6316 Questions and Answers about Combat Robotics
from Team Run Amok

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

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

Last month marked the 15th anniversary of the 'Ask Aaron' website. We celebrated by sponsoring a Combat Robot Mad Lib Contest with an unbelievably huge prize!

Congratulations to Ian McMahon for his winning entry! I can't wait to use it. Jump to our 15th Anniversary Contest Page to read all the entries.

Q: So I was wondering how vlad did so well. I mean most lifters don't do that well without another weapon. Is it the power of the lifter or the way the driver drives or Something else? (No offense to lifters.) Thanks! [M] [Lynn, Massachusetts]

A: [Mark J.] Two things:

  1. Watch some videos of 'Vlad the Impaler' and you'll see that it was not as much a 'lifterbot' as it is a powerful 'rambot' that happened to have a small lifter.
  2. Whaddya mean 'lifters don't do that well'? At the time Vlad was fighting (its last fight was in 2001) lifters were very successful. Ever hear of 'BioHazard'?

Q: So I saw overdrive S1 had some technology in it's wheels. What does this do? Thanks! [M]

A: Series 1 'OverDrive' was powered by direct-drive electric bicycle brushless hub motors, which place the entire driveline into the wheel. This greatly simplifies robot design and speeds construction, but also limits the ability of the builder to optimize the drive system.

Although the motors were placed in the smallest practical wheels, the drive provided considerably less torque than conventional combat robot drive systems. For series 2, a completely new version of 'Overdrive' wisely reverted to conventional drive motors.

Q: Do amp flow motors only work with amp flow escs? [Arlington, Virginia]

A: [Mark J.] AmpFlow motors may be controlled with any brushed motor ESC of suitable voltage rating and current capacity, and AmpFlow ESCs will operate any standard brushed motor within their current and voltage rating.

Q: Can you make a calculator for how much current a given motor will draw at a given load?

A: Permanent magnet direct current brushed motors have a linear relation between torque output and current. If you know the torque output required, the formula is very simple:

Current Required = Motor Stall Current * (Torque Required / Motor Stall Torque)
If you're building a combat robot drive train and don't know the torque output needed, the Tentacle Torque Calculator will evaluate the motor, drivetrain, and chassis elements for you and will model robot performance -- including maximum current draw. An example drive train analysis using the Tentacle Torque Calculator is available here.

Q: Would the ampflow a28 400 running a small disc be appropriate for a lightweight? If so, would the RageBridge 2 be a good weapon ESC?

A: Several comments:

  • I cannot make a weapon motor recommendation based on a weight class and a general description of the weapon type. We offer a collection of design tools so that builders may perform their own evaluations of combat robot system. These tools include two different programs that will model the performance of specific spinner weapon systems. You will do well to learn to use these tools.
  • You will also benefit from reading our Spinner Weapon FAQ to gain an understanding of the principles of spinner weapon design and performance.
  • The RageBridge 2 is a dual-channel brushed controller designed to control drive motors on opposite sides if a tank-steer robot. The special features it offers are intended to support drive train applications. Using it to run a single weapon motor would be a waste of its features.
  • The A28-400 AmpFlow motor is a big, heavy brushed motor -- less than ideal for a 'small disc' on a lightweight robot. Current weapon design favors much lighter brushless motors that are well suited to spinner weapon applications.
So... I'm gonna say 'no' and 'no'. Stop guessing and do your design homework.

Q: What makes the RageBridge 2 so good for lightweights? It’s amperage rating is so tiny.

A: Previously answered. Search the Ask Aaron Motors & Controllers archive for 'Ragebridge'.

Q: Would a sabertooth 2 x 60 driving 2 amp flow e30 150 motors be adequate for a lightweight? [Dallas, Texas]

A: [Mark J.] Motor current draw depends on voltage, wheel diameter and gear reduction ratio as well as motor type and robot weight. The Tentacle Torque Calculator can evaluate these factors and provide an estimate of maximum current draw as an aid in motor controller selection. It will also model the speed and accelleration of your design. Learn to use this valuable tool.

The Sabertooth ESCs have a poor reputation in combat applications. Builders using them have reported sudden and unexplained complete failure of the controllers following a high-energy impact. Although they work well in general robotics I no longer recommend their use in combat robots. In this power range I recommend the RageBridge 2.

