5102 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


Timing belt drive from hub to second wheel Q: hi mark ... how about a belt drive for the wheels of a robot. ppl generally use chains ... belts will also give good grip on the pulley .... I was planning to use it on my 60 kg robot ... [Delhi, India]

A: [Mark J.] A timing belt drive is fairly common on smaller robots, but is typically used to power a second wheel from a gearmotor hub. It is difficult to get adequate speed reduction from an ungeared motor to a drive wheel with a single-belt system, and multi-stage belt reduction systems have too many failure points to be reliable in combat.

Here are a couple of links to belt drive design and selection information for V-belts and timing belts.

Q: Hi, Mark. What's your opinion on the controversy on the battle between Lockjaw and Overhaul on Battlebots Episode 2? What probably caused that happen? Donald is very experienced and seemed won't make this kind of mistake. [Chinese Forum]

A: [Mark J.] Once the TV producers get hold of the tape and finish their edits there's little chance of getting to the truth by watching the show. But, I've talked with people who were there...

  • 'Lockjaw' put in a VERY late hit after the match was over -- later than was shown in the edited broadcast.
  • It wasn't a damaging impact, more like insurance to keep 'Overhaul' from righting.
  • Donald Hutson is a very 'focused' driver. He may have simply missed the (very loud) end-of-match klaxon. He says it was unintentional.
  • Regardless, Lockjaw's late hit was unsportsmanlike.
  • The reaction from the 'Overhaul' team was also unsportsmanlike and out of proportion.
  • Both teams' actions made the sport look bad, and the producers made it look worse.

While we're on the topic, I really didn't like the way the show producers cast 'Complete Control' builder Derek Young as some anti-social rule-breaking misfit villain. Derek is an amazingly inventive builder and an asset to the sport. I've always disliked the way Comedy Central portrayed the robot builders in the original series, and I like even less the way ABC is handling the new series. Ratings are dropping; I don't think the viewers are enjoying this schlock.

Q: Using 4 motors in an insect robot using 11.1v lipo on 2.875 banebots, would the servocity 624 RPM precision planetary gearmotor or the Kitbots 1000 RPM gearmotor be more reliable and/or give out better pushing performance? By my calculations the robot would be faster on the kitbots but stronger with the motors from servocity. With their performance so close (unless I'm wrong in that aspect) which ones would be more reliable; the battlehardened kitbots or the stock servocitys? [North Carolina]

A: [Mark J.] How are you 'calculating' the performance of the Kitbots motors? The only performance spec they provide is RPM @ 12 volts. No torque spec, no current consumption, no terminal resistance. That's not enough for me to model their acceleration, pushing power, or general performance in a specific robot. Note also that the Kitbot motors don't come pre-hardened -- the 'battle hardening' they describe is a do-it-yourself process that can be applied to any small gearmotor.

The ServoCity motors come with full specs. They're also a better quality motor/gearbox. Assuming a 3-pound robot (4 of either motor would be too heavy for an antweight) with 2.875" wheels at 11.1 volts, a 'bot powered by the ServoCity motors would reach a top speed of just under 5 MPH in about 6 feet. Peak amp draw would be about 2 amps per motor, and the motors would generate more than twice the torque needed to maximize the pushing potential of the robot. That's very good performance.

The Kitbot motors are an unknown quantity and the ServoCity motors are well documented. Given the choice, I'd use the Servo City motors.

Q: Has anyone ever used a vacuum to keep their bot stuck to the floor? Successfully? [New Jersey]

A: [Mark J.] Vacuum 'bots are very common in the heavier sumo robot classes, but quite uncommon in the combat classes. It has been tried on multiple occaisions, but usually with limited success. I recall a couple specific 'bots:

  • Jason Giddings' heavyweight 'Armorgeddon' fought in the second season of 'Robotica' and used a vacuum downforce system for the sumo-like 'Fight to the Finish' where it defeated 'Metalmorphis'.

  • European 150 gram antweight 'Vacuum' has had good success in the Antweight World Series (video).

Q: Hi, scoop guy again. I know you suggest using 4wd with less powerful motors but my tech teacher suggests the opposite. In our competition we can build robots up to 4 lbs and he said that there is no comparison between 2 pdx16s and 4 of the little motors from servocity. If you could explain which is better and why (maybe toss in some math), it would be greatly appreciated. It would change how we build our robots entirely.

Also is riobots right that 1/8 aluminum is a better armor than 3/8 lexan? If so, would you mount it all together by welding? Thanks as always. [Canonsburg, Pennsylvania]

A: [Mark J.] Four pounds, you say? Up until now you've been telling me 'beetleweight', which is three pounds. It would have been useful for me to know that you had an extra pound of weight allowance; it changes the calculations. Please do try to provide accurate info -- my advice is free, but that doesn't mean that you should treat my time and effort cheaply.

Your tech teacher is correct in that there is no comparison between the motors. A pair of PDX16s output 800 watts of power. In a 4 pound robot that's 200 watts per pound. That's race car power ratio that you're trying to control in a little bitty plastic box. I already 'tossed in some math' the first time you mentioned the PDX16s -- let me refresh your memory:

"Your traction is restricted by the weight on the robot's drive wheels, and the PDX16 motors would overcome that traction at well less than 10% of their output power."

Let me break it down a little more:

  • Two wheel drive; assumed weight on drive wheels: (0.7 * 4) = 2.8 pounds
  • Estimated coefficient of friction between tires and dusty arena surface: 0.7
  • Torque required to break traction, assuming 3" diameter (1.5" radius) wheels: (2.8 * 0.7 * 1.5) = 2.94 in-lb
  • Torque available from dual PDX16 motors: 182 in-lb
  • Percentage of PDX16 torque useable without breaking traction: (2.96 / 182) = 1.6%
With these motors in a four pound 'bot you'll hit the 'go' button, the wheels will break traction, one tire will get some grip, and the 'bot will dart off in an unpredictable direction into a wall. Uncontrollable. You want someplace around 35% to 50% useable torque for a robot in a small arena. The smaller Servo City motors will give you that amount of torque and will result in a powerful, controllable robot -- with a considerable weight savings.

About armor materials:

  • There are MANY different alluminum alloys. Some make quite good armor, some are junk. There is discussion about various aluminum alloys in the Materials & Components archive.

  • Lexan (polycarbonate) can be a very effective armor material if properly mounted and allowed enough room to flex on impact. This is also extensively discussed in the Materials & Components archive.

  • The suitability of one armor material over another depends as much on the armor design as on the material itself. It's an oversimplification to state that one material is 'better' armor in all cases than another material. It's like saying that pizza is always better than a cheezeburger.

  • Robot armor is generally mounted with bolts or machine screws. Much easier to repair/replace in a hurry.

Q: What's the advantage of Shuffle Moving Mechanisms? They are heavier and more complex than wheels, and 'Battlebots' don't give this kind of mechanism any weight bonus, but why still has robot using them ('Overhaul' in 'Battlebots 2015')? [Guangdong, China]

A: [Mark J.] I suppose I can give one more design secret away, but at least a couple teams are gonna be mad at me...

Team JACD has some prior experience with shuffledrive, and they learned a great deal about how it should be done. If you get the design just right a shuffler can deliver better grip than either wheels or treads on a smooth arena surface. The individual shuffle 'blades' actually strike downward as they come into contact with the floor, momentarily giving a greater apparent weight and traction boost. You can see the team go thru their design and show their shuffle mechanism in this video.

Personally, I think this falls into the 'too complex for the benefit' bin.

Q: Hi - So I'm building a bot, and for the drive I am using the Sabertooth 2x60 motor controller, I have 2 motors @24v each... now I bought 3 Lithium Iron batteries @ 12v 21ah, I was going to run these in series for 36v, but then realized the Sabertooth only takes in 30v max... should I only run 2 batteries in series for 24v for the drive? That would leave me with one 12v battery left, and I have a 24v motor for the weapon, what would be the best thing to do in this situation? Should I run 2 batteries in series for the drive, and connect the remaining battery in series to the weapon to provide 36 volts to the weapon and reduce voltage drop when I use drive and weapon at the same time? Thank you very much for your help! [El Dorado Hills, California]

A: [Mark J.] What on earth are you building that requires 21 amp-hours of battery capacity? That's gotta be way too much for a 3 minute match with any robot that might use a Sabertooth 2x60 ESC. Recalculate?

The Sabertooth 2x60 ESC has an absolute max 33.6 voltage limit. You really don't have a choice but to run just two of your LiFePO4 (12.8 volt) batteries in series for your drive motors. You haven't mentioned any robot details, so I can't comment on performance at 24 volts.

Although it's possible to add the third battery 'upstream' from the drive motors to give 36 volts to the weapon motor, it wouldn't help the voltage drop. Bumping the voltage to 36 volts would increase the current draw of the weapon motor by 50% without adding any additional current capacity to the circuit. The voltage drop would actually be larger. Consider just running two batteries.

Q: I was looking through your archives and stumbled across a post where someone had asked several satirical humorous questions. Questions like:

"How do I stop my robot's paint job from getting dirty or scratched?"

...and that sort of stuff. In his reply, Aaron said they would've been funnier had he not received at least one serious version of every question except the pleo one. This, I absolutely have to see. Could you give an example of the "serious versions" of those questions Aaron got asked? It seems too funny to be true.

P.S. Whatever its flaws, I'm super happy that Battlebots is back on air and did well in the ratings on its first show. [Oakland, California]

A: [Mark J.] Almost any question can be re-phrased to sound ridiculous. For example, your request can be reworded as:

"I'm bored and lazy. Why don't you go thru your 5100 robot questions and pick out the stupid ones so I can laugh at them?"

