5750 Questions and Answers about Combat Robotics
from Team Run Amok

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

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
In Memorium: Aaron Joerger, 1991 - 2013
The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary
- Mark Joerger   
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Preface: [Mark J.] I apologize in advance for the following set of three posts. They all have sort of a 'hater' theme and were all waiting in my inbox this morning. I considered trapping and releasing them to wander free over at reddit where the haters party all day, but I went ahead and answered them. I'm kinda sorry that I did.

Q: Should I be embarrassed for loosing to [name of furry antweight redacted]? [Arden, North Carolina]

A: [Mark J.] Why would you be? At its most recent outing [name of furry antweight redacted] marched thru the field and made it to the semi-finals. If you lost because you showed up with an unprepared robot then that's about you, not your opponent.

Q: Do you think all the hate towards 'Chomp' is kinda justified? I mean, I can think of arguements both ways... [Walnut Creek, California]

A: [Mark J.] Some people are so narrow-minded as to have a visceral hatred of anything that does not match their concept of 'how things oughta be':

  • A woman shouldn't front a robot team...
  • Combat robots shouldn't be tall and narrow...
  • Robots that don't match my design ideas shouldn't win matches...
  • Stop doing things that challenge my world view 'cause that makes me feel inadequate!
I first traveled to England compete at Robot Wars in 2001. At the time the show had been shown on only a few PBS channels in the US and I was not at all familliar with the competitors. I was walking thru the pits when I came around a corner and saw something very odd sitting on a work table. It was tall with a small 4-wheel base, and it had a little sawblade-like spinner way up at the top where it couldn't possibly do any good. The whole thing looked like it would fall over as soon as it moved. How could this be a combat robot? The next day I saw 'Pussycat' fight and everything made perfect sense.

Judge on performance, not on paradigms. Stop the hate.

Toy based on Robot Wars competitor 'Pussycat'
'Pussycat' Toy

Q: Whatever happened to the robot Run Amok? He never appeared after a while. I heard that he got depressed and jumped off a the golden gate bridge, but except unlike all the other robots who jump off and instead land on a cruise ship and wind up starting a new life, he just fell into the water and died. [Walnut Creek, California]

A: [Mark J.] Unlike the many robots that fought once, lost, and were never heard from again - 'Run Amok' had a celebrated career and is now retired.

Q: I don't see how you interpreted my comment about [name of furry antweight redacted] as "hate", though I can definitely say that text doesn't really convey emotion much.

A: [Mark J.] Wondering if you should be embarassed by losing to the specific robot mentioned implied that [name of furry antweight redacted] is itself an embarassingly poor robot. Granted, the robot in question is a 'novelty' entrant, but I don't like picking on someone who may not be here to defend themself. I might normally have let it go, but your question suffered from arriving at the same time as the others in today's group -- which I do think are 'hateish'.

Q: Hi. I'm soon going to compete at a ant weight competition, and this time I have two robots. So what would be the best, easiest way to switch between robots with one dx6i. Thanks [direct email]

A: [Mark J.] Your Spektrum DX6i transmitter has a function called 'MODEL NAME' that can save the set-up info (mixing, trims, channel reversing, exponential response, dual rates...) for up to ten different robots. Check your transmitter manual for how to set this up -- I think its on page 35. Enter and save the set-up info for the two 'bots under two different model names and switch between the two models when swapping 'bots. If the two bots should happen to use identical transmitter settings this is not needed, but optimum settings for different robots are unlikely to be the same.

  • If you have two receivers: Bind both receivers to the DX6i. Select the correct model name for the robot you're driving, power up the 'bot and go.
  • If you have one receiver: This gets more complicated. You have to unplug everything from the receiver, move the receiver to the other 'bot, plug everything back in, tie down the receiver, and remember to switch model names in the transmitter. You may not have enough time between matches for all that plus the other things your robot may needin the limited time you have between matches when running two 'bots in the same weight class.
I strongly recommend a receiver for each robot.
Q: I plan on using a 7.4 v lipo to power two FingerTech 22:1 motors on my future antweight. Will my bot be able to stand against all the other ones that seems to use 3s lipos? Am I just over analyzing everything again? [Quebec, Canada]

A: [Mark J.] You're not over thinking -- it's a reasonable question.

More speed isn't always a bonus, particularly in a small arena. A pair of 22:1 Silver Spark motors @ 7.4 volts have plenty of power. For an 8 foot arena you'll want to run them with about 2.5" wheels. You'll be fine.

Q: I saw in my beta question you don't like electric hammers, what about pneumatic? in my first look at the math, it seems like pneumatic hammers are pretty tame too. Unless I did the work math wrong, you'd need a gargantuan cylinder like chomps to even break 1 KiloJoule which even some nastier 3 pound spinners beat. [Dublin, Ohio]

A: [Mark J.] Since you didn't include your calculations I can't check them, but a pneumatic system can provide much greater force than an electric system of comparable weight.

