5612 Questions and Answers about Combat Robotics
from Team Run Amok

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

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
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   
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Q: Hello again Mark, Sheffield, England here again. In all honesty, I'm not an experienced machinist, but I have been drawing robots ever since the first Robot Wars appeared on British television. This included batteries, and funnily enough, drivetrains. But this robot is still going through it's design phase over and over, because there are always new ideas that pop up in my head, but the measurements are not final. But I have a couple more questions for you now, so please bear with me. I also understand that drawing robots and building robots are two completely different things, but it's better to get the experience than not do it at all, right?

Idler shaft and sprockets for two-stage chain reduction A: [Mark J.] You have to dive in someplace, so let's go...

1) For the drive train itself, would pushbike sprockets work well enough for the drivetrain? Or would I have to go for a thicker material? My worries with such sprockets is that they're flimsy as all heck. Same goes with the axles too, what would be a decent thickness and material for a drive and/or idler axle?

A: I usually avoid giving specific material advice because there are so many possible variables in any given design that might invalidate my assumptions (see Frequently Asked Questions #17). I typically advise looking at designs similar to your own and seeing what has worked in real applications. However, in this case I have that similar design in my workshop.

'Run Amok' still has the dual-stage drivetrain used by 'Run Away' so I went down and took a photo and a few measurements:

  • Bike sprockets are flimsy and difficult to mount. Run Amok uses 3/8" pitch sprockets and matching #35 chain.
  • Both the drive and idler axles are 7/8" solid steel. The idler could be smaller, but it was convenient for me to keep them the same size for interchangeability of parts.

2) Sticking with the drivetrain, to help with the spinning of the idler shaft and the wheel axles, would it be better to go with bushings first, just to start off with? Or would it be better to go with bearings? I know both sides of the argument have pluses and minuses, but I would love to get your input on it. And hopefully a way to mount them too, as I've been looking all over the internet to see how I can mount such pieces, and all I get redirected to is how to mount them into cars and the like. Addition to this part: I have checked one of the questions, but all it talks about is putting the axle into the bearing, and I would like to know how to put such bearings/bushings into an axle mount.

A: Run Amok uses bronze 'oilite' bushings for the drive axle and ball bearings for the idler. The axle bushings came with the assembly I scavenged from the junkyard, but I found it easier to locally source bearings for the idler assembly. Use what you find convenient.

The drive axle bushings are flanged and simply drop into mounting holes in the chassis. They are held in place by the wheels, which are attached by bolts that run thru holes in the live axle.

The idler axle is also 'live'. The bearings have extended flanges with bolt holes that are used to mount them to the chassis. The bearings also have extended inner races drilled and tapped for set screws that secure the axle to the bearing races. The idler sprockets are also secured to the axle with set screws that nest into drilled recesses in the axle.

For additional options on shaft mounting see section 4.2 of the RioBotz Combat Robot Tutorial.

3) Battery packs. I was initially planning on running both the Bosch motors with two 24v NiCad battery packs (one pack for each motor), but I can't seem to find such packs in the UK. And I know for a fact that buying drill battery packs are a pretty bad idea because some of the battery packs could be of low quality and or bad efficiency. But I've also heard about NiMh battery packs. But it's the same problem with the finding of such packs. Would the best way of getting such battery packs be to just bite the bullet and get 4 (2 spares) from the robotmarketplace along with a decent charger? I know for a fact that it might cost an arm and a leg due to shipping and such, but it's better than having to jerry rig battery packs. And is it possible to link both batteries to one speed controller, or would that make the ESC go boom?

A: NiCad and NiMh are old school, which is probably why you're having trouble finding them. The currently favored battery technology is Lithium Polymer (LiPo). Smaller, lighter, and less expensive than NiCad or NiMh, they are a bit trickier to look after but provide very high performance. Lots of Q&A about LiPo batteries in the Ask Aaron Radio and Electrical Archive.

If you're going to run a pair of Bosch GPA motors in a 250 pound robot I think you'll need to run them at 36 volts in order to provide enough power for adequate performance. At 36 volts I calculate that your 'bot will consume about 4 amp-hours of current in a three-minute match. You'll want at least a 20% capacity 'cushion' to avoid dangerously over-discharging the battery. A pair of LiPos like the Turnigy 5000mAh 5S 30C Lipo Pack wired in series will give you the required voltage and capacity to run both motors. You'll want two chargers specifically made for LiPo batteries -- one for each pack.