That would qualify it as a prime example of the type of question you're asking about. It's much funnier when it's not your own question. I don't know which specific posts Aaron was referencing, and if I did I'd prefer to not further embarass the original question submitters.

If you'd like to search for yourself, I suspect that many of the questions that inspired the satire might be found in the Team Run Amok and Friends archive. You'll also want to read Aaron's Greatest Hits.

P.S. -- Aaron was a much nicer human being than I am. I'm trying to follow his kind example, but I do better some days than others. I'm sorry if you find my response 'cheeky'.

Q: Hi me with my beetle fbs and scoop. My group came up with a few good questions the other day and I we cannot find an answer to them.

1. Is a more horizontal scoop (like you pictured when we first discussed this idea) better than a vertical scoop like punjar or ice cube? This is against a field of drumbots and 8in vertical flywheels with about 18 teeth.

2. For a breaker box is a overpowered 2wd system better than a 4wd with appropriate motors for the weight class? We would be using 2 hd planetary precision 612 rpm motors or 4 of the 624 rpm precision planetary motors. If 4wd, how could we make the left side motors spin in sync?

3. For a fbs, how are they driven effectively? Specifically, how do drivers know which way is forward if they can't see the wheels? Also is there a specific tactic useful against vertical spinners and wedges?

4. What belt should be used to spin the robot? I suggest a v belt for slippage but they want to use timing belts. The spinning weight will be about 2.5 lbs and the rpm about 4000 with a 1:5 gear down from an inrunner brushless that spins at 2000 kv. We will be using an 11.1v lipo battery.

Thank you for your advice. [Canonsburg, Pennsylvania]

A: [Mark J.] All good questions.

  1. A more vertical scoop lile 'Ice Cube' uses is a reasonable general purpose weapon. If you're targeting spinner weapons a laid-back scoop like 'Breaker Box' uses is superior.

  2. 'Breaker Box' builder Jim Smentowski knows what he's doing. His four wheel drive adds stability, control, and pushing power. A two-wheel drive 'bot - no matter how 'overpowered' - will not match a four-wheel drive 'bot in these areas.

  3. Full Body Spinners don't require a complex driving tactics. Spin up, position in the center of the arena, move calmly toward your opponent. An FBS will have some visible que showing the rear of the 'bot': a small flat 'tail' attached to the chassis that sticks out under the back edge of the shell, a light that is visible thru holes or slots as the shell spins, or a 'flagpole' extension of the stationary weapon axle.

  4. A V-belt is a good choice, but you're going to have a very tough time finding either the belt of pulleys in so small a size. I don't believe they exist. For off-the-shelf components that will operate at such high RPM you're probably stuck with a timing belt. It'll be fine.
I'll note here that a 2.5-pound shell on a 3-pound robot is very heavy. A typical FBS shell might be half the weight of the robot.
Lightweight spinner robot 'Ziggo'

'Ziggo' has a flagpole

250 pound class BattleBot 'Bite Force'

2015 BattleBot competitor 'Bite Force'
Q: What do you think of Bite Force? DO you think its magnet is useful? [Guangdong, China]

A: [Mark J.] What isn't there to like? Powerful, controllable, modular weaponry, and based on a proven design. I'm not usually a fan of treads, but Paul Ventimiglia did them right. Every other tread has a magnet that pulls the gripping surface down to the steel arena floor and aids traction. From the ease with which it pushed its opponent around the BattleBox, I'd say the magnets are VERY useful!

[Mark J.] I'm very pleased to have a guest commentary on the topic of brushless outrunner shaft replacement -- see the post about five down this page. Mike Jeffries from Near Chaos Robotics has experience in replacing and modifying brushless outrunner shafts and offered to share his knowledge. The spinner mini-drum weapon he describes for 'Algos' rides on a dead shaft and incorporates the 'ring of magnets' cut from an outrunner can. The stator bearings are removed and the bore is drilled out to slide over the dead shaft. This cross-section drawing of the weapon assembly may help you visualize this. I think Mike underestimates his builder chops!
For the most basic replacement option (same diameter shaft, better material) you can often do this with just a press of some sort. Care needs to be taken to avoid warping the motor can if you do this, but it's a very easy process if you can buy shafts that are the right diameter and temper. I've had decent luck doing garage heat treating and tempering, but for some of the tougher steels you need to temper the shafts at a temperature well above what most household ovens can reach so when possible buying pre-hard shafts is the better option. The added bonus there is you don't have to worry as much about the treating process warping the shafts as they should be in decent shape when you get them.

On the more complex side of things, upping the size of the shaft takes a bit more effort. On my 1lb bot Algos I replaced the 3mm live shaft with a 1/4" dead shaft. The process for doing this was as follows:

  • Remove bearings from stator
  • Drill out stator to accept new shaft diameter (I went for a snug slide fit)
  • Remove shaft from motor can via press or similar
  • Cut away most of the end of the motor can, taking care to leave the portion supporting the magnets
  • Press remaining portion of can into the weapon assembly
  • Reassemble motor with new dead shaft using small outer diameter shims to maintain intended weapon location

Assuming the weapon itself is made to fit the weapon motor, the process at least can be done without all that much equipment. I used a lathe to shave down the motor can, though a steady hand and a cutoff wheel could do the same. The portion of the assembly done with an arbor press could also be done with a vise in a pinch.

Antweight combat robot 'Algos'
Spinner weapon from 'Algos' showing magnet ring'
Thanks, Mike. I second Mike's suggestion that you find pre-hardened shafts if at all possible. I personally wouldn't try to use a vise instead of a press to insert the new shaft, and under no circumstance should you try to 'tap it in' with a hammer! I know how you guys think...

If you aren't familiar with the Near Chaos Robotics website, do yourself a favor and take a look.

Q: For a design like Ice Cube (scoop robot) is it better to have the rear wheels extend over the rear of the robot or not? I ask this only because it seams that it makes his fully frontal attacks ineffective because he gets flipped over due to the large wheels hanging over the rear (Video). With out the wheels extending past the rear of his robot though he could get stuck on the back unable to move. Thank you for your time. [North Carolina]

A: [Mark J.] 'Ice Cube' is designed to allow contact between drive wheels and arena floor from nearly any orientation. To take it off its wheels you'd have to lift the tail and balance it up on the scoop. Designing to be 'always mobile' is certainly a valid tactic. As long as you have a drive wheel in contact you can influence the match. If you don't you're just a passenger.

Those instances in the video where driver 'Fuzzy' Mauldin flips the 'bot over with a 'wheelie' are entirely deliberate. Look closely and the only times he does it are when the 'bot is upside-down. Flipping up back upright with a quick power blip is a very quick method of restoring the correct forward/backward control orientation that is reversed when the 'bot is inverted. It's a strength to be able to do that -- not a weakness.

Q: Would adding a tread pattern as described in the riobots handbook be useful in an insect weight class? Same with the wd 40? Finally would this be effective on banebots wheels too? Thanks.

A: I have great respect for Team RioBotz and the effort they put into their combat robot tutorial. That said, I don't agree with all of their advice.

  • I've never found that cutting grooves into tires improved their traction on the smooth and uniform surface of a typical combat arena. Maybe they have some particularly funky arenas in Brazil?

  • Cleaning your tires is a very good idea. I don't use WD-40, I use lighter fluid and a clean rag. It cleans oily deposits well and evaporates quickly with no residue. Be careful with it and with the wiping rag -- highly flammable!

My advice: whatever compound the tires are, clean 'em but don't cut 'em.

Q: Sorry to bug you so much on a simple topic but I just thought how would you mount colsons to a 1/2 in keyed shaft and to 6 mm D shafts. Thanks again.

A: That depends on the size of the Colson. Small diameter Colsons have a 1/2" bore and can be mounted to a 1/2" keyed shaft by cutting a keyway into the wheel. A 6mm shaft can mate via an off-the-shelf hub. Larger Colsons generally require custom hubs.

Q: One final question for wheels and traction. For the insect weight classes are colson wheels really better than banebots and are duallys effective? Thanks.

A: That's two questions. What wheel is 'best' depends largely on your design. BaneBots provide good traction and have simple mounting options, but won't handle much abuse. Colsons are more durable, but heavier and more difficult to mount. Lite Flites are easy to mount and absorb damage well, but lack a bit in traction. Consider the exposure your wheels have in your design and pick accordingly.

Extra wide or dual wheels: some builders love 'em and claim increased traction. I think not. Most 'bots I see that have traction problems are two-wheel drive layouts that didn't put enough weight on the drive wheels. Pay attention to the center of gravity and you'll be fine with single width wheels. There are many posts on this general topic in the Materials & Components archive" -- search there for 'traction'.

Q: How fast should a Motor for an antweight spinner like DDT be? [Baden-wurttemberg, Germany]

A: [Mark J.] Not a simple question. G3 Robotics' antweight 'DDT' runs a Hacker A20-20 L brushless outrunner motor at 11.1 volts, giving just a bit over 11,000 RPM -- unloaded. There is some speed reduction built into the belt drive to the spinner blade; looks like about a 3:2 reduction ratio. Finally, builder/driver Jamison Go reports that he commonly operates the weapon at about half throttle, so the actual speed of the big horizontal spinner blade in combat is around 4000 RPM.

There a many previous questions and answers about determining correct spinner weapon speed and motor selection in the Ask Aaron Weapons Archive. You may also wish to read the Spinner Weapon FAQ.

