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4769 Questions and Answers about Combat Robotics
from Team Run Amok
Ten Years of Ask Aaron

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

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

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

Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok

Welcome to my website Recent Questions

Q: I bought the wrong BaneBots gearboxes! I have the 256:1 gearboxes which are far too slow for my purpose. Is there something I can do to change the gear ratio and speed up the gearbox? [India]

A: [Mark J.] BaneBots P60 gearboxes are modular. Your gearboxes have four 4:1 reduction stages housed inside a '4-stage' cylindrical ring gear. BanBots sells ring gears in 1, 2, 3, and 4-stage lengths. You can disassemble your gearboxes and reassemble them into one of the shorter housings while leaving out 1, 2, or 3 of the reduction stages. You'll end up with a 3-stage 64:1 gearbox, a 2-stage 16:1 gearbox, or a single-stage 4:1 gearbox. One of those ratios will likely be close to what you need.

Q: mark lucky here tell me exactly which type of bearing i should use in the end rings of my drum
drum diamensions :
od = 14 cm
length : 20 cm
weight : 14kg ( including tooth plate )
speed : 4500 to 5000 rpm
shaft dia : 3 cm
[Maharashtra, India]

A: [Mark J.] In general I'd use unmounted, shielded, pressed-steel cage, radial ball bearings with a 30 mm bore and 16mm width -- something like these: shielded bearings.

Q: Dear Mark,a question I've been wondering for quite some time:Why do featherweight class in America was rather unpopular when compared to lightweight and 15lbs class,thus made Robogames cancelled its fight some years ago,and why are they extremely popular in UK?Because of Robochallenge's promotion over the years? [Chinese Forum]

A: [Mark J.] Popularity had very little to do with RoboGames removing the 30-pound Featherweight class from their tournament in 2012. RoboGames removed the very popular 12-pound Hobbyweight class from the competition in 2008 even though it had more competitors than any of the other 'big' robot classes. The organizer simply preferred to have the heavier weight classes featured in the large combat arena, and removed the 'sub-light' classes from the event. The Hobbyweights continue to thrive, and the Featherweight class lives on in two popular 'flavors' at the Northeast Robotics Club (NERC) events -- regular and 'sportsman'.

I count ten(!) active weight classes in current US robot combat competition, and only three in the UK: heavy, feather, and 150-gram ants. I haven't followed the UK events closely enough to say why the competitions shook out to those three classes, but I think that concentrating in just a few classes has helped to strengthen robot combat in the UK.

Q: Mark,since RG was discontinued in 2013 and now America's only "major" event seems like is STEM Tech Olympiad,would you add the result of this year's STEM event to your "tournament tree" page? [Chinese Forum]

A: [Mark J.] It was several years after the start of Robo Games before I decided they were a 'major' event and added their tournament trees to the Team Run Amok Team Run Amok 'Who Won' page. One event does not make a major series. I'll give it some consideration.

UPDATE: I had a little time, so went ahead and worked out the STEM Tech Olympiad tournament trees. You can access them from our Who Won? page, or jump directly to the STEM Tech 2014 trees.

Q: I want to use ampflow e-400 motor for my drum,but as it is not available [in India], can i use 2 e-150 motors, one on each side of d drum?rpm would be same but will d hp and torque increase? [Maharashtra, India]

A: [Mark J.] Yes, you can use a pair of AmpFlow E30-150 motors to power your drum. Performance comparison below -- and I'll add a third option:

 E30-400 (x1)
@ 24 volts
E30-150 (x2)
@ 24 volts
E30-150 (x1)
@ 36 volts
Price (US)$109$158$79
Weight5.9 lbs7.2 lbs3.6 lbs
Stall Torque1500 oz-in1420 oz-in1065 oz-in

As the table shows a pair of AmpFlow E30-150 motors will have performance quite similar to a single E30-400 motor, but at greater cost and weight. An attractive option is to overvolt a single E30-150 by 50% to 36 volts. I might not recommend overvolting the E30-150 this much for robot drive use because of the high and sustained torque loading imposed on drive motors, but for a weapon that spends much of its time at low loads it is an option that you might consider.

Q: Hi, this is Chaitanya again, from India [scroll down about 10 posts to see Chaitanya's previous post] thanks for your guidance. As you mentioned, you want ratings of pmdc motors to calculate battery , they are
Stall torque: 50 kg-cm
stall current : 54 amp
rpm: 260
diameter of wheel : 16 cm
as you told, we are thinking on drum of material EN41 metal, length 22 cm, wall thickness 20cm [you must mean millimeters], diameter [I hope you mean radius...] 6.5 cm (excluding teeth height of 1 cm). as you said, wall thickness is too much less for strength because we are inserting 1 cm of teeth height inside the drum. so please give some suggestions regarding drum design and teeth dimensions so that we can design in a better way? as you told, following are the metals available in India for teeth are SS304, SS316, HEHER D-2, HEHER D-3, HDS H-13, BN-24, BN-31, OHNS.
which one is best for teeth? Is there any need to harden teeth metal? if yes then up to what value it should be hardened in HRC ? Please guide us about drum teeth. We read all the riobotz combat pdf for height of teeth but calculations were nt match.
please help about required battery for drive motors for 5 min match.
Thanks. [India]

A: [Mark J.] Please re-check the pmdc motor ratings you provided. A 24 volt motor with a 54 amp stall current should produce much less than 50 kg-cm stall torque if ungeared, and should produce much more than 50 kg-cm stall torque if geared down to 260 RPM. You also failed to send me the gear reduction ratio of the gearbox. I need clarification on both of these. Might you send me a link to the motor and gearbox specs?