  • Are there any electric flipper robots out there? There are lifters, but there are no flippers -- an electric motor/actuator/solenoid cannot provide anywhere near the explosive release of power available from a comparable pneumatic system.
  • Also consider the great complexity, expense, and effort expended by Team Hurtz to construct their electric hammer. It is a beautiful piece of work, but a comparable pneumatic weapon could be built from off-the-shelf components at a fraction of the cost.
I'm not a fan of hammers in general, but if you're going to build one it makes sense to use pneumatics. Assuming perfect gas flow, a 3" diameter pneumatic actuator at 250 psi provides 3.14 * (1.5^2) * 250 = 1,766 pounds of accelerating force. Try to match that with an electric motor. The trick is in getting that 'perfect' gas flow...
Q: I have recently broken the plug off of a small brushless motor. I have re-soldered the plug back onto the wire and out of concern for all three leads coming from the motor being the exact same length, I shortened the other two leads ever so slightly to match the one I broke. They are all re-soldered but now when arming the ESC, the beeps are really quiet but it does arm. When I attempt to throttle up, the motor just twitches and never chooses a direction, kind of like the wash cycle in the laundry room. What is your opinion? I am at a loss. [Nebraska]

A: [Mark J.] It sounds like one of your re-soldered connections is damaged or has a high-resistance 'cold' solder joint. Check your connectors carefully to make sure they are engaging firmly and completely.

If you have an ohmmeter, check the resistance across each of the three possible pairings of the three lead wires. If one of the three readings is much lower than the other two, the wire not included in that low-resistance pair has the bad connector. If you don't have an ohmmeter sensitive enough to measure the leads, replace the connectors and/or re-solder them all.

Q: Can you suggest drive and weapon motors for 150g bot?

So general requirement I guess that they are around 15g each, and drive motors need to reverse and [brake], so obviously esc need to be availiable as well. I've heard that brushless are generally better and that series motor is better for spinning weapon, can you evaluate? I am quite new to this [Bristol, England]

A: [Mark J.] I don't follow the UK 150 gram 'antweight' class, and the US 150 gram 'fairyweight' class is not very seriously contended. I can offer some general suggestions for a new builder:

  • I don't recommend brushless drive motors for new builders. You have enough to worry about without having to deal with the tricky details of brushless ESC firmware and poor low-speed brushless torque. Leave that for a future project.
  • The 9.5 gram 'Sanyo-style' brushed gearmotors are popular in US fairyweights. Similar motors are available from many suppliers. The 50:1 gear ratio at 7.4 volts works well with 1.25" wheels.
  • The FingerTech tinyESC provides proportional forward/brake/reverse control at just 4.5 grams each. They would match well with the Sanyo-style gearmotors
  • I'm not familliar with 'series motor' for this weight class. US insect class 'bots generally use 'outrunner' style brushless motors to direct-drive a spinner weapon. Motors like the 13 gram AD-100L Micro Brushless Outrunner are typical. The 6 gram Turnigy Plush-6A ESC is a suitable controller for this motor.
Take a look at local 'bots to make sure these specs make sense with what's happening at the events you plan to enter.

Q: Thanks a ton, and would you use the same type of motor for driving and weapon loading (like the snail spring power flipper weapon)? As I understand they generate max torque at 0 speed.

A: Yes, a brushed motor is the correct choice for reloading a spring powered weapon due to its predictable torque curve. Maximum torque is produced at stall -- but because power is the product of torque and speed, a PMDC motor generates maximum power when loaded to half of its free-running RPM. For quickest reload speed you'll select a gearing that produces twice the required torque. The torque equations are covered back in that archived post on snail cam flippers.

Q: I had a random idea that I probably won't pursue, but with how drones are a thing at Battlebots now, what's your thoughts on an 'Air Superiority' drone designed to combat other drones? Again, not really something I'm planning on doing, just a random thought.

Also, off topic: I just noticed that the icon for the solenoid FAQ is the Elsy'ier sept symbol from Warhammer 40,000... [North Carolina]

A: [Mark J.] Flying a drone in the BattleBox is a real challenge; the pilots report that the ventillation fans make precision flying impossible. I don't think it's a good place to stage an air battle, particularly given that a lawn rake has proven so effective in swatting multi-thousand dollar drones out of the air. Plus, whatcha gonna do if your opponent doesn't have a drone for you to fight?

I suspect that drones are last year's robot fashion accessory. They've been added by teams desperate to gain one of the limited slots available on the show.

Builder: "I've built a robot with a mini drum that's a good fighter and the team wears funny costumes."

Producer: "We've got a lot of those. Anything else?"

Builder: "Well, I've got a drone with a flame thrower..."

Producer: "OK, you're in."

