6637 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email about designing and building combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

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

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.

Q: What exactly are the nuances of Battlebots' "Marvin"? I know Rucker's bots are always more engineering genius than meets the eye, but I'm just not seeing the competitive edge they get from their disc design choices. [Woburn, Massachusetts]

A: [Mark J.] Hal Rucker has a different paradigm for combat robot design than most builders. There is always an emphasis on survivability above destructive mayhem. The weapon may not do a lot of damage, but it's gonna get multiple shots at you. Here's what Hal said on Reddit about the key design elements of 'Marvin':

Marvin has a bunch of new design concepts we're testing out. It will be interesting to see which new ideas are good ideas, and which new ideas are bad ideas:
  • The big white flywheel is machined from a single block of UHMW plastic. Is it a good idea to make a kinetic weapon out of plastic?
  • The green teeth are hardened 4140 steel. The teeth pivot around big steel axles you can't see because they're inside the plastic. Gravity and/or CF hold them in the correct orientation for damaging the opponent. If the teeth hit a wedge, or a vertical spinner, they rotate around the axles and absorb the energy instead of throwing the whole bot up into the air.
  • Marvin features a friction drive system to spin the weapon. The goal is to score points on hits/damage and survive the full 3 minutes. Marvin is not a one-punch-knockout kind of spinner.
  • Marvin is invertible.

Q: Hi, I just did a search on this but I could't find similar past questions so I'm just going to ask it.

In a 4WD setup where the four motors independently drive the four wheels, does a forward leaning center of gravity (weapon and wedge being at the front, like in the case of a vertical spinner) usually cause the front motors to experience such higher loads that they would consistently fail earlier than the rear ones? If so, would a slightly more aggressive rear gear ratio/wheel diameter combo help mitigate this issue? Thanks. [New South Wales, Australia]

A: [Mark J.] There is a fair bit of confusion among builders about how the weight on a driven wheel affects the motor driving that wheel. The Ask Aaron Optimizing Combat Robot Drive Train Guide gives a step-by-step process to select the 'ideal' gearing for a drive motor based on the weight carried by the wheel it powers, but in your case different wheels carry differing weights. What to do?

Simply shifting the weight of a robot toward the front wheels of a four-motor four-wheel-drive chassis does not place significantly greater load on the front motors while the robot is freely moving; both the front and rear motors will contribute equally to the accelleration and movement of the the 'bot. The difference in load comes when the robot is pushing hard against a resisting/immoveable object and the wheels break traction and start spinning freely. The point at which that traction break takes place is a function of weight carried by the wheels. Here's an example:

A 30-pound 4WD four-motor robot we'll call 'Dave' has a significant forward weight bias. Dave carries 10-pounds of weight on each of the front wheels and 5-pounds on each of the rear wheels. All four of the gearmotors and wheels are identical.

Following the steps in the Optimizing Combat Robot Drive Train Guide for this bot's specific drive train, we calculate that:

  • The rear wheels will each require 8 amps of current to generate the torque needed to break traction and spin free;
  • The front wheels will each require 16 amps of current to generate the torque needed to break traction and spin free.
Double the wheel weight equals double the current load on the motor -- but only under hard-pushing.
If you decrease the gear reduction and/or increase the wheel diameter for the rear motors you can raise their breakaway traction point up to 16 amps as well, but that won't reduce the current load on the front motors -- they'll still break traction at 16 amps. You'll have added stress to the rear motors with no reduction in front motor stress, no improvement in pushing power, and less overall torque for acceleration.

Select wheels and gearing based on the more heavily loaded end of the robot to make their breakaway current sustainable for those motors. Keep gearing and wheels for all motors the same for balanced torque at all wheels. The front motors will have greater loading, but if you have failure issues with them you should reduce gearing at both ends of the 'bot.

Q: Probably just an insane idea but in theory if you stacked two horizontal spinning blades on top of each other and spun them in opposite directions? What, if anything, could one expect to happen when they hit? [Pittsburgh, Pennsylvania]

A: [Mark J.] It is fairly insane, Pittsburg -- but that hasn't prevented people from building such weapons. Their combat records are not great. We've discussed both horizontal and vertical counter-rotating spinners in the Ask Aaron Robot Weapons archive. Search there for hozizontal spinner 'Double Dutch' and vertical spinners 'Counter Revolution' and 'CounterStryker' for full discussions. Here's a sample:

An alternative twin-spinner system design is employed by 2018 BattleBots competitor 'Double Dutch'. Their design features counter-rotating bar spinners placed above and below the robot body. Although both weapon bars can attack an opponent at the same time, it's likely that one bar with strike before the other and throw the opponent clear before the second bar can impact. Almost certainly the two potential weapon impacts will strike different locations on the opponent, which will also limit their effectiveness.

Q: Hey It's me from Anacortes again. The development of my robot has led to a couple more questions. Shortly after I designed a major component of my robot around 2.5 inch fingertech foam snap wheels, they went out of stock. I signed up for them to notify me when it gets back in stock, but I don't know if need to start looking for an alternate wheel solution. By now over half of the snap wheels are out of stock. Has this happened before?

A: [Mark J.] Yes, they go out-of-stock from time to time. The 2.5" Lite Flite foam tires will fit the FingerTech snap hubs. They're just a touch wider (0.035") than the FingerTech tires, but they're in stock and work well.

