5875 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.

Indian combat arenas are not safe! Q: making a combat robot using wiper motors but the wheels are not moving straight ....if there a problem of ground coz the motor also not working together connected with a single battery....can u help me [Madhya Pradesh, India]

A: [Mark J.] I cannot accept any questions from India -- Why?

Given your description I can't figure out what your robot is doing, let alone propose a solution.

Try asking at the Facebook page for 'Combat Robotics India' -- perhaps they know what you're talking about.

Q: Hi Mark. A couple questions.

1. In some videos I saw on youtube I see a little LED light, what is that? Is that the receiver or something?

A: [Mark J.] Most tournaments require a visible 'power on' indicator light as a safety feature. If the light is on, the robot is active and dangerous. In small 'bots this function is often met by the indicator light on a speed controller, but in larger robots there is typically a dedicated power light.

2. What is/are the most common speed controllers used in heavyweight bots? [A series of tubes]

A: Speed controller requirements depend on the specifics of the drive layout as well as the robot weight. Robots with large weapons may have drive trains with just enough power to move the weapon into position, while robots that rely on speed and acceleration may have drive motors many times more powerful that require speed controllers capable of delivering huge current at higher voltages. No single speed controller is 'right' for all heavyweights.

Assuming use of brushed drive motors, here are a few speed controllers used in heavyweight robots:

Each has pluses and minuses. Selecting the right ESC requires analysis of the drive train and setting reasonable expectations. Examples of drivetrain analysis to select an ESC can be found in the Ask Aaron Robot Motors and Controllers Archive.
Q: Do you have any advice for someone who's about to be driven insane by how you have to compress the tires on Fingertech's snap wheels to quark matter densities in order to get the snap ring on? If I spend one more minute with these things I'm going to fall asleep and wake up in Carcosa, if you know what I mean (and you'll know what I mean if you've read The King in Yellow). [North Carolina]

A: [Mark J.] ...or if you've read Ambrose Bierce.

Gosh, it looks so easy in the FingerTech installation video! Here are some real-world solutions I've run across on the web.

Builder Jeff Gier suggests:

  1. Stack two quarters concentrically on each side of the foam tire.
  2. Smash the tire overnight under a stack of books, toolboxes, etc.
  3. The tire gains an indentation that lasts about half a day, making assembly easy.
Builder Matt Spurk uses a pair of spring clamps to hold the retainer in place while placing the snap ring (photo at right).

Builder Robert Masek made and sold a few customised Vise Grips with specialized jaw extensions welded in place (drawing at right).

Do not go together!
Do not go together!

Update: Builder Alex Horne wrote in with a link to his You Tube video showing how he uses a pair of slip joint pliers to quickly install snap hubs.
Q: I have a robot idea which involves a 4 wheel drive and a weapon that is both an axe and a flipper at the same time (ie, it is a flipper which is rear hinged but able go 180 degrees). Which would be the best motor for the weapon, and which would be the best motor for a 4 wheel drive if it is a heavyweight? [Uxbridge, England]

A: [Mark J.] Axe and flipper go together like anchovies and ice cream. An axe head needs to be out at the end of a long handle so you can swing it up to speed, but you wouldn't deliberately design to lift the entire mass of your opponent way out at the end of a long arm. Consider:

Do not go together!
  • The power profile of an axe weapon is very different from a flipper. An axe requires a drive that will accellerate the relatively light axe head on the end of a long arm to very high speed over the full span of the weapon throw, while a flipper requires enormous thrust to accelerate the entire mass of your opponent upward over the span of just a few inches. The same mechanism simply isn't going to do both jobs.

  • While an electric motor can drive the axe head on a long arm up to speed in 180 degrees of arc, truly huge torque is needed to lift (let alone flip) a 100 kg mass way out at the end of that same long arm -- enough torque to shatter a reasonably scaled gearbox. You can build an electric axe, but a decent flipper requires the explosive power release available from pneumatics.
As for the drive motors: selection depends on multiple factors about your design, pocketbook, skill level, and expectations that you have not shared with me. See Frequently Asked Questions #21. We gladly provide tools to assist builders in component selection but we won't do your homework for you.

Note: According to BotRank, there has never been a combat robot named 'Anchovy Ice Cream'. Help yourself.

Q: How to calculate spinup time for a weapon running on a brushless motor. I'm building a combat robot with an asymmetric single tooth disc running on two brushless motors. [Maharashtra, India]

A: [Mark J.] Scroll down the page 7 posts for our policy on questions from builders competing in India. Your question has been previously answered here at 'Ask Aaron'. You are free to search for it -- should you find the determination to do so. Look here for a start: brushless motor power curve.

Indian combat arenas are not safe!

