5571 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.

Q: Mark, what's your opinion of some teams using drones to assist their "main" part of the robot? Would that really be effective? Because I saw Basilisk's assist drone being quite useless in the preview episode of the second season of Battlebots. [Chinese Forum]

A: [Mark J.] BattleBots has changed. It used to be an open tournament that anyone with a robot and the entry fee could enter, and the robots were built to win matches. Now you have to apply to be part of the 'show' and the robots are evaluated on their entertainment value. Adding a drone might make the difference between acceptance and rejection -- useless or not.

I can say that you may be more impressed by drone performance later in the 'tournament'.

Correctly designed snail cam
Q: Mark, my old antweight robot has gotten so broken that I have felt that it is beyond repair and I should build a new one. So for my new robot I have gotten a basic chassis drawn up in CAD to try and reuse any parts that I can, but for the weapon, I wanted a spring loaded flipper but the spring compression and release mechanism has been a massive pain for me so just asking if you have any suggestions for designs? [Guangdong, China]

A: [Mark J.] Are you really in China or are you just routed thru a Chinese IP address? I'd love to see some photos or video of Chinese robot combat!

Take a look at this video of a cam-driven compression/release spring flipper. There's a discussion of the design in the Ask Aaron Weapon Archive. It is the simplest effective design I've seen.

Q: Hey Mark,
Are bearings absolutely necessary for a spinning disc/bar weapon? I was looking at Team Nightmare's build of Backlash (1st version) and nothing was mentioned about bearings. It's kinda hard to tell in the pics, but it appears that the disc is attached to the pulley and they both spin on a dead shaft. Are there some type of bearings in there that I'm not seeing or is it possible to spin a weapon on nothing but a greased up shaft? [A few miles south of here, Oregon]

A: [Mark J.] Take another look at the weapon pulley in the 'Backlash' build report -- that's a sealed bearing at the hub. The loading at the hub of a high-energy spinning weapon far exceeds the capacity of a 'greased up shaft'. On a good hit it would spall and seize. You can (and a fair number of builders do) use oilite bronze bushings in place of ball/roller bearings to support weapon shafts. They have a bit more friction but can survive enormous shock loadings. Keep them well oiled.

Q: So if I wanted to use something like a needle roller bearing, I would put it on the dead shaft and then attach the weapon/pulley to the outside of the bearing? If so, what is the best attachment method? I know set screws aren't great but could they work for a 12 pounder?

A: Not a great idea to apply significant point force (like a set screw) to the outside race of a bearing. A needle bearing would typically be installed in the weapon/pulley as an interference press-fit with thrust bearings and spacers taking up any lateral motion on the shaft.

Alternately, flanged ball bearings can be simply inserted into the sides of the weapon/pulley assembly to be held in place by the structure supporting the dead shaft. Washers or tubular spacers can take up any excess clearance.

Australian Featherweight 'Decimator'
Aussie 'Decimator'
Q: I've been noticing that some spinners that use outrunners to mount the motor pulley to the can of the motor and not the to shaft. Best example I can think of is the Australian featherweight 'Decimator'. Other than reducing height, does this offer any other advantages? Any significant disadvantages with this? [A few miles north of here, Oregon]

A: [Mark J.] A design like Aussie 'Decimator' (photo at left) mounts the weapon pulley on the can and tucks the entire weapon drive inside the chassis without requiring an internal bracket to support the weapon motor. It's clean and simple, and it does save a bit of height while still protecting the weapon belt. Simple is good.

The drawback is that the pulley is now located at the far end of the motor from the motor mount. This violates the general engineering rule of providing support as near as possible to the point where lateral force is applied to a structure. Decimator's design places great mechanical loading on the motor's internal bearing support compared to mounting the pulley in the conventional manner. The short height of Decimator's weapon motor keeps the load from becoming too great -- stresses on a taller motor would be worse. Can mount vs. shaft mount pulleys

Q: i designed an antweight wedge on sketchup and i put the receiver, the esc , the batterie, and the motor inside of the frame. i waana know how much room should i keep for the wire? [Quebec, Canada]

A: [Mark J.] Some of those components get hot in operation, and others don't like to be close to each other due to electromagnetic 'noise'. Space them out a bit. If they're so tightly packed that you're worried about room for wire the components are too close.

If you're not careful your wires can turn into a nasty tangle that can take up as much room as the other components combined (photo at right)! Give some thought to where each wire starts and where it ends, and shorten excessively long wires rather than just stuffing them into tight spaces. Do not shorten antenna wires!

Nasty tangle of wires in a combat robot.