Q: Also, I was wondering about the dimensions of medium nutstrip. All of the sites I can find online say that the threaded holes are spaced at 14.5 mm intervals. Is that measurement referring to the space from the center of each hole, or from the closest edges? [Anacortes, Washington]

A: The hole spacing is measured center-to-center.

Q: Hi Mark, conceptual questions about brushed vs brushless drive motors.

  • Two 4wd rammer bots go head to head. Which motor would have the advantage? At standstill, wouldn't the brushed have the advantage with the most of its torque being produced at low rpm? Most push attacks are already at some speed, and the brushless having a torque curve more similar to an ICE, wouldn't it be the better motor overall?

  • Do brushless motors also require less Ah battery consumption?

  • Would a brushed or brushless motor be better for running a weapon? Wouldn't the brushed be a faster spin up because of its torque curve, but a brushless less likely to fully stop because of its torque being highest in the low to mid range? Does one motor have a considerable advantage over the other?

Many Thanks! [Erie, Pennsylvania]

A: [Mark J.] You're asking me to compare an unspecified brushed motor to an unspecified brushless motor. The hamburger is bad.

  • Are they the same physical size/weight? The brushless motor will be more powerful.
  • Do they have the same wattage output? The brushless motor will be smaller and lighter.
Generally, builders replace brushed motors with brushless motors that are a fair bit smaller/lighter and with a big scoop of more powerful. Comparisons of the type you're asking don't really apply.

The decision over brushed vs. brushless power for a combat robot generally comes down to how much pain and frustration the builder is willing to suffer thru to get a brushless motor operating correctly for the intended purpose. For a brushless spinner weapon the level of frustration is low. The brushless motor and controller are performing a function similar to the hobby aircraft purpose for which they were designed. For a small robot spinner weapon it's possible to match up an off-the-shelf brushless ESC with an outrunner brushless motor and you're in business. Brushed spinner motors are very nearly extinct.

For a brushless drive system the level of frustration is much greater. You'll have to locate a brushless controller that either has firmware that can be modified to provide bi-directional control or which will accept replacement of its firmware with something suitable (not a simple task). Then you need to sort thru the firmware settings to set forward/reverse control and take a guess at pushing the start sequence and brake settings as far as possible toward optimizing low speed torque and driveability. If you guess wrong you remove the melted ESC (and maybe the melted motor) from your 'bot and start over. Brushless drive systems are popular and 'hip', but you can still be entirely competitive with brushed drive.

For purposes of answering your questions I'll assume we're comparing brushed and brushless motors with the same peak output power -- although that would be unusual in practice.
Pushing power - A properly geared and set-up drive motor won't be operating near stall in a pushing battle. You gear your drive motors to 'break traction' and spin the tires to raise the motor RPM up close to the power peak under heavy pushing. If you have the brushless ESC correctly set up your brushless drive will be running a peak power when pushing -- just like a brushed motor. No 'conceptual' advantage to either motor type.

Efficiency - Brushless motors are more efficient than brushed motors -- by a little. However, in practice the use of 'overkill' brushless motors more than erases that small advantage.

Spinner power - Weapon spin-up time is all about motor power all the way thru the RPM range. A brushless motor has less low-end torque performance and would never 'catch up' to the low-torque advantage of a brushed motor with the same peak power. But again, real-world builders go overkill with brushless motors and make this comparison moot.

Keep it spinning - Mid-range motor torque is not what keeps a spinner weapon spinning after a hit. The kinetic energy stored in the weapon is orders of magnitude greater than any difference in motor torque, and mid-range torque would be very close to equal in the 'equal peak power' motors we're comparing here. No 'conceptual' advantage.

I'd advise that you look to how brushed and brushless motors are being used in successful combat robots and use that as your guide. Any conceptual arguement you might make disappears in the sea of real-world design variables.

Q: Just a quick, somewhat random question: I was watching the footage of 'Wrecks' in [ABC BattleBots] Seasons 1 and 2, and I've noticed that season 1 'Wrecks' seems to control a lot better than season 2 'Wrecks'. Closer investigation reveals that the original 'Wrecks' was seemingly lower to the floor and although its leg had limited motion (in contrast to season 2 Wrecks's leg, which could rotate 360 degrees), it allowed season 1 'Wrecks' to move easier at the cost of having a harder time self-righting.

What do you think? Is there anything else the original 'Wrecks' had that made it better than season 2 'Wrecks', or is there something significant about season 2 'Wrecks' that makes it superior to its predecessor? [Newton, Illinois]

A: [Mark J.] The reduced maneuverability for season 2 'Wrecks' had very little to do with the changes to the pivoting 'leg'. Gyroscopic precession walkers operate on physics principles that are in conflict with the optimum performance of their spinner weapon. Trying to strike the best compromise between weapon power and robot maneuverability is a challenge. A reader from Ontario, Canada pointed this out to me when they wrote in to comment on this post -- now in the Ask Aaron archives:

"There is such a thing as too much gyroscopic force for a precession walker. The maximum pivot speed is expressed as (force of gravity) * (distance between centre of gravity and the point of contact with the ground) / (angular momentum of the disk). Which means if you spin the disk twice as fast you (very counter-intuitively) actually cause the robot to pivot half as fast. You can see this in spinning tops because as they slow down, the wobble speed increases.