Antweight combat robot 'DDT'

Q: [Chinese Forum] Hi Mark, as the season premier of new Battlebots ended, how would you rate it? And about the competitior's shown on the first episode I have some questions:

1. What do you think of Lisa Winter's Plan X's design? Does that "defense system" a good idea for armours?

2. Can Wrecks' be qualified as a walker by today's standard? And does it got some weight allowances because of that?

3. Seemed like Warhead is still way too hard to control. In my opinion that's how it lost, do you agree?

4. Why didn't Nightmare put that wheelguard on when fighting with Warrior Clan? That seemed can protect its wheel from being getting underneath by the flipper of Warrior.

A: [Mark J.] How do I rate the show? Sixty-minutes of television, ten-minutes of robot fighting. More time devoted to commercials than to combat. Lotsa fluff, and the technical commentary was incomprehensible.

  1. Lisa Winter and the 'Robot Action Team' have a long record of building 'fun' robots. They get the basics right and always add an element of whimsy. The lesson to be learned here: making certain that the essentials are well covered goes a long way toward building a competitive robot. The ablative armor elements qualify as 'whimsy'.

  2. 'Wrecks' is not a true walker under the current deffinition. A true walker must have mechanical isolation between the 'forward-backward' motion of the legs and the 'up-down' motion. Wrecks' leg motions are not independent -- it can't even back up.

  3. Team Run Amok's design philosophy: "A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use." The terrifying 'Warhead' is neither simple nor easy to use. It also was built 13 years ago and does not appear to have any updates.

  4. I suspect that 'Nightmare' left off the angled wheelguards because they were easy targets for the very low flipper lip on 'Warrior'. I would have done the same. Without the wheelguards it became more difficult for Warrior to find a good target -- but Warrior got lucky. Could have gone either way.

Q: How do you feel about Battlebots coming back on the t.v.? [Missouri]

A: [Mark J.] I'm a bit worried about it. The TV network has so little faith in the show that that they scheduled it in a spot on Sunday night between 'Celebrity Familly Feud' and a re-run of 'Castle'. That isn't a place you should put a show ment to appeal to a young male audience.

I'm also worried about the changes to the way the tournament is run. In the 'old days' BattleBots was open to everyone and hundreds of robots competed. Much of the audience appeal came from the idea that the viewers might build a robot and compete. The new tournament has only 24 teams and they were individually invited by the show producers -- none of them what you'd call 'novice builders'.

The way the new show is put together reminds me much more of televised robot shows that were not successful in attracting a fan base. If this show fails, it's going to be a very long time before we see another. Fingers crossed.

Q: I've seen some builders that are running direct drive off a brushless out runner replace the shaft of the old motor to something thats longer and, or in some cases, larger in diameter that the old shaft was. How hard is it to modify a brushless outrunner to accept a longer and/or larger shaft? [Aumsville, Oregon]

Typical brushless 'outrunner' motor disassembled A: [Mark J.] This video shows the process of replacing a typical outrunner shaft.

  • If you want a longer shaft, you'll need to make or find a shaft of the new length with the correct C-clip groove and flat. Difficulty level: easy if you have the proper equipment.

  • If you want a larger diameter shaft, you'll need the new shaft plus:

    • Shaft support bearings sized for the new shaft;
    • Machining the bearing seats for the larger bearings;
    • Precision boring the motor bell shaft hole for a proper press-fit; and
    • A new C-clip and washer to assemble the motor.

    Difficulty level: senior machinist with a shop full of tools.

Q: From recent photos I see Nightmare's spinning disc's shape changed a lot in order to compete in the all new Battlebots series. One thing I've been wondering is: does that change decrease the energy that blade would store greatly when spinning?

Thank you for taking your time to answer! [Chinese Forum]

A: Nightmare's old spinning disk was made from aluminum. The new 'butterfly' disc is 1/2" titanium -- denser than aluminum. By my calculations the new disc has at least as great a moment of inertia as the old disc, and it spins a bit faster. Jim Smentowski is a very experienced builder who knows exactly what he's doing.

Q: hi sir i had confusion with pulley motor pulley diameter 66 mm and my weapon pulley diameter is 44mm, .. whether its ratio is 0.68:1 or 1:1.45 which ratio is correct and how i have to calculate? [India]

A: [Mark J.] The reduction ratio is the relation between the diameter of the weapon pulley and the diameter of the motor pulley:

Ratio = driven pulley : driving pulley
44:66 = 2:3 = 0.67:1

I think you have the pulleys reversed for your purpose. If you're trying to increase the motor torque and get the weapon to spin more slowly than the motor, you want the larger pulley on the weapon and the smaller pulley on the motor. That would give you a 66:44 = 3:2 = 1.5:1 reduction ratio.

Q: Hi me with my beetle fbs and scoop again. I came across a few questions while redesigning the bots.

FBS: What is the best material for teeth? I cannot afford steel for weight and titanium is too expensive. My school has 6061 aluminum that I currently plan on using that is 1/4 in thick. For this application what would be the best material and what thicknesses? I have about a pound to work with for 2 teeth. The teeth also now over hang the top and connect on the bottom but not on the inside. They are currently 4 inches tall and 3 inches long to connect to the bot (leaving 2 inches of air between teeth and bot).

Scoop: Again, best material but for the scoop? For this I thought about 12 inch pvc pipe in 1 foot sections. The material is cheep and if one scoop is destroyed in a match then I have 3 replacements waiting in line. You suggest titanium but again really expensive and I am not sure which grade of aluminum or titanium and how thick.

Also design note. If i made a scoop and had the base plate extend out to the edge and got flipped over, could I just use really larger (like 6 inch or so) wheels so on one side i have a scoop and on the other I have a wedge? If not, then how do I fight with a c going against vertical spinners (I don't want to give them free aggression points).

Thank You so much. Your input has been quite helpful. [Pittsburgh, Pennsylvania] Bolt heads as impact teeth

A: [Mark J.] Glad to hear you find my answers helpful. I understand that this is a school project and that your resources are limited. Let's see what we can do.

FBS: One pound of your three-pound robot is devoted to the impact teeth? That's a lot of weight, and they overhang the shell WAY too far. You haven't told me about your latest design revisions so I don't know how fast you decided to spin the shell, but the impact the teeth must survive will be at least as great as you could deliver swinging a claw hammer as hard as you can. Large, overhanging aluminum teeth mounted the way you describe will very likely shear right off with the first good hit.

Take a look at successful FBS designs and how their teeth are designed. If you're spinning the shell at 5000 RPM, the teeth only need to stick out about half an inch. For strength they should bolt directly thru the shell wall.

General recommendation: aluminum makes a lousy impact tooth, but a vertical strip of that 1/4" 6061 aluminum bolted thru the shell with 3/8" steel bolts would be an acceptable design. The exposed bolt heads would be the impactors with the aluminum serving to space them far enough out from the shell to be effective. About 5 bolts per tooth for your 4" tall shell, with nuts holding them on inside the shell. Robot Dojo's 'CoMotion' antweight hack

Scoop: Not PVC! I'm sure those nasty vertical spinners you'll be fighing have hard and sharp steel teeth, and a plastic scoop is no match for that. The steel will dig in -- even with little bite -- and rip the plastic apart. You'll need to match the hardness of their teeth with your scoop. Steel is a good choice here. Make it as thick as your weight allowance permits. Spend some time looking around the tool section at a hardware store -- you may find something like the big curved chunk of steel that pictured beetleweight 'CoMotion' uses for its scoop.

Inversion: Yes, it's a good idea to be able to operate inverted -- but I don't understand how extending your baseplate gives you a wedge when your 'bot is upside-down. If you're mobile, charge back into the spinner and let then flip you back upright. You get aggression points for charging toward your opponent whether you do any damage or not.

Q: Hello again. On the teeth for the fbs I made them smaller in length (less overhang and material on body) and in with (1.5 in wide to 1 in) and now they weigh .383 lbs each compared to the .575 lbs they were. Long story short, the bot is just .25 lbs over weight.

Two quick questions for you now:

1. Is 1/4 in steel ok for a tooth design like that overhangs the body like we first talked about?

2. Is pvc an ok material for the body? I know you will recommend UHMW or a steel bowl but I don't know where to get a large tube of UHMW.

One question for the scoop. Do you know what is better or have a preference for the PK27 motor or the PD27M motor? Still looking for a powerful motor but adding the scoop will increase my weight by at least .75 lbs so I need lighter motors than the PDX16s. Thanks as always.

A: [Mark J.] I'm missing a great deal of information about your design for the spinner, which makes it very difficult to give advice on the selection of materials and the dimensions of impactors. I don't know how fast you plan to spin the shell or exactly what mounting method you have for the teeth. I'll do the best I can.

  1. I REALLY DO NOT LIKE your impact tooth design. A hit will place a very large rotational shear loading on the mounting points -- very poor engineering practice. I don't think you understand the magnitude of the impacts these teeth will experience. If you can't pick up the shell and hammer a 3" nail into a board with a tooth, it isn't strong enough.

    I'm also concerned that you think your 'bot is four ounces overweight and you're considering changing the tooth material from aluminum to heavier steel. A change to a more conventional tooth design could shave off that extra weight, and instead you're changing to a heavier material. There are very good reasons why no one uses a tooth design like you plan. Please reconsider your design.

  2. PVC is not commonly used in combat robots. There are many formulations for PVC that vary a great deal in strength and toughness, and it's difficult to tell which type of PVC you're working with. I do understand that you have limited resources, and a large diameter PVC pipe is inexpensive and easy to obtain - but a 4" section of 8" diameter schedule 40 PVC pipe weighs 1.8 pounds. Add that to the weight of your impact teeth and you've only got 7 ounces left for the rest of yout 'bot. I think your weight calculations are way off.