If there is another season of BattleBots (and given the ratings that's a pretty big 'if') I don't believe any sort of drone will help to get you on the show. Try something new.

Q: Sorry if this was asked before but are there alternative ways to control steering better than 'tank treads'? Most of the bots in my weight class seem to be jerky and hard to control. Thank you. [Bristol, England]

A: [Mark J.] The control problems you're seeing aren't the fault of the 'tank steer' method. The problem is that most builders do not take time or do not know how to correctly adjust the features of their radio gear to provide smooth and precise control.

It's important to know about the many available functions on a modern computerized radio transmitter. Which functions are useful in robot combat? How do those functions interact with each other? How do you adjust those functions to correct a control issue?

Team Run Amok has several guides to assist robot builders in the selection, set-up, and troubleshooting of remote control radio systems. Of special interest is the Transmitter Programming for Combat Robots Guide. The guide was written for a Futaba transmitter, but the functions covered are common to most computerized transmitters.

Another common problem: unreadable manuals that come with cheap Chinese radio systems. Many of these manuals are poor translations that make very little sense. If you can't understand the manual you have very little chance of setting up the radio properly. Here's an actual example:

The time-recorder is used calculating comparable bo stipulated time unexpectedly, or the possible time of flight under the state that the fuel fill it up with, it is very convenient. The pattern of the timer-recorder is the count-down. Pour time-recorder from set for time is it is it count to change, show surplus time at interface to begin.

Download the manual for any radio you are considering purchasing; if you can't make sense of it you should buy a different radio.

Q: I have a third grader trying to answer a science fair question about the effectiveness of vertical vs horizontal spinners. He's built a bot out of a Thames and Kosmos building kit but he can't get enough power out of the motor to get spinners to do any damage to a piece of styrofoam. Spinner just stalls when it hits the foam. That's our problem to deal with, but:
  1. Are there equations we can run to determine the answer to this question?

  2. Does the angular velocity of a spinner change if it's in the vertical position, if all other things are equal?

  3. Does gravity assist or impede?

My engineer-son is asking questions his English-major mother can't answer. [Raleigh, North Carolina]

A: [Mark J.] You want to run some angular momentum equations for your third grader's science fair project?!?! My third grade teacher was still trying to get us to stop counting on our fingers. I guess things have changed.

It was my third-grade son who led his biology-major father into this mayhem. I'll be pleased to assist as best I can.

  1. Read thru the Ask Aaron Spinner Weapon FAQ. The entire FAQ will prove instructive, but you may find particular interest in the large, friendly blue text box near the top of that page describes the principle of spinning flywheel weaponry:

    General Principle

    Spinning weapons are flywheels. They rely on rotational inertia to collect energy from a continuous power source (electric motor, internal combustion engine...) over time and store it as rotational kinetic energy. On impact, the flywheel releases the stored energy in a blow that far exceeds the energy directly available from the continuous source.

    From your description of your son's spinner stalling, it is apparent that it does not have adequate rotational inertia to store sufficient kinetic energy from the small motor powering it. You could use a more powerful motor, but as a display of physics it would be much more interesting to increase the rotational mass of the weapon and note the change in the performance of the spinner.

    Here are your equations: How to Calculate Rotational Kinetic Energy, and I think this explanation of Kinetic Energy and Mass Moment of Inertia in Combat Robot Weapons might fill in some of the gaps.

  2. If you look thru the equations referenced above you'll discover that nowhere in the calculation of momentum or velocity is there a mention of horizontal vs. vertical orientation; the energy of the spinner system is not changed by its orientation.

    However, going from a horizontal to a verical orientation does effect the performance of the weapon in another way. When a spinner weapon impacts the opponent there is both an action on your opponent and a reaction on your 'bot.

    • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. Since your 'bot is supported by the arena surface, it does not move and most of the impact energy is transferred to your opponent.
    • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of the impact is split between moving the two 'bots in opposite directions. The desired transfer of damaging impact energy to your opponent is much less efficient.

  3. A balanced spinning mass is neither assisted nor impeded by gravity. In a vertical orientation the effect of gravity on the rising side of the mass is perfectly offset by the gravitational effect on the descending side. In a horizontal orientation nothing is rising or descending.
I threw a whole lot of information at you, but I think you can pick thru it to find answers that make some sense to you. New questions will arise -- write back as needed.
Correctly designed snail cam Q: I am in 150g weight competition, I was wondering if its possible to make a spring loaded spike/ram with ability to reload it, do you have any resources I could have a look at? [Bristol, England]

A: [Mark J.] Take a look at this archived post describing a spring-powered flipper reset by a rotating snail cam. With a little imagination it could be oriented to reload a spike, although a flipper is a more effective weapon.

Comment: Thank you, i was in fact going to do a flipper inspired but that very video, there is a lot of usefull info in that other answer!