The Hamburger is Bad Q: Would 1mm thick, grade 5 Ti be strong enough to replace the 1/8" thick, mild steel alloy im using for my antweight wedge? Im doing the switch mainly for weight reasons [Quebec, Canada]

A: [Mark J.] See FAQ #17. I can tell you that the well-supported wedge on antweight 'Algos' is made from 1/16" (about 1.6 mm) grade 5 titanium, but as I know nothing about your wedge design and support structure I cannot comment on the suitability of any specific material for your purpose. The hamburger is bad.

Q: Hello Mark, it's Luke again with yet another new question. I was wondering if it would be a decent idea to use the Hexbug remote control 'Fire ant' as a fleaweight. I think that it would be a fun, cheap and overall enjoyable little robot to fight with, if it meets the requirements of course. Thanks for your help, Sincerely -Luke [Tennessee]

P.S The pan robot is nearly complete and it will be ready to fight in the next 'Robot Battles' event in Atlanta in May!

A: [Mark J.] The Hexbug 'Fire Ant' uses an infared remote control similar to a television remote. Line-of-sight control systems like this are difficult to use in a combat arena and are allowed only at the discretion of the event organizer. It's neither fun nor overall enjoyable to have control problems with your 'bot in a match.

It's also very little fun to charge into a custom 150 gram spinner robot with a $15 passive plastic toy. You couldn't hope to put up a good fight, and if I were the event organizer I wouldn't let it in the arena.

Best luck at Robot Battles!

HexBug 'Fire Ant' toy

Q: Hello!

I have a question regarding different configurations of outrunner brushless motors. Specifically, I seem to see some that are designed to attach the load (wheel, prop, whatever) directly to the rotating "can", and some that appear to have an output shaft exiting the motor from the opposite end (the stationary part where the wires attach). Finally, I've seen some that appear to do both. [example links deleted]

Am I correctly interpreting how these different designs work? And on the dual-output designs, is it possible to attach things to both ends at the same time?

Thanks for your help -- Ian M [San Jose, California]

A: [Mark J.] Yes, and yes.

Take a look at the photo at right. Outrunner brushless motors have two 'chunks':

  1. a stationary 'base' component with wires, coils, and a tube running thru the center with bearings on each end; and
  2. a rotating 'can' component that is attached to an axle at one end.
The length of the axle and its attachment point to the can determines if it sticks out one end or the other. Typically the shaft will extend out thru the 'base' end of the motor, and a bolt-on 'prop shaft' may be added to the 'can' end if needed -- but it is possible to replace the motor shaft with a longer shaft and position it to stick out both ends if that best suits your needs. YouTube has many outrunner shaft replacement videos.
Outrunner brushless motor - partially disassembled

It's good engineering practice to take power from a shaft as close to the motor mounting plate as possible. Forces applied at the far end of the shaft place additional strain on the bearing tube and can cause the mounting structure to flex. You might consider a support bearing block out at the end of the shaft if your design calls for a pulley or sprocket that far from the mounting base -- but you can take power from either or both ends of the motor.

Note: You mentioned a wheel in your question. You may attach a prop, sprocket, pulley, gearbox, or small spinner weapon directly to the motor -- but small brushless motors do not heve enough torque to allow you to directly attach a robot drive wheel.

Comment: Hello Mark, Ian M again.

Thanks for your answer, that helps a lot. For context, this is just for a design excercise I've been playing with that's unlikely to actually be built: an invertible HOG drive. In this case, a variable speed reduction is inherent to the design and the shafts would have extra support bearings, so that's not an issue, but having a dual-ended motor would simplify things a lot so it's good to know that's a real thing. In any case, thanks for the warnings about supporting the shaft and the speed reduction, even if they weren't needed.

Q: Because my design requires it, I have to use 4" liteflites for my beetleweight. The issue is that the larger liteflites have a larger inner diameter and are wider than the more common liteflites, so my usual choice of Snap hubs are useless. As I have to make custom hubs anyway, I'm wondering the best way of keeping compression on the hub washer; snap ring or shaft collar? [40 miles north]

FingerTech Snap Hub A: [Mark J.] FingerTech Snap Hubs use snap rings for a couple of very good reasons:
  • they weigh very little; and
  • they don't come loose.
If your hub design is along the same lines as the Snap Hub, use snap rings.

Q: can i damage my lipo if i use them in my bot until they run out? [Quebec, Canada]

A: [Mark J.] You will damage your LiPoly battery if you discharge it below about 3 volts per cell. DO NOT continue to use a damaged lithium battery! See the 'Recognizing a Damaged Li-Poly Battery' section of our Li-Poly Battery FAQ - and read the rest of the FAQ while you're there.

Most electronic speed controllers have a feature that shuts down the robot at around 3 volts/cell to protect your lithium battery from over-discharge. With this feature turned on it is safe to run yout robot 'til it stops. This is handy for practice, but you don't want your ESC to shut your robot off in combat! Check your user manual to learn how to turn this feature on and off.

