5339 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 the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution   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.
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   

 Recent Questions

Q: Hey Mark,
I've been using the Tentacle calculator for a while to optimize my drivetrain. I guess I'd just like a clarification..

I'm designing a MW (120) and I'm planning on having 6" wheels. I understand that designing so that the wheel-break torque at 1/2 stall is best from a motor optimization perspective. But I seem to be stuck in a strange balance. For example, using an F30-150 geared down 10:1 will give me top speed of 12.2 mph, acceleration of 1.75 sec, and break torque at 55.6 amp which is perfect for most ESCs on the market that can handle 60A continuous. But this current draw is only 18% of the stall torque of this motor. Does that mean it's extremely oversized? I think finding an ESC that can handle almost 150 A continuous would be a little absurd for my bot, which is a spinner and won't be pushing most of the time. Downsizing to an E30-150 would make my accel a bit sluggish.

I guess my question is.. does the F30-150 seem like a good choice here? Or would I be wasting money since I won't be coming close to the peak HP of the motor?

Thanks and happy holidays. - James [Long Island, New York]

A: [Mark J.] I really like to see this type of question. You've been thinking your design thru and something isn't adding up -- a perfect use of Ask Aaron! Let's take a look...

Gearing the drive motors for tire breakaway at about 50% of stall torque allows the motors to achieve maximum horsepower output while protecting them from excessive current consumption. That's 'optimum' for the motors, but if the motor power is not adequate for the desired acceleration of the robot in a specific arena it may be necessary to change the gearing to something less than optimal to compensate. Other factors, such as your desire to to use less expensive ESCs, can take you farther away from 'optimal'.

Assuming that you're building for the RoboGames arena, you'll have about 16 feet of space between the front of your robot and the center of the arena. That's a good distance to use for calculations. Here's what different gear ratios do to your maximum current draw and to the acceleration time and top speed in a 16 foot sprint:

Sixteen-Foot Sprint to Arena Center
RatioMax AmpsSpeed (MPH)Time (sec)

You can see that in this arena dropping the gear reduction down into the 6:1 to 8:1 range improves the practical top speed of your 'bot without significantly impacting the acceleration time. It may also improve the driveability of the robot by reducing wheelspin on acceleration. Pushing power is unaffected. The cost is greater current consumption by the motors, which requires more expensive ESCs.

Note: the Tentacle Drivetrain Calculator assumes four wheel drive in it's calculations. If you have two-wheel drive the reduction in weight on the driven wheels will cause them to break traction at lower torque levels. See the pop-up help files in the Team Run Amok version of the Tentacle calculator to learn how to adjust the calculations for a two-wheel drive robot.

Bottom line: your motors aren't overpowered, you're simply choosing to not use their full power in order to save on speed controller costs. The motors are a good choice for a weaponed middleweight. You just have to decide if squeezing extra performance from them is worth the added expense given your weapon design and attack strategy. The F30-150s do give a good performance boost over E30-150s even at the reduced current level.

We Don't Need Your 'Bot
Q: Hello, Mark, do you think it makes sense that the BattleBots rules banned organic substance? In the 'Prohibited Materials' , organic substances are banned, except wood, wood products and battery electrolytes. The purpose seems to make the arena easier to clean. However, the contradiction is 'Radioactive' contains lots of plastic but it was accepted. I think there's no need to ban organic materials. [Guangdong, China]

A: [Mark J.] I'm not going to comment on every oddity in the build rules for Battlebots ABC season 2. BattleBots started as an open combat robot tournament with hundreds of teams showing up to compete. With so much interest the organizers must have realized that they could do anything they like and the robots would still show up. If you're not willing to put up with their arbitrary rules there are plenty of teams that will, so builder opinions carry no weight.

Q: Hi I'm participating in robo games though I'm from India can u pls tell me any energy calculation software or the calculations required to design the best drum in a 75kg category robot.what motors do u smudges [suggest?] the for drive and weapon [Tamil Nadu, India]

A: [Mark J.] I'm very puzzled by recent questions from India. Safety concerns prevent 'Ask Aaron' from accepting new questions from builders competing in Indian combat robot events (click here for details) -- but the questions being asked are ones that have been answered many, many times here.