Q: You've got a recent post about a rubber band powered flipper ['Millitant' - five posts down the page] that looks interesting, but I don't understand how the servo release mechanism works. Can you walk me thru that? [Some guy that cornered me in a alley]

A: [Mark J.] I made an animation that should help. The flipper arm pivots on an axle that is seperate from but coaxial with the servo output. The green line is the elastic band (or spring). The red rectangle is the servo arm. The servo itself is not shown so you can better see the release/reset action.

  • To trigger the flip - the servo arm moves downward from its rest position to contact a metal pin extending from the catch arm.
  • The plastic catch arm deflects downward, releasing the flipper.
  • To reset the flipper - the servo arm moves upward to contact a metal pin extending from the flipper arm.
  • The servo continues upward, pulling against the elastic/spring and rotating the flipper arm back to the latched position.
  • The servo arm then returns to the rest position, ready to trigger another flip.
The mechanism is simpler than a snail cam, but the force placed on the servo increases greatly as it approaches the latched position. Moving the servo off-axis and adding a curve to the servo arm could help correct this drawback and reduce the required servo force.
Q: Hey, man! It's been quite a while since I last posted here, huh? Welp, I'm back with a bit of a featherweight conundrum. To give you the long story short [You call this 'short'?] I went to EOH [the Engineering Open House] at the U of I [University of Illinois] in March and watched the featherweight competition [Robobrawl] they had. After doing so, I've been thinking about getting a team together to compete for next year. However, there's a catch: I don't want to pitch anything at them, guns blazing: I want to have most, if not, all of the specs down and ready to present to a group of people as a document and a presentation (if needed) in order to heavily streamline work or maybe even get it done before the competition (seeing that the next competition isn't until March 2019, I can say I've got a bit of time on my hands). Not only that, but it will help our team get the money to participate both through iRobotics [website] (which gives a starting budget of $2500) and through local, smaller businesses that may want to help out.

My idea is to make a featherweight full-body spinner similar in shape to that of 'Ziggo' (the idea was that it would be themed around a land mine) or the only FBS that was a featherweight I could recall off of the top of my head: 'Badger' (from Australia).

There are five things I came here to ask, which I will lay out for you below:

First What would you recommend for weight distribution? Given that a FBS has the ability to add more weight to its weapon because the weapon is also the armor, what percentage should I try to aim for? Following along the 30-30-25-15 rule, it would result in the robot being able to use up to 55% of its weight towards the weapon and armor. However, that would mean the shell would weigh in at 16.5lbs, which I fear would be WAY too much for the robot. Using only 30% seems to small for a FBS becaus e that means the shell weighs in at only 9lbs. The largest spinner at EOH had a bar made of S7 Steel that was 8" in diameter and weighted in at 8lbs, and since I assume a full-body spinner would be a bit larger than a bar, that 9lbs would be a bit on the weak side. 40%-45% would put it at 12 and 13.5lbs, respectively, and I feel that those numbers are better than the other ones mentioned. However, I still wanted to ask you in advance on what you thought I should do.
Second I was trying to calculate the minimum amount of energy storage the robot should be able to hold. Going off of your statement that for every Kg of mass, the robot should be able to store 60j of energy, I crunched the numbers for the approximate weight in Kg of the robot: 13.6, 13.63, and 13.7Kg, respectively. I found that the ABSOLUTE MINIMUM (without multiplying by 2, as you recommend) would be 816j and the ABSOLUTE MAXIMUM would be 1644j. Here is the question that I have about these numbers: if my robot is, for example, storing 1644j in its shell, will the amount of force itself affect driving capability or would it have more to do with the shape and mass of the robot? Also, do you think 1644j is too much or too little for a featherweight FBS?
Third I was wondering what material you'd recommend to use. From what I was told, the main building block of robots that compete at EOH is Aluminum of a wide variety of grades. I was thinking to use something along the lines of either 6061-T6 Aluminum, 5083-H131 Aluminum, or 7075-T6 Aluminum. Which grade of Aluminum do you think would be the best bet? If, by some chance, we were able to stretch our budget and were able to buy Titanium or had enough weight to go for Steel rather than Aluminum, which grade(s) would you recommend?
Fourth Do you think it would be better to have bolted or welded teeth? I know this would vary depending on what material was chosen for the shell, but I'm afraid of having teeth that constantly need bent back into shape. I know the newest iteration of Captain Shrederator uses bolted teeth, while Megabyte uses welded teeth. Under what circumstances should I use one or the other, and which one do you think I should go for?
Finally How should testing be done? I'd want to be able to test driving with the robot both with and without an active weapon, but I have no clue what kind of area would be suitable for a 30 pound FBS.