Q: Is it necessary to do that "pre-running" for motors bought directly from Kitbots? [Rockville, Maryland]

A: [Mark J.] I recommend pre-running ALL brushed motors -- particularly small, inexpensive Chinese motors like those from robot component suppliers like Kitbots. Suppliers do not do this for you.

For more info on brushed motor break-in see this article at the RC Universe Forum. Note: I don't recommend the 'wet' break in process, and a few minutes of running-in is adequate.

Indian combat arenas are not safe! Q: Hey Mark can you comment on the vertical spinner of professor chaos. [Madhya Pradesh, India]

A: [Mark J.] I cannot accept any questions from India -- Why?

You are welcome to search the archives for answers to your question. There are a multiple posts about 'Professor Chaos' in the Ask Aaron Robot Weapons archive. Search there for 'Professor Chaos'.

Q: I have a heavily modded viper and I was wondering if a piece of 1 mm stainless would be good for the bot. We have no specifications except it holds up decks. How does that compare with average titanium. And if it wasn't obvious enough I am using it for an ant [Elkridge, Maryland]

A: [Mark J.] 'Stainless' is a designation for a steel alloy containing at least 10.5% chromium. The addition of chromium makes these alloys resistant to rusting, corrosion, and yes -- staining. Other materials in the alloy (nickel, manganese, molybdenum, aluminium, sulfur, titanium, carbon...) determine its physical characteristics: hardness, strength, toughness, malleability, machineability, and chemical resistance.

There are a great many steel alloys that qualify as 'stainless' that cover a very wide range of physical characteristics. The most common grade for purposes such as deck building is type 304. Compared to the usual grade of titanium alloy used for robot armor (Ti-6Al-4V) 304 stainless has about 1/4th the yield strength, 1/3rd the hardness, and 1/2 the toughness. It also weighs about 80% more. Let's just say that NASA doesn't build spacecraft out of 304 stainless.

Used for a wedge, it's gonna get bent up badly compared to other available alloys. 4130 steel alloy is currently popular for sub-light and insect weapons and wedges. It is hard, tough, can be heat treated for added hardness, and available in small quantities from internet suppliers at reasonable prices.

There are dozens of posts about titanium and steel alloys in the Ask Aaron Materials and Components archive.

Q: I have 1 more spec. It took 2 nights to cut with a cutoff saw going at approximately 20,000 rpm

A: If you're not going to believe me when I tell you it's not good, why bother to ask me if it's not good?

I don't know how to convert units of 'Cutoff Saw RPM - Nights' into traditional material specifications. As I said above, stainless steel alloys are not made to maximize properties valued in robot combat armor. Feel free to try it, but it certainly isn't going to perform like grade 6 titanium.

P.S. - You need a new saw blade.

Q: I'm looking to build a compact bot with a robust, redundant, and reliable drivetrain. Long story short, given my size constraints, something like Whyachi's M3M would be too big for the design, but their planetary P3 would be perfect! This design's drivetrain would be composed of four individual systems, each with its own wheel, gearbox, ESC, battery, etc. Do you see the P3, or other compact planetary gearboxes for that matter, as a viable option for drive? [Lihue, Hawaii]

A: [Mark J.] Anything Team Whyachi makes is 'viable'. I'd give the wheels some impact protection, or at least use squishy tires.

Builder Orion Beach ('Skorpios', 'Electric Boogaloo'...) had some trouble with the P3 planet gears locking on their shafts under loading. He recommends polishing the gear shafts/bores and replacing the lithium grease with a high-pressure molybdenum disulfide grease. He also suggests supporting the output shaft with an additional bearing or bushing if you plan to mount the wheel directly to the output shaft.

Q: Hey! I wired up my new beetleweight and it's acting weird. When I push the trottle stick forward the 'bot starts to go forward, but then one or both of the motors reverses and response gets really jerky. When I try to back up the motors start and stop a lot. What's wrong with my speed controller? [Not a specific builder, Everywhere on the 'net]

A: [Mark J.] Insect class builders continue to report problems like these when running inexpensive brushed 'eBay Special' motors with certain speed controllers. However, the problem doesn't originate with the speed controller!

'Net wisdom' recognizes that these cheap motors are generating a lot of electrical 'noise' that interferes with the speed controller electronics. The usual suggested solution is to add capacitors from each motor lead to the metal motor case in addition to the standard capacitor (often supplied with the ESC) across the motor leads. This treatment may supress enough of the electrical noise to allow the 'bot to function, but it ignores the true cause of the problem.

Where is all that 'noise' coming from? Power to the armature is transferred across the sliding connection between the brushes and the rotating motor armature. Efficient power transfer and correct timing of power flow to the armature coils depends on the motor brushes being correctly contoured and in full contact with the commutator. If the brushes are not in contact with the commutator over the intended full surface area there will be excessive heating at the commutator plus electrical arcing that generates the damaging voltage spikes.