Q: "Melty Brain" robots do not count as having an active weapon under current Battle Bot rules. Have you ever seen a melty brain style robot that was paired with another weapon? I imagine that the robot's rotation could add even more energy to a hit from a spinner. [Westerville, Ohio]

A: [Mark J.] No, and it won't.

The concept of a 'melty brain' spinner is that the entire mass of the robot becomes a spinning weapon. Stealing mass and energy from this very efficient primary weapon and trying to add it back with a secondary weapon will do no better than break even on energy, and will add undesired complexity. Use all your weapon weight allowance on a single weapon. Simple robots win.

Q: Hey,Mark i've watched some of Malaysian built bots and i they're not as exciting as US or UK combat robots and i consider joining one of the tournaments so do you know where i can find the rules or anything about those tournaments? [Selangor, Malaysia]

A: [Mark J.] Please allow me to Google that for you:

Be sure to write back if you find another way to take up half an hour of my time so you don't have to do a web search. Ah eh na!

Q: Hmm..its me the guy who asked the 'Malaysian combat robots rules' guy here. I searched it on google and the only one that i could find is a tournament that happended 3-4 years ago.I know all of the links that you suggested but i still cant find anything, oh well.

So here's my question, which robot used CO2 or gas for their weapon?

A: Your IP address doesn't match, but let's assume you are who you say you are.

You asked me for rules and information on US and UK tournaments. The links I provided above take you directly to pages that have rules and current details on major US and UK tournaments and schedules. You write back to tell me that you knew these links but cannot find anything?

Now you ask me to list the hundreds of robots that have used pneumatic weapons. I don't think that's a reasonable request. I don't seem to be able to help with your questions. Perhaps you could ask elsewhere.

Q: Oops! sorry! I made a mistake, what i meant to ask is which robot FIRST used Co2 or gas powered weapons.

A: Pneumatic weapons have been around from the start. There were three pneumatic weaponed robots at the first Robot Wars event in 1994:

1994 Middleweight Champ 'X1'

Q: what wires do we need for a combat robot ? [Pulau Pinang, Malaysia]

3 minutes later:

Q: what do we need to make a combat robot ? [Pulau Pinang, Malaysia]

57 minutes later:

Q: what do we need to make a combat robot ? [Pulau Pinang, Malaysia]

Read the FAQ!

A: [Mark J.] An engineering mindset, mechanical experience, and the ability to read all the way to the end of a sentence. While you're developing those skills, read the Frequently Asked Questions.

Q: Greetings! I am currently rebuilding my 30lb. sumo bot 'Dispatchula' for future event. Its performance was 'meh' in comparison to the other bots, but the fact that I got that plywood and scrap metal monster to win a few matches was fantastic! Now that I have more time on my hands to prepare, I hope to bring it up to scratch with the competition as well as add few bonus features. I was only about 12 lbs without ballast, so I have of weight to work with.

My plan is to increase the down force by a vacuum or downward thrust system to take advantage of my powerful drive and grippy wheels, but at the moment I'm unsure of the design to use. I've looked into possible several options, but don't know which would be most suitable. Perhaps a large ducted fan, with an open base to suck in as much air as possible, or a vacuum pump with a flexible skirt to conform to the floor? Maybe the ducted fan with the flexible skirt? I just don't know. My goal is to add at LEAST 10 lbs of additional downforce, but that is the absolute bare minimum and I want as much as I can use. I really don't know how far I can go with this, but if Terrorhurtz could use this technology to climb a wall...

INFO: I've equipped it with 6 Fingertech sumo wheels, 4 Banebots P60 38:1 gearboxes hooked up to 550 motors running at 14.8v, and a massive Breaker Box-style scoop. The matches take place on a smooth plywood ring 10ft in diameter, and are 6 minutes total if the entire 3 3-minute matches last their full length (not often). I've never come close to draining the battery after a fight, and should have plenty of power to spare for a vacuum downforce system. [Ontario, Canada]

Bottom plate on 'Suckbot'
A: I've been involved in the design of a couple 'suckbots'. The most practical performance comes from a ducted fan pulling air in underneath a large flat-bottomed base to create ground-effect downforce. No skirts are used because this type of design relys on fast moving air under the flat bottom to create downforce from the Bernoulli effect. Bernoulli downforce is less sensitive to changes in chassis height or imperfections in the arena surface than a skirted design.

There are a couple of discussions about 'suckbots' on the Pololu forum and blog:

If you want to try a smaller fan with a flexible skirt you might get some ideas from this post on Hackaday: Vacuum Micromouse

I don't have a tool to help determine how powerful a fan is needed to meet your goals -- I think this is probably a 'trial and error' project. Let me know how this works out for you.

Q: I had a physics question!