Q: I would want to build an antweight wedge powered by 2 sparks motor (22.2:1 gearing) powering 2 banebots 1 7/8 wheels. I wanna know if this 1000 mAH HobbyKing LiPo battery could be good for this use bot.
thx for ur answer, i hope the hamburger is not bad. [Quebec, Canada]

A: [Mark J.] The Tentacle Drivetrain Calculator estimates that the robot you describe could get along quite well with a 100 mAH battery -- providing enough juice for a full 3 minute match with a 100% safety margin. A 1000 mAH battery is much bukier and heavier than you need.

High-quality LiPo batteries with very low capacities are uncommon. If you are comfortable with HobbyKing products, I'd suggest this Turnigy nano-tech 180mah 2S 25~40C Lipo Pack.

Typical brushless 'outrunner' motor disassembled Q: how can i install a pulley on a brushless outrunner? [Quebec, Canada]

A: [Mark J.] Well, you could slide a nice FingerTech pulley onto the output shaft of the outrunner and tighten down the Allen screw. That works. I'd file a flat spot on the shaft to give the allen screw a better bite on the shaft, and I'd put a drop of threadlocker on the screw to keep it from loosening.

Q: Hello Mark, it's the Deadblow to beetleweight guy again. I was wondering this time if tempered glass used in certain screen protectors would be viable for armor? I plan to possibly use it for the bottom of my chassis. I would send you a picture but, I don't know how to copy and paste it using my phone lol. [Florida]

A: [Mark J.] A 'tempered glass' and plastic laminate screen protector works well on a smart phone. It has a hard surface that resists scratches, and when supported evenly across its length and width by the phone screen it is not subjected to bending or twisting forces. But take a look at the table below:

MaterialYield Strength (MPa)
Aluminum 2014-T6400
Stainless Steel 302520
Titanium Grade 5830
Tempered Glass210

You can see that tempered glass does not have great yield strength, but worse is what happens when the yield strength of tempered glass is exceeded. If you press aluminum, steel, or titanium beyond yield strength it dents, but if you press tempered glass beyond yield it shatters. You also can't cut tempered glass to size. Not what you want for armor; you'd be better off with just the plastic bits.

My choice for bottom armor in an insect 'bot is G10 garolite. It's light, impact resistant, and tough as nails. There are multiple posts about garolite in the Ask Aaron materials archive.

Q: Hi Mark,

I'm designing a 3-lb Weaponized 4WD setup. Because of the weapon, to save weight and space, I am looking at making some kind of pulley system for [the drive wheels]. I was wondering how that is typically done? In larger bots I have seen the back wheels powered and the front wheels on dead shafts connected by pulleys. My problem with this set up is that most motors that I am looking at (e.g. 1000 rpm motor) have too short of a shaft for this. Would it be better to have a driven pulley that connects to both wheels on dead shafts or could I alter the gearbox to have a longer shaft?

Also I'm having trouble wrapping my head around how to attach the wheels and pulleys to each other. Most of the wheel hubs that are available in this class either are meant to be supported only on one side (i.e. Daves Hubs) or are too small and flimsy to properly attach a pulley to (i.e. Snaphubs). Do most people with this set up make custom hubs? Because that is the only solution I am currently seeing.

Thanks for your help. [Ohio]

A: [Mark J.] Replacing the gearbox shaft would be a real pain, and indirectly driving all wheels with belts is unreliable. Go have a look at Bone Dead Robotics' build log for Wedge of Destruction v3. Brandon doesn't write specifically about his hub solution, but there are many photos that should give you a good outline of what he did. It appears that he bolts FingerTech Snap Hubs directly to FingerTech timing belt pulleys and uses the pulley hub to support both pulley and wheel on the short gearmotor shaft. This is preferable to indirectly driving both wheels with belts.

UPDATE: I wrote to Brandon for details on his hub solution. He tells me that there are no bolts holding the wheel hubs to the pulleys -- they're just superglued in place! He recommends boring a 6.2 mm recess into the pulley hub to accomodate the nub that sticks out of the wheel hub and using that to locate and support the wheel. I'd put a couple of machine screws thru the assembly in addition to any adhesive -- just because.

Q: Hi
This is a wonderful website and I have been lurking around for a little over a year trying to gather enough info to build my own robot.
Well that time has come and I am currently working on an ant weight vertical spinner and I am having a little trouble finding a suitable blade for my spinner. The best thing I have found so far is a 3" diameter saw blade. My only concern is the fact that the disc is pretty much smooth around the edges. I'm not sure if I would get enough bite for it to be effective. Do you have any better ideas or am I just over thinking it?
Thanks in advance,
Ant-Noob [Florida]

A: [Mark J.] 'Pretty much smooth' sounds kinda ineffective. I think you want something that at least looks intimidating. The prototypical antweight vertical spinner 'VDD Kit' used the 4-3/8" Makita carbide tipped saw blade pictured at right. If you want something a little smaller, there's a similar 3-3/8" carbide tipped blade. Either of these blades will fit the new FingerTech clamping blade hub.