The two packs in series will feed into the single, dual channel ESC.

4) Speaking of ESCs... Now I'm no electronic whizz kid, but I have been looking at reviews of some of the speed controllers from other places on the internet. Two of them have caught my eye. The Vyper 120A DC Motor Controller and the Sabertooth Dual 2x32A 6V-24V Regenerative Motor Driver. Would either of these work for the 250lb robot we plan on making?

A: The Vyper ESC is overkill for your 'bot. It's a 'single channel' ESC so you'd need one for each motor, and at $249 each that's getting expensive. Each of your motors will consume no more than 60 amps at maximum pushing power so a 120 amp controller isn't needed.

The Sabertooth ESC is right out. It has only 32 amps continuous current capacity, will not operate at 36 volts, and has a reputation of just stopping at awkward times in combat. Avoid.

The favored ESC in your ballpark is the RageBridge2 from Equals Zero Designs. Operates at up to 40 volts, has true current limiting to keep it healthy, and can provide up to 75 amps for a full 30 seconds to each of two motors. Reasonably priced and combat proven.

Q: I may end up sending more questions your way. I've always loved robots, and I've always loved to know how they tick.

OH, and forgive me for my clumsiness, but me and my friend send our condolences for your loss of Aaron. Taken from the world way, way too soon.

A: Thank you. I miss Aaron every hour of every day. A large part of me left with him.

Q: Hello,

So I just saw the new Battlebots episode and it really inspired me to try and build my own bot. Do you have any general tips for a first time robot builder? I figure that it should be something small and simple, but I don't know where to go from here. Any help would be apprieciated.

Thanks [California]

A: [Mark J.] Sure. Start by reading the first eight entries on our Frequently Asked Questions page. They're aimed at the questions new builders most ask and/or most need to know. You'll want to read the rest of the FAQ, but the first eight are a good start.

You may also be interested in a combat robot kit for your first robot. I keep a list of currently available small combat robot kits in the Ask Aaron Toy Hacks and Kits archive.

Then there are 5600 more questions and answers about combat robot design, construction, control, and events here at 'Ask Aaron'. If you don't find answers to your questions here feel free to ask new ones.

Q: Please tell me I'm not the only one facepalming at 'Obwalden Overlord'... [North Carolina]

A: [Mark J.] 'Obie' isn't much of a fighter, but BattleBots has a history of 'Artbots' displaying exotic robotic technology. Remember 'Mechadon' and 'Snake'? ABC BattleBots 2016 needed at least one oddball 'bot.

Bigger question: why did Team Razer turn fearsome 'Warhead' into a dragon?

Historical Note There was a robot similar to 'Obie' at 'Robot Wars Extreme Warriors' fifteen years ago. An American team brought 'The Wife', but they were unable to get it working and it didn't appear on the show. At least 'Obie' got to fight.

BattleBots: 'Obwalden Overlord' vs. 'Warhead'
BattleBots: 'Obwalden Overlord' vs. that 'Warhead'?

Q: How do I build a robbort? [Rossford, Ohio]

A: How to build a robbort

Q: Hi Mark. I plan on making a 250lb robot with a long time friend of mine. It'll be our first robot, but we don't exactly have the money to buy big name motors such as Ampflow. That and we live all the way in the UK. Which means getting their motors with gearboxes is a huuuuge no no. I already have a couple of ideas for the motors such as a Bosch 750w motor, but I have no idea on how to build a gearbox in the same vein as the Ampflow gearbox speed reducers. Is there any chance of showing how I could make such a gearbox with a good reduction for pushing other robots around? [From Sheffield, England]

P.S: Loved watching Run Amok run through the competition in Robotica when I was younger.

A: [Mark J.] Thank you for your rememberance of 'Run Amok'. Those were good times.

The Bosch GPA 750 has a long and honored history in combat robotics. Fifteen years ago it was one of the 'go-to' motors, particularly in Europe. Favored at Robot Wars by competitors and the 'House Robots', the motors proved themselves adaptable and reliable. Team Run Amok used them for both drive and weapon power in 'Run Amok' and its Robot Wars variant 'Run Away'. But, that was all 15 years ago and robot combat has moved on.

