Questions and Answers about Combat Robotics from Team Run Amok.
7864 Questions and Answersabout Combat Robotics
from Team Run AmokPrivacy Policy
Team Run Amok receives a lot of email about designing and building 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.
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.
'Ask Aaron' is a simple little question and answer page. It is not Reddit. Messages not asking substantive combat robot questions will not be published.
Polite and Respectful
Q: Hi Mark,
Just wanted to let you know that the Robotica page on the site currently lists the Combat Robot Hall of Fame voting period as extending to August 17th, but the CRHoF page itself specifies that voting will only be open until August 10th.
Cheers, Max [Port Townsend, Washington]
A: Mark J. Thanks, Max. There was a snip of old javascript code on the Robotica page that used last year's August 17th end date -- now corrected to read August 10th.
I'll point out that in general you're not going to get anywhere on the internet by being polite and respectful. It works here, but "annoyed and snarky" is in fashion in most quarters. Something like this might be more social-media correct:
Hey Mark,
Did you forget how calendars work, or are you just making up dates now? Your Robotica page says CRHoF voting ends August 17th, but the real page says August 10th. Maybe try proofreading your own stuff before posting garbage that confuses people.
Whatever, Max
A Different Ballgame
Q: can i have information on sumo bot too?? [Bengaluru, Karnataka]
A: Mark J. Robot sumo is a whole different ball game. We claim no expertise in robot sumo -- there are better places to ask your questions.
And a Lot More
Q: Can a xt60 connecter handle 30 volts. [Glassboro, New Jersey]
A: Mark J. XT-60 electrical connectors are rated for a maximum 500 volts DC at up to 60 continuous amps. For the short periods seen in robot combat matches they survive 120 amp peak current nicely.
You Control Only Some Variables
Q: For as long as I can remember, the main meta with spinner weaponry has been having "enough" kinetic energy and absolutely all the bite you can get. But I hate vibes based engineering, and designing my rotating deathblade based on some vague notion of "enough" is nauseating to my lizard brain that wants numbers and true/false dichotomies. Please try to bear with me here because I have zero physics and/or calculus background. Any and all physics concepts I employ here were researched on the spot. (Context: Building 3 lb horizontal)
Mark J. Here I have hidden a 1300 word thesis wherein the admitted zero physics background author of the question attempts a physics-based analysis of bite versus kinetic energy as it applies to a horizontal spinner weapon. Included are a ChatGPT analysis where the AI entirely misses the concepts of both 'bite' and 'spinner'. If you have finished your daily crossword and would like to puzzle this out the way I did, click the button below to reveal the full text. Alternately, you may skip down to the answer section where I will attempt a summary.
My initial conception was that since kinetic energy increases with the square of speed [1/2 (I or m) (v^2 or I^2)], and bite increases with a roughly inverse relationship to velocity (~some constant k/omega) - bite, governing the "effective mass" of the bar at impact - would lose out and transferred energy would increase roughly linearly with velocity. I did notice that vertical spinners such as Copperhead at a lower velocity would produce bigger hits, which I chalked up to an increase in impulse, the tossing force (F*deltaT) without necessarily increasing the destructive capacity of the weapon. On a horizontal spinner, I believed at first glance, the ability to puncture armor and overwhelm the material would necessarily be more important than the ability to impart momentum on it, given that Newton's Third Law states that any FORCE you place into a non-breaking opponent would thusly be reflected back at you. Additionally the 1/4" bind threshold given by Riobotz shed some light on the issue to me, but I still saw no issue. "Above 1/4", you bind. But below that, you would 'shred' rather than 'skate' as everyone claims, right? If the quadratic omega equation wins out over the k/omega bite equation, that is.
I pitched this to ChatGPT, and uncharacteristically of the normally sycophantic clanker, it immediately shot me down - claiming that...