Full noise suppression capacitors for DC motor

Brushed motors (paricularly small, cheap brushed motors) must be 'run in' for a few minutes at reduced voltage and loading to allow the brushes to wear away surface irregularities and contour themselves to the commutator radius. Exposing the motor to high voltage and loading before running the motor in may damage the commutator and brushes and cause permanent 'noise' problems. Here's how you do it:

  • Put a SMALL drop of light oil on the motor bushings to assure that they are lubricated. If a gearbox is attached to the motor assure that it is correctly lubricated and operates freely.
  • Connect the motor directly to a battery or power supply providing about 1/4 of the nominal operating voltage for the motor. Check that the motor is operating smoothly and running at a steady speed.
  • Run the motor unloaded for about five minutes -- monitoring for smooth operation without excessive heat. You may notice the motor increase in speed as the brush contact area increases.
  • Reverse the polarity and run the motor in the opposite direction for five minutes while continuing to monitor motor operation.
Your motor is now properly prepared to run at higher voltages under heavier loading. It will run more smoothly with less heat and electrical 'noise' than it would have straight 'out of the box', and your commutator and brushes will last longer and function better. Your speed controller will thank you.

Q: I know you aren't a fan of direct drive weapons on anything over 150g, but do you think they have a place in the newer plastic ant category? [Salem, Oregon - really?]

A: [Mark J.] I think direct drive weapons have a place in all of the lighter combat weight classes. People build combat robots for many reasons and with a wide range of skill sets. For some of those builders a direct drive weapon is a good choice. Follow along here...

Combat records show that the highest ranked spinner robots in weight classes above 150 grams are not direct drive. Restricting the materials from which the robot is made does not change the energy physics of the weapon system. If your goal is to build a highly ranked robot in a heavier weight class and you have the construction skill set required to do so, you are best advised to avoid a direct drive weapon system.

However, if your goal is to build a plastic ant to compete and have a good time you should build whatever will maximize your enjoyment. Only one 'bot wins a tournament but everybody can leave with a smile.

'Ask Aaron' assumes that our questions come from builders interested in designs, materials and components that will give them the best chance of constructing a winning combat robot. However, we recognize that builders have differing mechanical skill levels and that there are reasons to compete that do not involve winning.

The more you tell us about your expectations and skills, the better our answers to your questions become.

Indian combat arenas are not safe! Q: What kind of wheels are these? [photo of Brazilian robot deleted] Are they custom or got from someplace?

Does wheel selection have a huge advantage? Plus how do i reduce slippage? [Tamil Nadu, India]

A: [Mark J.] I cannot accept any questions from India -- Why?

You are welcome to search the archives for answers to your questions. There are a great many posts about tires and traction in the Ask Aaron Materials and Components archive. Search there for 'traction'.

Q: how does a spinneris attach on a ring spinner? i saw the ringmaster build photo... but how do it fit up there? [Quebec, Canada]

A: [Mark J.] The spinning ring on 'Ringmaster' is assembled in three parts. The impactor ring simply drops into position where it meshes with the drive gears. Two 'cowls' (dark grey in the photo) are then positioned to rest on the guide wheels above and below the impactor ring. The cowls are bolted to the top and bottom of the impactor ring, sandwiching the impactor in place and positioning it.

Ringmaster impactor ring Ringmaster ring support Ringmaster ring spinner assembled

Q: But is there a better way way to do it? like in a hobby or beetle?

A: Ring spinners aren't effective in any weight class. They have too little mass in the ring to be damaging and the structure is too complex to be reliable. People build them to show off their shop skills -- they are pretty!

The 'better way' is to build a conventional weapon.

Q: Why are all (as far as I've seen) 150g spinners direct drive? Are there any methods of gearing down at that weight? [Saint Stephen, England]

A: [Mark J.] The engineering answer to your question is somewhat complex. Indeed, books have been written on the subject of how proportion changes with scale. The best single sentance explanation I've read on the topic is from the overleaf of "On Size and Life" by Thomas McMahon and John Bonner:

"If elephants had legs proportioned like those of mice, they would be unable to stand without breaking them."

The smaller dimensions of the 150 gram class leads to weapons with a disproportionately small moment of inertia. To obtain the same ratio of stored energy to weight as larger 'bots the small weapons must spin faster. This does create problems with weapon 'bite', but the trade-off is acceptable in this class. If you're going to spin the weapon faster, you might as well save the weight of the gearing system and devote it to the weapon rotor.

Q: When it comes to Chaos Corps' Bombshell, there was always something that confused me. How the wheels are placed with how one set is closer to the center than the other. Are there any added advantages or disadvantages to this design?