D2 tool steel (HEHER D-2) has a good combination of the hardness and toughness needed for an impactor. I would suggest hardening and tempering to HRC 58/60.

Tooth height depends on robot attack speed, drum RPM, and the arrangement of impactors on the drum:

  • With 260 RPM gearmotors and 16 cm wheels, your max attack speed is a bit under 5 MPH (200 cm/second).

  • Allowing for mechanical and aerodynamic loss, your max drum speed is about 5500 RPM (92 revs/second).

    Weapon drum from Team Cosmos 'Solaris' robot

  • Assuming two full-width impactor bars on opposite sides of the drum, you have an impactor passing a fixed point on the radius every 5.4 milliseconds (1 / (92 revs/second * 2 impactors) = 0.0054 second).

  • At top speed your robot can move forward 1.08 cm in 5.4 millisconds (200 cm/sec * 0.0054 sec = 1.08 cm).
So, your optimum impactor height for a full-speed ramming attack is 1.08 cm. You got lucky in your guess of correct height.

Note: I've seen a lot of Indian robot matches and I have never seen a full-throttle, top-speed ramming attack by a drum robot. Indian combat arenas tend to be pretty small, and combat generally consists of the robots maneuvering into position and easing their drum weapon into the other robot with a short, darting attack. This is less than ideal and does not use the full depth of the impactor.

You have not mentioned the impactor design you have in mind. Something like the full-length impactor bars used by 'Solaris' (Team Cosmos - pictured) should do well for you. The Solaris drum has hardened steel bar impactors set into shallow grooves milled onto the drum that locate the bars and take the shear forces off the countersunk machine screws that hold it in place. There are a couple of additional impact teeth at the ends of the drum, but these were added to correct a specific weakness in Solaris and can be ignored for your application.

Q: hey, this is chaitanya again, so sorry about those mistakes, the corrections are as follows:
voltage: 24 volt
stall current:- 9 amp
gear box ratio:- 4.5:1

so can you now tell me about battery for drive , pushing power of bot etc. for any other info, refer previous question. as you mentioned in the answer of First question, please tell me how many batteries of 12 volt 9 amp can we use (for both drive and weapon) so that bot can work properly for 5 min match?

New drum dimensions are length 22 cm, radius 6.5 cm, thickness 22 mm , are this dimensions suitable for our robot of weight 66 kg? if not then please guide us. which metal can we use for drum between EN31 or EN24? Is there any need of hardening drum metal? if yes then up to what value in HRC? Can you please tell me about tooth depth also. (length from surface of drum ). which metal can we use for dead shaft and tell us its radius also.
thnxs for helping us.

A: [Mark J.] I think you have some more work to do:

Drive Motors: the spec numbers you've given for your PMDC drive motors are quite odd, but if you're sure they're correct I'll continue.

The motors are not adequate for your drivetrain. I recommend a bare minimum of 4 watts of drivetrain output power per pound of robot, and your four-motor drivetrain produces less than 2 watts per pound of robot. A typical robot in your weight class might have 10 or 20 watts of drive power per pound.

With 16 cm diameter wheels, your four drive motors combined do not provide enough torque for reasonable pushing power in a 66 kg combat robot. Maximum pushing power will be well under 30 kg -- less than half what a robot in your weight class might be expected to produce. Acceleration will be very poor, and the drivetrain will stall under moderate load risking damage to the motors. I STRONGLY recommend that you obtain more powerful motors.

There is little point in calculating battery requirements for these motors, as I'm rather certain that the motors would fail well before the battery is depleated. For what it's worth, it comes out to just under 2 amp hours of battery capacity to operate the drivetrain for 5 minutes with these motors.

Drum: your drum dimensions and materials can store a reasonable amount of energy for a robot in your weight class. I can't comment on its overall suitability for your robot, as the weapon design must integrate with the rest of your robot design -- about which you have told me very little. The drum itself appears to be fine.

The material used for the drum is far less critical than the material used for the impactors. Given your dimensions, any grade of mild steel would work well. No need for hardening on the drum material.

The calculations for tooth depth were included in my answers to your last questions -- please find your answer there. None of the drum changes have altered those calculations, although they may change when you select new drive motors.

Dead Shaft: as discussed in Frequently Asked Questions #4 and #17, I do not provide a free engineering service. I'm pleased to discuss materials and dimensions in general, but I cannot recommend specific materials and dimensions for specific applications. There are too many unknowns in your design for me to have faith in an analysis at that level. I can recommend that you research designs similar to yours and examine the materials and dimensions other builders have successfully used.

Order of magnitude estimate: 3/4" chromoly steel, very well supported as close to the drum bearings as practical. When in doubt go bigger.

Q: Mark,
What do you think is a reasonable/appropriate speed for a drumbot? Because I have heard claims of speeds upwards of 20,000 rpm and that seems ridiculously fast. At that speed isn't there no time for the robot to achieve bite? [New Richmond, Ohio]

A: [Mark J.] It's certainly tempting to spin a weapon up to stupid fast revs. Double the speed, get four times the energy storage -- awesome!

The problem, as you note, is that the faster the weapon spins, the harder it is to get the weapon to 'bite' into your opponent and get a powerful hit. A weapon without bite will just skitter across a smooth surface and do no harm at all. If you have no bite you must rely on your opponent to make the mistake of offering a sharp edge to give your weapon something to grab.

How fast is too fast? Depends on the spacing of the impactors and your attack speed. You can get away with greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to come by if you have multiple impactors and a timid attack.

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read.