Still, if run at 36 volts the performance in a 250 pound robot isn't bad. A pair of them geared 10:1 with 6" diameter wheels will push the 'bot to a top speed close to 11 MPH in 20 feet. Pushing torque is very good, breaking traction at a current draw less than 60 amps. Not so bad.

Now, gear reduction. If you were an experienced machinist you wouldn't need to ask me how to make a gearbox, so we'd best keep this simple. Take a look at a previous post in the Ask Aaron Design and Construction Archive on two-stage chain reduction drivetrains. 'Run Away' used a two-stage chain and sprocket drive to give a 12:1 reduction ratio, so it's certainly do-able without precision machining or great expense.

An alternative: the other 'go-to' motors back in the day were electric wheelchair motors, often scavenged from used wheelchairs. Some of these had power comparable to the Bosch GPA and had the advantage of being a simple bolt-up power unit. Overvolting a pair of wheelchair motors like the NPC T64s to 36 volts with 9" wheels can give speed close to 10 MPH in under 20 feet, breaking traction at less than 70 amps of current.

That gives you a couple of old-school options. Neither is 'state of the art', but either will get you out there fighting.

AmpFlow speed reducer

Q: do i need to strap my lipo in my antweight if there is almost no room for it to move, just enough to expand a little.. the chassis is a UHMPWE frame millled out of a single block [Quebec, Canada]

A: [Mark J.] What exactly is holding it in place? If it has its own compartment, that's great. If it's just wedged in with other components, a good hit can shove it so hard into those other components it can crush them, or shove them into the battery to crush it. Go 'belt and suspenders' and secure EVERYTHING!

You can use zip-ties for hard-cased things like receivers and ESCs, but narrow ties will cut into a squishy LiPoly and/or crush the internal structure on hard impact. Wide straps for the battery, please.

Q: Hi lipo guy again ; ) can i use these Velco battery straps to strap my battery securely to my chassis?

A: I don't know how large your battery is or what mounting opportunities your chassis allows, but you're on the right track to use a 20mm wide strap that will spread the load out and not cut into your soft battery. You can pick up Velcro straps like these at office supply stores -- they're used to bundle computer cables and keep them tidy. The buckles aren't needed and they take up weight and space; just fasten the strap to the chassis and wrap around the battery.

Q: Hi lipo guy again, i use this battery and the space is very tight. i was just wondering, i also use 2 tiny esc and a minimixer from fingertech. can they puncture the batery? or im i ok with the strap only and the other electronic resting on it or near it

A: I like to tie down everything. The tinyESCs and mixer are small enough to mount with squares of foam mounting tape which takes very little room or weight. Good for receiver mounting, too.

Why are you running a mixer? Does your radio transmitter not have mixing options? You can save space and weight if you can switch to transmitter mixing.

If you don't mount everything will something be damaged bu a big spinner hit? I can only say that I'm not willing to take that chance and I don't think you should either.

Q: so if i unederstand well, if everything is strapped, bolted, glued or smash so hard it become one with the chassis, i dont need any foam around the battery, just the strap wll do the job well?

A: No. Every component should be individually anchored to the chassis:

  • Nice wide straps are good. Use an appropriate number - size dependent.
  • Bolts assocoated with items that have mounting brackets are good.
  • NO GLUE! NO TAPE! NO ZIPTIES! Exceptions for foam mounting tape (servo tape) to secure light electronics and zipties for wires and very small electronics.
  • A good spinner weapon hit will generate HUGE G-FORCES. Just cramming multiple items into a tight space is not acceptable. Components will compact and move in unpredictable ways, placing stress on connectors and fragile bits. Tie individual items down!
  • Foam is OK if you have one item in its own enclosed space. That's not what you've got.
This is all general advice. An antweight is different than a heavyweight, and your particular arrangement may have oddities that I don't know about. It's the little things that lose robot combat matches -- take care of them.
Antwight combat robot held together with utility tape

Not how it's done!

Q: can i replace the battery i showed u before with this NiMH battery? it is listed as 9gram on other website and since its my first robot.. im a little bit concerned about lipo fire..

A: You should be a little concerned about LiPo fires -- enough to take precautions. Unless a LiPo is crushed, cut, improperly charged, or otherwise abused they're fine.

