Ask Aaron: questions and answers about combat robots

7876 Questions and Answers about Combat Robotics
from Team Run Amok

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 Messages not asking substantive combat robot questions will not be published.

Try Elsewhere
Q: What controller would be best for robot sumo? [Maharashtra, Bharat]
A: Mark J. From ten posts down this page:
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.
For robot sumo questions you might try r/robotics on Reddit. You'll need to include information like weight class and a link to your rule set to get useful answers.

The Return of YCOSV

Q: YCOSV guy back again with another question about engagement theory. Your breakdown of the moment of impact into a normal and tangential force vector -with the normal force vector being opposed by a kick back- was enlightening to me. However I'm stuck between two possibilities:

In idealized conditions (smooth non-wedged armor, constant closing velocity) at contact...

A. The normal vector component is the ONLY force relative to the opponent that matters for damage- so the optimal speed value is some undefined amount below the "skate" threshold dependent on some math I don't want to do right now.

B. If the normal force component exceeds the kick-back vector produced by the hard armor panel, then the weapon will be pressed or "engaged" with the opponent robot and then some of the tangential force can be dumped into the opponent - therefore the optimal speed is the highest you can go without skating, and energy transfer "cliffs" off after that point.

A: Mark J. Both of your scenarios are important to energy transfer, but which holds greater importance will be dependent on the weapon style.

A horizontal spinner will generally benefit from maximizing the forces you describe in your "A" scenario, so as to generate acceleration on a vector thru the mass center of the opponent rather than a tangential acceleration that would simply spin the them around.

A vertical spinner seeking to 'launch' their opponent will most greatly benefit from catching a sharp edge but, should none be available, the ability to engage as described in your "B" scenario comes into play.

Zorro Arcade

Q: I want to set up my Radiomaster Zorro for arcade stick style driving (throttle controls forward/back and steering is on the aileron). I have done single stick mixing before on a Flysky but I am still trying to learn the Zorro. I think I understand the steps in your EdgeTX Combat Radio Tutorial to set up a single-stick mix, but what would I need to change to make it dual-stick? [Somewhere in California]

A: Mark J. My new combat robot setup guide for EdgeTX uses a Radiomaster Zorro transmitter for its example. This Link will jump you straight to the single stick mixing section of the guide.

This is what the Mixes Screen looks like with a right single-stick mix using CH1 Aileron and CH2 Elevator:

EdgeTX single-stick mix

To shift forward/reverse control to the left stick we will remove CH2 Elevator from the mix and add CH3 Throttle in its place:

EdgeTX throttle on left mix

You'll also need to adjust your left gimbal to be self-centering. Here's a video: Zorro gimbal adjustment.

Once you get the mix set on your transmitter it may still need to be tweaked if the robot is not responding correctly. Maybe a forward command causes the robot to turn left? My Combat Robot Mixer Fixer will ask three questions about your robot and will provide a quick correction.

Setting Up Zorro

A: Mark J. I always recommend downloading the manual for any transmitter you are considering purchasing to see if the instructions make sense to you -- before buying the radio. Setting up a Radiomaster transmitter is a whole different world from FlySky.

FlySky uses a menu-driven system where you just check boxes and move some sliders to setup your transmitter. Simple.
Radiomater employs the EdgeTX/OpenTX firmware which is essentially a programming language. It's flexible, but challenging.

Fortunately, I've written a combat robot setup guide for EdgeTX/OpenTX that will walk you thru setting up single-stick control. The guide uses a Taranis Q-X7 transmitter for its example, but for the purpose of single-stick setup the Radiomaster differs only in the buttons used for menu navigation.

Once you get the mix set on your transmitter it may still need to be tweeked if the robot is not responding correctly. Maybe a forward command causes the robot to turn left? My Combat Robot Mixer Fixer will ask three questions about your robot and will provide a quick correction.

If you get stuck I'll be happy to assist with specific problems.

NOTE: The Zorro actually has a "Delta Wing Mix Wizard" that will do a quick automatic version of the single stick for you. You'll still need to go in and adjust the elevator settings to +/-100 to get full forward/reverse thottle response, so I personally find it easier to just set-up the mix by hand. If you want to give the wizard a try, here is a video: RadioMaster Zorro Setting up a Delta Wing in EdgeTX - Model Wizard.

Are you sorry you gave up on your FlySky yet?

Q: Hey! It's the guy with the Zorro. I'm actually looking for help with mixing to two sticks, where the left stick up/down would be forward reverse and right stick left/right would be steering. I think I understand the steps in your tutorial but what would I need to change to make it dual stick?

A: Sorry for the misread; you said "throttle" but then later mentioned "single stick" on the FlySky. My bad. I prefer throttle on the left stick and steering on the right stick as well. You have your choice of two easy ways to accomplish this:

You can modify the Mix Screen to use CH1 Aileron and CH3 Throttle instead of CH1 Aileron and CH2 Elevator. The new screen looks like this:

OpenTX throttle on left mix

Alternately, you can use the standard delta wing mix as given but switch the transmitter "Mode" from the default "Mode 2" (throttle on left/aileron on right) to the Euro standard "Mode 1" which moves CH2 to the left stick and CH3 to the right stick:

Power On: Turn on the RadioMaster Zorro.

