faster than 1000 RPM. Doubling the speed to 2000 RPM will give four times the energy storage, but will require four times as long to spin up to full speed. You didn't mention the weight class of your robot, but a useful spinning weapon will require at least 40 joules of energy storage per kilo of robot weight. Plan accordingly.

Q: was the vdd kit the first combat robot kit, or was it just the first notable one? [Nate F. in Pennsylvania]

A: [Mark J.] Exactly which combat robot 'kit' was first depends a lot on exactly how incomplete a package you're willing to accept as a kit, Nate.

Inspired by the successful antweight competitor of the same name from Team Think Tank, the V.D.D. (Vertical Disk of Destruction) antweight kits first appeared at the Robot Marketplace in late 2003. They were far from complete kits, and the instructions were more of a set of suggestions than a complete guide. For $100 you got:

  • 2 Tamiya drive gearboxes;
  • 10 carbon rods;
  • 1 bottle of rubber CA glue;
  • 8 yards of Kevlar thread;
  • 2 wheel adapters;
  • 2 foam wheels;
  • 2 brushed drive motors with capacitors;
  • 1 brushed weapon motor with gearbox;
  • 1 weapon blade hub;
  • 1 hex wench and hardware.
'VDD Kit

To complete the kit you needed: speed controllers, radio gear, servo leads, armor of your own design, a weapon blade, a 9.6 volt battery, and a battery charger.

The chassis was constructed by strapping the carbon rods to the gearboxes and to each other with the Kevlar thread and soaking the junctions with cyanoacrylate glue. This was not a task for an inexperienced builder, and I suspect that many of the purchased kits were never completed.

The Robot Marketplace had been offering 'Robot Starter Packages' in assorted weight classes for some time prior to the appearance of the VDD kit. These packages were complete sets of components -- motors, gearboxes, wheels, speed controllers, battery, and charger -- but they left the chassis design entirely to the builder.

Q: i got a helical gear of 11.5 cm outer diameter with face width of 2.1 cm , internal diameter of 10 cm,number of teeth as 22. in which automobile it is used, how much load can it transmit,its speed in rpm . [India]

A: [Mark J.] So, you pulled a random transmission gear out of a wrecking yard and based on a general description of dimensions and tooth count you'd like me to track it back to the vehicle it came from and produce unpublished performance specifications?

I don't know whether to laugh or cry. Robot questions, please.

Q: Did "Original Sin" the first time inside mechanical failure disabled(lost all tires are not included) in CC2012 by "Electric Boogaloo"?
By the way,happy Chinese New Year! in chinese,it called "Chunjie" (Spring Festival) [China]

A: [Mark J.] 'Original Sin' has proven itself to be a very durable competitor. I don't have a record of reason for loss in each individual match, but I've written to Original Sin's builder Gary Gin to ask him if the Combots Cup loss to 'Electric Boogaloo' was the first due to internal failure. I'll post his answer -- when available.

Update -- Builder Gary Gin replies: "The loss to EB was a tap out due to excessive external damage. At RG 2011 I had a loss due to receiver failure."

Q: Hi Mark, what do you think of abrasive disks as cutting weapons? [India]

A: [Mark J.] NO CERAMIC CUTTING DISKS!!! It doesn't take much of an impact to shatter the disk, sending sharp shards at high speed in all directions. Given the average combat arena in India, that's a disaster. No sane event organizer would allow you to use such a weapon, plus they simply aren't effective at doing significant damage to your opponent.

If you are intent on using a cutting blade, an arrangement like that used by 'Gloomy' is your best option: an all-metal circular cutting saw attached to a movable arm that keeps the blade out of harm's way until the opponent is pinned and the blade can be put to good use. See: Gloomy vs. Hyperactive.

Q: ...and also about how much pressure would be needed to make a decent lifter in midweight class.

A: The power of a pneumatic ram is calculated by multiplying the gas pressure by the area of the piston. A lower pressure simply requires a larger diameter cylinder to compensate. I suggest that you read the Team Da Vinci Robotics Understanding Pneumatics page to get a good overview of combat pneumatic weapons and how to calculate the force of your lifter design.

A typical 'low pressure' pneumatic lifter will operate at 150 psi. Commercial pneumatic regulators, cylinders, and other components certified for this pressure are widely available and are relatively safe to work with.

Our own heavyweight lifter 'The Gap' uses a 150 psi pneumatic system to power a 3" diameter actuator. The actuator provides 1000 pounds of force, which translates to more than 400 pounds of lifting force out at the tip of the lifter arm.

'The Gap' launches a trash can at the 'Da Vinci Days' robot festival.
Team Run Amok's
'The Gap'

Q: Hi Mark, I am in the process of designing my first robot (lightweight) and was wondering whether it is possible to use a thyristor as an esc, wikipedia seems to be ambiguous on this topic. [India]

A: [Mark J.] Silicon controlled rectifiers (SCRs, also known as thyristors) are commonly used to control AC motors and can be used to control DC electric motors(examples). However, they are far from ideal for the type of control circuits desired in combat robots. Combat ESCs are based on highly efficient MOS-FET devices.