There is one excuse for a hyper-speed drum weapon: when two drums go 'head-to-head' and their weapons meet, the faster drum wins. So, if you are expecting to fight a lot of other drumbots you might want to be able to reach for a few thousand extra RPM to see if you can launch them. The rest of the time you'd be much better off to throttle the weapon back and charge hard.

Q: Hey Mark, I've been planing a new beetle weight after my first competition (An 8"x8" plate of steel with chunky lite flites) and I was planing on using modded VEXtrollers with the kitbots 1000RPM motors. The problem is that VEXtrollers don't have BEC built in, and I really don't have much money to buy a dedicated BEC of reasonable quality, so I was wondering if I could an off the self Radio Shack voltage regulator. If I can, what should I look for, but if I can't, why? There must be a reason why, because if they worked easily, RC guys would be using them. [Woodburn, Oregon]

A: [Mark J.] If all you're powering at 5 volts is your receiver, you can certainly stop by Radio Shack and buy yourself a 7805 linear voltage regulator IC and wire it as shown in the diagram to power your receiver. The capacitors (35 volt rated) are optional, but will give cleaner power to your receiver.

So why don't R/C guys do this? Linear voltage regulators are inefficient. They waste a lot of power to do the job and lose capacity as the input voltage rises. That makes them a poor choice if you have multiple servos plus your receiver drawing power.

A 'dedicated BEC of reasonable quality' like you are trying to avoid buying is a switching power regulator. More complex but more efficient, a switching regulator also doesn't lose capacity at increased voltage inputs. Take your pick.

7805 voltage regulator circuit

Q: In a pneumatic system for flipper which valve should i use ? Should i use 5 ports or 3 ports ? [West Bengal, India]

A: [Mark J.] I suggest you read the Team Da Vinci: Understanding Pneumatics page for a full description of 3, 4, and 5 port solenoid valves and their applications, as well as general design information and illustrations of robot flipper systems. I also suggest that you read thru the many posts about pneumatic systems in the Robot Weapons archive for additional design considerations.

Basic Pnumatic System Diagram from Team DaVinci.

Note: questions sent in to 'Ask Aaron' thru our embedded webmail system do not use your email. If you write in with a question and ask for a confidential response, you must include your email address. Without your address I have no way to send you a confidential reply.

Q: A few months back I designed a beetle weight wedge with a polycarbonate shell over a 3D printed skeleton. The skeleton had nuts hot glued on the inside which the polycarb bolted to. The printer I had access to could print either ABS or PLA; I chose PLA because it prints better in that printer. By the end of the first event that I took it to, the printed substructure had broken in several places. Many of the fractures occurred at places that obviously should have been thicker and I also plan on reprinting with a higher fill percent.

Do you think the substructure would be more resilient if I printed with ABS instead of PLA? [New York]

A: [Mark J.] It's a little complicated. ABS is an overall stronger plastic than is PLA, so if you carved a chassis from a solid block of each material the ABS chassis would be considerably stronger.

Complication: PLA bonds to itself much better than ABS does when you're layering it up with a 3D printer, which means the ABS has a tendency to delaminate. This makes a structure subject to loading from all angles (like a combat robot chassis) much weaker.

The final verdict would depend on the intricacies of your chassis design, but in general PLA makes a stronger and more resilient 3D printed structure than does ABS.

Comment: Thank you! I hadn't thought about delamination. I think I'll stick with PLA and thicken the weak areas and print with a higher fill.

Q: should i buy colson wheels and live hub or directly ampflow wheels? m using local gearbox for ampflow motors [Maharashtra, India]

A: [Mark J.] The 4" thru 10" AmpFlow wheels are Colson wheels with a hub that mates with the AmpFlow gearboxes. If your 'local' gearboxes have output shafts identical to the AmpFlow gearboxes the AmpFlow wheel/hub combos will work very well for you. If your gearboxes have a different output shaft you'll need different hubs for Colson wheels.

Q: Hey mark, can u help me with designing flipper bot ? I am using 2 pneumatic rams. [West Bengal, India]

A: [Mark J.] Sure. My billing rate for engineering and design services is $60/hour. Send me an advance for the first 10 hours and I'll get right on it.

Alternately, see Frequently Asked Questions #4 -- and read the rest of the FAQ while you're there.

Q: is there anything such as cotton balls that would really get jammed into a pinch point of a drum and stop the weapon? can you attach things such as this to your bots armor or would it constitute an entanglement device aka against the rules?

A: [Mark J.] I don't know about cotton balls, but it's certainly possible to jam a rotary weapon with nets, ropes, and fabric. Take a look at 'Juggerbot 3.0' Vs 'Ultra-Violence' at Robotica. Entanglement devices were legal at Robotica, but the current Robot Fighting League rules prohibit "...nets, tapes, strings, and other entangling materials", so I think you're out of luck.

Q: doesnt the sticky stuff people paint on their foam wheels in the insect division violate the "no liquids" rule? Thanks again [San Diego, California]

A: The RFL rules prohibit liquid weapons, liquid that can spill out of a superficially damaged robot, and weapons that require significant cleanup. I don't think that tire traction goo falls into any of those categories, so its use becomes a call for the event organizer.

Q: Hi, this is Chaitany, from India, we are willing to design a robot same as tauro. We are using amflo f30-150 motor having noload current 2.5 amp and stall current is 375 amps for hollow drum having length 16 cm , diameter 14 cm and wall thickness 27 mm. We get the energy 4205 joules at 4347 Rpm at 3.02 sec. robot weight is 66 kg. Dimensions of bot is 60*70*14 cm. So the my question is how can we mount the drum on the shaft if dead shaft system is used ? Can i use bearings at edge of drum? any suggestions regarding drum and energy? We cannot increase the diameter.