Enter System Setup: Access the main menu and go to the System Setup.

Navigate to Radio Setup: Scroll to the "Radio Setup" menu.

Change Mode Selection: Select the desired "Mode" and change it to "Mode 1".

Whichever method you choose, you'll also need to open up the transmitter case and adjust your left gimbat to be self-centering. Here's a video: Zorro gimbal adjustment

Flashback: an archived post from 2021

Like Frozen Pizza

Q: Isn't there anything we can do about those stupid kit bots? They're ruining the insect weight classes! I really hate to see some guy show up at a tournament with a kit they put together in an afternoon and beat up real robots that real builders took a lot of real time designing and fabricating. Can't they just compete in a class for kit bots only? [Everybody on Reddit]

A: Mark J. Complaining about kit bots is like complaining that frozen pizza is ruining dinner because it tastes better than what you cook. If you can't beat a kit bot you need to up your game.

Buy a Better Battery

Q: Went to set up my new Viper V3 antweight kitbot today. Followed all the instructions but can’t get it to work.

I made it to testing with the 9V battery. When I power it on the ESCs lights are bright red but then instantly shut off, and the receiver slowly blinks red. No inputs do anything. Now after several tries no lights turn on at all. I checked all connections. [Social Media]

A: Mark J. How long has that 9 volt been in your battery drawer? The LED signals you see and the sudden complete electrical failure all point to a weak battery. Try a fresh battery.

Q: That got it! The 4th 9V I tried fixed this. Charging up a Lipo battery now.

Combat Drift Car

Q: I have an old RC drift car where I have this fun idea of turning it into a combat robot. I would like to keep the body intact if possible since I like Japanese sports cars and want to maintain that theme for my robot. Would this idea even work? And if so, how viable would it be in a competition? [Albuquerque, New Mexico]

Cheerleader A: Mark J. The drive train and chassis of your drift car are designed for very different conditions than are the drive train and chassis of a combat robot. Your car is made for speed and open spaces; how well could it maneuver in a small arena, and could it generate the torque and pushing power needed to hold its own toe-to-toe with a machine designed for combat? Can you acquire a supply of replacement parts to repair the extensive damage it will suffer and are you able to complete such repairs in the limited time between matches?

If your goal is to add a wedge and some armor to your drift car and make it legal for a combat tournament where it will be destroyed in its first match -- yes, you may be able to find a competition that would accept it as an entry. If you want to add active weaponry you would need to strip out your radio and electronics and replace them with gear that provides the required "fail safe" response on loss of signal. That would be impractical.

If you don't like my opinion you can always Ask the Cheerleader.

A Level of Achievement

Q: For the Combat Robot Hall of Fame, what would you do if you got a threshold number of votes for a name that many robots use, but the voters didn’t specify which one they meant? [Newton, Massachusetts]

A: Mark J. There is occasional confusion in assigning votes for the The Combat Robot Hall of Fame but we have never had to sort votes amongst more than two robots. A certain level of achievement is required for Hall membership and I cannot think of a case where "many" prominent robots share a single name.

There was one instance where two well-known robots of the same name but different global regions each received substantial votes for membership in The Hall. Our voters are generally well informed so most ballots specified which robot they were supporting by team name or country. We found it reasonable to sort the remaining votes by the region from which the ballot originated -- US votes for the US robot and UK votes for the UK robot. Unspecified votes from other regions were few in number and would not have swayed Hall status for either robot.

There are also cases where a team develops a design in one weight class and then carries over both the design and name to another weight class. As noted in our Hall Eligibility page, in a case like this the two robots may share a single listing in the CRHoF -- like SawBlaze/MegatRON -- but only if both have credentials to support membership. This year the CRHoF received enough votes to award 'Emulsifier' an Honorable Mention in the Hall. None of the ballots specified either the featherweight or the heavyweight version, but the five-time NHRL champion featherweight 'Emulsifier' clearly has credentials to join the Hall, where the 1-win / 3-loss heavyweight 'Emulsifier' clearly does not.

We do our best to interpret ambiguous votes fairly. We would prefer that voters cast unambiguous votes.

It Keeps Blinking

Q: I have been trying to pair my FlySky FS-i6 transmitter with a new FS2A receiver but am having difficulties. The FS2A has never been paired before and the FS-i6 has only been paired with the FS-iA6B receiver it came with (successfully).

The issue I have is when I put the receiver into pairing mode (fast blinking) and turn on the transmitter in pairing mode, the receiver immediately exits pairing mode and begins blinking slowly again -- it doesn't switch to a solid light to indicate pairing like the FS-iA6B did.