If you're not willing to spend five minutes searching the Ask Aaron Archives and FAQs for answers to these very common questions, I don't think you have the right mind set to build a combat robot.

I'll make it really easy for you this time -- Start Here.

Indian combat arenas are not safe!

Q: The build rules for Battlebots season 2 were released recently. One thing that intrigued me was the requirement for a minimum of two master switches in each robot, one for drive and one for weaponry. Surely this is a bit uneccesary? Other rulesets I've read only require one master switch for both drive and weaponry. Does this new rule make robots "safer", or is it just a new inconvenience for current robots with only one switch? [The University of Manchester, England]

A: [Mark J.] I've learned to avoid thinking too much about why BattleBots does what they do. There are a number of puzzling requirements and vague 'soft' limits included in the new ruleset that are causing consternation in the builder community -- and what is that 'telemetry package' they mention in section 12?

Referring to multiple switches controlling different circuits as 'master' switches is a misnomer. For maximum safety it's clearly best to have a single true 'master' switch/link that will bring ALL robot motion to a halt. A requirement for separate switches for weapon and drivetrain does not make for a safer combat environment.

There has been no explanation of the switch requirement from BattleBots and I don't expect that there will be. BattleBots appears to actively discourage builder involvement or discussion of their ruleset. On the ruleset cover page it states:

"No portion of these Design Rules may be published, reproduced, sold or distributed by any means, or quoted or published in any medium, including on any website, without the prior written consent of BattleBots, Inc."

I wonder if I'll get in trouble for quoting their restriction on quoting their rules? Corporate sewage like this is one of several reasons why our team has not and will not compete at BattleBots.

Q: What is the minimum energy for a drum for thrashing a 75kg robot?and what is the length of teeth I should use [Tamil Nadu, India]

A: [Mark J.] Two things:

  1. We request that you search the Ask Aaron Archives before submitting a question to see if it has already been answered. Weapon energy requirements and correct tooth length have been discussed here multiple times.

  2. As noted in the submittal box into which you typed your question, due to safety concerns Ask Aaron is not currently accepting questions from builders competing in Indian robot combat events -- click here.
Indian combat arenas are not safe!

Q: Could you explain gyroscopic precession battlebots like 'Wrex' from the new battlebots series? Gyroscopes are definitely one aspect of mechanical physics that I have difficulty wrapping my head around. If you could also link some other battlebots that use similar movement (from any weight class) that would be awesome, I tried looking but didn't come up with much. Thanks for the help. [Durham College of Applied Arts and Technology, Ontario, Canada]

A: [Mark J.] If we were in a room with a pad of sketch paper and a half-hour to spare I could fully wrap your head around gyroscopic precession and demonstrate how it can be used to 'waddle' a combat robot across the arena. Given the lack of interaction here at 'Ask Aaron' the process is considerably more difficult.

I think it's best if I try to explain the 'how' and we'll leave the 'why' for a day when we can discuss physics over tall glasses of red ale and you're buying. If you can't wait for that opportunity to learn 'why', you may be able to glean some value from these videos:

It's also worth taking a look at this webpage about the inventor of the walking gyro that features his explanation and design drawings.

Back to the 'how': It's easier to see what's happening in a smaller robot. Take a look at this video of Team Misfit's antweight 'Gyrobot'. The spinning weapon is mounted to a servo that tilts the weapon to the left and right relative to the robot chassis.

  • The gyroscopic forces of the spinning weapon resist the force attempting to tilt the axis of rotation of the spinning mass, resulting in the servo force lifting one side the chassis off the arena surface.
  • In resisting the gravitational force attempting to pull down the unsupported side of the chassis, the spinning weapon (gyro) causes the robot to 'precess' and rotate the raised side toward the front of the robot.
  • The servo can then move back in the other direction to raise the other side of the robot and cause it to precess forward.
  • Lather, rinse, repeat -- forward motion.

This video of a small non-combat gyro walker that uses a servo to tilt the whole robot left/right may give you a better view of the details of the walking motion.

It may help to purchase a small gyroscope and play with it for a bit to see what happens when it's axis is supported horizontally at first one side and then the other.