Summary of Bite vs. Skate for Spinner Weapons
Variables:
τ = weapon motor torque (N·m)
r = radius to tooth tip (m)
ω = bar angular speed (rad/s)
v_n = closing velocity normal to opponent (m/s)
n = number of teeth on the bar
k = contact stiffness (N/m)
μ_s = static friction coefficient
F_tan = tangential force at tooth = τ / r
F_fric_max = maximum static friction = μ_s · N_static
Key relationships:
bite depth b = v_n / (n · ω)
static normal force N_static = k · b = k · v_n / (n · ω)
max static friction F_fric_max = μ_s · N_static = μ_s · k · v_n / (n · ω)
A spinner weapon achieves maximum destructive effectiveness when it "bites" rather than "skates." Bite requires sufficient normal force from the tooth’s engagement depth to resist the bar’s tangential drive torque. As angular speed increases, bite depth (and hence static normal force) falls, creating a sharp threshold ω_crit above which the tooth can no longer grip and energy transfer collapses. Optimal design therefore tunes tooth count, contact stiffness, closing velocity, and motor torque so that the weapon operates just below ω_crit, maximizing energy deposition into the opponent without slipping.
However, I am loath to trust AI about a non-mainstream research topic such as this. Additionally, the AI also seems to have a different framework than the metagame does, which aims to increase bite far past the point where the tangential force is lower than the static friction maximum, often boasting about an inch of bite or more. The AI, however, claims that there is a drop-off cliff where more speed will immediately kill your weapon effectiveness.
This would be okay, however, if I could single-mindedly design around increased bite. But clearly this is not the case. Many people say to spin "vaguely fast" and let the bite do the work. However...
In Tombstone vs. Bombshell and Tombstone vs. Escape Velocity, Tombstone won in dominant fashion - but did not actually impart much impulse onto the opponent. His engagements were more of a shred than a bite.
Contrast this with Blendo vs. Punjar and Tombstone vs. End Game. In spite of the KE likely being less than 10% of what modern spinners put out, due to Blendo’s spinner only being somewhere in the neighborhood of 500 RPM (and Punjar likely being made of a soft material), it got a huge amount of bite on Punjar, sending it spinning out wildly to a much more extreme degree than Tombstone did Whiplash and Escape Velocity. In spite of the large amount of bite, Blendo’s comparative lack of kinetic energy and the recoil of those high-impulse shots resulted in it not being able to inflict serious punishment on Punjar and its engine dying out. Versus End Game, in the two collisions that occurred during the fight, Tombstone actually did get a good amount of bite on End Game, but as a two-wheel drive horizontal spinner against a stable four-wheel design, Newton was not on Tombstone’s side. Tombstone was entirely unable to penetrate the frontal armor of End Game, thus receiving full recoil.
Additionally, anyone who's seen a Vector kit compete recently knows that thing flat out does not have the firepower to play with the NHRL beetle demigods. D2 kits were so dominant for so many years, in part because the horizontals of the time literally could not scratch the titanium plow on the front of it.
Therefore, it's easy to see how the current approach to horizontal spinner design feels to me like I'm being asked to juggle Molotov cocktails on a unicycle blindfolded, except I don't know where the unicycle is and the building is on fire.
It's easy to say "Just have enough" Kinetic Energy and focus on bite and engagement geometry when the current dominant material is TPU. TPU is a compliant material that is incredibly sensitive to geometry, and not at all sensitive to imparted kinetic energy. I can beat a TPU box to death with a sledgehammer for hours and get nowhere, but slash it up in seconds with kitchen scissors. But the meta changes, and eventually this may not be the case. Perhaps everyone will soon be building titanium boxes that truly do require a set X joules to puncture - and I would feel much more comfortable having devised a "unified field theory" or unifying principle behind why some spinners hit hard, others glance off, and some just don't hit hard enough.
The amount of energy transferred, as ChatGPT confidently reminded me, does in fact have to accord with the conservation of energy, and it is therefore directly proportional to the work done on the bar at contact - how much it slows down.
Bite, of course, would increase this negative impulse and thus the work done on the bar. However, clearly at some point increasing the percentage of your pithy energy transfer would be meaningless: reductio ad absurdum - 100 RPM. So, from 100 RPM - where do we go, when your opponent armor can handle "X+1" joules relative to the amount of energy that you are transferring? Up. So you begin to increase the velocity, and your weapon thusly continues to inflict an increasing amount of damage... until at some point this damage begins to decline? Or is it a sharp drop-off in accordance with the tangential force overwhelming the static friction maximum as ChatGPT claims? If the latter is the case, wouldn't the inch-deep bites the meta bots boast be grossly wasteful of potential kinetic energy? If the former is the case, why? Consider my equation from before, in which the quadratic scaling of velocity would overpower the inverse effect bite has on imparted energy - even considering (as ChatGPT states) that a static frictional engagement is required to transfer any kinetic energy at all - would my relation not hold true below this threshold?