Current weapon design favors a spinner weapon with very few impact teeth, but I think either of these blades will gave adequate 'bite' if you spin them at a reasonable speed. If you want to upgrade to a one or two impactor disk later you can always make your own aluminum disk with a couple of bolt-on impactors, but I like the idea of using an off-the-shelf blade to keep your first 'bot simple.

Tech note: a large diameter vertical spinner has less trouble with multi-tooth 'bite' than a smaller vertical spinner. The impact angle is different, and it tends to pop the opponent up and into the next impactor rather than just tapping the opponent back.

Q: is a antweight wedge with an angle of 45 degree will be able to get under opponent? its a dragging wedge, since the bot only has 2 wheels [Quebec, Canada]

A: [Mark J.] That kinda depends on your opponent. A 45 degree wedge is generally more of a defensive design than an offensive weapon. Shallower angles are more effective at getting under opponents, if the leading edge is sharp and low enough.

A search of the Ask Aaron Design Archive for 'best angle' and 'lowest wedge' will find several posts on this topic.

Q: is it possible to convert a throttle signal to analog so i can use the trottle stick on my receiver to control something like a lifter? [Quebec, Canada]

A: [Mark J.] That's exactly what an Electronic Speed Controller (ESC) does -- it converts the digital output from the R/C receiver to an analog current level, with or without reversing.

To control a lifter you'd require that analog output, plus positional feedback. A standard R/C servo has that, plus a motor and geartrain all built into a single unit.

There are a variety of other R/C interfaces for other specialized purposes.

Q: Which is the more reliable hub, a set screw hub or clamping hub?

The application is for a wedge beetle using 4 drive motors. It specs out, breaking traction at approximately 1/3 of motor stall torque with a max speed of 6-7 mph in the 5X5 arena. I've only used set screw hubs in the past with less than ideal results. I was looking for your advice for which one is more advisable considering this statement made by the manufacturer:

"The [clamping hubs] offer more holding power in high torque applications. The disadvantage is that they are not as equally balanced for high speed use."

Is this a high torque or high speed situation? Does your opinion change if only 2 motors/wheels are used -- breaks traction at ~60% of stall torque, top speed ~5.5 mph? [Pennsylvania]

A: [Mark J.] You've got a high torque situation with abrupt and frequent reversing. As you have apparently learned, set screws suck in these situations. Two-wheel or four-wheel drive makes no difference. Although there are things you can do to improve the reliability of the set screw (threadlocker compound, machined 'divot' in the shaft...), all things being equal a clamping hub is much more reliable. But in this case, all things may not be equal.

The hubs you're looking at are made to mate to the Actobotics disk wheels -- which are not suitable for combat applications. From the speed and breakaway torque you specified, you're planning on 2" diameter wheels. The hubs would be difficult to adapt to combat wheels, so I'm going to suggest that you consider the BaneBots T40 wheels and matching T40 hubs. They aren't clamping hubs, but the T40 wheel/hub is far superior to the Actobotics wheel/hub combo. Use a little theadlocker on the setscrew threads and you'll be OK.

If your design has the wheels fully exposed to attack from spinner weapons, you'll need something a bit better suited to abuse. Consider the FingerTech Snap Wheels and Hubs. Again, use threadlocker on the set screws.

Q: how could i give a blue, energetic look to my flamethrower like th one used in complete control? [Quebec, Canada]

A: [Mark J.] We don't discuss flame weapons here at Ask Aaron. See Frequently Asked Questions #28.

Q: Were competing in a junior high battlebot competition. 50 pounds maximum on robot. Were wanting to do a spinning weapon like tombstone. Any thoughts on motor, maybe a cog belt. and pulleys? [Amarillo, Texas]

A: [Mark J.] Start by reading the Ask Aaron Spinner Weapon FAQ. The FAQ outlines the key elements in spinner weapon design and points you to tools to match a specific weapon design to a suitable motor and weapon drive.

There are also a great many posts in the Ask Aaron Combat Robot Weapons archive about weapon motor selection.

As for drive components, browse the Mechanical and Drive Components selection at Robot Marketplace.

I'm pleased to provide the tools needed to design your weapon, and when you have your design roughed out I'll be pleased to look it over and offer suggestions. I won't design your weapon for you, but I'll give you a start:

An AmpFlow F30-150 motor @ 24 volts spinning a 24" x 3" x 1" steel bar via a 4:1 belt drive reduction. The bar weighs 20 pounds, spins up to more than 1600 RPM in 4 seconds, and stores better than 4200 joules of kinetic energy at that speed.