I read that 'beta' uses a gearing system and then cams to keep the load on the motor right so that its always in its maximum power band. Neat stuff! My question is, etc. How would you begin to calculate such a thing? I'm guessing you calculate the weight of the hammer at the end of the head after gearing as the load to start with, but what would the load be once it is in motion so you could figure out the cams?

Not looking to build one I just thought it was a neat question. [Dublin, Ohio]

A: [Mark J.] I've vowed to avoid calculus in the answers I give here at 'Ask Aaron'. I'm severely tempted to break that vow for this question given that the answer involves a system of non-linear equations, but I think I can provide an approximation that's plenty close enough for combat robot purposes while leaving a trail of crumbs for the 'Sons of Newton' to follow if they so desire.

Calculating the acceleration of a directly-geared electric hammer weapon is essentially the same as for a spinner weapon, the difference being that the spinner has unlimited rotation and can build energy over an extended time period while the hammer is restricted to (typically) 180 degrees of rotation.

In 2010 I took apart the Team Run Amok Excel Spinner Spreadsheet and re-wrote sections of the code to calculate and graph the energy storage of a hammer weapon of known configuration powered by PMDC motor with a fixed gear reduction thru a default rotation of 180 degrees: the Run Amok Hammer Spreadsheet.The hammer spreadsheet guides the user to find motor gearing that places the motor power peak at the mid-point of the swing in order to optimize the power output. The spreadsheet accounts for gravity, but does not account for the rotational inertia of the motor armature because that information is rarely available. I took this opportunity to clean it up a little from the previous beta release 0.9 (no pun intended) and now call it version 1.2.

The calculations are relatively simple because the torque output of a PMDC motor is linear. Now you come along and want an explanation of how to design snail cams that make the applied torque non-linear and extend the period during which the motor operates near peak power. If I were still teaching at university I'd ask you to see me during my office hours and then keep my office door locked and the lights out for a week in hopes that you'd give up. Unfortunately, I no longer have that luxury.

The overall concept is to start with a high gear reduction to allow the motor to quickly spin up close to 50% of its no-load RPM -- the point at which max power output is achieved. Gearing reduction is then decreased as the snail cam rotates to keep the motor near that same RPM thru the rest of the hammer swing.

Take a look at this discussion about axe mechanisms on the Australian Robowars forum. The whole exchange is worth reading, but note the post where 'kkeerroo' discusses his approach to 'snail cam' design:

Snail drive for hammer on 'beta'

"Also I'm intersted in the snail cam on Beta. I used the time for the axe to swing as you showed me, broke down the equation for power and rearranged with respect to acceleration (power = force x velocity) so I had a = (p*t)/(m*s). I used pi for s as I wanted the answer to be repesented in angular acceleration measured in radians (pi = 180 degrees). Once I had the acceleration I multiplied it to the time to get the angular velocity at the end of the swing. I converted it to rpm and used it to find the best gear ratio. I used this as the gearing on one end of the cam. For the other end I decided to use it and the average gearing to find the other end so the average radius of the cam is equal to the average gearing. For one set of sums I ended with 13.5:1 for the average gearing, 7.7:1 at one end of the cam and 19.3:1 at the other."

You can use the hammer spreadsheet to aid you in finding a solution similar to that used by 'kkeerroo':

Note: this section was originally written for version 1.1 of the Hammer Spreadsheet. I've re-written it to conform with the newly released version 1.2 which better accounts for actuation speed in addition to energy storage.

  • Enter your hammer and motor specs into the hammer spreadsheet.
  • Stage 1: search for a 'Gear Reduction' that reports: Peak output is at 45 degrees of hammer rotation in the 'Results' box. This will be the initial total gear reduction for the weapon at startup. For the example given with the spreadsheet this ratio is 55:1.
  • Stage 2: search for a 'Gear Reduction' that reports: Peak output is at 90 degrees of hammer rotation in the 'Results' box. This is the second stage gear reduction for the weapon. For the example given with the spreadsheet this ratio is 44:1.
  • Stage 3: search for a 'Gear Reduction' that reports: Peak output is at 135 degrees of hammer rotation in the 'Results' box. This is the third stage gear reduction for the weapon. For the example given with the spreadsheet this ratio is 38:1.
  • Stage 4: search for a 'Gear Reduction' that reports: Peak output is at 180 degrees of hammer rotation in the 'Results' box. This is the fourth stage gear reduction for the weapon. For the example given with the spreadsheet this ratio is 35:1.
  • Design the radii of the snail cams to provide the initial gear reduction constantly for the first 45 degrees of hammer rotation to allow the motor to come up to speed, then transition smoothly to the second ratio at 90 degrees, the third ratio at 135 degrees, and on to the final ratio at 180 degrees.

This 'four stage' approximation will provide a good practical solution given the multiple unknowns and unattended variables. Calculus will give you a more accurate theoretical answer, but the real-world result will be much the same.