The NiMH battery you found is a poor choice for your application:

  • The Turnigy nano-tech LiPo battery you linked to earlier can supply a peak 7 amps, which is more than adequate to allow your motors to produce their full rated power. It takes up 7 cm3 of space and weighs 13 grams.
  • The 'Dynamite Speedpack' NiMH battery you found does not list a current output but the small size of its connector wires indicates a MUCH lower current capacity -- likely no more than 1 amp. It takes up more than twice as much space (16 cm3) and, in spite of that 'other website', weighs four times as much (at least 52 grams).
Take reasonable precautions and stick with the LiPo.
Q: do u have a diagram on how an horizontal spinner like tombstone or mechavore is mounted? [Quebec, Canada]

A: [Mark J.] There are two common methods to mount spinner weapons; 'Tombstone' uses one, and 'Mechavore' the other:

  • 'Tombstone' uses a non-rotating 'dead shaft' solidly attached to the chassis as a structural member. The weapon blade and drive sprocket are attached to a tubular hub with bearings on each end that ride on the shaft. You can see spare weapon blade/sprocket assemblies for 'Last Rites' ready to slide onto the shaft in the picture at right.
  • 'Mechavore' has its spinner weapon and drive sprocket attached directly to a rotating 'live shaft'. The shaft itself rides in bearings attached to the chassis.
There is a diagram of live and dead shaft designs with a discussion in the Ask Aaron Weapon Archive: Live Shaft vs. Dead Shaft.
Alternate weapon bars for combat robot 'Last Rites'

Q: hello im the guy that asked the question about the horizontal spinner mounting option. Do i need to put something between the bushing and the frame under the spnner? it seems like the friction at this point will be very high. Im sorry if my english was not really good ; )

A: Vous parlez mieux l'anglais que moi le français. Je parle le français comme une vache espagnole.

I assume we're talking about a 'dead shaft' design? Some bearings come with 'extended' inner races that will space the weapon hub up away from the chassis to prevent rubbing. If the selected bearings do not have an extended inner race, a small washer placed on the shaft will rest against the race and rise the hub up for clearance. I left the washers out of the linked diagram for the sake of simplicity, but then I left out the chassis as well.

Q: can i bring an antweight combat robot on a plane for a competition? [Quebec, Canada]

A: [Mark J.] The battery is the problem. On flights in the US you can carry small batteries, including LiPoly, in carry-on bags. Rules in Canada are different. The Canadian Air Transport Security Authority offers specific guidelines for air transport of batteries that vary by the capacity of the battery. The way I read it, insect-sized batteries should be fine in either checked or carry-on bags -- but the website warns:

Not all airlines allow lithium batteries carried in personal devices such as laptops or cell phones in checked baggage. Please consult your airline for more information.

So, pack your robot as carry-on and you should be fine, or check with your airline about packing in checked baggage.

Q: Hello Mark... or rather, Crusher of Dreams?

So I've re-evaluated my options a bit since you helped me to realize that building a ring-spinner akin to 'The Ringmaster' isn't the best first robot to build. I did indeed do a google search of Hal Rucker. I then proceeded to cry. Yeah, he's good. Really good.

But in all seriousness, I've scaled back a bit. Instead of a lightweight single-tooth ring spinner, I think I want to see how well I do with a beetle first. I've also realized that I'm just as happy with a Single-Tooth FBS instead. What's that you say? A FBS is unlikely to do well in a beetle match because of the small arena size and less time to spin up?

I know . I'm not in it to win it. I just like Full-body spinners and want to get my feet wet and simply...have fun!

So I was wondering. Aside from the fact that a FBS wouldn't be the best idea, given the beetle arena size, do you think a SINGLE-TOOTH full-body spinner would work decently and provide a little extra bite? And...have there even BEEN any single-tooth fbs bots that you know of in the bigger circuits?

Thanks Mark, David R. [Livermore, CA]

A: [Mark J.] I looked up the competition results for every beetle FBS and ring spinner at a major event for the last decade. Let the dream crushing begin:

BeetleFBSCircular DispleasureDaytona Area Robot Tournament200611
BeetleFBSCritical MassTexas Cup200602
BeetleFBSShell RevSpring HORD200902
BeetleFBSUltramicrobyteSTEM Tech Olympiad201412

What could be worse than these dismal records? Each of these 'bots fought at that single listed event and never appeared at a major tournament again. Their builders were apparently so discouraged with their performance that they completely gave up on them. Doesn't sound like much fun to me. Single-tooth or not, I really don't see a beetle FBS being viable. By all means build what you want to build, but remember that even nicely made lead baloons fly poorly.