Q: Do you know anyway to mount 3" colson wheels on a 4mm 'D' shaft. I know I could custom make a hub but I was trying to avoid that. Anything off the shelf or that can be put together easily? I'm thinking trying to make a mount like you would on a car but can't make the jump in my mind to a easily workable solution.

  • I'm thinking these but am not certain if they are large enough to mount around the bearing and cant think of an easy way to utilize the bearing in the mounting design.

  • Or these bastardized some how.

  • Or these, cut to get rid of excess length, and a new set screw hold tapped in compliance with these but then I would likely have to ask how to make the shaft longer on a gear motor or if there is a way to extent it unless you think the long hub would be stable despite not being mounted with only one set screw.

Sorry if the links are confusing... I saw a lot of possible options but none of them easy. I guess I'm looking for advice on which is the easiest and/or most secure of if this is even possible or if you know of some dark magic to make them stick. Thanks.[Connecticut]


Hubs take a lot of loading in a combat robot, and it's really embarassing to have your drive wheels fall off. None of the options you've found are suitable for a combat robot. Get them right!

It would be a great help to have some details on your robot (weight, drive layout, motors used...) so that I might better judge how much loading the wheels and hubs are likely to face. I will say that it is uncommon to fit such relatively large and heavy wheels to a gearmotor with a realtively small 4mm shaft. Proper Colson hubs typically start at about 3/8" shaft size. Why are you determined to use Colson wheels?

Have you considered the BaneBots T40 wheels and their ready-to-mount BaneBots T40 4mm hubs? Without details on your robot design I can't offer more guidance, but please don't rely on 'black magic'.

See also Frequently Asked Questions #8.

Q: Is this Park 370 Outrunner EFL-370-1360 Brushless Motor powerful enough to spin a 2.5 lb steel bar (overhead spinner curved down like that in one of the designs in the riobotz book) or a fbs shell? Live shaft most likely but dead shaft is possible. Using Timing belts and a gear ratio of about 3.5:1 with and rpm of about 4000. I would like to know if it is strong enough to spin the weapon up to speed in a reasonable amount of time under combat conditions (being hit while attempting to spin up). 11.1v lipo.

Or should I use this Feigo inrunner motor?

Thank you. I know my questions could probably be answered in the excel spinner spreadsheet but I am not very good at using that particular tool. Using the lighter motor would be amazingly helpful for weight but I fear it is too small to spin that much mass effectively under combat conditions and will get burned out. [Pennsylvania]

A: [Mark J.] I'd suggest you start by reading the Ask Aaron Spinner Weapon FAQ, wherein you will find out why a spinner weapon cannot be evaluated based only on the weight and material of the spinner, and why the dimensions of the spinning mass are critical in the calculations.

It's also not reasonable for me to comment on adequate performance of a spinner weapon without knowing in what weight class the robot will compete. Is this for a mantisweight, hobbyweight, BotsIQ, or maybe something else?

Neither of the motors you ask about are suitable for your purpose:

  • The Park 370 has a power output around 100 watts -- suitable for an antweight class spinner, but way too small for you.
  • The Feigo does not list any specifications except RPM per volt. All I can say is that it spins way too fast for your design.
If you aren't willing to do the calculations to pick out a suitable weapon motor, look at successful robots in your weight class with designs like yours to see what weapon motors they use. That will get you close.

Important note: you don't seen any successful examples of that 'overhead spinner curved down' design in actual combat because that particular rotor shape in unstable! See this post in the Combat Robot Weapons archive. Pick another design.

Q: Do you have any thought on the PERM PMG 132 motor vs a Motenergy ME0708 or ME1003? This will be for use in spinning blade or drum robots.

I understand the performance differences but wonder if there are any strengths or weakness in either of them that would be specifically important to combat robots. [Orange County, California]

A: [Mark J.] I'm going to defer to Ray Billings on this one. Ray and Team Hardcore have a whole lot of experience torturing big pancake motors in heavyweight weapons. They've gone thru a BIG stack of original Eteks, PERM PMG-132s, and Motenergys. None of them hold up for long, but if you need big weapon power you have to live with short lifespans. Ray estimates it costs him about $1000 a match for equimpment replacement -- mostly batteries and weapon motors.

His current choice - for reliability and power - is the Motenergy ME0708, also known as the 'Etek-R'. If Ray likes it, I like it.