Combat robots that use gyroscopic precession for locomotion are uncommon -- for good reasons. They are extremely slow, difficult to control, can't back up, and are incapable of motion if the spinning weapon stops. The only real advantage to the design is that some event organizers will give it a 'walker' weight allowance that can be used to construct a more powerful weapon. Technically a precession system is not a 'true' walker, but you may get lucky.

Gyroscopic Walker prototype

huge Swiss army knife Q: How do you do, what tools do you use to make and repair robots? [Jiangsu, China]

A: [Mark J.] The answer depends on your robot design and which components you want to make rather than buy.

  • Here is a 43 minute video showing the steps and tools needed to construct a simple combat robot.
  • Section 2.2.9 of the RioBotz Combat Tutorial has an exhaustive listing of tools and supplies that might be useful in construction of a combat robot, but many builders construct successful 'bots with only simple hand tools.
Whatever tools you have, the one that you'll need to repair your robot at a tournament will be the one you left on your workbench.

4 bar flipper animation Q: Hello I am working with a combat robotics team in California, and we are planning on building a 4-bar flipper robot. We were very happy to see that you were hosting [the T.i. Combat Robotics 4-Bar Simulator software] so that we can still download it. I was wondering, however, would it be possible to obtain the source code for the program, so that we can remake it to add functionality and make it run faster? [Beverly Hills, CA]

A: [Mark J.] I too have plans for the source code for the T.i. Combat Robotics 4-Bar Simulator but the location of said code is a bit of a mystery. Adam Wrigley believes the code is on a computer stored at his parent's home.

Adam has promised to look for it on his next visit. With the holidays approaching I'm hopeful that he may find himself in a position to conduct that search soon. I'll ask if he's willing to make the code public.

Q: What thickness of the polycarbonate wall is suitable for a 500g combat robot event? [Guangdong, China]

A: [Mark J.] Thickness of the polycarbonate arena walls depends on details of the arena design. I'd be comfortable with 3mm thickness assuming:

  • a solid frame supports the arena walls; and
  • a solid 'bumper' prevents direct contact of the robots with the arena wall.

Q: You seemed pretty pessimistic about the chances of Battlebots being renewed by ABC. Did the network's decision to renew it come as a delightful surprise? [Saint Mary's College of California]

A: [Mark J.] I have mixed feelings about ABC's renewal of BattleBots for a second season. The original series on Comedy Central was an important step in the development of combat robotics. The tournaments were open and attracted hundreds of builders to each event. The new series is limited to a few dozen competitors that are hand-picked to make 'good television'.

I'll watch, and I'll be entertained -- but I'm not convinced that it's a good thing for combat robotics.

Q: Hey Mark, I've been working on my latest ant, a front hinged lifter. The motors don't seem to have the moxie needed at 7.4 volts (22:1 Sliver sparks mated to 2.25" wheels), and I want to make the jump to 11.1 volts. Thing is, the servos electronics are rated to 9v maximum (the questionable HXT 12kg, linked to the lifter by a four bar assembly), and I want to ask whats the best course:
  • Replace the servo with a HV version (expensive, but more reliable choices)
  • Use a 9v regulator (adds another part in between to fail, but allows use of my current servo)
  • Lobotomize the servo and run off a spare tiny ESC (no fail safes, but allows more voltage than the servo could handle normally
  • Risk it and run the servo at 11.1 volts anyway (doesn't seem like the best option to me)
What do you think is the best way to go about things? [Portland, Oregon]

A: [Mark J.] Take a look at this previous post in the Ants, Beetles, and Fairies archive. 'Beetle Lifter Guy' is using two LiPo batteries to provide separate power to the motors and to his lifter servo. Note carefully the warning about the red wires from the receiver to the ESC.
3S lipo battery balance connector power tap diagram
If your LiPo has a balance plug you can do the same thing with a single 3S battery. Tapping into the balance connector at 'A' and 'C' (see diagram at right -- wire colors may vary) will give you a 7.4 volt output to power your receiver and servo, while you run the main power connector to the ESC for 11.1 volts at the motors.

  • No extra electronics
  • No voltage risk
  • Minimal added expense/weight/space
  • Retains normal servo operation
That seems like a 'best choice' to me.