On the topic of single-tooth FBS -- no, I can't think of a true single-tooth FBS that has competed in any major event in any weight class.

UPDATE I missed a few beetle FBS that had their weapons listed oddly at In general their records are more of the same, but in fairness there is one that did a bit better:

Beetle FBS 'S.S.O.D.' first appeared at the 2005 Motorama and had a typical beetle FBS result of 1 win and 2 losses. However, the builder kept working on the 'bot and it got better. Its final two events fell within my 'last decade' time frame, and it managed to win as many matches as it lost:

  • Motorama 2007: 3 wins, 2 losses
  • Motorama 2008: 1 win, 2 losses

Q: So in your response to the "Crushed Dreams" guy's question about beetle weight FBS's, are those same crushing stats true for most weight classes? And why do you think all those bots were failures? [Kansas City, MO]

A: [Mark J.] As a class, shell spinners don't do terribly well, but there are certainly exceptions: Featherweight 'Triggo', lightweight 'Ziggo', and heavyweight 'Megabyte' are all mentioned in the Combat Robot Hall of Fame. I note that those three shell spinners have a common design element, can you spot it? Maybe it's just coincidence, maybe it's not.

As noted in an earlier post in the Ask Aaron Weapon Archive the force vector of the impact from an FBS tends to be nearly as large a problem for the FBS as for the opponent. In a small insect arena this is a particular problem. Larger arenas are better suited to robots that turn into high-velocity hockey pucks. Beetle FBS are doomed by tiny arenas.

Talon SRX Speed Controller – Combat Robot Quick Guide

The Talon SRX from CTR Electronics is a robust and capable brushed motor speed controller that should be finding broad use in larger combat robots. The controller was developed for use by competitors in the FIRST Robotics Competition with interface and control options that far exceed the needs of combat robotics. All those options inflate the size of the 'user guide' to 40 pages plus a 'software guide' that runs another 140 pages. The size of those manuals make it difficult find the information needed for combat applications.

I consulted with builders who have used the Talon SRX in combat applications and edited the manual down to a 'Quick Guide' just 3 pages long. The quick guide contains only the information needed to install and set up the Talon SRX in your combat robot.

Talon SRX Quick Guide

Q: I recently saw a video on Youtube of a pair of (admittedly bad) combat robots built out of old printers. Do you think Printerbots (i.e. combat robots that are built only from the components of a household printer apart from batteries and electronics) would be a good idea for a new class? [North Carolina]

3-D Printed Combat Robots A: [Mark J.] I love events that encourage imagination and creative thinking, but I've seen numerous competitions based on "build this using only [fill in the blank]" and they all eventually end badly. Somebody will spend way too much time and effort to find a very specific and rare household printer that has far better components for a combat robot than any other. Everybody else then has to search high and low, bid up eBay auctions to crazy levels, and exhaust the market for those specific old printers just to be competitive. Rather than encouraging creativity, the event quickly becomes very tightly constrained.

Something with printers that might work out is the new 'Plastic Ant' class. Rules are standard except that the chassis and weapon must be 3-D printed from a select group of plastics. This gets rid of all the exotic materials and fancy machine work. You can create your own print file or download one supplied by generous designers. Use your own 3-D printer, borrow/rent time on one, or send your file into an on-line printing service. Sounds like fun to me!

Q: What is the point of running more than one wheel on a motor via a chain, or such? [Washington]

A: [Mark J.] All-wheel drive provides superior traction and pushing power by placing all of the robot's weight on driven wheels. Powering each wheel with its own gearmotor is one method of powering all wheels, but there are situations where this is not practical:

  • Designs with a large weapon may simply not have room for gearmotors to power the front wheels. A chain and sprocket from the rear wheel takes up very little room.
  • Large robots using high-output motors for which gearboxes would be heavy and expensive may choose a multi-stage chain and sprocket system that both provides gear reduction and distributes the power from a single powerful motor to all the wheels on one side of the robot. This can save both weight and expense.