Again, if I was designing a vertical spinner, I WOULD simply produce a massive amount of bite and be content flinging opponents around the box with 190J and massive impulse. But with an HS I actually have to bust THROUGH metal plating which requires some X amount of Joules, even with my sharpened and raked impactors designed to transfer such energy. [Reston, Virginia]
A: Mark J. You're finding 'squishy' advice on bite because the perfect amount of bite is situational -- there is no calculated value that applies to all of the possible combinations of variables encountered in robot combat.
You didn't mention the prompt you gave to ChatGPT that resulted in the response you provided above, but the AI clearly does not understand the situation. It defines "bite" entirely as a sliding friction event rather than the more useful displacement event that produces the most damaging weapon hits in combat. Equally alarming is its use of motor torque in the equations while completely ignoring the weapon's stored kinetic energy -- which is orders of magnitude greater.
Let's look at the variables ChatGPT considered that actually have an effect on optimum bite to see which are under your control:
Radius to Tooth Tip -- Does effect bite arc, but not in the context used by ChatGPT. It is under your control.
Bar Angular Speed (rad/s) -- Yes, the rotational speed of the weapon is important and it is under your control.
Closing Velocity Normal to Opponent (m/s) -- This is also improtant, but closing velocity will vary throughout the match. Sometimes you may be charging at your opponent as they charge full-speed at you. Other times you may have just turned around to find only a short run toward a stationary opponent. This is not a constant and is only partially under your control.
Number of Teeth -- Important and under your control.
Contact Stiffness (N/m) -- You get to pick the material from which your weapon is made, but the material on your opponent which you will strike is not under your control.
Static Friction Coefficient -- Like contact stiffness, not entirely under your control.
There are other factors in required bite not mentioned by ChatGPT -- notably the presence or absence of exposed sharp edges or small radius bends on your opponent, and your ability to move hard edges onto the path of your weapon via wedges or 'fingers'.
The point here is that you as the designer of your weapon system are not in control of the variables needed to calculate optimum bite versus optimum energy storage. The methodology is to set values somewhere in the middle of that nebulous grey probability cloud and use your weapon speed control to adjust on the fly:
If your weapon is skittering along your opponent with no bite, turn down the speed.
If you're getting adequate bite but not getting big hits, crank up the speed.
Now go show your lizard brain who's in charge and build your beetle.
P.S. -- You're simply not going to "bust through" metal plating on your opponent's beetle with your horizontal spinner. Not gonna happen. Tear off wheels? Sure. Accelerate them hard enough to rip their battery loose? Maybe. Shatter their vert eggbeater? Possibly. Have a good time? Absolutely.
Q: Thank you for taking the time to respond to my mini thesis. When dismantling the AI-generated segment of my argument, you stated that bite is a displacement event rather than a sliding frictional event. However, you also referenced the static friction coefficient in the "Parts of GPT's Argument That Aren't Total BS." What role does friction really play relative to displacement in a weapon engagement?
A: Hardened steel sliding along hardened steel doesn't generate much energy transfer via friction, particularly given that one of those steel parts has approached at a shallow angle (note my comment above on "Radius to Tooth Tip") that creates a 'kick back' force vector acting to prevent extended contact. This produces the 'skitter' effect of the weapon ticking along the opponent surface without significant energy transfer. The hardness of the two surfaces effectively prevents the impact from achieving the "grip" condition as defined by ChatGPT -- all you get is some degree of "skate".
I included static friction in the "non-BS" summary because you may be fortunate enough to land a glancing blow on some softer material that will allow enough surface deformation to elevate sliding friction to a level sufficient to transfer some small portion of your weapon energy. Given that your opponent robot will - in most circumstances - remain free to rotate and that the impact site will generally be a significant distance from their center of mass, the result of your horizontal weapon impact will simply be to spin them around. This is a much smaller impact than the displacement impact achieved from true "bite".
Note that energy transfer will be much greater if your sharp impact tip can "dig in" to a soft surface and gain purchase -- but that is no longer a sliding friction event.
About Eighty Grams Worth
Q: what motor to use for a beetleweight eggbeater robot [Barnet, United Kingdom]
Nominations for new members in The Combat Robot Hall of Fame open in August of odd-numbered years. The upcoming balloting period will run from August 1st thru August 10th of 2025.