Comment from Near Chaos Robotics: Hi Mark, figured I'd add a bit to the question on using the HXT12kg servo with a 9v regulator-

'Reptar', the antweight lifter in this video, uses a 3s lipo for the main power and the 9v regulator from FingerTech Robotics to provide power to the servo. It's a pretty nice match, though the servo gets a bit warm after a full fight. While testing it I was messing with the trims and rates to dial in the lifter and inadvertently stalled it against the frame without realizing it and fried the servo, so some caution does need to be taken when setting things up. With that said, the combo works quite well if you've got the space and weight to fit it. As it sits the bot is right at the 1lb limit and has very little air under the armor. [Georgia]

Reply: Thanks, NCR. It's good to have a 'real world' data point. In general I advise caution in bumping the voltage. I can't think of a single unsuccessful robot that was turned into a successful competitor by turning up the juice -- can you?

Q: I've designed and built a featherweight combat robot with a drum weapon.

The drum is mild steel, 75mm diameter with a 12mm wall thickness and 160mm long. End plates are 10mm aluminum and the full-length live shaft is 22mm aluminum [alloy unknown]. The weapon motor is an Ampflow E30-400 with a gear ratio of 0.6:1. Drum energy storage is 1100 joules.

I'm concerned that my aluminum shaft will not be able to withstand the impact of the drum and that it will crack and break. Would you advise that I keep it the same or change it. I only have 300gms left to add in the robots weight.

Please reply soon as the event is in 3 days. [Eastern Hemisphere]

A: [Mark J.] Three days 'til your event? Since you don't know what alloy your weapon shaft is, we'll have to resort to testing.

  • Find a safe testing environment.
  • Spin up your weapon and charge at full speed into an immoveable object.
  • Spin back up and hit it again.
  • If the weapon survives you're good to go.
  • If the shaft bends/breaks you've got three days to repair and
I know builders who throw their new 'bots off the roof to see how well they survive a good impact. Better to break your bot now than in the tournament. I suspect that a 22 mm shaft is adequate for an 1100 joule weapon, but I don't know the details of how your shaft is supported. Test it and find out.

P.S. - You didn't ask, but your drum design needs work. See the Ask Aaron Robot Weapons archive for pointers on drum design and discussions about 'bite' before you build your next robot.

Q: Mark,
For my ant-weight robot I am planning on having a 3D printed chassis wrapped in 4130 Chromoly. My question is that the 3D printers that I have access too are able to print both ABS and Polycarb, which do you think would be a better option? I know that Polycarb has better properties but it also has a tendency to be brittle and I was thinking that with the holes that I would print in there for the steel covering could lead to stress points. On the other hand I noticed in a previous post that you mentioned that 3D printed ABS has issues with the layers bonding to itself. So all things considered which do you think would be a better option? [East Cleveland, Ohio]

A: [Mark J.] Printed polycarbonate has delamination issues similar to ABS plastic, and it also tends to warp a bit. The problem varies from printer to printer, so you won't know if it's an issue until you try it.

The properties of ABS and polycarbonate are much different:

  • Polycarbonate (Lexan) is NOT brittle -- it will take enormous abuse of allowed to flex, but it will crack if your mountings prevent it from flexing and create local stress points. Mounting holes aren't a problem if the armor is loosely mounted with grommets and washers.
  • ABS is a stiffer material that is less sensitive to flexing requirements. Standard mounting techniques will be more successful with ABS.
The choice depends on your chassis design. If the steel armor is providing stiffness to the structure and will take most of the impact, I'd suggest the ABS. If the armor is individual floating plates, you'll be better off shock-mounting the armor to a polycarbonate chassis.

Note: I consider 3D chassis printing to still be highly experimental. Be prepared for several design generations of failure.


A: [Mark J.] Click here.

Indian combat arenas are not safe!

Q: Hi mark, my bot is from 120lbs category running on two e30_150g motors .plz suggest me the best speed controller in 200$ and can we use single channel controllers to contol drive [Bay Area, California]

A: [Mark J.] Your IP address whispers San Mateo, but your 'bot design screams India. I'm deadly serious about not answering combat robot questions for builders competing on the Indian subcontinent. Click here to learn why.

Indian combat arenas are not safe!

Q: [Chinese Forum] Mark,what do you think of the new FRA UK Heavyweight Champion TR2? Interestingly unlike winners in recent years, it's flipper's power is not that strong as Manta or Eruption, does that means a more balanced design is good?