We are using 4 pmdc motors for motion having rating 24 volt 9 amp. We are using 12 volt 9 amps battery, 2 for weapon motor and two for pmdc [drive] motors. So any suggestions about battery? If these batteris are use, then till what time robot work properly??? Is there any arrangement except dead shaft system?, if yes then guide us plz. Can we use carbide metal as a teeth? Plz help us. thnxs. [India]

A: [Mark J.] First, thank you for sending such complete information about your robot weapon. That makes it much easier for me to answer your questions.

One correction: the stall current on the AmpFlow F30-150 motor is misprinted as 375 amps on the Robot Marketplace site. Calculating the stall current from the spec numbers on the AmpFlow website gives either 296 amps (Volts/Resistance = 24/0.081 = 296 amps) or 294 amps (Stall Torque/Torque Constant Kt = 1370/4.66 = 294 amps) -- so let's split the difference and call it 295 amps.

Mounting the drum on a dead shaft: scroll down five posts and you'll find an answer to this same question asked a few days ago -- with a photo. Hollow drum weapons typically have machined 'end caps' that slip into the ends of the drums and are held in place by machine screws inserted thru the drum face into threaded holes in the caps. The end caps have a hole sized for a bearing to support the drum on a dead shaft.

Drum suggestions: why so short a drum? A 24 cm long steel drum 14 cm in diamter with a 16 mm wall thickness will weigh the same (12 kg) as your short drum and will store more energy (4750 joules) at the same RPM (4347). For maximum energy storage, make the drum as long as practical for your design and reduce the wall thickness to keep the weight the same.

Battery suggestions - Weapon: if you look a bit further down the page of the Run Amok Excel Spinner Spreadsheet that you apparently used to calculate your drum energy, just below the 'Results' box is another box labeled 'Battery'. This box reports that your weapon system might be expected to use perhaps 1.45 amp-hours of current in a typical 3 minute match. With a 9 amp-hour battery you have many times the power needed for your weapon.

Battery suggestions - Drive: to calculate the current consumption of the drive motors I need much more information about the motors -- their stall torque, stall amperage, gear reduction, RPM, and wheel diameter. With that info I can estimate their current consumption as well as the robot speed, acceleration, and pushing power. That said, I very seriously doubt that the motors will come anyplace close to completely draining a 9 amp-hour battery pack in a typical match.

Commonly, combat robots have a single battery pack that operates both the weapon and the drive system -- consider using only two 12 volt 9 amp-hour batteries for the whole robot rather than four. You may likely be able to reduce the size of the batteries even further, but I need that drive motor info to be able to tell you by how much.

Alternative to dead shaft: sure -- you can use a live shaft firmly affixed to the drum endplates and supported by bearings in the robot chassis. A live shaft is rarely used because a ridgidly fixed dead shaft can be a stiffening member joining the chassis supports together, but if a rotating live shaft suits your design you can certainly use it.

Carbide impactor teeth?: no! Tungsten carbide is very hard, but it is also very brittle and can SHATTER on impact and send sharp shards flying at high speed. US and European events typically forbid carbide impactors for safety reasons -- don't use them!

Q: Dear Mark,I think most US robot can't sustain the hit 'Touro Maximus' gave to 'Tanto',Maybe 'Tanto'just too bad luck.'Tanto''s Hardox fork wedge just bent a little bits,that's too much energy release in one little places.Maybe that's 'Tanto''s design problem,the design is bad at against a drum spinner.'Touro Maximus' also caused fire by that hit.Though 'Tanto''s battery dropped on it,I don't think battery outside can cause fire inside.The hit may also out of 'Touro Maximus''s design. [Guangdong, China]

A: [Mark J.] Many opinions, but no question.

Q: hi.. for amp flow -A150 motor what is requirement of battery,and what will be the requirement of amp. and max limit is 24 volts (i.e for drum motor) [Maharashtra, India]

A: [Mark J.] Briefly, the amperage requirement for any electric motor depends on the torque load placed upon that motor, and you've given me inadequate information to calculate that load.

To calculate the torque load on your motor you need to first calculate the moment of inertia of your drum. This requires specific information on the dimensions of your drum and the material from which it is made.The Team Run Amok Excel Spinner Spreadsheet can perform those calculations for you and estimate the battery capacity requirement for a match of given length.

The AmpFlow A28-150 motor at 24 volts can pull 385 amps at full stall, and the closer you can come to providing that much amperage the faster your drum will spin-up. If your weapon should stall against an imoveable object, your motor may attempt to pull that full amperage from the battery for as long as the weapon is stalled and may damage a battery pack that is not rated for that great a drain. There is no point to having a powerful weapon motor if you do not supply it with the full current it needs to perform properly.

I suggest you search for the many previous posts in the Robot Weapons archive covering the use of the Spinner Spreadsheet to calculate moment of inertia and the battery requirements for your weapon.

Q: Mark, what made 'Behemoth' so unsuccessful in the earlier TV era and what made it became very impressive since the TV era ended? Back then It could [sometimes] defeat some of the best robots like 'Hypno Disc' but it seems like it's always very unlucky. [Chinese Forum]

A: [Mark J.] Often it is not big things that separate a successful robot from an average 'bot, but a number of small elements. If the basic design is sound and you stick with it you can identify and change the weak elements to improve the robot over time.

A change in gearing, a shift in weight distribution, a more efficient pneumatic system, stronger drive components, new armor materials, more secure equipment mounts, better battery technology, and refined R/C control set-ups are all things that could greatly improve robot performance and not even be visible to the audience. Persist and improve.