BattleBot 'Bombshell' under construction A: [Mark J.] 'Bombshell' was designed to accept multiple modules in the central weapon bay. The outer drive pods have the drive wheels on one end set in conventional position toward the outside edge of the 'bot, while the other end has the drive wheels set well inboard -- actually encroaching on the weapon bay. This offers advantages depending on the weapon module used.
  • When the vertical mini-drum is attached the narrow-set wheels are 'forward' and they set in close to the weapon. This both protects the wheels and exposes the weapon to make it easier to hit the opponent.
  • When the large horizontal blade spinner is attached the wide-set wheels are 'forward' to provide a stable base and make room for the weapon blade.
  • When the overhead axe is attached either end can be 'forward' and the weapon can again protect the narrow-set wheels if the driver chooses to attack with that end.
  • We didn't see the lifter weapon attached, but I suspect that it would be run with the narrow-set wheels 'forward' to expose the lifter in the same way the mini-drum is used.
I didn't notice any handling problems resulting from the differing front and rear track widths, but there may have been some unusual R/C programming involved. I dropped a quick note to Mike Jeffries, and he was kind enough to make the following coments:

The layout choice was initially for aesthetic reasons, but as the design evolved some of the less obvious benefits became apparent and we were quite happy with the result

With the lifter it'd be run wide side forward. As far as handling, it actually drove quite well. We had plenty of drive power to turn when we wanted to, so one of the major bonuses of the odd layout was that the narrow side helped the bot track straight under acceleration.

-Mike Jeffries

Q: Hello Mark, I am pleased to tell you that my first robot is finally about 85% complete. The only thing that I need now is to attach a pan to my BattleBots Son of Whyachi toy. I already have the pan, it fits over the toy quite well and the toy can push it around.

So, here is my question: How do I properly attach the pan to the toy chassis?

Thank you for all of your help on this, and previous questions. Sincerely, Luke, from Alabama.

A: [Mark J.] Last I remember you had hacked the BattleBots toy chassis as described in the Robot Dojo BattleBots Toy Hack Guide, so it should be looking like the photo at right. We talked earlier about adding a wedge, but this is the first time you've mentioned a pan. I assume we're talking about a cooking pan? I'm generally not impressed by the offensive capability of kitchenware. How exactly do you plan on attacking your opponent with a pan?

You'll need to add wide spacer blocks of appropriate height to span the distance from the chassis up to the pan -- a tall block to the front and a shorter block to the rear. Pick locations where you can drill thru the pan, the block, and the chassis to run long bolts. Nuts on the bottom will sandwich the structure together. Additional bracing blocks from the front and back of the chassis attaching to the inside edges of the pan would be desireable.

The blocks should be some fairly light but sturdy material. Wood might do, but a resilient plastic like UHMW Polyethylene (used for kitchen cutting boards) would be preferable.

Hacked BattleBots toy chassis

New! Happy New Year! I spent some of the last few hours of 2016 adding a new feature to 'Ask Aaron': you can now directly attach a photo, drawing, or other file to your question. Type in your question and choose your file. Click 'Send' and you're on your way.

Q: Previously on the Q&A, there was mention of a certain bot (I won't mention the name due to rules) that had 4 wheel drive but used omniwheels on the back. It was said that this was to basically be as maneuverable as a two-wheel bot but have the stability and pushing power of a 4-wheel one. I was wondering why other robots never used this tactic. Was there a drawback like the wheels not being durable enough, or was is it just a less known tactic? I ask because I would think that ramming robots would want to be as maneuverable around the arena as they can be [Houston, Texas]

A: [Mark J.] Since we're talking about a design concept unrelated to a particular event I think we can avoid the mystery and let the readers know that we're discussing 'Razer'.

The drivetrain design is well known to builders but has limited utility. The omniwheels on the rear of Razer were of greatest importance in close quarters, allowing 'bot to pivot around the forward-set front wheels and bring the crushing weapon into play even when the opponent was already in contact. This specific type of 'maneuverability' would not be of use to a rambot, which requires rock-steady straight-line running ability. Although potentially useful for a spinner weapon, it would be difficult to place the front wheels far enough forward to make this a useful tactic for currently popular spinner styles.

Q: I'm building either a fairy weight or an ant weight bot similar to 'bad attitude'. What would be the best way to defend against most spinners? The bot will be metal (aluminum most likely a few mm thick) and I am thinking about using would blocks on the side to have the spinner (probably horizontal) damage itself. [Smithtown, New York]

A: [Mark J.] Spinner defense is best accomplished with perfectly smooth, hard, gently curved surfaces on the outside of the robot. No sharp edges to give spinners a place to 'bite', no soft materials for sharp spinner teeth to dig into -- so absolutely no wood blocks!