Hackbot 'Plane Jane'

Q: Hello Mark, I plan to go to "Bot Blast" in Pennsylvania, but my Deadblow beetleweight may not be completed in time due to money restraints. So, would turning my Son of Whyachi R/C toy into a wedge bot be, legal within SPARC rules, without removing the weapon motor? I know that it will be dead weight but, I don't think I have the engineering skills required to remove it properly. I also plan to use your R/C hack guide to do this. Thanks-Luke from Alabama. [Alabama Supercomputer Network]

A: [Mark J.] There is no weapon motor in the 'custom series' BattleBots toys, Luke. The weapon is powered from the drive motors via a gearbox. The weapon only spins when the robot is moving and therefore does not even qualify as an 'active weapon'. The gearbox itself is a simple snap-fit into the chassis and is removed at a very early stage in the Robot Dojo BattleBots Toy Hack Guide. You can leave the gearbox in place if you like -- it doesn't violate any rules.

Bot Blast allows use of toy radio systems for robots with no active weapons, so your wedgebot should be fine.

Q: Hello Mark, this time I have "hacked" into my Son of Whyachi BattleBots R/C toy using the Robot Dojo guide on the website. (The guide was very helpful by the way) For some reason every time I try to turn the bot on, it lights up and then shuts off. Do you think I missed a wire, do I need to solder anything, or what do you think I did wrong? Thanks-Luke

A: Step #2 in the guide:

Now pull the weapon gearbox straight out of the main chassis module. Its a snug snap fit. If you try and run your bot at this point, it won't work, as there is an interlock that keeps it from going unless this is in place. We'll bypass that in a bit.

I'm guessing that you removed the weapon gearbox but didn't follow thru down to steps #18 thru #20 where it tells you how to do that 'bypass'. You need to solder a wire across the two rectangular pads on the bottom of the motor driver card that made contact with the slider you removed with the weapon gearbox.

Each step in the guide is there for a reason -- don't skip any!

Q: Would a alien power system c8080 brushless motor work for a heavyweight vertical spinner? If not, what brushless motor would you recommend for a heavyweight vertical spinner? [State of Tennessee Department of Education]

A: [Mark J.] It's poor design practice to start with a weapon motor and build the rest of the robot around it. Design your robot to do what you need it to do, then build to meet that specification. All of the components of the weapon (and the robot) must work together.

That said, a 5000 watt brushless weapon motor like the C8080 is underpowered by current heavyweight standards. Design a suitable heavyweight-class weapon (think 'Tombstone') and plug in the performance specs for different motors until total weapon performance meets your goals. There are heavyweights that use Alien Power weapon motors, but not the cute little C8080.

Q: should i consider using xt60 connectors in a antweight robot? they seem a little bulky and i have never seen any antweight using them [Quebec, Canada]

A: [Mark J.] The XT60 plugs are overkill for an ant. I've always used Deans connectors in small 'bots and never had a failure. The Deans Micro Plug can handle 20 amps and is small enough to tuck into really tight ant spaces.

Avoid the 'look alike' knock-offs -- use real Deans. They're popular for R/C stuff -- your local hobby shop probably has them in stock.

Q: Hello,
Why are all full-body spinners' drive systems so slow? I know they need good stability especially with the gyro effect, but why can't they be fast? And would making the top part spin the opposite direction as the bottom tooth part eliminate gyro and maybe even help with the "throwback" con of those spinners?
Thanks, Missouri

A: [Mark J.] Let's talk a bit about the 'gyro effect':
  • The gyro effect -- more correctly 'gyroscopic precession' (video) -- is a force acting at a right angle to a force that's acting to change the direction the axis of a rotating body is pointing.
  • The axis of a vertical spinner weapon points left/right in relation to the robot. Turning DOES change the direction that axis is pointing. A vertical spinner (like a drum spinner) DOES exhibit a 'gyro effect' when turning that can lift one side of the robot off the arena floor.
  • The axis of a horizontal spinner weapon points straight up/down in relation to the robot. Turning DOES NOT change the direction that axis is pointing. A horizontal spinner (like a full-body spinner - FBS) DOES NOT exhibit a 'gyro effect' when turning.
Horizontal spinners don't suffer from the adverse 'gyro effect', but they can have trouble with simple torque reaction. Newton's third law says that for every action there is an equal and opposite reaction, so when you apply torque to spin-up a big horizontal spinner, an equal torque is applied that attempts to spin the robot in the other direction. This force greatly diminishes once the weapon is up to speed, but at weapon start-up it can be quite troublesome.
Gyroscopic precession force diagram

Now that we have the terminology sorted out, let's get back to your questions.