Q: mark i am a little confused about capacity of single tooth snail design drum of touro
i am thinking to try that in one of near by event
i know it is very difficult in manufacturing but i wanna try
please tell me what i should do [Tamil Nadu, India]

A: [Mark J.] RioBotz designed the "snail drum" for 'Touro Maximus' using the MatLab Genetic Algorithm Toolbox software. You can find the academic paper describing the drum and the design process here: Drum Shape Design and Optimization Using Genetic Algorithms. I suspect that the drum represents an investment of hundreds of hours and thousands of dollars. It's a whole lot of work for very little gain in drum performance -- largely an academic exercise.

A single-tooth drum is a fine concept, but it is MUCH easier for a competent builder to implement a design using a counterweight balancing insert on the opposite side of the drum from the uni-tooth. The guys at RioBotz were just showing off their technical muscle in designing the snail drum. You shouldn't try to duplicate that effort.

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

Q: I'm making a drum of 14 cm OD,& thickness of 15mm which makes ID of 11 cm.drum weight will be around 7kg
what kind of bearing should i use for rotation of drum on rod?even if i use 11cm OD bearing it will have a bore of 80-90mm that will increase the size and weight of rod... please help [Maharashtra, India]

A: [Mark J.] There's no need for a huge bearing. Drum weapons typically have machined 'end caps' held in each end of the drum by screws. The end caps have holes to mount bearings of suitable size for the supporting rod.

Weapon drum end cap from Team Cosmos

Q: Mark, is there any real poor sportsmanship in robot combat? I mean like the Battlerat team's reaction after the decision of their fight against Nightmare announced in Battlebots 4.0. Did they really disrespected judges or was that just the magic of editing? [Chinese Forum]

A: [Mark J.] The level of sportsmanship at combat robot events is generally quite high, but I don't believe that any competitive activity is completely free of poor sportsmanship. I wasn't present at the Battlerat vs. Nightmare match and so cannot comment on any editing of the post-match comments, but I have personally seen much worse behavior at combat events. On one noteworthy occaision police had to be called in to calm things down.

One event actually encouraged a bit of poor sportsmanship. Competitors at Robot Wars Extreme Warriors were asked to 'talk trash' and be openly confrontational with their opponents during interviews. The network (TNN) was trying for a tie-in with professional wrestling and had a well-known wrestler as the host. Some teams played along, some didn't, and a few actually got kinda nasty. It really didn't work, and the trash talk was toned down for the second season.

Cartoon Fight Cloud

Q: hi
i want to build a robot for robowars (of my college) the robot should be of 30cm*30cm and no height limit and the weight limit is upto 5kgs the battery shold be of 18volt 5amps
can you give me some advice about what type of robot should i build.
what should be the design [-- Location Confidential --]

A: [Mark J.] A robot must be designed to meet the specific challenges it will face. You've told me a bit about the limitations placed on the robot, but nothing at all about the actual competition and your experience in this area. It's a lot like designing a race car; you'd build a much different car for a quarter-mile drag race than for an off-road endurance event.

Some of the things to consider:

  • Is the event full combat, sumo, an obstacle course, king of the hill, or some other flavor of 'robowar'?
  • What is the design of the arena/course, and how large is it?
  • Are there limits on the types of weaponry that are allowed?
  • How is the event scored?
  • Are there multiple ways to win (immobilizing, pushing out of the arena, pining, accumulating points...)?
  • What is your budget for the 'bot?
  • Is this your first robowars robot?
  • Do you have experience driving remote control vehicles?
  • What workshop tools are available to you, and what is your proficiency with those tools?
  • Will the robot be radio controlled, or will you be using a wired control tether?
If you can tell me more about the event, I may be able to give some suggestions on design -- but I'm not going to design your robot for you. See FAQ #4.

Nickeloden Robot Logo Q: I remember watching Nickelodeon Robot Wars, and I remember seeing Nancy Rodriguez with Loanerbot, 'Tut-Tut'. And questions kept racing thru my mind. Did she and her dad thought about making their own robot for Robot Wars? [Cicero, New York]

A: [Mark J.] I wrote to Mike Morrow of Team JuggerBot who coached Nancy Rodriguez and her dad Carlos in the Challenge Belt competition on Nickelodeon Robot Wars. He tells me that Carlos Rodriguez was the lead technician at WCMS Motors, who supplied custom electric motors to Team JuggerBot. Mike brought them along as honorary crew members for the trip to London in 2002. The teams were encouraged to bring kids along to 'Robot Wars Extreme Warriors' to drive 'bots in 'Nickelodeon Robot Wars' which filmed just after the main competition.

I don't know if the Rodriguezes had any plans to compete with their own robot at a future event, but there have been father/daughter teams in robot combat. See, for example, Lisa Winter.

Q: VestedMadScientist here again, this time with a more technical question. I have a Spektrum AR6210 DSMX receiver and a lightweight battlekit with F30-150s (I also have a Vantec RFDR36R, but that's still in the mail), and I'm a bit concerned with whether or not the aluminum chassis will interfere with the radio signal. Is there an optimal place to mount the receiver, or does it really matter? [North Carolina]

A: [Mark J.] The Spektrum AR6210 manual provides advice on mounting and orientation of the main and remote receivers (yes, a two-piece receiver). Keep the receiver away from electrically 'noisy' devices like the ESC and motors, and don't tape the wire antennas down directly to the aluminum -- space them up on a foam strip. As long as the receivers aren't enclosed on all sides by conductive material (metal, carbon fiber...) you'll get a good signal.

For more radio help, see our R/C Radio Reception Guide: Radio Reception Guide

Q: Also, what do you think about a double-layer of 1/8" lexan (totaling 1/4") as armor? Should I try it or should I just use a single 1/4" thick sheet?