There is a shortage of motors suitable for beetleweight robots at the moment. Although still listed at Robot Marketplace, both the PK27 and PD27M gearmotors are out of stock and do not have an expected restocking date. Unless you have another source I wouldn't count on being able to obtain these motors. Either motor is overkill for a beetle, but both spin rather slowly and would need to be matched to uncomfortably large diameter wheels to provide decent speed.

  • The PK27 gearmotors weigh 9 ounces each. Two of them in a beetle with 6" wheels will deliver a top speed of about 4 MPH and accelerate to that speed in about 3 feet. That's pretty good performance in a small arena.

  • The PD27M gearmotors are more powerful. They weigh a little over 10.5 ounces each. A pair of these motors in a beetle with 6" wheels will accelerate to a top speed of 5 MPH in just a bit less than 3 feet.

You might be interested in these Servo City Precision Gearmotors that are in stock and available to ship:

  • The 520 RPM Planetary Gear Motor weighs just 2.9 ounces and spins fast enough to enable use of a smaller wheel better suited to a beetle. Two of these motors in a three-pound robot with 2.5" wheels will give a 3.8 MPH top speed, reaching that speed in 4.25 feet while maintaining excellent pushing power. They are currently on sale at 25% off the regular price.

I really do need much more complete design details for your 'bots to be able to give more exact recommendations.

Q: hello sir..sorry to say but i read the whole solenoid section..i think only 1 DPDT solenoid can be used to control both forward and reverse direction of motor..
plzz sir suggest the connection diagram for DPDT solenoid control for both drive motors and for weapon.. thanxx alot.. [Chhattisgarh, India]

A: [Mark J.] Sorry to say, but your thinking is incomplete.

A single DPDT solenoid can provide forward/reverse directional control of a PMDC motor, but there is no 'center off' position with a DPDT solenoid. Your robot would be continuously 'on' in one direction or the other. To get the required forward/off/reverse control you need the circuits already diagramed in our Solenoid and Relay Guide.

DPDT H-Bridge motor control circuit
Forward/Reverse - but no 'Off'

Q: Is the drum weapon shouldn't be oversize? I think the larger drum will store more energy. In fact, the ICE powered big drum weapon bot 'REDRUM' always get disadvantage when fighting with horizontal spinners.(It only has few videos,mostly versus spinner) The drum didn't give out much damage and its gyroscopic forces makes 'REDRUM' difficult to drive.However, everyone can see 'Touro Maximus' with a small snail drum won the STEM and RG2015 champion.Is that because size of drum shouldn't be as large as possible or it is because of the successful snail drum design or it just because of 'REDRUM's problem itself? [Guangdong, China]

A: [Mark J.] A great many new builders share a mistaken belief that the success of a combat robot depends primarilly on the design of the weapon. This is entirely false.

The weapon may be the least important system on a combat robot.
If you're not winning matches it isn't because you have a poor weapon.

Any weapon is only as good as the robot that uses it, and placing too much design emphasis on the weapon will detract from the overall performance of the 'bot. See this post in the Design archive for a discussion, and this post on the design of the 'Touro Maximus' snail drum.

About drum weapon design:

  • A larger diameter drum weapon will store more energy than a smaller diameter drum of the same mass at the same speed -- see the Spinner Weapon FAQ for examples.

  • A larger diameter drum will also exert greater gyroscopic force on the robot that may cause control problems. Total Insanity Combat Robotics has a tutorial on weapon gyroscopic forces that fully explains the issue.

  • If your attack strategy requires high-speed rushing attacks and quick pivoting turns, a small diameter drum spinning at high speed as used by 'Touro Maximus' is the correct choice.

  • If your attack strategy allows a slow and cautious approach toward your opponent without high-rate turns (common in Indian robot combat) then a slower spinning large diameter drum weapon is ideal.

Different combat environments require different designs. Do not blindly emulate a weapon designed for a combat environment different from the one in which you will compete!

Q: I am attempting to design and build a beetleweight FBS spin bot.
  • Base: 3/8 in Lexan, about 8 inches in diameter.
  • Walls: Stainless Steel Bowl or 3/8 in Lexan about 9 inches in diameter.
  • Top: Bolted to walls. Walls and top larger than base. Same Diameter as walls.
  • Teeth: 2 teeth, 1/4" steel and mount under the wall and be bolted on through the wall.
  • Motors: 4 motors and 1.5 in colson wheels or tracks.
The biggest problem I face is I have no idea what motor to use. I do not know how to properly select what motor to use and how to gear it down using belts when the robot must be perfectly balanced, what number to teeth pulleys to use, the final rpm of the bot, etc. Could I use a brushless motor used in 15 robots' weapon? I know you do know approve of this design for the weight class but the engineering aspect intrigues me. Thank You. [Pittsburgh, Pennsylvania]

A: [Mark J.] Take a look at the mini-build logs for these Full Body Spinner (FBS) Beetles:

  • 'Aggro' is an unusual design that uses a friction drive to transmit motor power to the spinner shell without belts. I'm not a fan of friction drive spinners -- preventing excessive slippage is harder than it appears.

  • 'Donalello' is a more conventional belt-drive FBS. It looks like all the spinner components were custom made. It's hard to build a small FBS with off-the-shelf components. Certainly not a task for an inexperienced builder.

I've also seen direct-drive spinners in the insect weight classes, with the shell directly mounted to an outrunner brushless motor. Motor shafts are not designed to take that great an off-axis load, and I really can't recommend such a design.

Read the Ask Aaron Spinner Weapon FAQ for information on spinner weapon motor selection. Calculating the power requirements is not a simple task, but the tools to help you are available here at Ask Aaron.

General comments:

  • Lexan is a very poor choice for a spinner shell. Lexan (polycarbonate) gains its strength thru its ability to flex under load and absorb impact. You do NOT want your spinner shell to absorb impact -- you want it to be very rigid to better transmit the impact to your opponent.

  • 'Tracks' add weight and complexity but offer no advantage in a robot arena. They also have a nasty habit of failing in combat. Avoid.

Q: Hi Aaron. I am building a powerful beetleweight pusher box. I am using 1/2 inch HDPE and 2 pdx16 gear motors. I have two questions for this design:

1. What battery should I buy for competition that will power these motors and may not need charging after every match.

2. There are controllers that use a trigger and wheel on the right to maneuver the robot. Is there a controller of the same design with the wheel on the left for an affordable price (not the 250+ reversible controllers) . Thank You. [Pittsburgh, Pennsylvania]

A: You're a very busy builder.

  1. You've got bigger problems than battery selection. The PDX16 gearmotors weigh a pound each: two pounds of motors in a 3 pound robot? Not gonna work. That much power would be wasted in a beetle anyway. Your traction is restricted by the weight on the robot's drive wheels, and the PDX16 motors would overcome that traction at well less than 10% of their output power. Too heavy and a waste of power -- pick more reasonable motors and try again. The Tentacle Drivetrain Calculator will help you in motor selection, and will estimate the battery power requirement of your robot. There are multiple examples using the Tentacle calculator in the Ask Aaron Archives. Left and right handed operation of pistol grip transmitter

  2. Inexpensive pistol grip transmitters all come in 'right-side wheel', but if you want to use your left hand to steer, you can just hold it horizontally in your right hand and reach over with your left to steer. See photo.

    Pistol grip transmtters are used by VERY few robot drivers. Few of them have the capabilities needed for robots (channel mixing, failsafes...) and those that do are MUCH more expensive than an equivalent twin-stick transmitter. I suggest you read our guide on Combat Robot R/C Features before making a radio decision.

Q: Hi, me with my beetle spinner again. Bottom line should I direct drive the motor or try to use belts or chains to lower the rpm? I can hardly see how effective a 25,000 rpm weapon is given it cannot make solid strong contact with the opponent. I am only doing a fbs because a horizontal spinner over arching the body is in excess of 5.25 lbs. If you have a better solution please tell me.

The most challenging competition are 8 inch flywheels with several teeth spinning vertically. The school that host this event in our area produces a fleet of such bots and are unstoppable. Strong push bots have done well against them historically so I though one on steroids might do the trick. (BTW should I toss a wedge, scoop, or leave the front flat against this kind of opponent?) I also thought that that a powerful horizontal spinner or fbs (since none of either style have been effectively produced) might be whats needed to knock out their weapon.

Finally, would a lifter (like biohazard or t-minus) (pneumatic or electric) be effective in this weight class against almost entirely vertical spinners. Thank You so much for the aid in redesign and construction. :-)

A: I believe I already mentioned that I cannot recommend a direct drive FBS. You could use an oversize low RPM motor to get enough torque for a quick spin-up, and compensate for high RPM with a single-tooth shell design -- but the loading on the motor shaft is still a vulnerability. The point I'm making is that a FBS design is NOT easy to implement, and NOT something I can recommend for a novice builder.

Second problem: your FBS design is poorly suited to match-up against a vertical spinner. Head-to-head, the large diameter vertical flywheel will make contact with your shell well before your undercutter blades can find a target. Outlook very poor.

Lifters have to get past a spinning weapon to access something to lift, and effective pneumatic components small enough for a beetle are non-existant. My advice is to keep your design simple. Scoop and dustpan designs have proven effective against vertical spinners.