A: [Mark J.] I've lost track of how many times I've commented on this topic. I direct you to this post in the 'Robot Design' archive that responds to an earlier question from the 'Chinese Forum'. Substitute 'Toon Raider 2' for 'Touro Maximus' and you have my answer.

Q: I am used to working with materials like wood, plastic, aluminum, steel, etc. but not titanium. I am building a beetle and want a titanium wedge on front that is 6"x 6". It would have 4 screws on each wall and two supports in the middle with again 4 screws. The support material would be 1/2" hdpe. Screws would be regular steel 1" long. Going against vertical spinners made of hardened steel, how thick should the titanium be? The slope of the wedge is about 35 degrees. [Pennsylvania]

Ask Aaron is not a free engineering service! A: [Mark J.] 'Ask Aaron' is not a free engineering service -- see Frequently Asked Questions #17.

I advise that you look to other builders with similar designs and learn from their experience. Assuming that you're using Grade 5 titanium, you might wish to use the optional titanium wedge for the D2 beetleweight kit as a design reference. It has a thickness of .080".

Note that the D2 kit uses countersunk screws to secure the wedge. I would advise that you use the same -- don't give those spinners anything to grab onto.

Q: What information would one have to provide in order to avoid FAQ #17?

A: As stated in FAQ #17, 'Ask Aaron' is not a free engineering service. No amount of information you can supply will avoid that.

If in some other reality we were a free engineering service, calculation of material thickness for a specific component would require a full drawing of the part, exact specifications of the material to be used (alloy, temper...), and details of the magnitude and vector of the force to be applied to it. That last bit is pretty much impossible to provide in combat robotics.

That leads us to the advice we offer on material selection: "Look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger."

Q: What type of material makes up the functional aspect of the transducer that creates the
   high-frequency sound? [San Clemente, California]

A: [Mark J.] Why do I think I'm doing your homework for you? You might at least put it in your own words and frame it as a combat robot question.

Please allow me to Google that for you: Ultrasonic Transducer.

Q: Hi Mark
Would you please make a report for the HW championship this year? Thank you! [A proxy server: maybe in Kansas, maybe in Virginia]

A: [Mark J.] Two problems...

  1. Which "HW championship this year" - Colchester, perhaps?

  2. 'Ask Aaron' does not now and has not ever provided fanboy event reports.

Q: Why doesn't Manta's flipper work well? [Chevy Chase, Maryland]

A: [Mark J.] Are you talking about the 'Manta' with these achievements?

  • Fighting Robots European Championships 2013 Runner-Up
  • Fighting Robots UK Championships 2013 Runner-Up
  • Robot Wars Winter Tour 2013 Champion
  • 3rd in Fighting Robots UK Championships 2014
  • Robots Live! - Whitwick 2015 Winner
  • 4th in Robot Wars World Championships 2015
Seems like a fine robot to me. Nothing wrong with their flipper.

Q: I've tried unsuccessfully to master 4-bar but cannot get a successful layout no matter what input values I use. For a beetle, what would some of the approximate dimension be? My overall dimensions are currently (for the robot, not arm length) 10" x 6" x 2.25". [Pennsylvania]

A: [Mark J.] Why do 4-bar questions always come in on weekends when I'm away from the computer that has the 4-bar calculator installed? I can at least give you a starting point...

Team Run Amok's beetleweight 4-bar lifter 'Zpatula' has a front bar powered mechanism with these dimensions:

  • Base (E): 145 mm
  • Front Bar (H): 90 mm
  • RearBar (F): 55 mm
  • Top Bar (G): 115 mm
  • Rear Bar base height: raised 20 mm
Proportions similar to these should give you a starting point in your calculations. Click the image at right for a full-size 4-bar output screen of Zpatula's lifter using 'inches' and 'ounces'.
T.i 4-bar calculator output

Click for large image

Note: the 'Total Insanity' website hosting the 'T.i. 4-bar calculator' has shut down. I've pieced together a mirror site to allow downloads of the calculator: T.i. 4-Bar Calculator. I'm also working on an updated version of the 4-bar calculator that will run with current operating systems -- stay tuned.