A: I use 1/8" Lexan on my antweights -- I think you're gonna need more than 1/4" for a 'bot 60 times that heavy. Just how much top armor you need is debateable because few competitors build overhead hammer/spike/saw weapons to attack from above. However, they are out there and if you run into one with poor top armor you can find yourself in serious trouble -- video.

Lexan is able to survive large impact because it deforms to absorb the energy. You need to leave considerable room for this deformation between the armor and what you're trying to protect. For your application I think I'd recommend something a little stiffer -- consider garolite. There are several posts about garolite in the Materials & Components archive. And yes, garolite is radio transparent.

Q: Was it difficult to make your robot waterproof for the waterfall at Robotica? [Watertown, Massachusetts]

A: [Mark J.] We didn't waterproof 'Run Amok' -- it wasn't necessary. The top armor was a single, solid sheet that shed water to the edges like a roof. The only vulnerable components (steering servo, receiver, and speed controller) were all tucked up against the top armor and had their own cases that would resist any random splash. None of the 'bots at 'Robotica' had any trouble with the water.

Q: Roughly what magnitude impact force does a powerful bot like Metroid impart on another antweight when it hits it several feet into the air? Is there a way we can estimate this by using the Joules of energy stored from your Spinner Spreadsheet (assuming both bots are very rigid)? [San Diego, California]

A: [Mark J.] I went thru the math for this in a previous post. Search the Robot Weapons archive for "brag about my spinner" to find the post with equations and a diagram.

The numbers from these simple equations overstate the real world performance of the weapon, since it is effectively impossible for a spinning weapon to transfer all of its energy directly into vertical acceleration of the opponent -- but they make for nice brag numbers.

Q: Thanks for referring me to the spinner brag equations. But I was really curious about getting a Force value out of it. Something you could apply to a finite element model or hand calc in order to determine the stress or displacement in your chassis or weapon mount.

A: [Mark J.] Unfortunately, joules of energy don't - in this case - convert directly to force. A heavy but slow moving object might just gently push your robot out of the way, while a fast moving lighter object with the same kinetic energy could deliver a damaging blow. Add to that the many different angles and places of potential impact and many possible values for inertia of the robot depending on the rotational axis imparted by the impact, and you'll get so great a range of theoretical force values (many highly improbable) that the analysis will be useless.

The classic combat robot design advice is to build it strong, and if it breaks make it stronger.

Q: If you need to bolt some things to your Lexan sheets (have read the many posts about only welding Lexan, never bolt), what is the best way to prevent a failure around the hole? should you use a large washer to spread out the load? i want to use about a 8" diameter Lexan piece and have 3 bolts connecting it to it in a triangular pattern near the edges. so it will still have some space to flex between the bolts and in the middle. [San Diego, California]

A: [Mark J.] Lexan plastic (polycarbonate) gains its strength from its ability to flex on impact. Conventional use of bolts will restrict flexing and cause cracking and failure around the bolt hole. General rules:

  • Use as large a diameter bolt as is practical and drill the hole oversize; at least a 1/4" hole for a 3/16" bolt.

  • Do not tighten the bolt down to squeeze the plastic -- insert a tubular spacer so that the bolt may be tightened without stressing the Lexan. The bolt should only locate the Lexan, not squash it.

  • I've had good success using rubber grommets to provide 'shock mounting' that allows the plastic to move a bit without creating local stress.

  • A good sized washer on one or both sides of the bolt hole will help.

  • Keep the mounting holes well back from the edge of the material. I recommend an inset of at least two times the diameter of the hole.
Rubber grommets
Rubber Grommets
Lexan is a wonderful structural or armor material if you recognize its needs and limitations.

Q: Mark, what caused Tanto to explode into many pieces when fighting Touro Maximus in the STEM Tech Olympiad in May [2014]? Does that prove that many robots who are fighting in England right now are not suitable to American competitions? [Chinese Forum]

A: [Mark J.] British heavyweight robot 'Tanto' simply took a good hit from a strong spinning weapon storing many thousand joules of energy and suffered an extensive structural failure (video). The main chassis lands out of sight of the camera. This demonstrates that 'Tanto' wasn't strong enough to survive an attack from one specific South American robot, but I certainly won't generalize that weakness to other Britbots.

British and American combat arenas differ in design, and those differences have a strong influence on robot design and combat style. We've discussed this in earlier posts, but in short:

  • British arenas have a large space between the barrier around the combat area and the arena wall which makes it practical and relatively simple to toss your opponent out over a low retaining wall for the win -- hence the large number of flipper designs in the UK.

  • American arenas generally have a small (if any) space between the combat barrier and the arena wall, making a toss-out a much less reliable tactic and leading to an emphasis on very destructive weaponry.

Many fine combat robots have travelled across the Atlantic - in both directions - only to find that their 'bot does not do well on the other side of "the pond". That isn't to say that either combat style is better than the other -- they're just different.

Q: Just thought about this when I saw the winning streak question. I remember before BioHazard's continous win which started in 2.0 of Battlebots it had already achieved an impressive winning streak from 1996 Robot Wars US season to the quarter final in Battlebots Las Vegas 99 -- am I right? [Chinese Forum]

A: [Mark J.] With an overall record of 35 wins and 5 losses, you might expect a few long win streaks. BioHazard won its first 13 matches in a streak that ran over four events, ending with a loss to 'Vlad the Impaler' at the '99 BattleBots event in Las Vegas. BioHazard's full match record at

Q: Hey Mark, I made a documentary about robot combat for my videography class a month ago, and I wanted to know what you think: Robot Combat: A Documentary [Havertown, Pennsylvania]

A: [Mark J.] You couldn't put in a clip of one of my 'bots, Nate?