Q: Also, I tried using the Tentacle Drivetrain Calculator to figure out if four Pololu 50:1 Micro Metal Gearmotor HP 6V with 1.25 diameter wheels on 7.4V and if it would work, but I am not sure what I should be looking for with the calculator. Would this set up be efficient for an antweight robot? What should I look for when using the calculator? Thank you, and I hope that the hamburger is not bad.

A: The hamburger is fine.

Once you've put the details of your drivetrain into the Tentacle Calculator the first thing to check is the Amps (per motor) to spin wheels. This value should generally be less than half the 'stall amperage' of the motors. Your design has the wheelspin amperage (0.23 amps) way under the stall amperage (1.97 amps). That's good, but being that far under the stall amperage may be a sign that the wheels are too small or the gear reduction is too great.

Next, click on the Acceleration Calculator button. Insert the width of the combat arena you expect to compete in and click 'ReCalc'. For an antweight the eight foot default is fine as a starting point. You'll notice that your design leaps to its top speed of 2.8 MPH in less than one foot! That plus the low amp draw noted above tells you that the gear reduction is too great.

Close the acceleration window and click on the Gear Ratio Tips button. You'll eventually want to read the whole document, but for now scroll down to the 'Adjusting gearing for special conditions' tab. You can follow the instructions there to try the different gear ratios available for the Polulo motors and different wheel sizes to optimize the robot's performance in an arena of any given size.

A quick recalc changing to the 30:1 Pololu Micro HP motors shows that the maximum motor amp draw is a very comfortable 0.45 amps while the robot now accelerates to 4.4 MPH in a workable 3.5 feet. That's a considerable gain in performance over the 50:1 motors for a six or eight foot arena. Now that you know what to look for you can evaluate other motors for your design.

Q: is it me or in the first season of battlebots on abc, stinger can have the lifter of sewer snake on the front, or swap it to one who looks more like evil plunger? i saw the tested video, but im still unsure [Quebec, Canada]

A: [Mark J.] All of Team Plumb Crazy's heavyweight robots have weapon receivers that accept a variety of attachments. 'Stinger' has a robust front bar for a pivoting passive wedge/scoop, and an active lifter with a square tube receiver designed to quickly add or change between multiple forks and probes. I assume that the lifter receiver is sized to accept the bolt-on attachments that were available to their earlier robots as well as new attachments built for 'Stinger'.

Stinger: the Killer Bee

Correctly designed snail cam
Q: Mark, my old antweight robot has gotten so broken that I have felt that it is beyond repair and I should build a new one. So for my new robot I have gotten a basic chassis drawn up in CAD to try and reuse any parts that I can, but for the weapon, I wanted a spring loaded flipper but the spring compression and release mechanism has been a massive pain for me so just asking if you have any suggestions for designs? [Guangdong, China]

A: [Mark J.] Are you really in China or are you just routed thru a Chinese IP address? I'd love to see some photos or video of Chinese robot combat!

Take a look at this video of a cam-driven compression/release spring flipper. There's a discussion of the design in the Ask Aaron Weapon Archive. It is the simplest effective design I've seen.

Reply: Thanks for the design. It really is brilliant and I'm going to try and construct it. And about the competition in China, well I'm in China, but I'm not competing in China, so I will ask around in the community for videos or pictures of the competition. May/June I think.

Q: Hello Mark, this time I was wondering if there is anyplace in the world that sold wheels for the BattleBots R/C toys. I know ebay sells complete robots but, those get really expensive, really quickly. The reason for this question is the fact that I am trying to use a 'Son of Whyachi' toy in combat without wheel armor. If there is a better suggestion than using those type of wheels. I'm all ears. Thanks-Luke [Kentucky]

A: [Mark J.] The BattleBots R/C toys had several wheel styles, but they were all custom made for the toys and were never available as spares. The white plastic wheel hubs are a non-standard size, but if you remove the screw in the end of the hub and pull it off you reveal a 6mm diameter white plastic sleeve with a 3mm steel shaft inside. That sleeve is supported by a plastic bushing in the gearbox cover. We can work with that!