You'll want to make your wedges out of harder material than aluminum. A hard, curved scoop is the preferred shape for spinner defense. Keep it between you and your opponent, and watch that they don't get to your wheels. Practice your driving until you can do this without even thinking about it.

Q: Hi it's the wood block guy and I read your previous message sent to me (I know not to use a wood block now I just wanted you to know it's me) but anyway I wanted to know if this same design/strategy would work against drums, eggbeaters, and vertical discs/bars.

A: This type of scoop-bot is known as a 'spinner killer'. It is very effective against vertical spinners, assuming that the curve radius of the scoop is at least a little larger than the radius of the opponent's spinner weapon. Be very careful with the vertical edges of the scoop when facing a horizontal spinner: pay attention to the direction the weapon spins and keep the edge of your scoop away from the incoming weapon strike.

Q: The wood block guy (yes again!) and I was thinking about an interchangeable weapon so that it it would look similar to Tornado form Robot Wars (a spike on each corner with one large one in the middle for the front). Would it be an effective weapon against any specific type of bot or should I just not bother and focus mainly on the wedge/scoop?

I have a little driving practice with the Battlebots Hexbugs however I'm not sure if that helps (you said to practice driving in a previous question).

A: Spikes are very useful against robots armored with:

  • fruit
  • butter
  • wet mud
  • balloons
  • wine corks
  • paper mâché
  • marshmallows
  • and maybe thin ice
If you're facing an opponent made of any of those materials it would be handy to have some spikes avaialble, but that situation will probably not arise.

Every robot drives a bit differently and it is cruicial to finish your 'bot well before the tournament so that you can practice and adjust the response of the robot to your liking.

  • Aaron listed his favored driving exercises in this post in the Ask Aaron archives.
  • The radio transmitter you will use for a combat robot is much more complex than the transmitter for your Hexbug toy. You'll need to spend some time adjusting the transmitter settings to make the radio interface comfortable for your driving style. In combat your nerves will be on edge and a too-responsive robot will become impossible to control. See our transmitter selection and set-up tips: Combat Robot Radio guides.

Q: After some thought I think I want to make an undercutter type spinner either 12 or 30 pounds. I came up with the attached sketch, nothing is to scale, which is my question.

First does this make sense? I was thinking that making the part as close to the blade as strong as possible was the way to go, and tapered bearings are stronger than regular ones as long as the bolt is tightened right.

Second, how do you figure out the proper size of everything? I have no idea how big of a shaft I should be using, how big of bearings to use, how close I am to the blade cracking because I put too much stuff through it, ect. [Baton Rouge, Louisiana]

Questionable Undercutter design...

A: [Mark J.] Multiple suggestions:

  • Scroll down the page about four posts to find the undercutter layout question with an attached diagram. The two weapon bearings must be well separated to eliminate destructive 'moment loading'.
  • Tapered bearings are not commonly used for spinner weapons. The primary bearing loading when the weapon 'hits' is radial -- the lesser axial loading is accomodated by the freedom of the robot to move along that axis in response to the impact force. Standard ball or roller bearings will do nicely.
  • Find some 'build logs' for robots of your proposed style and weight. Like it says in the FAQ: "Look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger."
  • Grade 8 bolts are very strong in 'tension' -- stress applied along the length of the bolt -- but bolts in general are not designed to withstand lateral 'shear' forces of the type your design places on the weapon shaft. An undercutter in the sub-light weight classes should use a 'tougher' steel shaft that can absorb shock loading without breaking.
  • The hub and pulley must be attached to the shaft in a manner that prevents rotation. Set screws suck. Ideally the pulley, hub, and shaft are all broached for a keyway. Keyed shafts @ Robot Marketplace. Examples of keyways and other attachment options are available in the Ask Aaron archives.
  • The fewer holes you put on your weapon blade the better, and minimal localized stress on any hole that does exist is critical. The preferred blade hub design is a 'clamping' style that holds the blade in place without adding bolt holes thru the blade itself. There are examples of clamping hubs in the Ask Aaron archives.
Don't try to re-invent the wheel. Find examples of successful weapon mounting designs, study them, and base your design on what you have learned.
Q: I have desgined (not yet built) a beetle weight robot with a drum that lights up (with LEDs for entertainment purposes) but what if I spin my drum like so fast that it wont cause damage? I'm sure I'm going to get 'The Burger Is Bad' or 'Check the Archives'. [TCRM, Malaysia]

A: [Mark J.] The hamburger is good and you don't need to check the archives. Read thru the Ask Aaron Spinner FAQ. The 'Rotational Speed' section of that FAQ discusses the factors involved in getting the weapon to 'bite' into your opponent rather than just 'skitter' across them without doing any damage.

Read the whole Spinner FAQ while you're there -- it'll save you a lot of time and trouble.