Starting with the 2025 ballot there is a hard limit of twenty-five robots that may be nominated per ballot. See: Useful Information for Combat Robot Hall of Fame Voters. I find the new rule to be clear and unambiguous, but it seems that some of the more fictional friends of 'Ask Aaron' remain confused by the balloting process...
Q: Some familiar faces have drafted emails to me about the Combat Robot Hall of Fame. I keep explaining that I'm only responsible for the '25 robot rule' and am in no way affiliated with the CRHoF. Hopefully you can sort their questions out...
- See you August 1st, Iceywave : ) [West of San Antonio]
Q: Can I bypass my frie... I mean, Iceywave’s rule by saying, "one vote per bot"? [Kevin's Bots & Stuff]
A: Mark J. Assuming "one vote per bot" means "one vote for every bot that ever fought in a combat robot tournament anywhere", no. The whole concept of a Hall of Fame hinges on the idea that not everyone gets in. You may vote for no more than twenty-five robots -- by name.
Q: AM I CRAZEE?!:;/=+*% [Mistro Deb Hamper]
A: Robot questions, please.
Q: May I place an exact vote for my own sandwich press? [SpoopyVex029]
A: You may put your sandwich press on your ballot, but if it is not a robot that has fought in a recognized combat robot competition it will not be counted.
A: You may put American writer/directors and Latvian filmmaker/animator/composers on your ballot, but if they are not robots that have fought in recognized combat robot competitions they will not be counted.
Q: I'm thinking of a robot called Bag O' Ice! I don't know what it looks like, but can I put it on my ballot? [CRAIGYBOT]
A: You may put Anything You Like on your ballot, but if it is not a robot that has fought in a recognized combat robot competition it will not be counted.
A Rare Exception
Q: i am making a 15kg bot in which i have decided the specs for the bot should be:
Rhino 12V DC 100RPM 40Kgcm IG32 Heavy Duty Planetary Geared Motor
Rhino MDD 20Amp 6V-30V Dual DC Motor Driver (2 Channels)
FlySky CT6B 6ch 2.4GHz transmitter & receiver 1Km range
Astro Heavy Duty 75mm TPR Rubber HIgh Endurance Wheels (1"Wide)
300W 20A DC-DC Step Down Converter Module High Power Buck Voltage Regulator 6V-40V to 1.25V-36V
TecoKart Battery Cut Off Switch 12V/24V
24V DC Motor 2500rpm
i have decided to make a spinner weapon bot where the 24v battery would drive the spinner and the 12v are for wheels the buck converter would step down the voltage from 24v to 12v to drive the motor which would be connected to motor driver. Now i cannot find a compatible esc for 24v motor which would have braking functions. i can find the motor speed controller but they change the speed by potentiometer of spinner weapon but i want it with pwm compatible. What other alternative could i try for the spinner weapon and any other suggestion would be helpful. [Maharashtra, Bharat]
A: Mark J. 'Ask Aaron' has a general rule that we do not answer questions from competitors on the Indian subcontinent (
why not? )
- but I'm making a rare exception this time because:
You're building a fairly small robot;
You're not asking about spinner weapon design; and
You're going to have a very disappointing robot if you go ahead with those components.
Maybe I'm getting soft in my old age, but let's have a look at your parts list:
GenX 6S 3300mAh 40C/80C Lithium Polymer Battery -- You've told me too little about your spinner weapon for me to calculate the actual battery capacity requirement. The chosen drive motors will use very little power so this may be larger than needed. Assuming that you have a proper LiPo charger this should be OK.
Rhino 12V DC 100 RPM 40Kgcm IG32 Planetary Geared Motor -- Find larger motors This is a small RS-385 brushed motor with a peak output of about 15 watts. Rule of thumb has the ABSOLUTE MINIMUM drive power needed for a 15 Kg robot at 120 watts (8 watts per Kg) and a pair of these motors provide only 30 watts. With the 100 RPM output gearbox and 75mm wheels, the top speed of your robot will be a VERY slow 1.3 KPH (0.8 MPH).
I also worry about the durability of these gearmotors. The output shaft is only 6mm diameter, which is considered marginal for direct drive to a wheel in the 3-pound combat class -- it is far too small to survive in 15Kg combat.