Q: Hey Mark, I'm the beetle spinner guy again [Oregon] and I have a few more questions:

1: I know you recommended ball-bearings but I've been wondering about bushings. Should I consider them? I know they have significantly more friction but I'll take reduced performance if it means I can improve dependability.

A: [Mark J.] Correctly selected small spinner bearing failures are uncommon. Bushings are fine if well aligned (and well oiled) but if they are knocked out of alignment they will bind. Unless you're very confident that your weapon shaft support is rock-solid, use the ball bearings.

2: After seeing asymmetrical spinners (such as Decimator and Banana Spliter) I want to try to fashion my own. Dimensions for the one toothed spinner would be 6"X2"x.125" of 4030 [steel] with the striking edge hardened. I do not have access to CAD software capable of finding center of gravity, so I want to know the best way to find the spinners center of gravity manually and figuring how to get it as close to the location the blade hub as possible either through calculation of a sketch, or building the weapon and adjusting it?

Finding the center of gravity for a blade A: A blade is easy:

  • Cut out your blade, but do not drill the hub mount.
  • Balance the blade across a sharp vertical surface (see photo). A ruler edge is better than your finger.
  • Mark the balance point and drill your hub mount.
  • See this post in the Ask Aaron Robot Weapon archive for instructions on getting the fine balace correct.

3: What's the best way of locking a spinner dead solid? I was considering a heavy duty bicycle lock, but what other ways are good to keeping a spinner locked?

A: Locked, as in preventing rotation in the pits? Why so important to make it 'dead solid'? I don't know enough about your design to come up with something clever, but often a simple zip-tie will do nicely for an insect class 'bot.

4: For when the Small Johnson motor becomes to weak to progress (either through stress or natural selection) what typically works better for spinners: Sensorless Brushless Outrunner or sensored brushless inrunner?

A: Inrunners will typically have stupid high RPM, so outrunner is my choice.

Sensored have a faster spin-up than sensorless, so sensored is my choice.

Of the choices you offer, outrunner sensorless is the better combination.

5: I've noticed that some one-piece spinners have are notched on the striking surface(Tombstone's red blade is a good example). Does this improve performance to any significant degree or is it just for show?

A: That's not a 'notch' -- the ends of some blades are ground to from a wedge shape at the end. That makes a sharp edge to cut into softer armored opponents. Against hard 'brick' opponents that edge is quickly dulled, so any advantage depends on the opponent you're fighting. We discussed blade choices in response to one of your earlier questions -- see the post about Tombstone's blades near the top in the Ask Aaron Robot Weapon archive.

I'm receiving a growing number of 'offline' requests for robot combat arena guidelines and specifications. I know of two sources:

  • The Robot Fighting League (US) provides an RFL 'Resources' page (archived) with drawings and a materials list for an arena designed for 60 pound robots. The page also provides guidance on robot safety inspection and match judging.

  • The Fighting Robot Association (UK) has a comprehensive FRA Guide to Event Safety (PDF) that includes arena specifications and procedures for events of differing weight classes and weapon types.

Q: Hi, Mark, I want to ensure that will it be safe to use a 1 MPa ram in a 7 MPa pneumatic system? If not, what kind of ram the competitors are using? I didn't find any ram is designed to hold such pressure. [Guangdong, China]

A: [Mark J.] 

Running 7 MPa pressure (70 Bar, 1015 PSI) with a 1 MPa rated ram turns it into a bomb -- metal shards embedded in the walls and in anyone so unwise to be present. Don't even consider such an action! Your 'competitors' are most certainly not using 1 MPa rams and valves at 7 MPa.

'Ask Aaron' does not offer advice on the construction of pneumatic systems exceeding 10 Bar pressure. 'Full pressure' pneumatic systems are extremely dangerous for inexperienced builders. Even with the correct components a small mistake can be fatal. There are components that can operate at 70 Bar, but for safety reasons I'm not going to provide info on where to find them. By the time you're experienced enough to know how to use them you won't have to ask me where to get them.

Read the Team Da Vinci pneumatics guide for information on combat robot pneumatic systems.

Q: Sir I want to know which motor is good for rotating an 2-3 kg weapon? [India, masked as a SoCal ISP]

A: [Mark J.] Click here.

Indian combat arenas are not safe!