I've spent some (too much) time trying to figure out gyroscopes myself for future bots and I'm not an expert but I figured I can still help with the info I somewhat understand.

The 'Hyperphysics' site explanation on spinning top physics is probably one of the most complete sources for this info that isn't completely indecipherable."

I believe that season 2 'Wrecks' had increased the weapon's energy storage which resulted in a slower pivot rate; they reduced maneuverability for increased weapon power.
Q: on the excellent spinner spreadsheet the formula for getting a rough calculation of the stall torque is given as ((1352/Kv)*(Volts/(mOhms/1000)))/141.69

i think i must be mixed up, because it seems like this formula is producing some crazy numbers - so maybe i'm doing something wrong and you can help me out. for example, if you look at the spec of a brushless motor like the Scorpion HKII-7050-330KV and plug in the numbers:

Volts = 50
internal resistance in mOhms = 5
Kv = 330

if you use the spreadsheet formula, you get a stall torque of an incredibly powerful 289 N-m!

but if you calculate torque using some other formulas that i've learned about, like Kv/Kt where Kt is 9.549/Max Amps or using the formula PeakPower/w, where w is given by (2*pi*Max RPM)/60 then you find that both of those formulas get you a more reasonable number of 8 N-m of torque, which is way different than 289 N-m.

so am i misunderstanding something fundamental here about brushless motors and torque? did i mess up the numbers or formulas somehow? am i being dumb? i think i need some help.

-josh in salt lake city, utah

A: [Mark J.] Let's start by applying the formula from the spreadsheet to a motor with known specs. The brushed AmpFlow A28-400 will do nicely:

  • 24 volts
  • 44 mΩ
  • 205 Kv
The spreadsheet formula gives a stall torque of: (1352/205)*(24/0.044)/141.69 = 25.16 N-m. That's really close to the published 25.13 N-m stall torque for the motor -- the formula is correct.

The Scorpion HKII-7050-330KV can deliver 15,000 watts of output power while sucking down 300 amps of current. If you pay $999.99 for a brushless motor you're entitled to 'incredibly powerful' results, but does it actually deliver the 289 N-m of stall torque the spreadsheet formula says it will? Absolutely not, because brushless motors are different than brushed motors in how they operate at low speeds. The concept of 'stall torque' really doesn't even apply to hobby brushless motors. See this archived post about brushless hammer drive for greater detail on low-speed brushless torque.

So, why do the other formulas you cite give a much different torque number?:

  • Torque = Power / Angular Speed -- calculates torque at a specific speed where true power output is known. You're assuming 'Peak Power' and 'Max RPM', which never occur together in an electric motor. Stall torque occurs at zero speed, where output power is also zero, giving a 'division by zero' error; the formula cannot directly calculate stall torque.
  • Torque = Kt / Kv -- it's 2 AM and my brain has gone to mush, but this method of calculating stall torque isn't making any sense at all to me. This webpage may offer some additional help on the relationship between Kt and Kv: Learning RC - Brushless Motor Kv Constant Explained.

I don't think that you're using the most recent version of the Run Amok Excel Spinner Spreadsheet. The current version (19-D) is optimized for brushless motors and has a more sophisticated method for approximating true low-speed torque that takes motor controller software into account. Download it from the Run Amok tools page.

Q: thanks for the explanation and the pointer to the new spinner spreadsheet. great stuff (!!) but i think the new spreadsheet seems to still have a major limitation related to brushless motors, and i think it might be because there is no ability to plug in the maximum amperage (maybe?) and so instead it goes to some unrealistic theoretical maximum which is so unrealistic as to not be useful to model - or maybe i'm screwing something up.

for example, if you take the stats from that fancy scorpion motor:

50 volts
330 Kv
5 mΩ
10 magnet poles

and say we use a gear reduction of 5 to 1 and then let's say our horizontal spinner is made of steel, 0.5 meters long, 0.125 meters wide, 25mm thick, which is about 12kg (about 27 lbs).

so the results from the calculator are that this blade achieves 3,000 rpm in less than 0.3 seconds (my gut tells me that this is probably off by a factor of 10 or more) and that it will peak at 8,500 amps, which at 50 volts is almost half a megawatt of power. i doing something wrong, or is this spreadsheet unable to model a big brushless motors like this very well?

thanks again - josh in salt lake city

A: You're planning on buying a $1000 brushless weapon motor capable of 20+ horsepower output and then strangling it with an inadequate power supply? Well, to each their own...

The spreadsheet assumes that the weapon motor will be provided with all the current the brushless controller chooses to pass to it, without voltage sag. There is no adjustment for a fixed 'current maximum' because brushless controllers generally do not have true current limiting, but the spreadsheet is capable of making specific adjustments for changes in the controller firmware that limit control the motor speed at which the 'soft start' feature cuts out and the maximum power pulse percentage during that soft start period.

In SimonK firmware these features are known as 'Commutation Max' (commutation time in u-sec below which 'soft start' restriction is active) and 'Power Max' (maximum power pulse length during 'soft start') but other firmware will typically have similar features. You can 'play' with these settings in the spreadsheet to see their effect on current consumption and performance, then transfer the settings to the controller firmware to implement those power restrictions.