I know that that's a lot of info, and some of the info is missing because an exact size hasn't been chosen yet, but I hope you can help me out! As always, if there's anything else you would recommend I am always happy to hear your advice! Thanks again for the advice and all of your help! [Champaign, Illinois]

A: [Mark J.] New question length record - 847 words I think I can answer your questions more concisely.

The featherweight FBS that you should 'recall off the top of your head' and emulate should be:

Combat Robot Hall of Fame - Honorable Mention 2015

Tetanus / Triggo - Team Brain Damage owns the featherweight class in the northeast US. Full-body spinner 'Tetanus' went 21-2 with five titles before being updated with a stronger shell, renamed 'Triggo', and adding on a 20-6 record and three more titles to date.

Go to the Team Brain Damage website and read all three of the build logs for 'Tetanus'. Zac understands FBS design very well and his build logs will answer most of your questions.
  1. Full Body Spinners commonly place 50% or more of their weight into the weapon shell and drive system. It isn't too much. If you're gonna go for a big weapon you should go really big.
  2. FBS are all about energy storage. I would say that 1600 joules for a feather FBS is conservative. Keep the center of gravity low and you should have no driving problems.
  3. Shell material is not critical - 'Ziggo' used a modified cooking pot. For a feather I'd suggest making it easy on yourself and use steel; chromoly is tough, widely available, and inexpensive.
  4. At this energy level you can get by with welded teeth if using a steel shell. For an aluminum shell, recess and bolt steel teeth thru the shell to a backing plate.
  5. Safety first! Check with iRobotics to see if they make their Robobrawl arena or another facillity available for testing -- they should.
  6. Zac's first FBS was unsuccessful. Most FBS have losing records -- they are not an easy style of 'bot to build. I don't recommend an FBS for your first move into heavier weight classes.
  7. Landmines are not amusing. They maim and kill innocents. Pick another theme.
Update - the links to the 'Tetanus' build logs on Zac's website are down, and they are not available on the Internet Archive. I've written to Zac to see if he has that info available someplace. Stay tuned.

Q: So I had a simple (or I think simple) idea of a bot that had a rotating spike. Is this a good design or does it need some work? By the way ty for wandering all of my questions! :) [Lynn, Massachusetts]

A: [Mark J.] I'm certain you have a clear idea in your mind of 'a bot with a rotating spike', but your description leaves too much to the imagination for me to share in your idea. Read: The Hamburger is Bad.

Q: Heres a cad of the idea. The spike is the spike that spins. Is there anything you would add to make it better? Thanks! :)

A: So... it's a spike... on the front of the robot... that spins like a drill bit.

I don't understand what you expect the spike to accomplish, and I don't understand why you think spinning the spike helps. The spike WILL help wedges to slip under your bot, the spike point IS highly vulnerable spinner bait, and a sharpened point - spinning or not - will do no damage to any 'bot armor.

My suggestion to make the 'bot better is to remove the spike and rotating mechanism and use that weight to make a stronger wedge.

Here's the cheerleader button if you need to press it:

Q: Hello Aaron. It’s my first time in your wonderful world of robots, and I hope you can help me with a problem.

I have a Sabertooth 2x32 controller attached to two 200 watt motors. The system runs on 24 volts and is controlled by a wired analog joystick. The thing is that the motors are running weaker and slower than I expected. Checking the output from the controller to the motors with a DVM shows 12 volts instead of 24 volts. I checked the parameters with the software, and everything looks OK, but maybe I overlooked something?

The joystick signal is 2.5 volts in the center, 5 volts at full forward, and 0 volts at full reverse. I don't know if that's OK or not - I thought it should give 5 volts in every direction.

I appreciate your help in figuring this out. Arik [Tel Aviv, Israel]

A: [Mark J.] Hi Arik. The Sabertooth 2x35 controller is made for general robotics applications; its functions are much different than the dedicated R/C Sabertooth controllers used in combat robots. Still, I may be able to help find a solution to your problem.