  • The cheap and easy solution is to slide a BaneBots 6mm aluminum hub over the sleeve and tighten down its set crew (1/16" hex key required). Then slip a BaneBots 2-3/8" wheel or 2-7/8" wheel onto the hub and install the retaining clip (a little tricky).
  • For a little more money you can install a pair of 6mm Dave's Hubs over the output sleeves and fit Lite Flite tires to them. They require a hex key for the set screw as well. Lite Flite tires are popular with exposed wheel designs like yours because they are very 'spongy' and will absorb great impact. This would be my choice.
  • Robot Dojo used Tamiya 70145 narrow tires on some of their toy hacks (picture at right), but installation requires cutting away most of that while plastic sleeve but leaving enough to ride in the support bushing. The Tamiya wheels are just a press-fit onto a 3mm shaft -- not very secure. I'd use one of the other options.
Hacked BattleBot R/C Toy

Q: Mark what do you think about the new Robot Arena game? I've played it for a while and it seems to be worse than RA2. Didn't expect much due to the potential player base of the game being quite small, but this is not what I expected for a game in 2016. Still seems to be the only dedicated robot combat game right now. [Guangdong, China]

A: [Mark J.] The 'early access' version of 'Robot Arena 3: Create.Destruction' rushed out two weeks ago (May 26, 2016) is clearly a beta release: incomplete and bug-riddled. There have been two updates already. I suspect the developers were under pressure to get something out prior to the start of BattleBots season two on ABC.

Releasing the clearly unfinished game has led to really awful reviews and public doubts that the game will ever be playable. I can't recommend purchasing at this time, but I'll reserve judgement and wait for a 'final' version of the game before I comment further.

Q: Hello Mark, this time I was wondering, just what is the best way to test a transmitter? The best solution I could come up with is sticking the robot in a fridge or cabinet and have it drive into the door so you could hear whether or not the controller penetrates the surface. Is there anything more reliable than that or no? [Kentucky]

A: [Mark J.] As much as I enjoy the thought of a combat robot going berzerk inside a refrigerator, you need to test your radio in a place where you can see what it's doing. An impact 'clunk' won't tell you if it was running straight and steady or lurched sideways into the door. Try puting your 'bot outside and driving it around while you watch thru a window.

Some radio transmitters have a special function to test reception. Spektrum transmitters call this 'Range Check' but other manufacturers may have a different name for it. This feature reduces transmitter power output to simulate difficult reception conditions. Check your radio manual.

Our Radio Reception Problems in Combat Robots page gives guidance on ensuring good reception and curing problems that do pop up.

If you decide to test the 'bot in your refrigerator, latch the light 'on' and put a video cam in with it. If it goes crazy you'll be the King of YouTube!

Cartoon fight cloud

6-pound dumbell Q: I've got a hockey robot (basically, a 12 pound combat robot without an active weapon) that I threw together in a weekend for an upcoming event later this month, but it turned out to be over six pounds underweight. Unfortunately, I can't really do much to the design of the robot without starting from scratch, so do you have any possible suggestions? I know this is a vague question but I'm out of ideas. [North Carolina]

A: [Mark J.] Ballast, or just run it light.

Q: Respected Sir,
I have a pair of Victor Talon SRX's to control the drive motors of my robot.
I would like some guidance on how to connect the receiver to the driver such that one channel is for steering and the other for throttle. [Kerala, India]

A: [Mark J.] Controlling your robot using one channel for steering and one for throttle is done by 'channel mixing'. This is not a special connction of the motor controllers to the receiver, but a programming function in the transmitter usually called 'Elevon Mixing'.

Turning on elevon mixing is specific to your transmitter make and model. Consult your radio manual for instructions on enabling mixing, then refer to How to setup Elevon Mixing for a Combat Robot to complete the set-up. Read the rest of the guide to radio functions while you're there -- you might find other useful advice.

Policy Note: 'Ask Aaron' is closed to questions from India and other global locations where robot combat arena safety is dangerously inadequate. I've made an exception in this specific case for my own reasons, but the policy remains in effect.

Q: does aaron like justin bieber? [Malta]

A: [Mark J.] Aaron's musical tastes were from an earlier time. He enjoyed Ray Charles, The Blues Brothers, Jimmy Hendrix, and Roy Orbison.

Remembering Aaron... 

Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.
Killer Robot drawing by Garrett Shikuma

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

It's a mystery!
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