Note: all this fancy machining and extra moving parts are extra pretty and quite fascinating, but due to the torque characteristics of a PMDC motor the impact on energy storage by the hammer is not all that impressive -- about a 20% theoretical increase over an optimum fixed reduction ratio. It is much easier to use a more powerful motor to start with and save yourself the grief.

Comment: Thanks for the answer to my 'beta' question! Also, on your tools page you don't have your hammer calculator spreadsheet!

Reply: [Mark J.] You're welcome. I haven't advertised the hammer spreadsheet for a couple of reasons:

  1. I wrote the spreadsheet to answer a specific question from a builder, but I didn't take time to document or polish it for general use. Although I cleaned it up a bit yesterday to better answer your question it still wasn't ready to display alongside the other Team Run Amok tools.
  2. Electric hammer 'bots are fun but they really aren't competitive -- except perhaps in the 'Sportsman' classes. I'm hesitent to encourage builders with a tool that will help them to build 'bots that won't win.
Given the current interest in hammerbots I reconsidered and took some (too much) time and re-structured the graphics to better match the spinner spreadsheet, tweeked the gearing guidelines to optimize actuation time, re-formatted the results output text so that it might make sense to someone other than me, and documented the spreadsheet so tha I might remember what I did here. The new 'version 1.2' now is available on our Combat Robot Design Tools page.
Comment: China fighting robot championship FMB (Fighting My Bots) coming soon. We got a preview video, drum spinner and flipper. I build both of them. : ) This video is very exciting, I hope you will enjoy it. [Yunnan, China]

Reply: [Mark J.] You've been very busy! The robots look great. I recall the tournament is soon -- September 15th if I remember correctly. Looking forward to more video.

The preview video wouldn't open on my android, but I had no problem opening it on my PC.

Q: What are the rules of the 'fairy weight' competition? [New Jersey]

A: [Mark J.] Most events for the 'insect class' robots:

  • 150-gram 'fairys'
  • one-pound 'ants'
  • three-pound 'beetles' and
  • six-pound 'mantises'
as well as the heavier classes follow the SPARC Robot Construction Specifications.

SPARC also provides guidance on robot combat tournament procedures, match rules, and match judging. Links to these files can be found on the SPARC Documents page.

Rules may vary from event to event so make sure to check with the organizer of the specific event you plan to enter for any special allowances or restrictions.

Q: Hello! I am considering competing in Battlebots. The idea of my robot would be similar to 'REDRUM', but it would have a 50 pound hollow drum, a vertical 2-wheel drivetrain, and have ball transfers as the back wheels (so that it doesn't drag and is also revertible). However, I had a few questions in regards to the design:

1) How well do you think a vertical drivetrain would do?
2) If the drum is gonna be as big as the robot itself, is the hollow drum a good choice?
3) Are there any other recommendations you have?

Please let me know what you think! -Blockhead Robotics [Illinois]

A: [Mark J.] I recommend caution in building to compete at BattleBots. The competition lives and dies on television ratings and the ratings weren't good this year. It's unclear if the show will be back for a third season, and if it is renewed you still have to gain approval from the show producers in order to compete. I know of several established and competitive teams that failed to gain acceptance to this year's tournament. Don't get your hopes up.

Back to your questions:

  1. I'm not sure what you mean by 'vertical drivetrain'. Is it something like the design of Robotica competitor 'Fury', with dual 'high and low' drive wheels on each side to give invertability with a large vertical spinner? That type of design could work well for a big drum.
  2. Yes, hollow is the appropriate choice for a large drum in order to store the most energy for its weight. See the Ask Aaron Spinner Weapon FAQ for examples of mass placement and energy storage in drums.
  3. I'm not a fan of ball transfers. They tend to jam from arena debris and turn into clunky impediments that are worse than tail dragging. If you don't have much weight on the tail I'd suggest replacing the ball transfers with simple skids made from low-friction UHMW or Teflon. The nose of Team Run Amok's heavyweight lifter 'The Gap' rides on a small Teflon roller that works very well.
'Fury' sans armor
Note: 'REDRUM' had a much heavier drum than you're planning -- more than 35% of the weight of the 'bot. That's equivalent to a 90 pound drum for a 250 pound robot. If you're trying to get accepted to BattleBots with a BIG DRUM then you'd better build a REALLY BIG DRUM. Best luck!
Q: Hi Mark, I have a variant of this question [from the Ask Aaron 'Design and Construction' archive]:

Q: Hi mark. I am building a 4lb wedge bot and am indecisive on a final fact. I am indecisive on using 2 wheels with drive motors or 4. With 2 drive motors my dimensions are 6" wide X 7" long. With 4, 6" wide X 10" long. The area my bot takes up is a concern as the arena is only 5' X 5'. I am wondering if the approximate 3.6 lb of pushing force of the 4 motors is worth the extra 3 inches when compared to 2.25 of the 2 motors. Pushing force and maneuverability are key but would the shorter length and less surface area be better given it has less pushing force but still breaks traction (4 motors break at 30% 2 motors break at 50%)? Thanks. [Warrington, Pennsylvania]

A: [Mark J.] Fairyweights (150 grams), antweights (1 pound), kilobots (2.2 pounds), beetleweights (3 pounds), mantisweights (6 pounds); aren't there enough insect weight classes? Who decided we needed a 4 pound class???