  • Your joystick signal voltages are correct - zero to 5 volts with 2.5 volts at center is what the Sabertooth expects for analog input.
  • While you have your Digital Volt Meter handy, verify that the B+ to B- terminal voltage on the Sabertooth is 24 volts. If you wired two 12 volt batteries in series you will have 24 volts, but if you incorrectly wired them in parallel you will have only 12 volts. Please check.
  • Check that the DIP switch settings on the Sabertooth are correct for bi-directional analog control: switches 1, 2, and 6 must be 'ON', while the other switch positions vary with your requirements. See your Sabertooth 'Quick Start Guide' for settings.
  • In analog mode, a voltage input to the 'A2' connector on the Sabertooth sets a maximum output to the motors. Nothing should be connected to 'A2' for your application.
  • If you have used the 'DEScribe' software interface to adjust controller settings you may have incorrectly set the 'calibrate' function. Restore all the parameters to default settings.
If none of that gets you anywhere, it's time to contact support at Dimension Engineering.

Q: Hi Mark, thanks a lot for your last answer! I sent the question to the controller company support, hope to find an answer shortly.

I have another question. The platform that I am designing must carry in total 160 kg, and must move at up to 18 km/h. The system supplies 24 volts to two motors (200 watts DC brushed motors for now). The motors drive the wheels with a belt direct from the motor shaft in a ratio of 7 to 1. I don't know the motor torque. Are the motors OK for my design or do I need to find new motors? If so, what do you recommend?

A: Without full motor performance specifications I have no way to calculate the performance of your platform when powered by your proposed drive train.

With complete motor and drive train specifications, the Tentacle Drivetrain Calculator can provide a very good estimate of performance on flat terrain. For example, a pair of 200 watt motors with a no-load speed of 3500 RPM and a stall torque of 0.55 N·m driving a 160 kg platform via 8" diameter wheels thru a 7:1 reduction has a calculated top speed of 19 km/hr, but acceleration is so poor that it would require a run of hundreds of meters and several minutes to achieve that speed. Further, the platform would lack the capacity to climb even a very slight incline. A total of 400 watts motor power is MUCH less than you will need to achieve reasonable performance, and your motor controller may need to be upgraded as well.

'Ask Aaron' specializes in questions about combat robots, which have very different requirements from the platform you are building. We are not a free engineering service. I think you will do better to find a general robotics forum to supply advice for your project.

Q: Am I allowed to ask as many questions as I want (That are robot combat related)? Thanks! :) [Lynn, Massachusetts]

A: [Mark J.] You may ask as many questions as you like, and I will answer as many as I please.

There are a few off-limits topics, identified on the page you see after submitting a question. There is more guidance in our mission statement:

Mission Statement
  • The Ask Aaron site exists to support builders of combat robots with information, design tools, and advice based on our robot competition experience. We are not a free engineering service, and we won't do your homework for you.
  • As a secondary service we are pleased to share our knowledge about what goes on 'behind the scenes' and 'in the pits' at competitions, our views on issues important to the sport, and information we have uncovered while researching the history of robot combat.
  • Questions on robot topics outside these areas will be answered on a lower priority basis.

Mouseover to zoom
Q: Hello. Glad to see the site is still going strong.

I am in the process of building a beetleweight robot, and I decided to try making a rubber band powered flipper. I have been insipred by a mechanism used by a UK antweight named 'Militant', as well as footage of a couple of Lego robots that weigh nearly as much as a beetleweight.

Essentially, I'm aiming to build a beetleweight replica of Robot Wars series 10 champion, 'Eruption'.

I have a servo which has 33kg/cm of torque, which I'm hoping will be enough to crank the flipper into position. The same servo will also be linked to a release mechanism which will fire the flipper.

I have a 2kg (4.4lbs) box of dumbbells ready as my practice opponent, thinking that if it can flip this, it can flip other beetles easily. (I know it sounds excessive, but I couldn't find anything weighing exactly 3lbs that would be suitable for the job.)