Q: What was the first "typical beetledrum" (2wd beetleweight, outboard wheels with wrap-around guards, centerline mounted drum directly mounted to the [longitudinal] frame rails) design? At the moment the earliest ones I can find are 'Weta' and 'Grande Tambor', but I get the feeling that the design goes back even further... [Asheville, North Carolina]

A: [Mark J.] Trying to determine 'who was first' gets very sticky very fast. The answer really depends on how far from 'typical beetledrum' you're willing to stray.

  • 'Grande Tambor' made its first combat appearance at the Schiele Museum Clash of the Bots event in July of 2010. That first version of the 'bot had no wheel guards, did not use UHMW polyethylene frame rails, and had significant rear overhang not seen in a current 'typical' design.
  • 'Weta - God of Ugly Things' made its first appearance at that same event. That first version of the 'bot had a belt-driven drum, lacked 'anti-wedge' fingers, and used an atypical 'inrunner' weapon motor.
The two builders learned from the strengths of each other's 'bots and both evolved toward the pervasive current design. If you are looking for earlier roots of current design you might need to drop down to the antweight class. 'Poco Tambor' made its first combat appearance in December of 2007 and had all the key design elements later upscaled for 'Grande Tambor'.

Q: Hi it's me from Anacortes again. It has been a week since my first competition with the beetleweight wedge I've been working on. 'Firecracker', as it was called, had its first battle against 'Dark Pummeler', and the short fight ended with my robot in five pieces. This being said, thanks to the kind and encouraging attitude of the other builders, it not a discouraging experience. I've begun a redesign of Firecracker, and along with the new ideas, I have some new questions.

I am looking into the prospect of making the front and back wall of my robot out of aluminum (6061) but I don't really have a solid idea of how strong it is in the world of combat robots. I know this is not a lot of info to work off of, but do you think that a 1/4 inch aluminum plate reinforced by nutstrip would hold up somewhat well as a front wall/armor? Ideally most of the hits would be taken by the wedge, but I've quickly learned that things don't always go exactly as planned... [Anacortes, Washington]

A: [Mark J.] A first-round draw against any of Russ Barrow's 'bots is a tough break -- initiation by fire. Glad to hear you weren't discouraged.

I'd be fine with 1/4" 6061 alloy for front and rear panels in a beetleweight, but I'm not a fan of relying entirely on nutstrip to hold the corners together. Machine screws are made of material and temper to optimize tensile strength -- not resistance to shear forces. Take a look at this video from Robert Cowan for details. You can use Robert's hardened pin technique, or you can 'slot and tab' (see illustration) the panels to take some of the shear load off the screws. That will greatly reduce the number of pieces you'll need to pick up after a match.

Q: Hi Mark, I'm trying to spin a bar with impactors with a moi of 0.055 kg*m2 with a Scorpion HKIV-4025-1100KV outrunner. I believe it will spec out to be 13,200 rpm with 13.9 Nm of torque at 12V. I was going to gear it down 3:1 so the weapon would spin at about 4,000 rpm maxed out. My question is about the amp peak when using the spinner tool. I read in the forums about your explanation of ohms law saying this was only theoretical but it still concerns me.

Is my design flawed and is that causing the high peak? Should I be using a brushed motor instead? What ESC should I be using? Would the current setup do - instantly fry, twitch back and forth then fry? I hate electronics so your help is greatly appreciated. This is an overhead spinner for the 12lb class. [Rock Hill, South Carolina]

A: [Mark J.] Sounds like you're in way over your head here. Take a deep breath, relax, and try to forget all about those disturbing numbers.

The Run Amok Excel Spinner Spreadsheet is a complex tool that requires the addition of a healthy scoop of 'real world' to temper its results.

The most effective use of the spinner spreadsheet is to model an existing spinner weapon system that is known to function well, then model a new design to see how the outputs differ. You can then make a reasonable judgement about the viability of the new design.
That 1400 amp current spike you're looking at is a theoretical value which assumes that the weapon motor is the only resistance in the circuit. Your ESC will add in at least as much resistance as that particular motor, and the internal resistance of your battery is somewhere in the same ballpark. Put that all together and your real world peak current tops out at around 300 amps and falls off quickly with increasing RPM. Feeling better now?

Part two: since amps equal torque and that high current spike is theoretical, that 'wishful thinking' 13.9 Nm of stall torque goes away as well. Because of the black magic any specific brushless ESC goes thru to get its associated motor to spin up from a stand-still you'd never see that amount of torque even in theory. The concept of 'stall torque' really doesn't even apply for hobby brushless motors; spin-up time for a brushless spinner will be longer than the theoretical output of the spreadsheet.

Lets take a look at some of the specifics of your design:

  • You didn't share actual dimensions, but my best guess says a bar with a Moment of Inertia around 0.055 Kg*M2 will weigh about seven pounds. That's way too much for a 12-pound robot. Spinning that heavy bar at 4000 RPM will store about 5000 joules of energy. That's healthy for a featherweight -- overkill for a hobbyweight. Excess ≠ Success
  • You've picked a $200 helicopter outrunner motor for your weapon. Nice motor, but it's designed to run at twice the voltage you intend to supply. That's like buying a Ferrari and sticking a block of wood under the gas pedal to limit output to 25% power*. If you're gonna do that you should just buy a Hyundai to start with.
* Electric motor power increases with the square of voltage: double the voltage equals four times the power.
Standard Advice If you don't know what you're doing, copy a successful design. Look around at hobbyweight spinners and find motor/ESC combinations actually in use that are powering effective bar spinners. Size your bar reasonably, and make improvements over time as you find weaknesses.