Q: Hi, Mark. Could we turn a Viper Kit spinner into an overhead spinner? We have the V2 because V1s are discontinued as you know. Well anyway, my viper spinner keeps pulling a 'Don't Ask' (moving itself impossible directions like sideways ad by sideways I mean my bot is pointing forward and it is going sideways). Please do not make this go to 'Hamburger is Bad'. Ok, well here is the main question: (drumroll) will it work? [Herndon, Virginia]

A: [Mark J.] Well, you could just turn it upside-down and...

Q: P.S by turning it into an overhead spinner I do not mean flipping it upside down. I mean making it an overhead spinner.

A: You mean like...

Q: P.S.S *cough* Hazard, Brutality, and Justice *cough*

A: So a longer bar-blade centered on the top of the 'bot high enough to clear the wheels? I see two major problems with that:

  • You don't have enough weight allowance on the Viper for an effective spinner bar that long; and
  • Using the direct drive method of the Viper for a blade with that great a moment of inertia would both strain the motor and give a very slow spin-up.
I'd suggest finding out why your undercutter is behaving so strangely -- I've seen many Viper spinners and never one that acts as you describe. Your weapon motor may not be aligned correctly or the weapon blade may be imbalanced. Work on getting your current weapon working correctly before moving to something new.

Q: In reply to the Viper kit owner, I've had similar but less pronounced issues with 'Original Robot Do Not Steel', where the robot tends to want to drive in a slightly diagonal line when spinning up. It's probably something to do with the large contact surface between the ground and the blade hub. It's probably something to do with the large contact surface between the ground and the blade hub.

The bottom hole of the blade hub is threaded, so you might want to put a machine screw in there. [Black Mountain, North Carolina]

A: FingerTech's blade hub webpage recommends installing a screw in that threaded hole:

"The hub has a 6-32 threaded hole so that your horizontal spinner robot can rest on a screw head and not be pulled in different directions when the blade spins."

I assumed this was standard practice. I'd recommend a round-head nylon screw if you have one.

All horizontal spinners will face some newtonian 'action/reaction' deflection force as they spin-up, but once up to speed there should be little interference with mobility unless there is poor weapon alignment or poor balance; the blade should be perfectly parallel to the arena floor.

FingerTech blade hub with screw head

Q: I am the viper kit guy and I forgot to tell you that we are still working on an arena. And in the meantime we use a cardboard blade to practice. Plus we don't plan to fight the bot until spring fling 2017 so while we practice we use tape instead of the screw.

A: It seems the hamburger was bad.

  • You failed to mention your cardboard blade that will deform at speed and create odd aerodynamic effects.
  • You didn't think it was important to tell me about the the piece of tape that moves the blade hub contact point away from the center of the spin axis.
No wonder the 'bot behaves oddly! Get the 'bot in full fighting trim before you complain of poor performance.
Q: I am using two E30-150 ampflow motor for my mobility and Kavitsu KA-02 planetary gearbox for reduction. The problem is that the reduction provided by the gearbox is too much and so my mobility speed is a bit slow (Gearbox is attached to motor by gear meshing which further reduces rpm by half). The gearbox is also quite heavy which is causing problem in weight management. So i was thinking of making a custom gearbox for my E30-150 with decent speed and torque. Can you suggest me what type and what material etc. should I use in making my gearbox?

Another question is can you tell at what rpm I should rotate my drum and what diameter and material I should go for my drum? Can you suggest some motor alternative to A28-400 ampflow motor for rotating the drum as it's too costly.

P.S My current drum is about 13 kg with 18cm diameter and 4800 rpm rotated by E30-400 ampflow motor. I am going for 120 lbs category [Tamil Nadu, India]

A: Although I very much wish to support the technical aspects of robot construction in the energetic and expanding Indian subcontinent, I am also greatly worried that I may be contributing to an extremely dangerous situation for both builders and spectators. This has brought me to a painful decision:

The 'Ask Aaron' website is closed to questions from builders competing in India

Aaron certainly wouldn't approve of continuing to support reckless endangerment of life and limb from the operation of high-energy weaponry without adequate arena containment. Until there is a significant and universal change in Indian arena safety and/or rules to control dangerous weapons, I cannot in good conscience accept new questions from the region.

I will point you to the Ask Aaron Spinner Weapon FAQ for guidance in the design of drum weapons and weapon motor selection. I will not, however, make specific recommendations.

I can also warn you that the construction of a reliable custom gearbox is well beyond the ability of the average robot builder. You may find some useful information on multi-stage chain and sprocket drivetrains in the Ask Aaron Design and Contruction archive.

Q: Hi Mark,
I am the Indian guy who asked the question. I think you are still under the wrong impression that arena used in our competitions are unsafe. Now most of the places in India use polycarbonate sheet walls for the arena. The good teams don't participate in the competitions whose arena are not safe for fighting due to obvious safety reasons. The sheets in the image are 10mm thick.