My questions are:

  1. How many rubber bands will be required for the flipper?
  2. Do I have the sufficient amount of torque required to crank the flipper in place?
  3. If rubber band power isn't strong enough, will the servo itself be enough to be used as a quick lifter instead?
[Perth, Australia]

A: [Mark J.] G'day, Perth. I'm sorry, but 'The Hamburger is Bad'. The answers to each of your questions depend on details of your design that you have not given: layout and dimensions. I think I can give enough guidance for you to evaluate your progress as your design develops. I'll address your questions in reverse order, as the discussion of the later questions helps to explain the earlier questions.

  1. Assuming that you have a simple lever arm attached to your servo, the lifting capacity decreases with the length of the arm measured from the servo axis. Quick torque calulations show that a servo capable of 33 kg-cm torque at stall could lift a 3-pound load out at the end of a 4.75" arm while maintaining half of its no-load speed. A 3-pound load on the end of a 9.5" arm would stall the servo and you would get no lift.

  2. The force required to stretch a rubber band increases linearly as the band is stretched -- up to a point. Assuming a design like 'Militant' where the servo is coaxial with the flipper pivot, the torque required by your servo will be the force supplied by the bands at the 'latch' point of the flipper times the distance from the servo axis to the band attachment point on the flipper arm.

    Example - If your rubber bands exert a 10 kg force when the flipper arm is latched, and the distance from the flipper pivot to the rubber band attachment is 2.5 cm, the torque required by the servo to latch the arm will be 10 kg * 2.5 cm = 25 kg-cm. That's far enough below your servo's 33 kg-cm stall point to achieve reasonable reset speed and keep the servo healthy.

  3. Rubber bands come in all sorts of sizes; width/length/thickness. Different sizes will give different force profiles. The better question is, "How much elastic force is needed?" With a flipper arm as shown in the photo, the force out at the end of the flipper arm is:
    Elastic Force * (Pivot to Band Attachment / Pivot to Flipper Tip)
    Example - Bands exert 10 kg force at latch; the band arm is 2.5cm, the flipper arm is 10cm. Force at the end of the flipper arm is 10 kg * (2.5 / 10) = 2.5 kg.
Is that enough to deliver a good flip? There are variables I left out of the calculations (like elastic pre-load) to keep this from becoming a graduate level thesis, so my best advice is to build a simple mock-up of the flipper arm and play around with critical lengths and forces to see what it takes to deliver a good flip, then run the numbers to see if you can reset it with your servo.

Note - Don't copy 'Militant' too closely; it has far too little weight on the drive wheels.

Q: I am currently working on my next hobbyweight and have some questions on press fitting. I recently landed a job at a CNC shop and am able to achieve tight tolerances. I am wanting to press fit my pulley onto my motor for the weapon system. I previously used set screws, which worked most of the time but also failed, as set screws often do in this sport. I have watched several videos about press fitting , but (inconceivably) they weren't combat robot related. My question is, is press fitting the best way to go for attaching pulleys? Or will it not hold up?

BTW I have asked you several questions over the last few years and your answers have been spot on. I truly appreciate the time you take to offer us advice. [Albany, Oregon]

A: [Mark J.] Thanks, Albany. I got a lot of help from some very generous people when I started to build combat robots, and I'm just trying to pay back my debt to the community.

You and I have previously discussed the merits of non-circular shafts and matching hubs for small shafts, as well as clamping collars and the problems with set screws. Readers can catch up on that post in the Ask Aaron 'Ants, Beetles, and Fairies' archive (link).

I'm not prepared to say that an interference 'press-fit' is categorically the 'best' method for attaching a hub to a small motor shaft. Press fitting is something of a black art, particularly then using dissimilar materials like steel into aluminum. You can try an on-line calulator to assess the torque capacity of a specific press-fit junction but, as you point out, combat robots place odd and unpredictable loads on structures.

I'm willing to say that I'd much rather go into combat with a correctly press-fit pulley than with a set-screw pulley. The only down-side I see is the difficulty of pulley replacement in the pits if you do have a failure at an event. Having spare complete motor/pully assemblies in your kit would be the solution.

I've used a lot of press-fit gears on small motor shafts over the years and never had a failure. Make up a hub and do some testing.