Q: Hi Mark, I had a follow up question about the spinner spreadsheet. Your explanation was very helpful and brought much comfort. I know the 7lb bar is overkill but I wanted to try something like that at least once just to see what would happen in the area upon contact. Overkill is underrated. Maybe...

You said ballpark, the resistance would be about 3x the amount listed. This changes the calculation some on the battery requirements, right? Couldn't I use a much higher voltage battery with a much lower mAh? I ask because as always weight, but particularly space, is an issue. Many thanks!

A: My resistance explanation above was intended to explain why the big start-up current surge the spreadsheet shows isn't really that big in 'real world' terms. As the weapon speed starts to rise the electrical characteristics of the motor start to change. Here's a more complete explanation.

At theoretical 'stall' the combined resistance of this particular motor and a typical controller and battery is about three times the resistance of the motor alone -- but it doesn't stay that way. The internal resistance of your battery pack and controller do not vary with weapon speed, but the effective resistance of your motor increases linearly with speed due to the rise of back EMF generated by the motor. With the weapon spinning at full speed the effective resistance of the Scorpion motor will have risen from 0.006 ohm to nearly 2 ohms and the added 0.012 ohm resistance from the battery and controller will have long been rendered insignificant.

For purposes of the Spinner Spreadsheet you should still enter the resistance value for the motor alone because the spreadsheet takes into account the rise of back EMF with increasing RPM. The additional resistance at start-up lengthens spin-up just a bit and the huge theoretical current spike at start-up is diminished, but the total draw on the battery to spin the weapon bar up to 5000 joules is essentially the same. Just remember that even though the big current spike at start-up isn't really as big as the spreadsheet shows the total current consumption is close to correct. There's no such thing as a free lunch; you still need a battery with the same watt-hours of capacity.

Q: Btw, what ever happened to the Texas bar spinner?

A: The conversation I had with Tex broke off abruptly and I have no more recent news. Here's where the 'Texas Spinner' page leaves off:

It's been about a week since I last heard from Tex. I have, however, heard from a number of applicants for the second season of BattleBots on ABC about their rejection calls. One note was from a BattleBots veteran who's design was rejected for not having enough 'bling':

ABC was picking robots based on appearance. We got thrown into the bar spinner category, and Trey [Roski] said they had a ton of entries that looked exactly like Tombstone...

Given that Tex was pitching a Tombstone clone I suspect that his 'hook' wasn't tempting enough to get a positive response from BattleBots. I hope he builds a 'bot anyhow.

Q: Hi Mark,
Probably a dumb question but I figured I'd have to ask. So you know how a 540 size motor attaches to a p60 gearbox to drive a bot... why isn't a similar set up used for powering the weapon? Attach a motor to a gearbox and use that to drive the weapon. It would certainly be easier to mount and you'd have a keyed shaft instead of a 'D' profile. It seems like a no brainer in the hobbyweight since you usually take that size or similar motor and gear it down 3/4/5 or so. Then you could do a 1:1 v belt or flat belt from keyed shaft to weapon to allow for slippage. Does the gearbox get too hot for this? Couldn't you put a fan on it if it does? I can't imagine I'm that smart and would like to know where my thinking is wrong. Many Thanks! [Raleigh, North Carolina]

A: [Mark J.] I've seen this done, Raleigh. It's not a horrible design, but you lose more than you gain:

  • Simplicity Your design still needs a belt to save the gearmotor from destructive shock loads on weapon impact. If you do the reduction with the belt drive you leave out complex parts that could become failure points.
  • Efficiency Gearbox heat comes from internal frictional losses. Leave out the gearbox and that lost energy can be spinning your weapon quicker and faster.
  • Weight A 4:1 P60 gearbox weighs in at 6.5 ounces -- plus grease. Scrap the gearbox and you can put that weight directly into the weapon where it will do some good.
  • Cost A 4:1 P60 gearbox runs about $50 -- delivered. A larger driven pully to get that reduction costs much less.
Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

Q: I've seen plenty of cheap lipo chargers on the market that you just plug the balance connector into and it starts charging. Yet with most higher end chargers they let you adjust the current going into them relative to the capacity of the battery. I assume thats in the best interest for the cells lifespan, but is there any risk in damaging the battery short term by using the first type of charger? [Roseville, California]

A: [Mark J.] Setting the charge rate is important for reasons beyond battery longevity. The safe charge rate for most LiPo batteries is '1C', which is one times the capacity of battery in amps. Charging a LiPo at a higher rate risks overheating and combustion of the battery; ALWAYS make sure that you charge your battery at a safe rate.

These inexpensive 'plug-n-play' LiPo chargers have a fixed charge rate: typically about 1 amp. This rate is optimal for 1000 mAh batteries.