A: I follow Indian robot combat closely. I'm one of the moderators for the 'Combat Robotics India' Facebook group and I review the arena construction details of all the events that post there. I also scour YouTube for video from recent Indian combat events.

I can agree that Indian robot combat arenas have improved in the last few years, but there are still MANY events run with completely inadequate containment. The current Indian standard for a 'safe' arena for 120 pound robots would not be viewed as adequate for 12 pound robots in the US or Europe. Your current drum weapon - as described in your post - stores more than 10,000 joules of energy. On a 120 pound robot that's more than enough to breach 10mm polycarbonate walls, let alone the more common Indian arena containment.

Modern Indian robot combat arena
VIT Chennai arena

I receive many requests from Indian builders who are interested in building more and more powerful weaponry for their robots even though the arenas barely contain the current weaponry. I have no way of knowing if the question comes from a 'good' team that will compete in a 'safe' arena, or what their workshop and testing safety practices might be. I don't want to be involved in a death or maiming resulting from design assistance I might provide.

I believe I understand Indian combat safety quite well. If you want information on making events safer I'll be glad to assist. If you want information on more powerful robots for your current arenas the answer is 'no'.

Q: do you have a drawing on how an undercutter blade is fixed to a bot? i know u did for a fbs and a horizontal spinner. [Quebec, Canada]

A: [Mark J.] The mounting is pretty much like an FBS turned upside-down. The weapon shaft is rounded on the ends and usually sticks out both below the blade and above the chassis to allow the 'bot to invert.

Undercutter blade mounting

Q: Hi, its me again. I wanted to know if using 22:1 fingertech silverspark motor at 7.4v with 1 5/8 wheels is ok for an antweight? i put all the spec in the geartrain calculator and the number didnt means anything to me.. could you help me with that? thanks a lot your work is appreciated. [Quebec, Canada]

A: [Mark J.] The Tentacle Drivetrain Calculator you referenced is a valuable tool in the selection of drivetrain components and robot design. Your understanding of the calulator's output might be improved by reading the Example Drivetrain Analysis using the Tentacle Torque Calculator that I published a few months ago.

For the gearmotor, voltage, and wheel diameter you propose: the calculator shows a too-slow top speed around 2.5 MPH with a maximum current draw near 33% of stall. Acceleration is OK with top speed is reached in 2.5 feet from a standing start. A wedge in a small arena needs better performance than that.

Most builders run the 22:1 Silver Spark gearmotors with 3-cell LiPoly batteries (11.1 volts) and wheels around your size range. At that voltage the performance of your drivetrain improves greatly: 3.8 MPH top speed, maximum current draw at 22% of stall, and top speed still achieved in just 2.5 feet. Those a good numbers for an antweight wedge in a small arena. Bump up to a 3-cell battery.

Q: hi, my antweight "wedge" is design so i can swap the attachment i put on it, fot now, i have these 2 fork as the main one, but i want to have a solid wedge to fight against spinner. But, cant it be just a flat piece of steel at an angle of about 35 degree, or do i need to add the little wedgelet thingie on the side like algos and many other bots have? is it really necessary? i joined a pic of the design as it is for now [Quebec, Canada]

A: [Mark J.] Top-ranked combat robots like 'Algos' do not have design features present just to look cool and waste weight allowance. In this case, the turned-down wedge end caps serve two important purposes:

  1. Spinning weapons get excellent 'bite' on any exposed sharp edges they can find. Leaving sharp edges on the ends of your wedge where spinners can easily get to them is suicide. Builders call exposed edges like those "spinner bait". The gently curved-down ends on the 'Algos' wedge move the edges back and to the side where a spinner weapon will have a much more difficult time taking advantage of them.
  2. Extending the wedge around the side of the robot helps deflect spinners and other weapons away from your vulnerable exposed wheels. A wedge that ends right at the side of the robot gives a clear path for an opponent to 'feel its way' right down the side into the wheel. Save your wheels -- offer them a little protection!
An 'Algos' style wedge is very effective. I'd suggest imitating it closely.
Quebec's antweight
Quebec's spinner bait design


Q: i recently bought my first transmitter, a spektrum dx6i, and i found out the antenna on the receiver is really long.. what should i do with it to make it more compact? [Quebec, Canada]

A: [Mark J.] Several different receivers can be used with the DX6i transmitter and you didn't mention which receiver model you have. Perhaps it's the Spectrum AR610 with two antennas: one short and one a few inches long?

Do not cut the antenna wire. Try coiling the part of the longer antenna between the receiver and the shrink-wrapped 'amplifier' around a short piece of plastic soda straw. A piece of tape will hold the coil in place.

You may want to read our guide on Radio Reception Problems in Combat Robots for more antenna tips.

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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