I'm always happy to see outreach that may generate some interest from the public. I like your selection of combat clips, and the editing is quite professional. I also like the breakout of the video into sections on history, people, and robots. The only thing I might have wished was that your interview subjects had been a bit more animated -- you needed to interview Stephen Felk!

Q: Am using 3HP amp flow motor for my drum bot but weight of drum is 7-8 kg maximum and the length of drum is 14-15 cm..please suggest me the drum diameter and thickness of drum..which stored maximum energy.. [Maharashtra, India]

A: [Mark J.] I've spent considerable effort to put together the Team Run Amok Excel Spinner Spreadsheet that allows builders to model the performace of spinning weapons, but there still seems to be confusion about the basic physics of drum design and moment of inertia. I'd suggest that you study up on the topic before continuing your design efforts -- it's more than I can teach here.

The variables that determine the energy that a spinning drum holds at a given RPM are: mass, material density, diameter, and length. You've specified mass and length, and I'll assume that you're using steel. With mass, length, and speed held constant the energy storage will increase with increasing diameter. Examples - for a bare steel tube (no end caps or impactors):

  • 15 cm long - 15 cm diameter - 14 mm thickness: mass is 7 kg and it stores 2900 joules of energy at 4000 RPM

  • 15 cm long - 20 cm diameter - 10 mm thickness: mass is 7 kg and it stores 5600 joules of energy at 4000 RPM

  • 15 cm long - 25 cm diameter -  8 mm thickness: mass is 7 kg and it stores 9200 joules of energy at 4000 RPM
If your primary design consideration is greatest energy storage, make the drum as large in diameter as is practical for your overall design. There are other elements in drum design that should be examined before you get too excited about maximum energy storage -- see recent posts in the Robot Weapons archive on drum design.

Q: Has any bot in history had a winning streak longer than 'Hazard's? [Havertown, Pennsylvania]

A: [Mark J.] Middleweight 'Hazard' ran up an impressive 17 match win streak before losing to flipper 'T-Minus' at BattleBots 5.0, but it isn't the longest streak.

  • Hobbyweight 'Helios' completed a 19 match win streak with its championship victory at the 2004 Robolympics.

  • Antweight 'Dark Pounder' ground out 22 wins before losing to 'MC Pee Pants' in 2007.

  • 15-pounder 'Humdinger' cruised thru a 23 match win streak in a one-year period spanning 2006 and 2007. Team mate 'Humdinger 2' retired with an 18 match win streak, going undefeated in 2008. Maybe he'll make a comeback?

  • But the current streak record belings to beetleweight 'Gutter Monkey', who put together an astounding 25 consecutive wins over six events in 2009 - 2010.
Middleweight robot 'Hazard'
Middleweight champion 'Hazard'
Other memorable streaks include lightweight 'Wedge of Doom' who ran up a 15 match streak that ended at BattleBots 3.0 with a loss to 'Gamma Raptor', and heavyweight 'BioHazard' who would have had a 20 match streak if the judges had given it the controversial decision in the championship match at BettleBots 3.0 against 'Son of Whyachi'.

Q: Mark, I know lighter weight is always a big disadvantage in Robotica (due to less traction it will provide), but what makes Deb bot go so far in season 2.0? It's literally a middleweight robot. [Chinese Forum]
A: [Mark J.] Several light robots did well at Robotica. Season 1 champion 'Run Amok' was nearly 40 pounds under the weight limit. 'Deb Bot' weighed less than half the max weight, but it had a small frontal area and a lot of power. Its small size allowed it to pick its way effectively thru the rubble in 'The Gauntlet', its nimble performance allowed it to pick out a few key obstacles in 'The Labyrinth', and its low wedge was able to get underneath a tall tankbot in 'The Fight to the Finish'.

Traction was generally difficult to find at Robotica. The special effects fog machines coated arena surfaces with an oily mist that caused trouble for many types of tires. The slick surfaces leveled the field for large and smaller 'bots -- you needed more than just weight to get pushing power.

Deb Bot - too light or too fast for Robotica?
'Deb Bot' at Robotica

While I was looking for video of 'Deb Bot' for the question above, I came across this video:

Alexander Kerr's 'Tribute to Run Amok - Robotica Season 1' (link corrected)

Never had my own video, and with an Ahmet/Dweezil Zappa soudtrack! Thanks Alexander...

Q: A general competition-related question, but besides Vlad the Inhaler were there any builders that entered under a nickname or alias? -- VestedMadScientist [Asheville, North Carolina]

A: [Mark J.] Builders are generally proud of their accomplishment and pleased to have their name associated with their robot. 'Vlad the Inhaler' wasn't actually trying to hide his identity at Combots Cup VII. Ownership of the entered 'bot was well known, so perhaps they were just having a bit of fun.

I don't know of other instances of a driver alias being used at a robot combat event, but it isn't something that I track.

Q: mark how the touro get its gyroscopic effect of spinnin? [Poona, Maharashtra, India]

A: [Mark J.] I'm not sure I understand the question. A vertical spinning weapon will exert a force that will attempt to lift one side of a turning robot. The more powerful the weapon, the stronger this gyroscopic force will be. It's not something you 'get', it just happens.

Q: hey hi mark how r ? lucky here i hav a doubt about supply input and output of motor . motor 's power is max at near about half of actual speed rpm so i thought couldnt we make make arrangement to make it to rotate at this rpm?
i am talking about ampflow e30 motor [Poona, Maharashtra, India]

A: [Mark J.] I'm rather sad, Lucky -- but thanks for asking.