Rhino MDD 20Amp 6V-30V Dual DC Motor Driver -- I don't know of any combat robots using this driver. Appears to be adequate to control the RS-385 motors, but not much more. You'll need an upgrade when you go to larger drive motors.
FlySky CT6B 6ch 2.4GHz transmitter & receiver -- Upgrade to the FS-i6 The CT6B is a very basic and difficult to adjust R/C system. For very little more you can purchase the FlySky FS-i6 system which is much easier to program, has many more control options, and does not require the added expense of a programing cable.
Astro Heavy Duty 75mm TPR Rubber Wheels -- These wheels will not attach to your motor output shafts without modification. They are designed to spin freely on an unpowerd shaft; the bearing must be removed and an adaptor hub fabricated to mate to your very small 6mm gearmotor shaft. That's a fair amount of work.
300W 20A DC-DC Step Down Voltage Regulator -- This is not needed A proper transmitter (like the FS-i6) can be programmed to provide reduced power to the drive motors via the motor controller. As your motor driver is rated for your full battery voltage you can use this trick. See this archived post for specific instructions using the FS-i6: you'll decrease the CH2 mix setting from 100% to 50% to get half-voltage.
TecoKart Battery Cut Off Switch 12V/24V -- WAY bigger and heavier than you need, but it will work.
24V DC Motor 2500rpm -- You have spec'd all the other components, but all I get here is a voltage and an RPM. I can make no comment.
Do you require "braking functions" on your 24 volt brushed motor controller to meet some weapon spin-down time requirement? I have never heard of such a requirement in Bharat. As you have provided no motor specs I can make no specific ESC recommendations, but I will mention that if you use a bi-directional ESC you can GENTLY apply a small reverse current to create a braking force and decrease the spin-down time.
All Fall Down
Q: I am making a 150 g grabber robot that clamps onto other robots and pushes them into a pit, and since our competition mostly consist of zippy vertical spinners, i am using two 1200 rpm motors to catch up with them. To prevent the robot from tipping while driving fast, I have placed the battery near the front of the robot. However [I'm concerned that] when I drive the other robots into the pit, I also fall in due to the far-forward center of mass (I think). How can I optimize my robot for not falling into the pit? (My robot consists of a bent metal box with wheels and two metal forks.) [San Diego, California]
A: Mark J. The description of your robot is quite sparse. You say it is a grabber, but only mention two metal forks, a metal box, and an unspecified number of wheels. Is there a servo powered clamp? I'm imagining a design something like 'Jawbreaker' from 'Robotica' (pictured below).
A few general comments:
You probably do not want or need to mount the battery in the front of the robot. See Where Does it Go? two posts down this page for guidance on weight distribution to prevent nose lifting under acceleration.
If two robots go into a pit together, most competitions award a win to the robot that was in control when they went in. A two-wheeled clampbot will most often follow its victim into a pit.
A longer four-wheeled clampbot can have a center of mass far enough back to allow it to safely approach a pit and drop in an opponent without tipping itself in as well -- if that's what you actually need.
I'll need more information about your 'bot to give more specific advice.
Reply: Thank you for your feedback! Here is a sketch of my robot:
A: The render helps. Your design has very little weight on your drive wheels. When you add a good chunk of your opponent's weight onto the front forks you're going to have very little traction to push your opponent around and real difficulty in executing a turn. I would suggest smaller diameter wheels moved well forward so that an opponent clamped above them could add weight and improve traction. You might need to extend the forks upward to keep your trapped opponent away from the wheels if the new wheels extend too high. That should give you enough maneuvering control to set them right on the pit edge and drop them in.
I'd also add a rubber tip to the "grabber". Your motor is unlikely to have enough power to get that sharp point to dig in and hold.
Reply: According to SPARC, if the robots are stuck together when entering a pit they will be separated and taken back to the arena. If I am still clamping the robot on my dustpan will the count as us being stuck together?
A: There are two paragraphs in the SPARC Match Rules covering this situation. The first says whoever goes in first loses:
If the arena is equipped with a Death Zone/Pit/Push-out or similar hazard a robot entering this area in a one-on-one match will result in the end of the match and a loss for the robot that first entered the area.