Not all FBS are slow, but most builders skimp on the drive system in order to put additional mass and power into the weapon system. Let's face it, a FBS is all about the weapon. The attack strategy doesn't depend on speed, and any side of the robot is as dangerous as any other so maneuverability isn't an important factor either. All you need to do is move toward your opponent and wait for contact. Adding a powerful drive system steals power from the weapon and doesn't significantly add to the effectivness of the robot.

Splitting the shell into two counter-rotating components adds significant complexity and fragility to the weapon system and results in less energy storage in the part that actually hits your opponent. It could eliminate the torque reaction on spin-up, but the 'throwback' on impact is a of Newton's third law and the horizontal impact vector. You can't cheat Newton.

Q: So the rotational energy from the counter-rotating top wouldn't stop the bot from spinning away from impact?

A: That's right -- Newtonian counter-reaction trumps counter-rotation every time.

Q: What I'm trying to figure out is how to not get beat up from super-low wedge spinner bots. If they come in slow enough to move the gyro they can get under you. Unless you have a fast drive you can't get where they can be hit.

A: Like I said, not all FBS are slow. If you're willing to trade off some weapon power for robot speed there's nothing in the design to stop you.

Q: How do you defeat a bot that has a knife edge wedge and titanium spinner on top of that?

A: People beat wedges all the time. Suggest you watch some video of matches with this type of opponent to see how it's done in whatever weight class you're building. Modify your design and strategy accordingly.

Q: Hi Mark. First, thank you for your website and efforts in educating the rest of the community that desires to learn and compete. Spending countless hours on the website I have grown to appreciate the awesome task you have undergone and the vast wealth of knowledge you possess.

If you don't mind, I have a question on behavior of servo lifters (this is for the beetle weight class). I most likely will be short on details but will try to be through without adding too much extra detail. Imagine a wedge bot (in my case a box with only one wedge). Then split the front plate into thirds, securely fastening the outer two leaving only the middle plate. Attach this plate to a servo in the traditional mounting method (no four bar lifter, just a bar going straight to the servo and attaching securely). Now in this configuration I would like to know what happens when the middle section is attacked directly by a vertical spinner before I am out a few hundred bucks and waste alot of time perfecting a flawed conception. If the section is grabbed by a vertical spinner will it be tossed like it was hinged or would it remain fixed like it was just a solid piece in the front or somewhere in-between? Would the attack cause any damage to the servo (I'm particularly worried about striping the gears despite the servo I have in mind being titanium)? Would the same be true if the plate was attempting to go down while being hit by a spinner forcing it in the upward direction?

Thank you for your time. If you could please explain the reasoning why I'd be grateful and don't want to keep asking questions that would be child's play if I understood how a servo works and why (as I assume at some point it will break and I'll attempt to frantically repair it during competition). Thank you for the difference you are making in the world. [Greenville, Pennsylvania]

Servo Saver A: [Mark J.] You're correct to be worried about what happens when a servo takes a sudden impact. An R/C servo is designed to hold the controlled surface firmly against external force -- to the point of breakage. The force of an impact from a beetleweight spinning weapon would be passed along your simple linkage and shatter gears and the housing of the servo. The problem is the 'attaching securely' bit.

Going to a four-bar mechanism offers a couple of advantages over "just a bar going straight to the servo". The path of the lift can be designed to lift in a curve forward and away from the robot, but equally important the four-bar structure itself can direct some of the force of an impact away from the delicate servo and into the more robust hinges attached to the chassis. But the servo itself is still vulnerable because of its unyielding geartrain.

The R/C car racers had this same problem when their cars ran into unyielding objects like curbs. They developed a device called a 'servo saver' to keep such impacts from destroying their servos. A servo saver contains a spring mechanism that provides allows a certain amout of 'give' to the system that absorbs some of an abrupt impact, transmitting less of the force on to the delicate servo geartrain.

I suggest that you run a web search for 'servo saver' to find several different designs. Evaluate those designs to see how well each might fit into your lifter concept. Do also consider a four-bar mechanism -- it helps.

Q: Greetings:
I was wondering how effective the following weapons would be on a robot. Note that I've ordered these roughly from "least likely to work" to "most likely to work", and am only seriously considering building the last three.
  1. A vertical spinning weapon on a pneumatic piston
  2. A horizontal spinning weapon on a pneumatic piston
  3. A drill/'Niterider'-style weapon on a pneumatic piston
  4. A pneumatic piston on a horizontal spinning weapon
  5. A lifter that deposits the opponent onto a spinning weapon on top of the robot (if you need a better description I can link you to a picture.
  6. A horizontal spinning drum weapon
  7. A "face spinner" (think 'Invertibrat' from Season 4)
  8. A front-hinge flipper a la 'Firestorm' or 'Cassius', except the lifting arm is replaced by a blade to potentially damage the opponent (obviously this would have to be a high-powered pneumatic flipper)
And yes, I know your "efficient, simple, easy to use" mantra, I just wanted your opinion on this. [Portland State University]

A: [Mark J.] I don't consider any of the weapons to be practical, so let's talk about the designs purely from a conceptual standpoint.