  • Batteries with greater capacity may be charged at this rate without additional risk, but will take proportionally longer to charge. You probably don't want to wait three hours for your 3000 mAh battery to be ready to go.
  • Small insect-class LiPo batteries with less than 1000 mAh capacity should definitely not be charged at a 1 amp rate! Doing so is inviting explosive combustion.
I'll add that you should never attempt to charge a lithium battery that shows signs of physical damage, including any swelling or 'puffiness'. It's also an excellent idea to charge your LiPo batteries in a flameproof container such as a LiPo charging bag.
Q: I've got a 3D printed plastic antweight with a direct-drive drum weapon. The brushless outrunner weapon motor is pressed into the drum. The motor gets so hot after a couple minutes of operation that it melts the plastic and the drum starts to slip out of position. Do I need to put something between the motor and the drum or is there some way to cool the motor? [On-line Forum]

A: [Mark J.] Small hobby brushless outrunner motors are made for model aircraft and are designed to operate with a strong airflow to dissipate the considerable heat that they generate. If you cut off all airflow thru the motor by stuffing it into a blind plastic tube you're going to rapidly generate a lot of trapped heat -- more than enough to melt a PLA printed drum.

Drill some 1/4" holes from the outer surface of the drum into the central void of the drum. As the drum spins the holes will pull air out of the drum. Keep the vent holes in the motor base uncovered and you'll generate cooling airflow thru the motor.

Q:How are robo claw 60A solo ESC for the drive 30+ kg robot [Kathmandu, Nepal]

A: [Mark J.] ESC selection depends on much more than the weight of your robot. From Frequently Asked Questions #21:

Q: What Electronic Speed Controller (ESC) should I use?

A: 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).

Once you have settled on motors and drivetrain details, the Team Tentacle Torque & Amp-Hour Calculator will calculate the peak amperage the drive motors can consume with the robot pushing at full throttle against an immoveable object [Amps (per motor) to spin wheels]. Use that number to select an ESC with suitable capacity. Give yourself a little extra capacity (~20%) to allow for unexpected conditions.

I can comment that the Robo Claw 60A Solo is not widely used in robot combat. Robots requiring ESCs in that current range generally use the RageBridge 2 ESC.
Q: Okay, so I've noticed that the teeth of the Saifu Antweight kit can be moved around. Normally, the teeth are placed with one side having the teeth in the far left and far right holes, whereas the other side puts the teeth in the middle left and middle right holes (the way I'm describing it makes sense, right?). I was wondering if there would be any advantage against any robots to place all four of the teeth either in the farthest holes or in the center holes. I know that this would fiddle with the way energy is distributed in the drum, but I didn't know if it would be worthwhile, or just an unnecessary thing to play around with.

AS ALWAYS, THANKS! [Chester, Illinois]

A: [Mark J.] Don't play with it! The 'normal' tooth arrangement as you describe it is the correct and most effective arrangement of the moveable teeth for all situations. Each of the four teeth has a clear path in its rotation which assures that the 'bite' of each tooth is maximized. It also maintains both static and dynamic drum balance.

I suggest that you read sections 6.3.1 and 6.3.2 of the RioBotz Combat Tutorial for a complete description of 'bite' for spinner weapons. Read the rest of the tutorial while you're there.

Q: I am running a Dr. MadThrust 1700 kv motor for my weapon on my 15 lbs robot. I need to find a battery pack or make my own that will be able to run that motor and I cannot use Li-Poly batteries. I know that I need a pack around 3000 mAh but I don't know about the discharge rate. Do you have any advice? [Dublin, Ohio]

A: [Mark J.] The standard solution for robot events that do not allow Li-Poly batteries is a switch to safer and universally allowed lithium iron phosphate packs -- also known as 'LiFe' as a contraction of their LiFePO4 cathode material.

LiFe packs have lower discharge rates than Li-Polys, so you'll want to go with larger capacity packs to obtain higher amp peak drain. A pair of 3S 30C 4200mAh packs in series will get you in the ballpark. LiFe cells have different charge requirements than Li-Polys, so make certain that your charger has a specific LiFe setting.

The Dr. MadThrust 1700 kv motor is rated 105 amps of current, and the 30C 4200mAh packs are rated for 126 amps continuous current output with a 168 amp peak. The peak current draw of your weapon motor under load depends as much on weapon size/weight/gearing as it does on the motor but you gave no weapon info. I think you'll be fine -- LiFe packs are more forgiving than Li-Poly packs, and a spinner weapon will pull high current only briefly while spinning up before settling at a much lower level. Best luck.

Q: I'm looking through your info on spring flipper mechanisms and it's very well done & informative. My question is, how would I adjust a "choo choo' mechanism to make it shoot down, like for an axe? [Woburn, Massachusetts]

A: [Mark J.] Thanks. I'm happy to hear that you like our guide to spring flipper designs, but I must admit that I'm surprised that you looked at the four mechanisms on that page and decided that the 'choo-choo' would be a good choice for an overhead axe weapon.

The demands of flippers and overhead axes are quite a bit different. An axe can build energy gradually over a long arc before striking its target, while a flipper needs a big burst of energy released instantly. A modified flipper mechanism is not your best choice to swing an axe, which is why you haven't seen them used for that purpose.