I've previously answered your question. Search the Motors & Controllers archive for "typical PMDC" for the explanation of why this doesn't work.

Q: I noticed a lot of comments on the net thought The Big B was robbed of it's lightweight title during the [BattleBots] 4.0 final, my opinion is that Ziggo showed enough strategy to win that 24-21 decision. Your thought towards this fight, Mark? [Chinese Forum]

A: [Mark J.] I won't re-judge a match based on edited television video. I've been present at a number of matches that were later televised. It was easy to agree with the judge's decision having seen a match live, but working only from the video I might have come to a different conclusion. Unless I saw the match live, I'll always give the judges the benefit of doubt.

Q: hi mark if i want 5000 joules energy at 5000 rpm what thickness should be drum's outer dia and thickness if mass is limited upto 13 kg?
i am confused about it i cant exceed outer dia more thn 15 cm please help me [Tamil Nadu, India]

A: [Mark J.] There seems to be a lot of confusion about stored energy in rotating weapons. Stored energy in a cylinder rotating around its radius center is a function of:

  • Rotational Speed (RPM)
  • Drum Diameter
  • Drum Length
  • Drum Wall Thickness
  • Material Density

You've given me a desired output and only two of the five variables (rotational speed and diameter). By selecting values for material density and drum length, I can give a design solution for any wall thickness to meet your criteria. For example:

  • A steel cylinder 15 cm in diameter, 100 mm in length, and solid to the center will weigh 11.5 kilos and will store 5000 joules of energy at 5000 RPM.

  • A steel cylinder 15 cm in diameter, 300 mm in length, with a 6.7 mm thick wall will weigh 7.1 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 300 mm in length, with a 31 mm thick wall will weigh 9.6 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 2600 mm in length, with a 2 mm thick wall will weigh 6.7 kilos and will store 5000 joules of energy at 5000 RPM.

I suggest that you download the Team Run Amok Excel Spinner Spreadsheet to model the performance of various spinner dimensions and designs.

I also suggest that you browse thru the Robot Weapons archive for several recent posts about drum weapon design.

It is quite odd to specify a weapon diameter, speed, and energy storage as starting parameters, and to then back into the other dimensions. In particular, specifying such a high rotational speed is detrimental to the overall performance of a spinner weapon. Were it my weapon, I would design it to maximize energy storage at the slowest possible speed -- something like:

  • A steel cylinder 15 cm in diameter, 380 mm in length, with a 10 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 3760 RPM.

A 5000 joule weapon does you no good at all if it's spinning too fast to have decent 'bite' and the ability to transfer that energy to your opponent in a single, huge impact. Slow it down a bit. A larger diameter drum would be able to store the same energy at an even slower speed -- example:

  • A steel cylinder 20 cm in diameter, 390 mm in length, with a 7 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2780 RPM.

Or even better:

  • A steel cylinder 25 cm in diameter, 360 mm in length, with a 6 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2175 RPM.

Don't compromise weapon performance with dimensional restrictions that you can avoid, and design for the lowest weapon speed that will store enough energy to be effective.

Q: Then another question about your team in Robotica. Who would be the bigger threat to 'Run Amok' in the Fight to The Finish: 'Mini Inferno' or 'Killer B'? From your previous answers it looks like Jason's bot would be much less dangerous, I think. [Chinese Forum]

A: [Mark J.] Jason Dante Bardis was building a 'bot from scratch to compete at Robotica, but he ran out of time and decided to substitute a very light 'bot that was essentially the chassis for 'Dr. Inferno Jr.' with a carbon fiber wedge added. Although the drivetrain was very powerful (four DeWalt 18v gearmotors running at 24 volts), the robot weighed weighed only 37 pounds. At the weigh-in, Jason walked onto the scale carrying 'Mini Inferno' and was still under-weight! Powerful though it was, Mini just didn't have enough pushing power to be a threat against any of the robotica finalists in a sumo match.

Jason did finish his original Robotica entry and fought with it at BattleBots seasons 3.0 thru 5.0 with limited success. He remarked on a forum post that after he finished 'Towering Inferno' he realized that it would not have been a successful Robotica competitor.

'Killer B', on the other hand, was a very dangerous sumo competitor. Had they put the blunt end of the robot into use instead of the wedge end, they could have used their powerful drivetrain to simply shove 'Run Amok' off the platform. Lucky for me, they didn't choose that option.

Q: From your point as a Robotica reigning champion, do you think 'The Killa Gorilla' (aka 'Silverback' in Battlebots) built by Rob Farrow is a good design to compete in Robotica, Mark? Because from the only episode in the net including it (2.0 Final) it seems like it's very hard to drive,and will fall off platform by mistake very easily. And which kind of stage layout is more challenging - the new format in later seasons or the 1.0 format in which your team competed? [Chinese Forum]

A: 'Robotica' called on competitors to answer several differing performance challenges, making it difficult to put together a single optimal design. Rob chose a short wheelbase and a responsive, torquey drivetrain to overcome the 'Gauntlet' and 'Labyrinth' challenges. Although those design choices made the 'bot difficult to control, its maneuverability and power made it a strong competitor in the non-sumo parts of the competition.

I didn't have the opportunity to drive the courses for Robotica 2.0 and 3.0, but given the success of competitors returning from Season 1.0 I'd have to guess that the level of challenge was similar.

Killa Gorilla - got banana?
Killa Gorilla

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.

More of Aaron's Poems

Aaron's Minecraft High Dive Video

Killer Robot drawing by Garrett Shikuma

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