Q:I'm having trouble with a pair of 550 series motors I purchased recently. At full throttle the motors spark and sputter with the power dropping off and surging. They do this even when powered directly from the battery (3-cell) so it isn't the electronics. I've never had this trouble before. What's going on here? [Northern Ohio]

A: [Mark J.] This sounds like motor brushes that aren't seated properly. Inexpensive brushed motors like your 550s are manufactured by several different sources, so even if you're buying from only one distributor you may be getting variation between one batch and the next. The care taken by the manufacturer to assure that the brushes are correctly contoured to match the commutator is one of the things that can vary.

Prior to subjecting to full power, brushed motors (particularly small, inexpensive ones) should be 'run in' at low-speed/no-load to allow the brushes to wear in and match the curve of the commutator. Failure to do this may result in electrical arcing that can damage the brushes and commutator.

  1. Put a small drop of lubricating oil on the motor bushings/bearings.
  2. Secure the motor and apply about 1/4 to 1/3 its rated voltage.
  3. Allow the motor to run unloaded for 10 to 15 minutes. Monitor for heat build-up and pause for cooling if needed.
  4. The motor speed should be constant and stable at the end of this period - if not, continue until it is.
  5. Reverse the polarity and run in the opposite direction for 10 to 15 minutes, as above.
A properly run-in motor will run cooler, produce more power, and be more reliable. It's well worth the time and effort.
Q: Hi Mark,

I was doing calculations for our competition and was wondering about the relationship between bite and energy storage. What is the tipping point for bite to energy storage? For our 5 inch radius spinner we can spin it to 16,000 rpm with a bite of 5 mm at 12,000 joules stored. We can also spin it to 8,000 rpm with a bite of 11 mm with 3,500 joules stored. At what point is bite more important than energy storage, and in what situations? Does this change for bot designs and weight classes?

Many Thanks. [Pittsburgh, Pennsylvania]

A: [Mark J.] Take a look at the 'Rotational Speed' section of the Ask Aaron Spinner FAQ. The 'bite' required is highly situational, and it does depend on your weapon design, your opponent's design, and the influence of the strategy employed by both you and your opponent as it effects the true maximun closing rate of the 'bots. As it says there:

There is no single 'right balance' of bite and speed for a given weapon. The balance is situational:
  • Fighting a hard-surfaced opponent with no sharp edges calls for all the bite you can muster.
  • A drum head-to-head against another drum requires maximum speed and can dispense with bite.
  • Small arenas and close fighting call for big bite, while larger arenas and higher closing speeds need less.
  • When your opponent has only soft exposed surfaces it may be better to ignore 'bite' and switch to sharp 'shred'.
  • If you have a ramp to help your vertical spinner get a shot at your opponent's sharp front under-edge you need very little bite.
Stay flexible. Design for ample bite and adequate energy storage, keep some extra RPM available for special cases, and be prepared to throttle back your weapon if it's just 'skittering' across the hard surface of your opponent. Consider swapping in a sharp edge blade for 'soft' opponents.
You'll also want to read this post in the Ask Aaron Weapons Archive that discusses design effects and special situations for 'bite' adjustment.
Q: Does the shape of the bar on a bar spinner make any difference? Thanks! :) [Lynn, Massachusetts]

A: [Mark J.] Yes, shape and size make a huge difference in spinner weapon performance. Read the Ask Aaron Spinner Weapon FAQ and then play with the Spinner Weapon Kinetic Energy Calculator. You might also be interested in the extended discussion I had with a builder designing a large bar spinner: Texas Bar Spinner.

Q: Is there a way to vote for Robot combat hall of fame robots?

A: Yes, see this post in the Ask Aaron archives.

NameWeightWin Rate
Tombstone250 lbs91%
Tombstone340 lbs64%
Last Rites220 lbs61%
The Mortician120 lbs60%

'Original Sin'

Q: So you said simple robots win. What are some examples of simple robots that did well? Heres one I like... [Lynn, Massachusetts]

A: [Mark J.] 'Tombstone' is likely the simplest 'bot at the BattleBots reboot and it has the best record at those events -- but the producers of the show only accept complex robots with big weapons that generate a lot of destruction for the cameras. Truly simple robots aren't allowed to compete, so this can't really be considered 'open' competition.

The 250-pound version of 'Tombstone' does very well against the artificially complex robots at the new BattleBots: a 91% match win rate! However, Ray Billings' other bar-spinner robots don't have the same success rate against more competitive 'real world' combat robots.