More to the point: I can't recommend trying to 'home brew' an electronic speed controller. A full-featured ESC is a terribly complex design challenge (example). I've seen many attempts by builders to wire up their own controllers. The best of them were disappointing, and the great majority were entirely disastrous. You can't afford to have poor, unreliable motor controllers in your combat robot -- use a reliable and proven ESC or take up a different hobby.

Q: Hey mark,
I have a basic wedge shaped bot with 4 48 VDC motors comnected directly to the tyres. I am planning to make a wired combat bot and have 4 push buttons in my remote, can you tell me how to connect the motors so that the front two buttons move the bot forward and rare buttons move it backwards? (I have taught of shorting the right motors and the left motors) please tell me how could i do the connections?
Waiting for your reply.
Thankyou [India]

A: [Mark J.] You'll need four double pole double throw (DPDT) momentary contact push-button switches. The switches must be rated for the voltage and amperage that the motors will use. You didn't mention the stall amperage of your motors, but I think you're going to have trouble finding push-button switches that are capable of handling that great a load.

Assuming that you can locate adequate switches, the diagram at right shows the circuit for one side of the robot. Duplicate the circuit for the other side.

Forward/off/reverse control of two motors with DPDT switches

Q: what is the best possible weapon against 'Tauro Maximus'?? [India]

A: [Mark J.] Let's look at the fight record. In US competition from 2009 thru 2014, heavyweight drum robot 'Tauro Maximus' lost 11 of its 32 fights:

  • One loss to 'DXTER' - a hinged wedge
  • One loss to 'Polar Vortex' - a lifting wedge
  • One loss to 'Last Rights' - a horizontal bar spinner
  • Two losses to 'Original Sin' - a pivoting wedge
  • Three losses to 'Great Pumpkin' - a simple wedge
  • Three losses to 'Sewer Snake' - a forked wedge/lifter
Ten of those eleven losses were to some form of wedge. A smooth low wedge or scoop surface gives no edge for a drum impactor to strike against, neutralizing the drum weapon.

Q: In a PopSci article titled "A Brief History of the Demise of BattleBots", author Kelsey Atherton claims that BattleBots was killed by boring, invincible wedges. Is that true? [Colorado]

A: [Mark J.] From the inagural competition in Long Beach (1999) thru the final televised 'Season 5.0' (2002), fourteen different robots won class titles at BattleBots events. By weapon type, there were:

  • 1 horizontal blade spinner;
  • 1 clampbot with hammers;
  • 1 vertical disk spinner;
  • 2 full-body spinners;
  • 1 overhead axe;
  • 1 thwackbot;
  • 2 sawbots;
  • 2 flippers;
  • 3 lifters;
  • and ZERO passive wedges.
I believe that adequately refutes Mr. Atherton's premise.

Q: Hi Mark, Sage here. I hope everything's well.
I was almost gonna buy the Sabertooth 2x60 ESC but just then some builders said that it's impact sensitive. They said that whenever they got hit, the ESC reset which made them lose the stick centre position. That's really bugging me because being a spinner killer, my bot would definitely need to tolerate a lot of abuse and losing the stick centre in the middle of a match isn't acceptable.
Now I'm considering a pair of the Talon SRX single channel ESCs, sold at the VEX robotics site. What bugs me again is that they were specially designed for FIRST use. I'm a bit doubtful about their effectiveness in combat. Are they good, or should I stick with the Sabertooth?
And yeah I decided to go for pair of E30-400 gearmotors instead of the hydraulic power pack ones, they were just too slow.

A: [Mark J.] As I recall, you're building a lightweight (~60 pound) robot with AmpFlow motors geared 8:1 and 6" wheels. Your calculations indicate a maximum amp consumption of less than 30 amps per motor before wheel spin limits greater current use.

I keep hearing about Sabertooth controllers giving trouble in combat, but the claim seems to come from reports of a very few US builders that have been picked up and broadly repeated 'second hand'. I've not heard of any such problems reported directly by Indian builders. I'm really not sure how big a problem this is.

The Talon SRX controllers are fairly new, but have been tested in lightweight combat robots using AmpFlow motors with good results. You should not dismiss these controllers just because they were built for FIRST competition. Note that the Talon ESC is not plug-in ready for R/C control -- it needs to have a standard connector added and may require a firmware update. See the User's Guide for details.

The choice betwen the two controllers is not simple:

  • The Talon SRX has many features that will not be used in robot combat, and lacks a few features that could be very useful -- like current limiting.
  • The Sabertooth 2x60 does have some known callibration quirks and limited reports of resetting after impact, but it is rich in combat features.
I'm a bit worried about using an ESC without current limiting with motors that can pull more than twice the peak current rating of the controller if the drive train jams or is otherwise restricted. If you do pick the Talons, I would suggest using them with a battery pack with a low 'burst current' rating that may help limit amperage spikes.