  • The reaction a flipper 'bot experiences from the quick upward burst of energy just presses it down onto the arena surface where it is well supported. Everything remains stable.
  • That same quick burst of energy applied to a downward striking axe weapon will produce a large reaction that lifts the front of your 'bot and tries to flip it over backwards before the axe even strikes your opponent.
You've got plenty of 'swing' to build axe speed with a simple gearmotor powering the axe directly -- don't add the complexity of a spring load plus a mechanical linkage to wind and release it.That said, it is certainly possible to orient a 'choo-choo' to pull back and release a spring loaded hammer: see sketch at left for a simple solution. It will work, it just isn't a good idea.

Q: what kind of motor is best for... (deleted) [Maharashtra, India]
The 'Ask Aaron' website is closed to questions from builders competing in India.

The best enclosed arenas in India would be considered inadequate for 30 pound robots in Europe or the US but are hosting events for much heavier 'bots. Aaron certainly wouldn't approve of the reckless endangerment of life and limb, and I will not contribute to the development of combat robots in any region where the arenas are not universally able to safely contain them.

Q: This isn't a techy question and it unapologetically fits in the cheerleading section but I just had to ask it anyways. I've been debating asking this question on this site for weeks now and I just had to ask. If it's a garbage question through and through just toss it lol!

Okay, so I've read this website for a while now and I am aware of the cheerleading section and this belongs there but if you have the time I would love your opinion. Ima cut right to the chase and say this: Weaponized Turbojet Engine Afterburner.

You see all of these crazy jet engines on youtube that spit white hot flames and enough noise to wake the dead and we know from bots like ICEwave that crowds love loud bots. Now there are MANY design challenges flaws for using a jet engine in combat robotics. From the reliability of a 100,000 RPM engine when hit by a massive spinner to the weight of the jet not to mention the heat problem... In my mind the weapon would either be a automotive turbo outfitted with a flame tube or a custom made turbojet (which would package a lot better). Maybe even an EDF throwing air through to the afterburner instead of the heavy jet engine (although it would sound like a vacuum). It would be very hard to get working right, as I'm sure most non electric powered weapons are, and the robot wouldn't ever be competitive but what are your thoughts on a weapon like this?

Have a great one guys! [London, Ontario]

A: [Mark J.] The next time you're on YouTube looking for dangerous machines you might pop 'Suvival Research Laboratories' into the search box. SRL puts on destructive machine performance art shows featuring devices that include a big pulse jet very much like you suggest here. Their outdoor shows are the right place for these creations; a robot combat arena is not suited to such displays.

  1. In the very few events that allow robots to carry flame weapons there are strict fuel volume limits. BattleBots allows a 0.5 litre tank of propane or butane, which I suspect would would power a worthy turbojet for perhaps 10 seconds.
  2. A heavyweight-class combat arena is a very expensive item made largely from thick sheets of very meltable plastic. Imagine what happens to the remains of your white-hot jet engine when a spinner weapon hits it with enough energy to rip a water heater in half. No event organizer is going to allow your turbojet inside their precious arena.
Book a flight to the next SRL performance and watch their pulsejet melt a Volkswagen. That oughta get this out of your system. If not, here's the button:
Q: Hey there Mark, I was hoping to ask you about two seperate but semi-related questions concerning some bots I'm building (beetle and feather), both using HDPE.

1) All of my bots have been made of HDPE from 5 to 20mm thick, cut with a mixture of a handsaw, a jigsaw, a chop saw, and a circular/skil saw. I notice however with making curved cuts, the resulting cut tends not to be right-angled, getting more and more drastic the further you go up in thickness. The jigsaw gives cleaner finishes but bends more often, and the hacksaw gives squarer finishes but tends not to curve as well and gives a rougher finish. Do you have any tips for getting cleaner curved cuts with plastics like HDPE/UMHW that don't involve machining?

A: [Mark J.] Given your selection of tools, I think the jigsaw is your best option. Several tips:

  • Buy a brand new blade for your jigsaw and use it only for soft plastic.
  • Several manufacturers offer special blades for soft plastics that reduce heating and make for smoother cuts -- example.
  • Blade tooth count: about 10 TPI works well for HDPE.
  • For tight curves: rough cut about a centimeter from your final cut line, then come back with a second cut to trim away the thin edge.
  • A little wax on the blade helps reduce friction and heating.
  • Try lower blade speeds, and keep the material moving.
  • Square up edges with a sanding block.
2) My second question concerns a snail cam design for a spring flipper. I'm aware that you've been asked questions concerning this topic multiple times and have read through all relevant posts, but for my design I've attempted to adjust the parabolic spiral to allow for a continuous length for the final 1/4 of the rotation to allow for leeway when winding and to prevent misfiring. The motor still seems to struggle to wind spring loads it should be perfectly capable of, and while there may be other issues in the design that could cause that, I believe the cam contributes to the problem.

I've included a render of the cam in question, winding down springs 15mm from their original position. The tolerances in the drawing (eg. the cam not making contact with the spring when the flipper is fired) is to prevent shock damage to key components when the weapon is fired. The cam was also drawn by hand essentially, with me trying to compact the parabolic spiral over 3/4's of the circle. Would you know of an equation that would allow me to draw a more accurate parabolic spiral to fit these tolerances, and would a cam with it's last 1/8th being even put less stress on the motor than the current design?