  • Avoid exposed sharp edges for their spinners to bite;

  • Keep the radius of the scoop at least as large as the radius of the largest spinner;

  • Worry less about weaponry and concentrate on strength and reliability;

  • Get the 'bot built early enough that you get plenty of driving practice;

  • Learn about the advanced features of your transmitter and use them to make your 'bot more controllable.
Flywheel vs. Scoop
Impactors get no 'bite' on a scoop
Dustpan design combat robot

Q: When did UHMW start to become common in combat robots? [Havertown, Pennsylvania]

A: [Mark J.] I recall Ultra High Molecular Weight (UHMW) Polyethylene first appearing in machined motor mounts around 2001 and quickly finding acceptance in sub-light robots as structural and armor pieces. By the time Team Run Amok's beetleweight 'Zpatula' used UHMW sheet for drop skirts at the 2004 RFL Nationals, the material was common enough that it did not garner any particular attention.


A: [Mark J.] Designing to maximize energy storage is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. Please read the FAQs before asking a question.


A: The relationship between weapon speed and bite is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. PLEASE read the FAQs before asking a question.


A: Nobody has asked for an explanation, but yes I can provide.

  • 63% of Max: generating that pretty chart in the Run Amok Excel Spinner Weapon Speadsheet that shows weapon speed over time requires a fixed point of known speed-time. The formula to calculate that 'time constant' gives an answer for 63% of max RPM. The 63% is a logarithmic thing that would take too long to explain here; ask your physics professor. Since I have to generate that time constant anyway, I include it in an output field as a clue to math boffins that I know what I'm doing.

    Once I have the time constant, a simple formula converts it to the spin-up time for other percentages of maximum speed:

    Time to Speed = Time Constant * loge(1 - (speed / max speed))

  • 95% of Max: given mechanical and aerodynamic loss, the weapon will never reach 100% of the unloaded motor speed potential, so I cut off the calculation at an arbitrary 95%. In the real world, the weapon is unlikely to reach even that 95% figure, but it serves to point out that 100% will not be obtained.

Q: hi i am making a bot of 10 kg of drum type which type of tyre should i use [Pune, Maharashtra, India]

A: [Mark J.] Soft rubber, not pneumatic. Durable wheel. I recommend the round ones. Something like these.

Q: which is the most successful design in combat robot history till now ? wedge,scoop,drum bot, vertical spinner or any other [India]

A: [Mark J.] The answer depends on how you define 'successful'.

  • If you're looking for the weapon class that has the greatest chance of producing a winning record, then passive weapons (primarily wedges) are generally best. Take a look at the analysis I did for 'bots in North America: What Weapons Win.

  • If you're looking for the type of weaponry that wins the most tournaments, the question becomes more complicated. That answer varies with weight class and with the design of the arena.

    • In arenas where it is possible to eject an opponent for an instant win - common in Europe - flipper weapons dominate heavier classes and vertical spinners rule the lightest classes.

    • In 'closed' arenas the heavy class winners are commonly wedges and lifters, with vertical drum and disk spinning weapons becoming more dominant as the weight classes grow lighter.

  • If you're looking for the weapon class used by the robots with the highest 'History Score' over their careers, there is no clear trend. The top ten history scores for heavyweights include three lifters, two full-body spinners, a wedge, a horizontal spinner, a drum, a flipper, and a pneumatic overhead spike.

Note: as we've said several times here at 'Ask Aaron', "The weapon may be the least important system on a combat robot." See this post in the Ask Aaron Archives for a full explanation. Success in combat robotics depends much more on the experience, skill, imagination, and resources available to the team than on the type of weapon.

Q: Hi sir, i have bought two ampflows E30-150G and three pair of 5"wheels in 120lbs category with 600*600mm overall dimensions. what gear reduction is needed for Indian arenas and plz specify me a proper breadth and height as well as best angle for the wedge. are there any difficulties in using six wheel drive using sprocket chain mechanism compared to four wheel drive. thanks for the help!! [India]

A: [Mark J.] First, what do you expect to gain from 6-wheel drive? You'll not gain appreciable pushing power over four-wheel drive and it adds complexity in the drive train. Exposed chains are a weak point in a combat robot drive train -- you want as few of them as possible. I'd suggest sticking to four-wheel drive unless you have a very good reason to go to six. There is an advantage to six-wheel drive but it isn't traction.

A typical Indian combat arena seems to be about 6 meters square. Indian builders have a preference for slow robots -- perhaps from the old cable-control days?

  • If you're building a slowbot, a chain drive reduction of 2:1 on top of the Ampflow gearbox reduction of 8.3:1 will give a top speed around 5 MPH, accelerating to that speed in about 4 feet. Slow, easy to control, boring.

  • If you want a more effective, faster charging attack wedge I'd suggest a 1.5:1 reduction. Top speed now about 7 MPH, reaching that speed half-way across the arena. Not screaming speed, but about all you're going to get from a 120 pound bot powered by two E30-150 AmpFlows.

A wedge is typically full-width, so in your case 60cm. Height depends on how tall your robot is. It should go at least to the top of the 'bot, and a little higher if the 'bot is very low profile -- you don't want your opponent to be able to just drive 'up and over' your wedge. straight wedge and curved scoop

Angle for a simple wedge should be no steeper than 45 degrees. I'd suggest laying it down a bit more if possible to perhaps 30 degrees. 'Best' angle is not a single angle at all, but a curved 'scoop' as shown in the drawing. A scoop is MUCH more effective against the drumbots that are so popular in India.

Q: About the all-new Battlebots series starts late this month [June, 2015], what do you think of this single weight class format with 250 pounds as weight limit? Would that be good for the development of robot combat in America and around the world? Looks like Gage Cauchois doesn't like it from this report.

And do you think the invitational format is good? Or was that just for TV? I do see lots of famous builders back, but I don't think that's appropriate. [Chinese Forum]

A: [Mark J.] I'll wait for the show to air before I make any comments regarding its impact on combat robotics. Without seeing the finished show I'd just be speculating -- and there's more than enough of that in the media already.

General observations:

  • Builder Gage Cauchois is unhappy that the new show will feature less robot fighting and more 'backstory' about the robots and builders. I don't know why the producers believe this is a good idea, as other robot combat shows have tried such a format without success.

  • The robot forums have been discussing the new 250 pound weight limit. Some think it was raised to allow returning 'veteran' robots a little extra weight to bring them up to modern standards without having to start over. Some think it's a waste of time to try and figure out a reason for anything BattleBots does.

  • BattleBots hasn't been a true 'open tournament' for a long time. The producers have favored successful and popular builders by offering them 'seeded' starting positions within the tournament that gave a tremendous advantage by allowing them to skip the opening rounds of the tournament and face opponents already damaged by early battles.

  • The new 'invitational' format gives the show producers even more control over the show content, but eliminates the 'everybody can play' popular appeal of the original tournament structure. It will certainly cost less to organize and film a tournament with 24 robots than one with several hundred.

BattleBots has been trying to return to television for many years, and they may have signed away their soul for this opportunity. The new format sounds much more like other robot shows that have tried and failed to find an audience. We'll see.

Q: Hey Mark, I've got a few questions that might or might not be related [Charlotte, North Carolina]:
  • I'm planning to sharpen my ant wedge, and my event organizer (rightfully) requires something to cover any sharp edges to protect blades and builders alike. Whats a good, cheap material that I can quickly take on and off my wedge blade edge that will provide good protection while not dulling my edge?

Sheath to protect sharp wedge blade A: [Mark J.] Keep it simple. The razor-sharp front wedge on our antweight 'Rat Amok' is protected by a simple strip of heavy poster board folded in half to form a shallow pouch that slips over the blade. A strip of strapping tape around the length holds the ends closed and makes for a snug fit. Make it a bright color so you'll remember to take if off before the match starts!

  • Also, I found a small, tank steered RC toy called an Airhogs Hyper Trax. With some modification, would it be suitable to open fairy weight combat? What mods would you do to mod it for combat?

The Air Hogs Hypertrax is marginally light enough (about 140 grams) to be a fairyweight, but you'd need to lighten it further to allow for a passive weapon. No active weapons with the included radio!

It has a number of problems that would be difficult to overcome:

  • The Hypertrax are geared for speed, not pushing and controllability in a small arena.

  • The deep-grooved vinyl treads are not well suited for grip on a smooth insect arena floor.

  • If you replace the treads or coat them with a higher-traction material (like silicone rubber) the motors will likely stall when pushing.

  • The treads offer a very inviting target for spinner weapons.

If you just want something to throw in your pit box to enter the fairyweight class for fun, I suppose a Hypertrax would do, but I don't see any reasonable way to overcome its problems and turn it into a serious competitor. Give it a try and prove me wrong.

Q: Theoretically, driving as fast as you can into the enemy increases the bite your spinning weapon will have. While I see drum-bots flying full speed across arenas, I am not aware of a shell spinner that drives even half so aggressively. Is there a particularly good reason for this? I ask because I want to size the thickness of my teeth, and my drive-train to be optimal for the speed that I will really operate the robot. [Palo Alto, California]

A: [Mark J.] First, I know I'm being picky but that's my prerogative:

  • Increasing bite with speed isn't theoretical, it's demonstrable fact.

  • The other 'bot isn't your enemy, it's your opponent. There's a difference.

So why don't Full Body Spinners (FBS) charge aggressively? Two things to consider:

  1. The large diameter of the spinning mass allows a FBS to store huge potential energy at a relatively slow spin speed. Slow spin speed will give good 'bite' even with a slow approach speed. This allows the designer to use a less powerful (slower) drivetrain and shift that mass into the weapon to increase the destructive potential even further.

    Small diameter drum weapons must spin at high RPM to match the energy storage of a FBS. The high weapon speed requires a high attack speed to match the 'bite' of a slower-spinning FBS.