It's just like a sumo match: whoever first touches outside the line loses and the match is immediately over. That is the common rule in robot combat and it makes universal sense. However, two paragraphs farther down in the rules is a paragraph that confuses the issue:
In the event that both robots enter the death zone simultaneously they will be returned to the combat area and the match will resume. A robot that places its opponent in the death zone must be able to do so without also becoming stuck itself. If it is not able to separate
from the other robot this will be treated as simultaneous entry.
Different event organizers interpret this paragraph in different ways. Some interpret this rule as only covering cases where the 'bots are literally stuck together. Others will reset anytime both robots end up in the pit. It varies from one event to another so it pays to clarify this with the organizer of your specific event.
Precious Little Distance
Q: For those flipped xl belt designs now prevalent in the modern beetleweight clsss (w/the smooth side wrapped directly around the outrunner can as a slip clutch) how am I supposed to gauge the correct distance to get the belt to not slip off and also not be vulnerable to snapping? Like how undersized should the belt be? Say for example I'm in the 50T range, do I want my weapon to require a theoretical 53T belt distance and then buy a 50T belt so it stays on? Or a 51T belt distance?
Should the motor be mounted on oval screw holes such that it can be slid back and forth to determine the perfect tension? [Hidden in an iCloud]
A: Mark J. I'm not a big fan of this approach to weapon belt drive.
Timing belts have effectively zero 'stretch' and brushless outrunner motors are not designed for lateral loading on their cans. This combination leaves precious little distance between "too loose" and "can deformation destroys motor". Adding or subtracting a single tooth changes the circumference of an XL belt by 0.20", which is far too coarse an adjustment for a friction-only belt drive.
The preferred method of getting the tension right is to gather the weapon components (hub/motor/belt) and mock up the system on your workbench. Snug it up, measure the distance between hub center and motor shaft center, and transfer that distance to your weapon mount CAD.
Yes, do use elongated holes for motor mounting to get your final adjustment after testing. I add hot-glue to fill in the elongated holes once the screw positions are set to prevent unexpected tension failure if the screws loosen.
Where Does it Go?
Q: You've said that a two wheeled robot should have 65% of the weight on the drive wheels. My CAD program gives me the location of the center of mass but where do I place the mass center to put the right amount of weight on the wheels? [West Sacramento, California]
A: Mark J. For best traction you want as much weight on the drive wheels as possible, but leaving too little weight on the front of your 'bot will allow it lift up off the arena floor under acceleration and make it vulnerable to attack. Designing for 65% of the robot weight on the drive wheels gives good traction and leaves enough front weight in most cases.
To place 65% of the weight on the rear wheels:
Measure the distance from the weight-bearing point of contact at the front of the robot back to the axles of the drive motors;
Move components as needed to place the mass center at a point 65% of the way rearward along that measured line -- see illustration above.
Note 1 - Robots with unusual layouts or odd dimensions may require different axle weights. Page 46 of the RioBotz Combat Tutorial has a discussion on calculating optimum placement of the center of mass based on traction and the height of the mass center.
Note 2 - Magnet downforce may be used to correct traction and lift problems in a steel-floored arena. Combined gravity and magnetic weight of 65% on the drive wheels will still be a good starting point.
The Tools You Know
Q: I'd like to get started in combat robots by building an antweight. I've asked about getting started on [social media site] and people there tell me to use a 3D printer. I don't have access to a 3D printer so is there another way to build a simple robot? [Southern Germany]
A: Mark J. Builders who regularly use a 3D printer get that method stuck in their head.
"If the only tool you have is a hammer, every problem looks like a nail."
- Abraham Maslow, Psychologist
The above saying describes the tendency to rely too heavily on a familiar tool or method, even when it's not appropriate for the situation. It highlights a cognitive bias where people apply a single, known approach to various situations, potentially overlooking more effective alternatives.
3D printers have a large up-front cost in terms of both skills and expense. Once you have paid that price the printer becomes a quick and effective tool. But if you have not paid that price it is more effective to use techniques and skills with which you are comfortable. You can build a simple robot simply by gathering compatible components, bolting them down to a stiff baseplate, and adding protective structures and offensive capability as you see fit.
I'll also point out that there are combat robot kits -- with or without a chassis -- that allow you to quickly construct a functional combat robot you may later modify as you see fit.