A: I've been trying to avoid banging out equations for parabolic snail cams, but I suppose I've put it off long enough. Let me find a pad of scratch paper and have a go at it...

One hour passes

OK, I've got it. The general equation for a parabolic sprial cam is... wait... wait... %#!@*&%$!!!

Another hour passes

Right! It's important to get the shape of the parabolic spiral correct to even out the torque load on the motor. The spring becomes harder to compress as it compresses farther, so the cam has to provide a lot of compression at the start of rotation (half the total is in the first 25% of rotation) and taper to less compression as the load increases (the last 12.5% takes the final 25% of rotation).

The formula for the cam in polar coordinates is: Radius = k ∑ θ 0.5 + minimum cam radius where 'k' is a growth variable that defines the lift of the cam; a 'k' of 0.0527 will give one unit of lift over 360 degrees of rotation. Here's an example for a small snail cam with a 5mm minimum radius and 15mm total lift:

Radius = (15 * 0.0527) * θ 0.5 + 5

0 degrees rotation
(15 * 0.0527) * 0 0.5 + 5 = (0.79 * 0.0) + 5 = 5.0mm

90 degrees rotation
(15 * 0.0527) * 90 0.5 + 5 = (0.79 * 9.5) + 5 = 12.5mm

180 degrees rotation
(15 * 0.0527) * 180 0.5 + 5 = (0.79 * 13.4) + 5 = 15.6mm

270 degrees rotation
(15 * 0.0527) * 270 0.5 + 5 = (0.79 * 16.4) + 5 = 18.0mm

360 degrees rotation
(15 * 0.0527) * 360 0.5 + 5 = (0.79 * 18.9) + 5 = 20.0mm

Note The mathematically correct spiral (below left) has far too steep a drop and rise at the start of its lift to allow a cam follower to drop in and correctly follow the profile. The exact needs of that 'landing area' will depend on the details of your lifter mechanism, but it will require some modification (below right) to be mechanically functional.

Mark gets himself a celebration beer

So far, so good. Now, the torque calculations I've given in previous snail cam posts assumes that spring compression occurs evenly throughout the full 360 degree rotation of the cam. You've modified that in two ways:

  1. Compression reaches maximum at 270 degrees of rotation, and
  2. There is no compression during the first ~60 degrees of rotation due to the cam follower being held away from the cam.
The combination requires the motor to fully compress the spring in about 210 degrees of rotation, which requires ( 360 / 210 ) - 1 = 71% more torque from the motor. No wonder the poor thing is struggling. Yes, reducing the no-lift zones on the cam will reduce the motor torque requirement and result in smoother operation.

How to get a proper cam profile that includes a no-lift 'flat sector' at the end of rotation? Increase the growth variable until you get the full lift you need at the degrees of rotation you want your flat sector to start and freeze the radius at that point. The smaller the flat sector, the less torque your motor will need to put out to wind the spring.

3) Finally, for a question that ties everything together, what would be the best way of cutting a cam like this out of 10mm HDPE be? I believe my first attempt was too rough, as it seems the cam gets caught on the smallest of differences during the winding. I'd love to try to make a good cam myself without having to resort to machining.

Thanks for reading. [Galway, Ireland]

A: I have concerns about 10mm HDPE being too thin for the purpose, even for a beetle. A wider cam will spread the loading over a larger area and prevent the cam follower from deforming the surface and increasing turning resistance -- that may be one of the reasons your motor is struggling. Take a look at the width of the snail cam on 450-gram antweight 'Jšnnš' (video) for comparison. You might also consider a larger 'flat' cam follower to provide greater surface area that would be less affected by surface irregularities.

I think you can make an entirely satisfactory cam by following the jigsaw tips I gave above. A little work with a sanding block can remove any troublesome irregularities, and a flat cam follower will be less sensitive to catching. Best luck!

Comment: Hey again Mark. Cheers for the equations, that's a great deal of useful information! I've attempted a new updated design for the cam based on the recommendations made, with a better gradual increase, less of a loss of torque, and a thicker cam with a recessed hub to compensate for the size increase (space is at a premium within the mechanism). Comparing the old cam design to the new one, I noticed that where the difference between the two was at its greatest was where the gears in my motor sheared, so your theories on load seem bang on! The fact that it's not a direct pull down is still an issue, but that would require more adjustments to equations, and the old mechanism was able to wind that part fine with a similar profile, so I should be alright there. I'll also keep your tips for HDPE in mind when working on the new cam.

Cheers again, if I'm ever in the area I reckon I'd owe you another celebratory beer!

Update In response to a bushel of off-line questions on this topic I've put together a new page devoted to all things snail cam: The Snail Cam Files. In addition to sections on cam profiles, drive motor torque, spring rates, and control circuits -- the page links to a Google Files spreadsheet I authored that will assist in designing your mechanism.

Remembering 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   

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.

Aaron's Greatest Hits! More of Aaron's Poems Aaron's Minecraft High Dive Video Aaron's World of Warcraft Player Guide

We welcome combat robot questions. Please search the Ask Aaron Archives first to see if we've already answered your question. Recent Q&A are posted above.

Type your question in the box, attach files if needed, then click: 'Send'

Why is there a 'Cheerleader' button?

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

Copyright 2009, 2019 by Mark Joerger
All rights reserved.
how to design and build a battlebot