  2. The result of an impact with a horizontal spinner is much different than the result of a vertical spinning drum impact:

    • The drum impact will impart an upward motion on the opponent, with an equal but opposite Newtonian reaction pressing the attacker downward. As this downward reaction is countered by support from the arena floor, the attacking drumbot is relatively unpreturbed.

    • An impact from a horizontal FBS will impart a lateral (sideways) motion on the opponent, with an equal reaction throwing the attacking FBS laterally in the opposite direction. A high-speed attack that lands a glancing blow on the opponent can send a FBS caroming around the arena. Each time it strikes the wall the impact from the shell can add more speed and prolong the mayhem. A slower and more planned attack run can minimize the 'pinball' rebound issue and help keep the FBS under control.

General advice: be cautious with your FBS attacks or you can do more damage to yourself than to your opponent.

Q: i have seen most of the bot's teeth are not welded in the drum.....i know they are not strong enough to bear the impact instead of welding they uses some types of screw (may be allen bolt) can you please explain which type of bolt they use and how they manage to fit in the drum....and are they strong enough to bear the impact [India]

Cross section of bolt holding impactor into groove in drum weapon A: [Mark J.] Impactors are typically set snuggly into a machined groove or pocket in the drum. The groove supports the base of the impactor and prevents the tooth from moving laterally. Bolts run radially thru the impactor and into tapped holes in the drum to hold the impactors in place. A very thin drum may use nuts or a nutstrip inside the drum to give enough thread depth for the bolts.

Do not attempt to simply bolt the teeth to the surface of the drum without providing a groove -- bolts are very strong in tension along their length but are not good at resisting 'shear forces' they would face if the tooth is not well supported.

The bolts themselves are usually coarse thread and as large a diameter as is practical for the impactor design. High strength 'grade 8' bolts are favored by some builders.

Strong enough? Depends on the materials and dimensions of your design. See Frequently Asked Questions #4.

Q: how to determine the tooth height of a drum drum is 160mm in dia and 20mm thick

A: Optimum tooth height does not depend on drum dimensions. Use the search box at the bottom of this page - immediately below the box where you entered your question - to search the Ask Aaron Archives for "tooth height" to find an explanation and an example calculation.

Q: sorry to say but sir i have seen most of the archives but i didn't get the topic of tooth height please help me [India]

A: You're building a combat robot but cannot search a website? Please allow me to spoon feed this to you:

Tooth Height Post in the Ask Aaron Weapon Archive

There is also a complete discussion of tooth height in section 6.3 of the RioBotz Combat Tutorial.

Q: When was Googlw invented? [Waterville, Maine]

A: [Mark J.] Googlw?? I think you just invented it.

Q: I have a question about metal case on bottom layer of Sabertooth 2x60 motor controller, can it be disassembled from PCB without damaging the controller? [India]

A: [Mark J.] Mark Harrison gave me a photo of the Sabertooth 2x60 circuit board with the bottom plate removed and some information about it. The plate is held to the board by the two fan screws and a bead of adhesive. If you're careful you can remove the fan screws and pry the board free without damaging it, but I don't know why you'd want to.

Sabertooth 2x60 ESC with bottom plate removed

Q: Hello Mark. It's been a long time since I asked the last few questions. Thank you for your patience over the years in answering my (and our) questions!

Recently the full episodes of Battlebots have surfaced on the internet, and in watching it I've got some questions. Thank you for your kindness to answer these questions. [Chinese Forum]

Lisa Winter's 'Mecha Tentoumushi' lightweight combat robot 1. Is Lisa Winter's Mecha Tentoumushi's "smothering shell" a sound design? Surprisingly it fared well when competing against Lightweight bots like Wedge of Doom and Dr. Inferno Jr. However it looks like that shell could raise the bot's center of gravity heavily when raised, thus caused its knock out loss to Wedge of Doom in 4.0. But when it worked it looked impressive.

A: [Mark J.] The clever thing about a 'smotherbot' is that once the opponent is concealed by dropping the hollow shell over them it becomes very difficult to determine which 'bot is in control of the match. Is Tentoumushi moving its opponent toward the arena hazards, or it it being dragged toward them?

Judging criteria have changed, and 'control' points are no longer awarded. The day of the smotherbot has passed.

2. In 5.0 Vladiator won over Pro-Am in a very close fight. After the match [Pro-Am's builder] Mike Konshak thought Vladiator's higher speed may have made the judges think it was more aggressive, thus causing his loss in this close decision. Is that possible?

A: 'Vladiator' was (and is) a wildly overpowered 'bot that fights like a bull in a china cabinet. I think speed and great acceleration does influence judges on aggression points.-- and I think it should. Sometimes a close decision comes down to which 'bot you'd rather see fight another match and exciting, explosive speed is something you'll want to see again.

And then some questions about recent fights:

3. Is Herr Gepounden now officially the oldest American bot still competing? It still won some matches in this year's Robogames. Twhack bots are [generally] uncompetitive in today's matches, so how can it still win fights?

A: It's very difficult to determine 'oldest'. 'Herr Gepounden' had its first competition appearance at BattleBots 3.0 in May of 2001, and its most recent (as of this question) at RoboGames in April, 2015. It has made appearances at combat tournaments every few years during that long period, and it remains largely unaltered in appearance. I would say that it has a good claim to the title, but so do other 'bots...

'Mouser-Mecha Catbot' (see next question) started its career as a combat robot in June of 2000 at BattleBots 1.0, nearly a year earlier than 'Herr G.' However, MMC took a 13 year-long break between BattleBots 5.0 and its appearance at RoboGames 2015. Does that very long break disqualify it? MMC hasn't won a match in more than a decade, does that rule it out? I don't think I can make an 'official' determination.

'Herr G.' is still competitive for a couple of reasons:

  • Its 'thwackbot' drivetrain incorporates a functional 'Melty Brain' (AKA 'Tornado Drive') system that allows the 'bot controlled motion while spinning. This high-tech trick allows the robot to persue its opponent while in 'spin mode' and keep racking up aggression points from the judges.

  • Its high profile and large ground clearance does not give much of a target for modern spinner weapons seeking low, sharp-edge targets to dig into. Herr G. can't do a lot of damage, but opponents can't damage it much either.

Veteran combat robot 'Mouser-Mecha Catbot' 4. Mouser-Mecha Catbot had a brief return to combat events as well, but for a bot that hardly changed since TV era is that a good choice?

A: See my comments about 'Mouser-Mecha Catbot' in the previous question. Was it a good call to pull a 'bot out of a 13 year-long retirement to fight again? If the team had fun, then yes!

And about TORO's rather surprising loss to Phrizbee Ultimate in Battlebots 5.0 -- does that "run away and wait" strategy work when facing spinning blades? [Toro's builders] Inertia Labs got booed by the audience, but I think had they attacked earlier that might have worked for them?

A: In the 'early days' it was fairly common for high-power spinners like 'Phrizbee-Ultimate' to lack the stamina for a full three-minute fight. Battery power could run low and diminish their attack power, particularly if you could run them into the wall and force them to spin back up to speed a few times.

You'd lose aggression points and the crowd really hated it, but if you had a 'knock-out' punch like 'Toro' had it was a workable strategy -- IF you got the timing right!

Q: If thick walled drum weapons are not effective, why is the 'snail drum' on the heavyweight champion 'Touro Maximus' built to this design? [India]

'snail drum' weapon for heavyweight combat robot 'Touro Maximus' A: [Mark J.] Team RioBotz know what they are doing. They fight in large arenas and their attack plan is to charge at high speed into their opponent -- much higher speed than Indian robots can use in their small arenas. If the attack speed is high, the drum can rotate at a higher speed and still retain good energy transfer ('bite') into the opponent. Double the attack speed allows double the weapon RPM -- and double the RPM gives four times the energy storage in the weapon!


  • A specific robot with an attack speed of 3 MPH finds that it must the limit weapon to 2000 RPM in order to maintain good weapon bite. At 2000 RPM its weapon can store 1500 joules of energy.

  • If the attack speed of the robot is increased to 6 MPH, the weapon RPM can be raised to 4000 RPM and still retain the same 'bite'. At 4000 RPM this same weapon can store 6000 joules of energy.

  • If the attack speed of the robot is increased to 12 MPH, the weapon RPM can be raised to 8000 RPM and still retain the same 'bite'. At 8000 RPM the weapon can store 24000 joules of energy.
So, at 4 times the attack speed, the weapon can store 16 times the energy and still have the same ability to bite into the opponent without 'skittering' off. When you have the capacity to store that much energy in the weapon, you can afford to decrease the storage capacity a bit to make the weapon more durable and better able to survive the massive hits it will deliver.

Recap: the snail drum on 'Touro Maximus' is a thick-walled small-diameter drum in order to better survive the ultra-high energy impacts it delivers because it spins at very high RPM. It is able to spin at such high RPM because 'Touro Maximus' has a very high attack speed, which gives the weapon good 'bite' even at such high RPM.

This works in arenas that are large enough to allow robots room to accelerate to high speed. In a small arena it's much better to concentrate on larger, thinner walled drums to store high energy at lower RPM due to their greater rotational inertia. Don't blindly copy the snail drum if you're going attack at low speed!

Q: hello aaron can i use 304 [stainless steel] as drum tooth???? if yes then it should be hardened or not because i heard that [stainless steels] are not hardened.... [India]

A: [Mark J.] The 300 series stainless steels are Austenitic alloys that CAN NOT be hardened by heat treatment. They are soft and ductile: not what you want in an impactor for your drum weapon.

Impact tooth material has been discussed many times here at Ask Aaron. There is general advice on impactor material selection in the Spinner Weapon FAQ and there are several discussions of specific materials in the Ask Aaron Materials and Components Archive.