Horizontals Stay Flat
Q: Hi - I'm building my first UK antweight robot, which is a giant 50g bar overhead spinner, with inspiration from robots icewave, moros and bloodsport. It's 2WD, with the brushless motor in between the two drive motors, and a screw in the back to put the blade at an angle. My problem is so: When testing the drive, it's completely fine, when testing the weapon, it's also fine. When I drive and spin the weapon, the robot is incredibly unbalanced, and pings around my test box like an air hockey game.
Could you give me some tips on how to make it balance better? (Also the weapon bar itself is balanced.)
Cheers! [Eaton, England]
A: Mark J. That's a very pretty 'bot with a lovely spinner bar, but you've changed a critical design element from the robots you credit as inspirations. That change is causing your problem.
The spinner bars for 'Icewave', 'Moros', and 'Bloodsport' are all precisely horizontal -- the rotational axis of the weapon remains pointing straight up when the robot turns. You have elected to place "...a screw in the back to put the blade at an angle." That angle causes the direction the weapon axis points to change when the robot turns. Combined with the huge rotational inertia of your weapon this causes a strong gyroscopic reaction, which raises one side of the 'bot, which then causes additional changes in the weapon axis, which then... you get the idea. This is why you don't see angled horizontal spinners.
There is no fix other than tilting the spinner axis back upright and making sure it stays like that. 'Bloodsport' and 'Icewave' are four-wheeled chassis to help keep their blades 'flat' while 'Moros' is two-wheeled but has the center of mass well back toward the trailing rear skid for stability.
Yes, setting your bar fully horizontal raises the weapon up too high to be terribly useful. Call it a rookie mistake. Do not be tempted to 'droop' the impactor end of the bar -- that causes a different type of instability.
I Still See It Clearly
Q: Does any footage of your insectweight bots like 'Rat Amok' and 'Zpatula' exist at all? [Havertown, Pennsylvania]
A: Mark J. Our insects all fought before video was easy. The only footage runs in my head, and you don't want to go in there.
Until Its Hat Floats
Q: 'Ramfire 100' won [the heavyweight class at the] 1994 US Robot Wars, then fell off Earth as far as I know. Do you know if ANYTHING else happened to it? [Lehi, Utah]
A: Mark J. Michael Sorenson's 'Ramfire 100' fought two matches at the 1994 US Robot Wars and won them both to become the heavyweight champion. Depressed that there were no more opponents to defeat, 'Ramfire 100' made its way to the center of the Golden Gate Bridge and leapt off - only to land on the deck of a freighter bound for Finland. After hitching a ride into Helsinki, 'Ramfire 100' worked as a forklift in a paper mill for several years and saved every penny until it had enough money to open a small cigar shop. It married a lovely girl named Helmi and had four children: a girl, two boys, and a fax machine. The family is doing well.
A: Oh, that's completely different! Embarrassed to learn that its direct-drive spinner weapons (that work pretty well in 1-pound robots) instantly self-destruct in the 250-pound class, 'Chrome Fly' heads home from the 2016 BattleBots tournament but makes a left turn off I-15 at Provo and drives west until its hat floats. It sank into the deepest part of the Great Salt Lake, which isn't all that deep. The end.
Resetting the Rat
Q: Has anyone tried to make a robot like 'Rat Amok' with a 'Snail Cam' (second of four designs on our Spring Flipper Designs page)? A rat trap with a near infinite number of resets sounds very effective.
- sincerely, Iceywave [West of San Antionio]
A: Mark J. I'm quite proud of 'Rat Amok' -- she is a one-of-a-kind antweight that won her first tournament and was later victorious at the "King of Robotica" match between myself and season two champion Mike Konshak with a clean OOTA ejection. Her one-shot weapon struck fear into her opponents, and the suspense of "when will it fire?" added much to the drama of her matches.
A 'snail cam' mechanism is quite bulky due to the large spiral cam. It would be difficult to translate the linear motion of the cam follower into a 180 degree rat trap reset. I suspect this is why no one has attempted a snail cam version of 'Rat Amok' -- but your question got me thinking...
I just now took Rat Amok off the shelf and measured the torque required for a reset against the torsion springs. A full 180 degree reset requires an initial torque at the axle of 12 kg-cm that rises to 29 kg-cm at the end. That is well within the capacity of high-torque R/C servos that weigh about 2 ounces and have up to 270 degrees of motion.