  1. A spinner weapon gets much better 'bite' if the closing speed on the opponent is greater. In close quarters, having the ability to quickly thrust the spinner forward is conceptuallly valuable.
    Conceptual score: A
  2. Same benefit as #1, but horizontal spinners throw both the attacker and target in opposite directions. Less desireable.
    Conceptual score: B
  3. 'Niterider' had a 'disembowler' spike intended to penetrate another robot then twirl some stiff wires around to chop up internals. Would work really well if combat robots were armored with cardboard and filled with eggs. Adding pneumatics won't help.
    Conceptual score: F
  4. Whaaatttt?? How does that accomplish anything except sudden imbalance?
    Conceptual score: F
  5. I really hate dual-weapons. Decide whether you want to flip 'em or shread 'em, then put all your weight allowance into a single effective weapon.
    Conceptual score: D
  6. OK, maybe this one is practical. The drum stores more energy than a bar or disk, so it has some promise. Keep the drum fairly short and with a large diameter and you might have something.
    Too practical for a conceptual score
  7. 'Invertabrat' was an unsuccessful 'flipper' robot that violated the 'two-weapon rule' and added an ineffectual milling head to the back of the robot. Spinning at full speed the milling head might possibly leave a nasty scratch on aluminum or plastic. Enlarged, one edge goes up (good) and the other edge goes down (bad). Hit with the wrong edge and YOU fly upward.
    Conceptual score: F
  8. Blades can't cut modern robot armor, and a sharp edge makes it easy for your opponent to slip off to one side or the other. If you wanna flip 'em, flip 'em. If you want a pneumatic pickaxe, build that. Points for visual appeal.
    Conceptual score: C

Q: Dear Aaron Archives,

Just writing to update on the progress. [See: previous post from Team Schmidt]
Robot Solutions replaced the controller and we have not had any problems since.

We competed in Robogames, the kids learned a ton from building the robot and competing.
We are making plans for next year.

A: [Mark J.] I'm happy to hear that the motor controller issue was resolved, and I'm VERY happy to see you and your boys out competing. You held your own in combat, gained good experience, and you must have had a good time. Nice job! Things like getting the start proceedure right will come with a little more experience.

The Schmidt boys with their robot

Featherweight drumbot 'Proteus'
Drum spinner with machined pulley groove
Q: Hi Mark! I have a quick question on drumbots. How is a drum usually mounted and supported on a dead shaft, such that it can handle direct impacts and keep the shaft from bending? [Midvale, Utah]

A: [Mark J.] Usually? Sturdy endplates, great big bearings and a big hardened steel shaft anchored securely to the chassis as close to the bearings as possible.

Q: Hi Mark, I'm following up on my drum spinner mounting question. What if the drum was split in half with the pulley mounted in between? In my case this would be necessary given the compactness of the bot and motor mounting restrictions. So the shaft would either be 1.5" thick titanium or 1.1" thick steel anchored on the far left and right sides of the bot, and the drum itself is about 2" thick with a 6.3" OD not including teeth. It just has that vulnerability in the middle where the drum is split to be able to mount the pulley in between. Is this a viable option if built as robust as possible? If not, another option could be using the drum itself as a pulley? (Assuming the gearing could still be made correct)

A: I really dislike splitting the drum right at it's point of greatest structural weakness. Adding additional bearings near the center to support the split would load the shaft at its weakest point -- a poor engineering choice.

Your drum is amply thick to machine a v-belt groove (or two) into the drum. That will weaken the drum a bit, but not nearly so much as splitting the drum to add a central pulley.

Q: Hi Mark. That sounds good. I was leaning towards a built-in V-groove as well. Tolerances will have to be very high on the machining though, since the plan is to have a single tooth and a counterweight spanning across the entire drum. So the V-groove will have to go through those two AND the drum, misalignment between the the three could cause troubles. It should work out fine though.

I could also just split the tooth and the counterweight in the middle to allow the pulley to go around just the drum. Is there a disadvantage to splitting a tooth into two pieces? When the drum hits it can potentially put all of the load on just one tooth.

A: You haven't mentioned the length of your drum. Assuming that it's fairly long I'd split the impactor tooth and the counterweight. At the energy levels a large drum weapon generates even a tool steel impactor will elastically deform enough on impact to localize the loading, so there is little benefit to a long-span tooth.