Q: Hello,Mark. What made horizontal gripping/crushing weapons like 'Tough As Nails' and 'Kan-Opener' still being rather competitive even after the TV era? They can easily be the targets for flippers, but they still could win some fights. [Chinese Forum]

A: [Mark J.] We have often remarked here at 'Ask Aaron' that the weapon may be the least important system on a combat robot. A well designed, well built, well driven robot with reliable components can win matches regardless of the weapon choice. Most people refuse to believe this statement, but the evidence is on our side.

'Kan Opener' combat robot
Q: And Mark,do you think british robot Gabriel's tactic of setting extremely high ground clearence to make flippers have no chance to get a good position to flip is a sounding design? That looks pretty interesting and beat some flippers.

A: If that's Gabriel's strategy, it doesn't always work very well (video). The actual strategy seems to be surviving long enough for their opponent to make a mistake and drive into the pit.

A 'reaction hammer' style weapon cannot deal out any real damage, and I expect the British teams to quickly develop 'anti-Gabriel' attachments to their weapons that will provide enough extra height to deal effectively with the big-wheel tactic.

Q: hello sir, i m looking forward towards a vertical spinner. I just want to know which is the better option to go for, smthing like "Electric bogaloo" or "the witch doctor". What are the +ve and -ve aspects of these bots??? please help me out. [India]

A: [Mark J.] Click here.

Indian combat arenas are not safe!

Q:Where can one get really good beetle motors for less than $20 a motor? The ones from [supplier name deleted] I have found to be subpar and the ones on [supplier name deleted] are quite expensive at almost $30 each. [Pennsylvania]

A: [Mark J.] I'm not a fan of mixing 'cheap' and 'combat robot' -- "really good beetle motor" and "less than $20" don't really go together.

  • The best I can do at your price point is the Kitbots 1000 RPM Beetle motor. Many builders like them for their speed and torque, but they require a bit of preparation to be tough enough for combat use. Follow Kitbots' recommended 'Battle Hardening' process.

  • There is also a large selection of cheap 1000 RPM motors on EBay that have found some application in beetles, but it's a real grab bag. Maybe you get something you can use, maybe you don't. You'd get better odds at a Native American casino.

Try the Kitbots motors. If you don't like 'em you should find a cheaper hobby.

Q: Respected sir,😅
We are planning to build bot based on titan 2 by riobotz. Using a ampflow E30 400 for weapon. Can you tell me some drawbacks involved with the design. We have included 4 WD instead of 2 WD. [India]

A: [Mark J.] Click here.

Indian combat arenas are not safe!

Q: Hi servo lifter guy again. I am trying to settle down finally on a servo and am stuck between the HS-7980 and the HS-7954. I need to know exactly how bar length is calculated... is it point of rotation to point where the servo bar attaches to the scoop (middle of scoop) or is it the point of rotation to the point of the scoop (the endpoint scraping the ground)? Can a servo be effective if operating at 80% of its lifting capacity (obvious slowing it substantially but how slow is too slow?) Your answer may save me $80. Thanks. [Massachusetts??]

A: [Mark J.] The bar length is measured in a straight line from the point of rotation at the servo to the point where the weight of your opponent will be applied. In most cases that will be the far end of the bar.

How much weight will actually be on the bar is another issue. If you have a short 'scoop' that will only penetrate far enough underneath to lift one end of your opponent, the lifter will - for practical purposes - only have about half your opponent's weight on it. If your lifter is designed to get far enough underneath your opponent to lift the entire robot you will need to plan for their full weight.

How quickly you need to lift will also depend on your design and your strategy:

  • If you're building to lift your opponent up just a bit to break their traction, you'll only be lifting their weight a very small height and speed is not really an issue.
  • If you're trying to tip a 'bot over from the side, you'll be lifting higher and longer while the other side of the 'bot still has traction. Speed is critical.
  • If you're getting underneath and lifting them clear of the floor, you'll need greater lifting power and a rapid start to the lift and be lightly enough loaded to hold them for an extended period without frying the servo.
In actual combat you're never sure what lifting opportunity may present itself which is why I design for adaptability using the 60% load factor discussed in our previous post (now top post in the Ants, Beetles, & Fairys archive). Think about your design, the likely way you'll be using the lifter, how high you'll need to lift, and how long a heavy load will be applied to the servo.

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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