Q: what is "Demonic Teeth" in india most the bot uses it for drum tooth. [India]

A: [Mark J.] Unknown. Ask an Indian robot builder, and then scold them for making up 'cute' names for materials.

Q: i am having drum of 110dia (OD) and thickness is about 40mm and length is 20cms and using bearing caps at end i want to participate in a 60kg robowar dose it store enough energy at 3000 rpm [India]

A: [Mark J.] Have you been asleep in the back of the classroom?

There are at least four other posts on this page asking me to calculate the energy storage of drum weapons. My answer to all of them has been the same. Take a look at this post which makes the same error of data omission you have made.

Read the Ask Aaron Spinner FAQ. It will tell you how to calculate the energy your spinner weapon should store, and it gives you three options on how to calculate that figure.

Q: ...and also in your spreadsheet how can i give detail of tooth...thankyou

A: Ahhh! So you have been paying at least a little attention -- you know about the Run Amok Excel Spinner Weapon Spreadsheet. Good.

Impactor teeth and bars come in all shapes and sizes. Calculate the mass of the teeth (volume * density of tooth material) and increase the value for the drum radius until the mass is increased by the mass of the teeth. This will give you a good estimate of the contribution to energy storage the impact teeth will make.

Now, stop being lazy and run the numbers for your drum. You'll find that your drum is too small in diameter to store a reasonable amount of rotational energy. A larger radius and a thinner wall will greatly improve energy storage with the same mass.

Q: Thank you for i am making a new drum 15cm DIA and 25 cm length with thickness 35mm for 60kg weight limit ...but i am little bit confused about the diameter of dead shaft can you suggest me the dia of dead shaft

A: Is this a steel drum? Have you noticed that the drum will weigh nearly 25 kilos? That's a very heavy drum for a 60 kilo class robot, and it doesn't store a lot of energy for its weight. You may wish to work on the design a bit more. Here is your current design:

Wall Thickness
3000 RPM
24.7 kilos
Energy Storage
4420 joules

Reducing the wall thickness to 20mm and increasing the speed to 4000 RPM will significantly reduce weight and improve energy storage:

Wall Thickness
4000 RPM
15.9 kilos
Energy Storage
6074 joules

Better still, increase the diameter to 20cm, reduce the wall thickness to 11mm, and keep 4000 RPM:

Wall Thickness
4000 RPM
12.7 kilos
Energy Storage
10,063 joules

Thick walled, small diameter drums simply aren't effective at energy storage. Think bigger!

Back to your question -- the required diameter of the dead shaft depends on many factors:

  • shaft material
  • solid or hollow
  • energy storage capacity of the drum
  • distance from drum bearings to shaft support
  • strength of shaft mounting to chassis...

Assuming solid hardened steel alloy shaft and the drum design you've mentioned, 20mm to 25mm is a good starting point.

Q: I am trying to find a compact drive motor that can move 100+ easily do you have any ideas (Of cores high torque). [Moultrie, Georgia]

A: [Mark J.] Sorry, but 'move 100+ [pounds?] easily' does not give me enough information to recommend a motor. The hamburger is bad. See this post in the Ask Aaron archives for an explanation of the problem and a list of the information needed. There are also a great many posts about motor selection in the Motors & Controllers archive.

If you write back with complete information about your robot and the performance expectations you have, I may be able to make a motor recommendation.

Q: I'm expecting it to go at least 3 mph and be somewhat small and good for a battle bot competition in the USA not UK.

A: That doesn't really qualify as 'complete information' -- but I'll do what I can.

If you want to build a competitive combat robot for US competition you're going to need better than 3 MPH. You're also going to need enough power to accelerate to that speed quickly and provide good pushing power. Just how much speed and acceleration you need depends in part on your attack strategy; a ramming or wedge robot will need more speed and power than a robot carrying a large destructive weapon.

There's also the matter of how many motors your robot will use. Most have two drive motors: one on each side of the 'bot. Some have 4-wheel drive with four motors. A few have more.

Then there is the drivetrain. You can't just bolt a wheel directly to the shaft of a motor without gear reduction. The gear reduction ratio needed depends on the motor characteristics, the wheel diameter, and the weight of the robot. Since you're looking for a 'compact' solution, I'll assume you want a gearmotor that needs no external drivetrain components.

Using the Tentacle Drivetrain Calculator to model performance of a 100 pound robot with two motors:

  • The DeWut 3 Speed Gearmotor provides a compact and durable package ready to accept a drive wheel directly on the gearbox output shaft. With 4" wheels, a 100 pound robot powered by two of these motors will accelerate to a top speed of about 7 MPH in seven feet. That's good performace for a general purpose combat robot.

  • The AmpFlow E30-150 motor with speed reducer is another compact and durable gearmotor with enough power for your application. Top speed with 4" diameter wheels would be a bit over 7 MPH, with top speed reached in eleven feet.
That's as good as I can do with the info you've given me.

Q: I am joining the new battlebot competition in the middle white class so most likely it will be 115 pounds and i will be using five motors in all four for the drive and one for the weapon. the weapon consists of a fly wheel and steel bars with padlocks used as hammers. My design is some what like Mauler's but my body will be square. The wheel diameter will most likely be 3-5" tall and 2-3"wide all of the armor will be steel. The wheels will be boxed in. All of the armor will be 1/8". 18v motor. This is all of the information i have for now.

A: You haven't done your research -- BattleBots has changed:

  1. The 'new battlebot competition' (2015) has no middleweight class. It's a single class with a 250 pound weight limit.

  2. The 'new battlebot competition' is no longer an 'open' tournament. Competitors are invited by the show producers. If you aren't already a successful, established combat robot competitor you don't get to 'join'.

  3. Steel 1/8" armor and padlock hammers were OK twenty years ago, but they won't do at all for current competition.
Read thru the Frequently Asked Questions for some help in finding and competing in local/regional combat tournaments. You'll do well to start in a lower weight class and work your way up to the 'Big Time'. Write back when you have a new plan.

Inside Sewer Snake Q: hllo sir..
would you tell me how to calculate the motor specification to use it as lift motor like that is used in sewer snake..
thankyou.. [India]

A: [Mark J.] You may have noticed that there are VERY few electric lifters like 'Sewer Snake'. The problem isn't the motor -- lots of motors have the power needed to hoist your opponent off the floor. The problem is finding a gearbox that can take the huge torque loading without exploding! 'Sewer Snake' uses an AmpFlow A28-150 motor connected to an industrial 25:1 gearbox that feeds a 3:1 chain reduction [photo at right]. The torque available at the lifter axle is a whopping 10,650 kg-cm!!!

Here's the formula for the torque needed for a simple single-pivot lifter like Sewer Snake's:

Torque Required (kg-cm) = Weight of Opponent (kg) * Distance from Pivot to Tip of Lifting Arm (cm)

So, for a 60 kg class robot with a 50 cm lifting arm the torque required is:

60 kg * 50 cm = 3000 kg-cm

Now, your motor has to provide that much torque while still spinning at a reasonable speed, so you can't use motor 'stall torque' (zero RPM) as a starting point. A PMDC motor develops max horsepower when loaded to 1/2 stall torque, so we use 1/2 the motor stall torque in the next calculation.

Required Reduction Ratio = Torque Required / (0.5 * Motor Stall Torque)

Example: an AmpFlow E30-150 motor has 51 kg-cm stall torque. If you require 3000 kg-cm output torque at half motor stall torque, the reduction ratio required is:

3000 kg-cm / (0.5 * 51 kg-cm) = 118:1

When loaded to 1/2 stall torque, PMDC motor RPM is 1/2 the free-running speed. So the output RPM under load is:

Lifter Output Speed (Loaded) = (Unloaded Motor RPM / 2) / Reduction Ratio

The E30-150 motor has a free-running speed of 5700 RPM @ 24 volts, so:

(5700 RPM / 2) / 118:1 = 24 RPM = 2.5 Seconds per Revolution

You need less than 1/4 of a revolution to lift your opponent, so that's reasonable lifting speed. So the motor is suitable, but the problem is finding a 118:1 gearbox that can survive 3000 kg-cm of torque! A BaneBots P60 gearbox is rated for less than 500 kg-cm.

Q: how can a material be hardened like tool steel A-9 like hrc 50 because in india most the people don't know about hardening process [India]

A: [Mark J.] Heat treatment of metals is a highly specialized process best left to commercial facilities. The details of the process vary from alloy to alloy and often involve very high temperatures, controlled atmospheres, and carefully monitored heating and cooling rates. Attempting to do your own 'back yard' heat treatment will very likely yield warped/distorted parts with unknown properties. I HIGHLY recommend that you find a commercial heat treating facility to do your hardening.

Read the FAQ! Q: in your spreadsheet it shows the weapon has 109 joules of energy is it sufficient for a drumbot of length 22cm, 11cm dia and 30mm thickness and it is hollowed so that a dead shaft is inserted and bearing at end plates....if the energy is not enough then what should i do so that it can stores more energy i am using ampflow e30-400 [India]

A: [Mark J.] The Ask Aaron Spinner Weapon FAQ answers all of these questions. Read it.

Q: hello aaron, please suggest some ways how to make a drivetrain with chain and sprockets or some links may be..
motor - ampflow E30-150
gear reduction - 20:1
robot category-50kg [India]

A: [Mark J.] A chain drive at a 20:1 reduction requires a two-stage reduction thru an 'idler shaft'. The idler shaft is supported by bearings and has sprockets of differing size fixed to the shaft. To get a 20:1 reduction, the 'first stage' from the motor to the idler might be a 5:1 reduction (9 tooth sprocket on the motor to a 45 tooth sprocket on the idler, for example) with the 'second stage' from the idler to the wheel a 4:1 reduction (maybe 10 tooth on the idler to 40 tooth on the drive wheel). The overall drive reduction is the product of the first and second stages (5:1 * 4:1 = 20:1).