Q: Hey Mark, Sage again.
Actually the number of Indian builders facing the issue of resetting after impact in the Sabertooth 2x60 is substantial. I've even seen videos of matches which were lost due to this problem.
So, if not the Talons, how about a pair of the Holmes Hobbies BR-XL ESCs? The site says they're rated for 80A continuous but I'll take it with a pinch of salt. Nevertheless, they have been used in the 30lb class with good results so I guess they'll just work out for me. What do u think?
Another ESC I'm considering is the Ragebridge (I'll install a fan myself) which has some great combat features. I hope it doesn't have issues like the Sabertooth. What do you say? Ragebridge, Sabertooth or Holmes?

A: Forgive my scientific skepticism, but I believe that what you've seen are videos where it was claimed that the match was lost because the ESC glitched. If a specific controller gains a reputation - deserved or not - for transient impact failure, it becomes very tempting for a builder to blame a 'bot failure on the easy explanation. How many of these claimed failures are actual ESC glitches, I can't say.

I hesitate to recommend a specific ESC because there are elements in your design that I just don't know about that could greatly influence that choice. The best I can do is to share some comments that may help you in your decision:

  • If you believe the Sabertooths are flawed then you don't need to ask me about them -- they're out.

  • In your last post you were concerned that the Talon SRX wasn't a real combat ESC, but now you're considering the Holmes BR-XL -- a hobby R/C controller? Their specs are good and their feature list is OK, but they have far less robot heritage than the Talons. I'd be willing to use either the Talon or the Holmes, but they are both vulnerable to unexpected current surges from a big motor like the AmpFlow E30-400 if anything goes wrong. A little risky.

  • Ragebridge? Designed specifically for combat with true current limiting. Simple to use, no frills, correct fail-safe response, and excellent motor control feel. Downsides: big footprint, no protective case, requires special mounting care. If you have room to properly mount it, this ESC could be a very good option.
I can't tell you much more than that, Sage. As in many engineering choices, there may not be a clear 'best'. Pick your poison -- best luck.

Feedback: Thanks a lot Mark! Your reply was bang-on. It was all I needed! I'm going for the Ragebridge...
Your help has been invaluable to me!

Q: I have an e30-150 ampflo motor. I am wondering what the radial load rating is. The website does not say. I do not want to use a gear box. [Texas]

A: [Mark J.] No gearbox? Doesn't sound like a robot application. Suggest you contact AmpFlow for guidance on shaft radial loading limits.

Q: hi mark...i am having drum bot when i give a big hit to opponent my shaft bends inside. first i tried 20mm shaft in MS it bends. and then i used 25mm shaft at EN24 It also bends , what can i do at which mm and which material i have to use? [India]

A: [Mark J.] You've told me almost nothing about your robot, but you want me to tell you how much larger/stronger a specific part must be to keep it from failing again? See Frequently Asked Questions #17.

Ask Aaron isn't an engineering service, but if I were spec'ing a weapon shaft I'd need a lot more information:

  • Shaft length?
  • Drum weight?
  • Weight class?
  • Peak drum speed?
  • Live shaft or dead shaft?
  • Dimensions of the drum?
  • Where exactly did the others bend?
  • What is the design of the shaft support?
I suspect that the problem isn't the size of the shaft or the material from which it is made. You didn't mention if the shaft was hardened, but a 25 mm EN24 shaft is difficult to bend. If I had to guess I'd say the problem is a flawed design in the support and bearing placement for the shaft, but I can't confirm that because you haven't told me anything about your design. The hamburger is bad.

Q: What would be better as armor in 30 kg and 60 kg category: 202 stainless steel or 304 stainless steel? Any other suggestions? [India]

A: [Mark J.] Stainless steels are not commonly used for robot armor. Stainless alloys are fomulated for corrosion resistance and high heat performance -- attributes not valued in robot armor. Both of your suggested alloys are relatively 'soft' and deform easily (low yield strength), and neither can be heat treated for improved hardness. Of the two alloys, 202 would be my choice for armor due to its slightly greater hardness and yield strength.

I don't know the full list of materials available to you, but other Indian builders are using EN24 'chromoly' steel. EN24 is much better suited to use as robot armor and should be considerably cheaper than the stainless alloys.

Q: hi .... I would like to add to the 202 armour question . can u plz suggest some good options for armour. So i can see if the material is available to me . I have used 202 and it worked good . Titanium is out of budget . any other Bite proof (lol) good material ? [India]

A: See FAQ #17. Although I'm pleased to comment on specific materials, I don't think it's reasonable to ask me to list and evaluate all possible metal alloys, polymers, and composites so that you can pick thru them. It's better if you do your own homework on this.

I suggest that you read Chapter 3 of the Riobotz Combat Tutorial for an extensive listing of robot structural materials and for guidance on evaluating the specific materials that are available to you. Read the rest of the tutorial while you're at it.

Tip: don't rely purely on the properties of the material. Great armor depends as much on design and mounting as it does on the material itself. I built two champion robots armored with wood.

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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