Rather than a snail cam, I believe that the 'Servo Latch' (fourth of four designs on our Spring Flipper Designs page) would be more easily modified to reset the trap, latch it, and trigger the release. Might be fun!
Are Smaller Bots Weaker?
Q: I saw a discussion on [social media] where somebody said that multibots don't work out well because pound-for-pound the smaller robots don't have as much weapon power as larger robots. They said they didn't know the math but that's how it works out. I figure you know the math. Is there an explanation for this? [iCloud Private Relay]
A: Mark J. I'll guess that 'the math' referred to here is the the Square-Cube Law which points out that directly upscaling an optimized design requires disproportionally greater mass to be allotted to structural components, which leaves less mass available for things like weapons. But I think the poster has it backwards -- smaller robots require less proportional mass for structure which leaves more mass available for weaponry.
The chart below is from our Spinner Weapon FAQ. It shows typical spinner weapon motor weight as a percentage of robot weight for 1-pound thru 220-pound robots. A typical 60-pound lightweight weapon motor weighs about 30% greater per pound of robot than does a typical heavyweight weapon motor.
However: If you're going to toss four 60-pound robots into an arena with a 250-pound robot, you'd best build those lightweights with thicker armor than they would need when facing robots of their own weight. I figure the added armor mass will just about negate the weapon weight bonus from the square-cube law.
There are good reasons multi-bots aren't generally effective, but a reduction in the total available weapon power of a multibot swarm compared to a single heavier robot is not one of them.
I Know a Trick
Q: I'm building a combat robot for a local competition, but it is very difficult for me to get suitable motors and electronics in my country. The only high currrent brushed motor controllers available to me are the BTS7960 H-bridge motor drivers. The BTS7950 accepts pulse width modulated signals for variable speed cotrol but I cannot figure out how to set up my FlySky FS-i6 radio to get both forward and reverse response from the motors. Can you help? [Location Withheld]
A: Mark J. The BTS7960 motor drivers are inexpensive and claim a continuous bi-directional output of 43 amps at up to 27 volts. They are a little bulky, but appealing for controlling brushed motors in intermediate weight combat robots.
The problem is that the BTS7960 is designed to be controlled by an Arduino microcontroller board rather than directly by R/C receiver output. There are seperate input pins for forward and reverse motor rotation, and each pin expects a full 0-100% duty cycle PWM signal. The FS-i6 just can't manage that and still provide single-stick control -- but I know a trick.
It just happens that the circuit board in an analog servo takes standard receiver output and splits it into the two PWM signals the BTS7960 is looking for. You'll need a couple old servos and a 5 volt power source:
Take apart an old servo and strip out the control board.
Connect the control board leads that went to the servo motor to input pins 1 and 2 of the BTS7960.
Provide +5 volts to pins 3, 4, and 7, with a ground connection to pin 8.
Plug the servo lead into the appropriate receiver port and your BTS7960 will respond just like a regular ESC.
Below is my circuit diagram with an inset showing the details of the input pin wiring.
An explanation of why this trick works gets technical. If you want the details write back -- if not just be glad it works.
No Steering Wheel
Q: drive train [Madhya Pradesh, Bharat]
A: Mark J. I've never actually tried, but it shouldn't be very hard. You can only go where the rails go, so just give it a little throttle and toot the whistle once in a while.
They Don't Get It
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
A: Mark J. My robots don't care. My robots don't spread, suffer from, or die from Covid-19 -- but you can. Don't be selfish. Follow the science. Stay safe.
Remembering 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
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.
We welcome combat robot questions. Please search the Ask Aaron Archives first to see if we've already answered your question. Recent Q&A are posted above.
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.
Q: Can a xt60 connecter handle 30 volts. 
A few general comments:
A: Mark J. I'm not a big fan of this approach to weapon belt drive.
Q: 'Ramfire 100' won [the heavyweight class at the] 1994 US Robot Wars, then fell off Earth as far as I know. Do you know if ANYTHING else happened to it? [Lehi, Utah]
However: If you're going to toss four 60-pound robots into an arena with a 250-pound robot, you'd best build those lightweights with thicker armor than they would need when facing robots of their own weight. I figure the added armor mass will just about negate the weapon weight bonus from the square-cube law.
An explanation of why this trick works gets technical. If you want the details write back -- if not just be glad it works.
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