Why not as good? Because simple robots are tougher competition than complex showboat 'bots. Which heavyweight robot has the best record against 'Last Rites'? That would be an even simpler robot: the fearsome wedgebot 'Original Sin'.

  • 64 wins and 15 losses overall (81% win rate);
  • 12 wins and 7 losses vs. Last Rites (63% win rate);
  • An unmatched seven RoboGames heavyweight titles;
  • An unmatched four ComBots Cup championships; and
  • Full membership in the Combat Robot Hall of Fame.

    Simple 'bots win

Note - 'Tombstone' is actually a deceptively simple robot. The design appears to be straightforward, yet numerous builders have tried to copy Ray's bots with very little competitive success. There is more to the Team Hard Core 'bots than meets the eye.

Q: So I'm only a kid and I love deigning combat robots but I don't have the time, parts, money, parents permission, etc. to make one. Is there any other way I can find my way into robot combat? [Lynn, Massachusetts]

A: [Mark J.] There are some options:

  • Consider joining the 'Combat Robotics' group on Facebook. I'd suggest just lurking and soaking up some of the information that flows thru the group. Don't believe everything you hear there. You may find out about upcoming events in your area.
  • If you can get to a robot tournament, do it! Being there is a whole different experience from watching the videos. You might like it a lot, or it might scare you off. Either way, you'll find out what combat robots are really like.
  • If you have access to a Windows computer (I think you're on a Mac) you can run Robot Arena. Design and build unique combat robots limited only by your imagination, then fight them against other robots in wild and imaginative arenas. Play against A.I. opponents, go on-line to challenge your friends, or compete in open tournaments.
Aaron and I played the older 'Robot Arena 2' for countless hours. I haven't tried the newer 'Robot Arena 3' that was released in 2016, but it seems to be at least as much fun. At about $20, it's a whole lot cheaper than having your real robot torn to bits.
Q: So I had an idea for a bot that is baisically a full body spinner mounted on a wedged body. You said simple designs win. Is this a good idea or totally dumb? [Burlington, Vermont]

A: [Mark J.] There is a lot of space between 'good idea' and 'totally dumb'. People build combat robots for all sorts of reasons:

  • Some enjoy fanciful designs;
  • Many want to impress their fellow builders;
  • Others like to show off their machine skills;
  • Lovers of destruction enjoy building huge weapons as dangerous to themselves as to their opponents; and
  • A precious few actually want to win matches and maybe even a tournament.
'Ask Aaron' is dedicated to that last group -- builders who accept the challenge of building machines that will win combat matches.

I have evidence that simple robots win. In 2006 and again in 2016 I sifted thru the results of robot combat tournaments and compared the results obtained by robots with different categories of weapons. You can read the results here. The results may surprize you.

Build what you like, but if your idea of a 'simple robot' is to take a deceptively complex robot of a design that rarely wins and mount it on top of another style of robot that nullifies the only advantage the weapon has, you're asking for advice at the wrong place.

Now press the cheerleader button; it'll make you feel better:

Q: What if a bot had a hammer but the blade could split in half and turn into a spinner? Would this be a good idea? What are some of the pros and cons about this? By the way thanks for answering my Bronco pneumatics question! Thank you! :) (M) [Lynn, Massachusetts]

A: [Mark J.] You may not have thought this through. Perhaps I don't fully understand your design, but it makes very little sense to me.

  1. A conventional hammerbot swings an arm with a weight on the end in a half-circle arc from the rear of the 'bot to impact (hopefully) your opponent out in front. If you miss your opponent, the hammer hits the arena floor. How exactly do you split that hammer arm in half and have enough clearance for the resulting weapon to spin without hitting the floor?
  2. Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

    Design Philosophy

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

  3. The three primary types of active weapons have very different design priorities to accomplish their tasks. Compromising the weapon drive system to bridge the needs of two weapon types will result in a weapon that is not good at either task.
    • A spinner stores kinetic energy in rotational inertia that is transfered to the opponent on impact. That energy is accumulated by the weapon as it spins up to speed over some period of time. More energy storage is generally better, but there is a trade-off between increasing rotational speed and the ability to obtain enough 'bite' to effectively transfer that energy.
    • A hammer also stores kinetic energy as it accelerates from its resting position toward impact, but it has much less time-distance to accumulate that energy. It's nigh impossible to get levels of energy storage in a hammer weapon comparable to a spinner weapon. Get as much as you can and hope its enough. Heavyweight hammerbot 'ßeta' manages about 7 joules per kilogram.
    • A flipper does not store kinetic energy to impact the opponent. The flipper mechanism applies force generated from stored potential energy directly to the opponent to accelerate them upward. Measuring the 'static' force of the system does not equate to the net force applied while the system is in motion.
  4. Finally, do not overestimate the importance of the weapon system.