You haven't given me much about your design or what rules the event will run, but in general I like a large footprint on insect 'bots. Team Run Amok's successful six-wheel drive beetleweight lifter 'Zpatula' is 12" wide by almost 16" long!

Conventional design theory says that a smaller robot is better because the armor can be thicker with less area to cover. This is true, but the larger footprint allows for much greater turning torque, plus the larger 'bot will react less to weapon impacts. Judges also generally respond better to a larger robot; the size differential works in your favor.

Go bigger and dominate the small arena.

Beetle, wedge. Front steel wedge (1/16) at 35 degrees. 4 drive motors (970 rpm servocity econ motors). 2.875" Banebots wheels outside if it matters. Aluminum base. My question refers specifically to the base plate to relative dimensions. Do you think an overall size of 8" wide x 9" long with 1/8" aluminum is better than 1/4" aluminum with dimensions of 6" wide by 9" long?

Thanks [Pennsylvania]

A: [Mark J.] I don't have much to add to the previous answer. I like large footprints for insect class 'bots for the reasons given.

You haven't given me enough structural detail about your chassis for me to comment on the adequacy of the baseplate thickness, but as a general comment 1/4" unspecified aluminum alloy is a very thick baseplate for a beetle.

See also: The Hamburger is Bad.

Q: Hey Mark,

I've noticed that many, if not most, insect class drum bots use screws for their weapon's teeth. Aside from the teeth being easily replaceable, what advantage would this give you that you couldn't get with a single, long weapon tooth that you would normally see in bots like Poison Arrow or Minotaur?

Thanks, David R. (Livermore, CA)

A: [Mark J.] The advantage is that you get to eat this month instead of spending your rent money on custom software and machining.

Read thru this PowerPoint presentation that outlines the design process for the single-tooth drum used by one of the robots you mention. Then consider the machining required to manufacture said drum.

For the average thin-walleted insect-class builder who has access to perhaps a lathe and a drill press, a few hardened screws threaded into a simple symmetric aluminum holder is very appealing.

'Saifu' antweight drum

Q: Are magnets the good solution to reduce the effect of Gyroscopic Forces in vertical spinners? Are there some robots use magnets to do that? [Guangdong, China]

A: [Mark J.] Seems like a good idea, doesn't it? Many builders have tried to use magnets to increase traction or fight gyroscopic lift, but over and over it's been proven that chassis downforce magnets are much more trouble than they are worth in combat robots.

  • They collect debris and interfere with maneuverability;
  • They clamp onto arena side rails and hold fast;
  • They loose attractive power with the cube of the distance from the surface, so when they let go they let go abruptly; and
  • They supply no help at all in arenas with non-magnetic surfaces.
There have been successful insect class robots using magnet wheels to greatly improve traction, but these have mostly been rambots and lifters. Attempts to use magnet wheels or treads in larger robots have not met with success. In general, it's best to leave magnet downforce to sumo robots.

The Total Insanity Gyroscopic Force Calculator can help adjust robot design to better cope with the weapon gyro forces.

Q: Has there ever been a counter rotating vertical spinner in a robot? I'm thinking of two large disks parallel and close to each other on the same dead shaft. Only one would have teeth and they would have one or more small perpendicular drive wheels between them causing the counter spin. The reasoning behind building this would be to lessen gyroscopic effects. Thanks! [Minnesota]

A: [Mark J.] I know of two big 'bots that were designed to nullify the annoying gyroscopic forces associated with vertical spinners. Both used mechanically simpler solutions than your proposed co-axial counter-rotating disks:
  • Team Boilerbots built 'Counter Revolution' to compete at BattleBots. The twin counter-rotating vertical disks are not co-axial, but the counter rotation largely cancels the net gyro effects when the robot turns.
  • Richard Chandler campaigned superheavyweight 'Strike Terror' at BattleBots 4.0 and 5.0 with a vertical spinning weapon that was free to pivot in the longitudinal axis. This allowed the weapon to twist near-horizontal when turning without effecting the chassis, and then re-establish a vertical spin when turning ceased.
Neither 'bot was particularly successful. I'd recommend against adding the mechnical complexity your design requires. Simple 'bots win.

'Wedgemaster Wedge' writes in to remind us of 'CounterStryker' -- a 6-pound 'mantisweight' with counter rotating vertical disks built for Bot Bash:

Comment: Zac O built and documented this bot which is pretty close to what that dude wanted.