Do you have any idea why this happens or how to fix it? [Boardman, Oregon]

A: Mark J. Your FS2A receiver is binding correctly, but its LED display is a little different than the FS-iA6B you previously bound.

When the FS2A in bind mode (rapid flashing) recognizes the FS-i6 transmitter in bind mode it will bind and revert to slow flashing -- exactly what you are seeing. Turn off the transmitter and power-down the receiver. When you power back up (transmitter then receiver) the FS2A will correctly display "solid on" indicating that it is bound to your transmitter.

The Joy of Discovery

Q: Watching Robotica again (getting close to finishing S3) and I have a few questions:

1) I noticed that while 150 points total are available in the Gauntlet, the highest one bot can get is 105 (80 for getting all 8 obstacles first - which would probably require a Piece-de-Resistance-tier bad opponent and/or an opponent's immediate breakdown, 10 for first up the ramp, and 15 for the bonus pane) Do you think this is worth noting? [Somewhere in the Willamette Valley]

A: Mark J. Your math is correct, but as none of the competitors reached that 105 point limit ('Viper Revision 2' came closest at 95 points in S3) I don't find it vital to mention on my rule summary. Besides, publishing the limit might prevent someone else from experiencing the same joy you found in your discovery.

2) What happened to the teams behind *deep breath*...

Spring Breaker, Grimlock, Hot Wheels, Boelter Beast, Solar Flare, Crisis Management, Hamerschlag, Krypler, Evil Beaver, KaNuckle Buster, Tetanus, Scarab, TakaTakaTak, Armorgeddon, Metalmorphis, Skewer Rat, Ill-Tempered Mutt, Wizard of Sawz, Shannonagans, Monkey's Wrench, Northern Fury, Thor, Scrap 8.2, Buzz Bomb, Pangolin, Nemo's Nemesis, Zeus, Particle Accelerator, Denominator, and Fury?

Did any of them compete in anything else, or did they all leap off the nearest bridge, end up on a freighter to Uzbekistan, settle there and marry a local, and raise a boy, a girl, and an automatic circumciser?

A: Many (possibly most?) of the 65 teams that competed at Robotica also competed in other combat robot events, but as best I know the 30 you list here all jumped off that bridge and found other things to do with their lives.

As an aside I will mention that although the 'Evil Beaver' team itself did not appear at other events, team member Camp Peavy had previously competed successfully at the US Robot Wars and continues to be involved in a wide range of robot competitions.

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 17^th end date -- now corrected to read August 10^th.

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)

Bite (grip) occurs if:
F_tan ≤ F_fric_max ⇔ (τ / r) ≤ μ_s · k · v_n / (n · ω)

Skate (slip) occurs if:
F_tan > F_fric_max ⇔ ω > (μ_s · k · v_n · r) / (n · τ)

Critical spin speed ω_crit = (μ_s · k · v_n · r) / (n · τ)

Brief Thesis Writeup

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 important, 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.

Q: On the skitter effect seen when attempting to hit a smooth steel plate with an over-speed weapon, you write:

Hardened steel sliding along hardened steel ... given that one of those steel parts has approached at a shallow angle (note my comment above on "Radius to Tooth Tip") ... 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.

But for a compliant material, (such as those TPU unibodies that are everywhere in the insect classes) wouldn't that kickback simply not occur because the force of the blade goes into deformation of the material rather than being reflected? So therefore, wouldn't I be able to spin my weapon arbitrarily fast against such materials without being bite-limited? Silent Spring in 2017-ish used to have a saw tooth disk to slash up foam tires pre-TPU, but perhaps the 95A hardness mainly used does provide enough of a kickback to bite-limit my weapon? Of course, in reality, I am acting on your advice of the practical match strategy being to modulate RPM adaptively - but I'm speaking from a purely theoretical standpoint here. [Reston, Virginia]

A: Mark J. Yes, the softer the material your impactor encounters, the more surface deformation you get. Greater bite is still preferable -- inserting your opponent farther into the weapon path will give a steeper impact angle that provides greater velocity into the material as opposed to along the surface. The illustration below may aid in visualizing these impact angles. Kickback still occurs, but less of the impact energy goes into that kickback and more into deformation.
Weapon Bite Angle

About Eighty Grams Worth

Q: what motor to use for a beetleweight eggbeater robot [Barnet, United Kingdom]

A: Mark J. Big weapon? Small weapon? Direct drive? Belt drive? Budget? Battery voltage? See: Frequently Asked Questions #29 and The Hamburger is Bad.

A Rare Exception

Q: i am making a 15kg bot in which i have decided the specs for the bot should be:

GenX 22.2V 6S 3300mAh 40C / 80C Premium Lipo Lithium Polymer Battery
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). Combat robot 'Jawbreaker' 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: 150g clampbot

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] Inside-out XL timing belt on weapon hub 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.

Correct mass center placement for two-wheeled robot

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.

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.

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