    Aaron's Wisdom  I've said this often but builders don't want to believe me:

    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.

    Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: So I had an idea that I have no idea if it is good bad or dumb. A spinner that can change from vertical to horizontal. If this were to compete, what would it's advantages and disadvantages be? Thanks!

A: Let me say this again:

Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

There have been many attempts at robots with multiple/interchangeable weapons, and very few have done well. You may be interested in reading up on 'Bombshell' -- a heavyweight robot designed around a modular interchangeable weapon set. 'Bombshell' was constructed by a large, well-financed, very experienced team -- this is not an undertaking for an inexperienced builder.

Build a simple robot, strong enough to hammer down a door. If a part breaks, make it stronger for the next fight. Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose.

Twin Broncos

Q: How does 'Bronco' make his pneumatic flipper so powerful? I heard pneumatics are normally slow and not that powerful. Thanks! :D (M) [Lynn, Massachusetts]

A: [Mark J.] You heard wrong. Inertia Labs is one of many teams on both sides of the Atlantic that have been building lightning quick and terrifying pneumatic flippers, hammers, and crushers for the last 20 years.

I'll warn you that performance like you see in 'Bronco' does not come from 'off-the-shelf' components. Most if not all of the pneumatic components in the twin Broncos (yes, there is a complete, ready to run, spare 'Bronco') are custom made at no small expense. Pneumatics are dangerous even if you know what you're doing. Don't try pneumatics on your first robot... or your second... and probably not your third.

Q: How did 'Pork Chop Jr.' end up fighting in China? What happened to it there? [East Texas]

A: [Mark J.] After losing all of its matches at Rocket City Robot Assault, 'Pork Chop Jr.' fled to India where it competed in cage matches for thousands of rupees. After crippling a professor at IIT Goa, it got a job on a Chinese TV show where Zhang Yimou drove it directly into the arena hazards. Depressed and now missing a wheel, it went drinking at a disreputable KTV where it met a girl from Xinjiang. Accompanying her home, 'Pork Chop Jr.' settled down to life as a goat herder.

See also: The Many Lives of Pork Chop Jr.

Q: Hello! I’d love to know what motor(s) did Paul’s bot “Brutality” from Aptyx Designs used, and how was it configured? [New York]

A: [Mark J.] The heavyweight version of 'Brutality' was thrown together on short notice, largely from spares for the earlier middleweight 'bot of the same name. Paul Ventimiglia was with Team Demolition at the time, and the 'bot was later campaigned by Paul for WPI Robotics. Aptyx Designs is a more recent team created by Paul for the BattleBots reboot.

Brutality's drive motors are short MagMotors connected to Team Whyachi parallel-shaft gearboxes powering the front wheels. A long chain runs back on each side to drive the rears.

The weapon is powered by two AME D-Pack motors driving a right-angle Team Whyachi M3R2 gearbox up to a final stage spur gear at the blade. Similar in size and performance to the AmpFlow E30-400 motor, the inexpensive AME D-Pack was widely used for weapon power until supplies dried up several years ago.

'Thank-you' to Paul for quickly returning my message and verifying what I gleaned from the photo -- he's pretty busy right now.

Q: how would a circuit of a rotating weapon robot with a brushless motor in the weapon and two DC motors in the wheels? [Michoacan de Ocampo, Mexico]

A: [Mark J.] See Frequently Asked Questions #19.

Q: as would be a circuit of a combat robot with a brushless motor on the weapon and 2 engines brushed on the wheels.[Michoacan, Mexico].

A: If the diagram in FAQ #19 isn't what you're looking for, I'm not sure what you want. Maybe this will help:

Image courtesy xkcd - Animation by Team Run Amok

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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