Thanks, Wedge.

Comment: Thanks Mark and Wedge that is exactly what I was thinking about. I found a video of CounterStryker fighting and it seemed to handle the turns well.

A: 'CounterStryker' has a good record: 2nd at Bot Bash '15 and 3rd at Bot Bash '16. I'm not a fan of friction drive for weapons -- Zac took care with the design and it works well in this insect class 'bot, but I wouldn't try it in a larger 'bot.

Q: With season one of the new Robot Wars complete, I gotta ask: What did you think of it? Personally, I really... (expunged) [Roseville, California]

A: [Mark J.] My apologies, but the topic is off limits. A number of years ago we were beset by particularly odious Robot Wars hooligans and I swore that never again would Robot Wars content appear in Ask Aaron. See Frequently Asked Questions #37.

Q: Hey Mark, I've got some paranoia with my first beetle build (2 wheeled horizontal drum) and I'm hoping that you could perhaps ease my mind with something.

In episode 5 of Robot Wars (2016) there was a 2-wheeled vertical spinning bot called Infernal Contraption. Due to the weapon's placement being close so close to its high-torque drive system, it would easily invert itself by just driving forward (a la any overhead thwackbot).

Is there a rule of thumb or formula I could use to ensure that I can maintain good torque in my drive system without ending up flopping around like a Magikarp-out-of-water?

Thanks! David R. [Livermore, CA]

A: [Mark J.] Getting a 'bot to do the whole torque-reaction overhead hammer thing requires some deliberate design work to set the center of mass close enough to the drive axle and eliminating rear overhang. The chance of doing this inadvertantly is vanishingly small, but getting the center of mass 'just right' does require some planning.

Have a look at section 2.7.7 of the RioBotz Combat Tutorial for the math on where the center of mass should be placed in a two-wheel 'bot to get optimum traction without risk of nose lift.

I like the 'Magikarp out of water' analogy. I'll save the image and use it to adorn unworkable or needlessly awkward solutions. Watch for it.

Q: How do horizontal spinners keep their weapons off of the frame? I know for example the most iconic horizontal spinner Last Rights/Tombstone has an adjustable height blade, meaning it isn't riding on the bottom frame. Is the friction of the bearing on the shaft enough to keep the blade from shifting during big collisions or is there something more to it that I am missing because that doesn't seem adequate? [Cleveland, Ohio]

A: [Mark J.] Typically the weapon hub and pulley/sprocket fill the entire space between the frame members. They ride against the inner bearing races or against 'thrust bearings' that take displacement loading during a 'hit'. The diagram shows a 'live shaft' arrangement -- in a 'dead shaft' design where the shaft does not rotate the bearings are incorporated into the weapon/pulley hub and the spacer is part of the hub assembly.

If the design allows additional space between the weapon bearings for blade height adjustment, tubular spacers (orange in the diagram at left) slide over the weapon shaft above and/or below the weapon hub to raise or lower its position.

Q: How does a horizontal full body spinner like 'Barber-ous' work? Is it a shell spinner like 'Ringmaster' on its side? [a server in California]

A: [Mark J.] First, a little terminology clarification:

  • Spiners are classified by the direction their impact, not by axis orientation. 'Barber-ous' is a vertical spinner and 'Ringmaster' is a horizontal spinner.
  • A 'shell spinner' has the entire exterior of the robot spinning. 'Ringmaster' is not a shell spinner -- it is an example of the rare and complex 'ring spinner' where only the outer edge of the body spins, leaving the wheels exposed at top and bottom to allow inverted operation.
Team Rotractor's original Barber-ous webpage (archived) has the worst build report I've ever seen, but I'm still grateful that it exists. The chassis photo at right came from another source. The shell and electricals have been removed and you can see that the layout is not nearly as complex as Ringmaster's:
  1. The weapon motor is mounted on the central chassis that also carries the batteries and electronics. The entire central chassis is concealed by the weapon shell when assembled.
  2. Non-rotating stub axles come off each end of the chassis. This version of 'Barber-ous' has worm-drive gearmotors bolted to the ends of the stub axles to power the drive wheels. The gearmotors are obscured in the photo by the wheels and hubs.
  3. Laying on the floor is one of the two large weapon hubs. With the gearmotor removed, the free-spinning hub slides onto the stub axle and a drive chain connects the hub sprocket to the weapon motor. The weapon hub on the far side is already in place. With the weapon hubs in place, the weapon cylinder slides on over the hubs and chassis and is bolted to the hubs. The drive-wheel assembly can then be re-mounted.
'Barber-ous' went thru many revisions and updates to the chassis and drive motors, but the weapon drive principle remained the same.

Remembering Aaron... 

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

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

Robot haiku:

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

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