The upper diagram at right shows a two-stage chain and sprocket reduction with two idler output sprockets driving two wheels on one side of the robot.

The lower diagram shows the two-stage chain reduction in a BattleKit Modular Drive Unit with the idler shaft in-line between the motor and wheel.

Two stage chain reduction diagram

BattleKit modular drive unit

Q: is NPC-41250 Motor is good for wedge robot(25-30KG) for 60kg events having following specs
4 wheel drive with 1 motor each channel
6inch wheel
arena size (5X5)m
thanxx in advance [India]

A: [Mark J.] First, why are you building a '25-30KG' robot for '60kg' events? That's like showing up with a knife at a gun fight. Give yourself a chance and build to the weight limit.

NPC-41250: heavy (7.5 pounds), slow (3 MPH @ 24 volts w/6" wheels), awkward mounting, cast aluminum gearbox housing is weak. Suggest that you use the Team Tentacle Drivetrain Calculator to evaluate motors for your application.

Q: hi sir
whether12V 5310 RPM "CIM" Brushed DC Motor
  • 2.5-inch CIM brushed DC motor
  • Stall torque: 2.42 N-m
  • No load RPM: 5,310
can be used for combat if it is shall i buy this to run my 4kg drum [Tamil Nadu, India]

A: [Mark J.] The CIM motor has been used for years in the FIRST robotics program, and is certainly capable of being used in combat. However, it weighs a lot (46 ounces) for the amount of power it produces (about 1/2 horsepower). Compare that to the RS-775 motor That weighs 12 ounces and produces 3/4 horsepower.

Should you use the CIM motor for your 4 kg drum weapon? You didn't tell ne enough about your weapon for me to answer that question (see the Spinner Weapon FAQ), but YOU can use the Team Run Amok Spinner Excel Spreadsheet to model the weapon performance with the CIM motor to see if it meets your needs. This is the same solution I offered to the two posts immediately below. I'm sensing a theme here. Maybe that spreadsheet is useful?


A: [Mark J.] See post immediately below.

If you are unable to use the Team Run Amok Spinner Excel Spreadsheet, you do have other options:

Q: my drumbot length is 20cms and OD is 100mm and ID is 75mm and dead shaft is inserted in drum and at the ends bearings is mounted how much is energy it is storing ar 3500 rpm [India]

Ask Aaron is not a free engineering service! A: [Mark J.] 'Ask Aaron' is not a free engineering service. We provide tools and information to assist combat robot builders in the design, construction, and operation of their machines. We don't do your design work for you.

I spent a good deal of time and effort to create the Team Run Amok Spinner Excel Spreadsheet to assist builders in modeling the performance of spinner weapon designs. The spreadsheet is mentioned prominently several times on this page, in the Frequently Asked Questions, and in the Ask Aaron Robot Weapons Archive. Just a few questions down from the top of the Weapons archive is a detailed example of the use of the spreadsheet in calculating the energy storage of a drumbot.

I will occasionally work thru the process of calculating some performance factors for a specific robot as an educational example. However, having already performed that exercise and having provided you with the tools required to calculate the energy storage capacity of your drum weapon, it does not serve the purpose of 'Ask Aaron' to perform that calculation for you.

I will point out that you failed to mention the material from which your drum is constructed. Without that information it is not possible to calculate the mass of the drum, and without the mass it is not possible to calculate the energy storage. I could not answer your question if I wanted to. Be so kind as to run the numbers yourself -- you might learn something.

Q: hi sir considering single teeth drum and snail drum two different drums which one will give best hit? [India]

A: [Mark J.] There is a previous post about the 'snail drum' -- search the Robot Weapons archive for 'snail' to find that post.

The snail drum does not store more energy or deliver a better 'hit' than a single-tooth drum with comparable dimensions running at the same speed. The snail is tremendously difficult to design and construct, and the primary benefit appears to be the durability of the integrated impact tooth. The only real reason to build a snail drum is to impress your opponents with your machine shop skills. Avoid the temptation.

'snail drum' weapon for heavyweight combat robot 'Touro Maximus'

Q: Dear Mark,How to attach two pieces of PC(Polycarbonate) sheet in T shape(one piece's side attach to another's face)?I tried self tapping screw but it is difficult to tight and damage the sheet.Should I try ordinary screw?(I think team Hurtz did so) Another question, can I disassemble the attached PC and attach them with no damage? [China]

A: [Mark J.] Team Hurtz's preferred method of joining polycarbonate was to chemically weld the parts together with a methylene chloride solvent, although other specialized adhesives are available. This spreads the load evenly over the entire area of the joint and minimizes local stress -- a key in joining polycarbonate.

If you require a joint that can be disassembled, you should use course-thread machine screws. The fastening holes should be pre-drilled and the piece to be joined should be tapped with a proper thread cutting tool. You didn't mention the plastic thickness, but in general the screw diameter should be no greater than half the thickness of the plastic. Use plenty of screws.

Do not over tighten the screws! Tighten only enough to pull the pieces together. Polycarbonate absorbs impact forces by flexing, and areas of high local stress will develop around the fasteners. If the screws are already creating strain by being over tightened, the risk of fracture failure at those points is increased. A large thin washer under each screw head is a good idea to help spread the fastener loading.

Q: Hi Sir, I am making a combat robot with specs: Diameter Of Wheels: 12cm, 2 motor for drive, 1 for weapon- rotating heavy weapon, Arena= 10*10 meters, Twin Weapon System- Lifter wedge and rotor on piston controlled arm like cutt-off saw machine, External AC power supply 220V - 10A. Max Weight of Robot =60Kg.
My question is :
What are specifications of all motors we should use?
What kind of drive we should use: four wheel or two wheel? [India]

A: [Mark J.] External 220V - 10A AC power???

  1. Re-check the rules for your competition. I'm told that AC power was outlawed in Indian robot combat years ago. I don't believe it's currently allowed in robot combat anywhere. Far, far too dangerous! The Robot Fighting League caps voltage at 48 volts DC without special permission.

  2. My experience in motors for robotic operation is entirely limited to Direct Current Permanent Magnet (PMDC) motors. The torque and power curves for AC induction motors are quite different and I am unable to comment on AC motor selection.
Beyond suggesting that you not participate in such a needlessly dangerous activity, I can't help you.

Q: Sorry Sir but I want to correct the previous question about AC supply in competition. I'm pretty sure about the rules. We can use either AC or DC supply. We wanted to use AC so that we can save the cost of Batteries as will be using Hydraulic pistons for our weapons that are quite costly and we have to outsource it. We will be using simple on off switches for driving (Wired Control). What we have planned is to convert AC to DC and all the motors will be DC so I Want to specifications of motors like Horse Power, Torque and rpm for the same above conditions stated in previous question? and the type of drive(four wheel or two wheel)?The robot will be 70*50 cm size and maximum height can be 1m. We want to use a heavy rotor, so motor must be powerful.

A: [Mark J.] Several problems:

  • Combat robots use batteries for good reason. A one horsepower PMDC motor -- typical for drive motors in your weight class -- will require about 65 amps of current at 24 volts to produce that one horsepower, and twice that current to produce maximum torque. Two drive motors at max horsepower = 130 amps of current @ 24 volts. How do you plan to convert your 220V 10A AC to 24V 130A DC?? And that's just for the drive motors.

  • Gear reduction from the example drive motors and your wheels should be around 8 to 1 for a balance of acceleration, speed, and pushing power.

  • Weapon calculations require more specific descriptions than 'heavy rotor' in order to determine an effective motor and gearing. Suggest you start your design thinking by reading the Spinning Weapon FAQ.

  • The switches for your 'simple' wired control are going to need to handle very large currents. You might be interested in reading the Solenoid Guide for some ideas on drive and weapon control options.

  • Hydraulic pistons to control your weapon position are expensive, heavy, add complexity, and are not needed. Recommend using a fixed position weapon and adding the weight saved by scrapping the hydraulics to the weapon mass. In my experience 'complex' and 'first time builder' do not go together well, plus simple robots win tournaments.

  • Robots with large, destructive weapons do not rely on four-wheel traction and pushing power. I would suggest two-wheel drive to avoid adding additional complexity.

Would you be so kind as to give me the contact information for the organizers of this event? I'd very much like to have a safety discussion with them.

Q: i was looking for alloy steels and which one will be good for drum teeth
en 18 ,en24 vs h12 ,h13(hot worked tool steel) [India]

Ask Aaron is not a free engineering service! A: [Mark J.] The topic of steel for impact teeth has been discussed MANY times previously -- see the Materials and Components Archive.

'Ask Aaron' is not a free engineering service. I'm not willing to compare every possible subset of materials on a 'which of these is best' basis. Here are guidelines for impactor material selection:

  • Desirable properties for impactor teeth are hardness and impact resistance. Both are required.
  • These properties vary with the heat treatment (hardening, tempering) applied to the material.
  • Heat treating is mandatory, but do not sacrifice impact resistance for great hardness.
  • Look up and compare the properties (Rockwell hardness, Charpy impact score) of the materials available to you.

Equally important to material selection is the design of the impactors. The teeth must be attached to the drum in a way that minimizes localized stress on the impactor or they will fail. I'd much rather have a well designed impactor made of mild steel than a poorly designed impactor made of the finest tool steel.

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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