Q: Drum guy here again, thanks so much for your help so far! The total drum length is 11", but it actually won't be split directly down the middle, based on the latest design... more like into a 3.5" piece and a 7.5" piece. By the way, just wanted to mention that with the current materials and dimensions the drum will generate just under 20KJ at 90% RPM.

A: That's a lot of energy for a weapon that size. An aluminum drum with the dimensions you give plus steel tooth & counterweight weighs about 33 pounds and needs to spin close to 8000 RPM to store that much energy. Balance is going to be critical. Best luck.

Q: Drum guy here again. Thanks for the input! You are exactly right about the total weight and RPM. I'm curious though, what dimensions did you estimate the steel tooth would be, and how much it extends out of the drum? I've been trying to balance how far it extends out vs how far it is supported in the drum, and have yet to determine the sweet spot.

A: I didn't assign specific tooth dimensions. I just added a bit of mass to the outside diameter of a drum with the dimensions you gave to bump up the weight and get the RPM for 20K joules to come out a round number. There is a formula to calculate the maxiumum useable tooth height for a specific weapon at a specific closing rate:

Tooth Height (inches) = Closing Speed (inches per second) * 60
Weapon RPM * Number of Impactors

The formula requires some assumptions on closing rate, but does at least provide some guidance. How deep to set it into the drum? Best engineering judgement.

Q: would u recommend spark motor for a 1 lbs wedge bot? are they fast and powerful enough? [Quebec, Canada]

A: [Mark J.] Yes. The Fingertech 'Silver Spark' motors are widely and succesfully used in antweight wedge robots. They come in a range of gear ratios. The correct gear ratio will depend on the tire size, the selected voltage, the number of motors you will use, and the size of the arena. Example:

  • A one-pound robot with two Silver Spark motors and 2" diameter wheels running 7.4 volts in an 8 foot arena will perform best with the 22:1 gear ratio.
The Tentacle Drivetrain Calculator is the tool of choice for calculating robot gearing.
Q: On a heavyweight robot, would you recommend a 2 wheel drive setup or 4 wheel drive? I'm leaning toward 2 right now but want to be educated. [Tennessee State University]

A: [Mark J.] I like people who want to be educated rather than just be handed an answer. It takes longer for me and for you, but the result is more rewarding. Here we go:

When you go out to buy a car, do you start by deciding how many cup holders the car should have? Unlikely, because the number of cup holders is not critical to meeting the needs you have for a new car. Likewise, when designing a combat robot you don't start with the number of wheels. Start with the specific type of attack you wish to use and the specific method by which you will execute that attack. Example:

You want to grab other robots and hold them immobile while you saw thru the top armor. You plan to execute this attack by clamping your opponent with a pneumatic pincer and lowering an arm with a circular saw down from above.

Now you can design a chassis to support the weaponry required for that sort of attack. In this case:

The weaponry will be fairly heavy and the attack does not require great speed or pushing power, but maneuverability is critical. A two-wheel 'dustpan' style chassis is appropriate.

Start with exactly what you want the machine to do and design toward that goal. The number of cup holders will become obvious at some point in the design process.

Q: You've made some passingly negative comments about the new Battlebots show, would you care to elaborate? [Mount Laurel, New Jersey]

A: [Mark J.] I think I've been clear in expressing my concerns. I don't think the approach the BattleBots re-boot takes in selecting competitors is good for the sport.

The original 'Comedy Central' era BattleBots events were 'open' tournaments, unique among televised combat robot events. Anyone with the bug could build a robot, travel to the next scheduled event, and compete. Hundreds of thousands of people watched the show, and hundreds of them built robots and fought in the tournaments; 396 robots fought at the Season 3.0 event alone. Jay Leno built 'Chinkilla', Homer Simpson built 'Chief Knock-A-Homer', audiences were involved! The robots themselves developed at a frantic pace with new designs spawning and being tested at each event. That was great for combat robotics.

Now BattleBots is back, but the tournaments are no longer open and inclusive. Builders must 'apply' to gain one of a very limited number of spots at the event. The application process is lengthy and difficult with robots being judged by the producers NOT on how well they will fight, but on how they will look on television. The robots selected look like they were built by NASA contractors with major DARPA funding. A parent and child sitting at home watching the show are not going to think that they could strip down that old lawnmower in the garage and turn it into a robot.

Team Simpson and 'Knock-A-Homer'
Team Simpson

The new format presents combat robotics as a closed, elitist activity. The brain trust behind BattleBots has long pushed to develop the sport into a larger enterprise like NASCAR, but that's not what the original format was about. Going upscale will not engage the audience. I had hoped for better.

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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