5316 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) requested 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   

I cannot accept any questions from India  Recent Questions

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]


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 (about seven posts down the page). 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.

The 'Ask Aaron' website is closed to questions from robot builders in India. Why??

Q: Respected sir,
can dewatt motors can be attached with banebots P60 gearbox?
Because It has 15 teeth in its pinion as in rs775 motors
thanks [Bangalore, India]

A: [Mark J.] The DeWalt 396505 series motors do have 15-tooth pinion gears, but those gears have a non-standard pitch that will not mesh with BaneBots gearboxes.

It is possible to remove and replace the DeWalt pinion with a standard pitch gear, but it isn't easy. The mounting holes for the DeWalt motors are also spaced differently than the mounts in the BaneBots P60 gearboxes -- they aren't gonna fit without extensive modification.

If you want to use the DeWalt 396505 series motors in a robot drivetrain I'd suggest using the DeWut?! gearmotor kits that are designed for that purpose.

Q: Hello sir,
We are making a wedge robot for 40KG category using two Banebots rs775 18V motor with 64:1 gearbox and wheels are 6 inch. Its a 4WD by chain and sprocket. The weight of bot will be around 38 KG.
My question are
1) Is the motor performance is good for effective wedge?
2) How much can I overvolt the motor?
3) In events there are many drumbots so what is the best strategy for competing those?
Thanx sir [Karnataka, India]

A: [Mark J.] Your questions have been discussed previously here at 'Ask Aaron':

  1. The Tentacle Drivetrain Calculator allows builders to model the performance of specific drivetrain designs with robots of any given weight. A great many examples of its use are featured in the Ask Aaron Motors and Controllers archive. I strongly advise all builders to learn the use of this tool. Very generally, your proposed drivetrain seems adequate for a wedge robot in a small Indian arenas where the robots tend to be rather slow.

  2. Motor overvolting is discussed in Frequently Asked Questions #25. Speed (19,500 RPM) and current consumption (130 amps @ stall) on these inexpensive motors are already quite high, making operation at higher voltage unreliable. I recommend staying very close to 18 volts. If you go higher, pack spare motors.

  3. Scroll down this page about 12 posts to find recommendations on wedge attack strategy against Indian robots with 'wedge skids' that applies to drumbots. Additional discussion on driving strategy for various designs may be found in the Ask Aaron Design & Construction archive

Q: I am building a hovercraft [HOVERING COMBAT ROBOT], i want to know that can i connect two different ESC's on two Different channels like one on throtlel channel for hull EDF (upward thrust) and one on elevation channel for forward thrust. [San Jose, California]

A: [Mark J.] Yep, that's how it's done. Your rudder servo will connect to (wait for it...) the rudder channel and you'll be ready to hover away.

Q: Hey Mark, I need ur help , I'm confused about the motors which I'm using for drive and wapin...
For drive - planetary encoder geared motor 24v 175rpm 100 watt and for wapin I'm using amflow A28 150 and drum of 18 kg [Maharashtra, India]

A: [Mark J.] I'm confused as well -- what exactly is your question?

Q: try to attach Bite Force forward Drills

How come that I so much like your machine

you have super [Prague, Czech Republic]

A: [Mark J.] Ummm... thanks. I'll pass on your comments to Paul Ventimiglia.

Q: i am using 500 rpm motor and 12v battery what will be the torque can you please tell,i am beginner to robo combat [Tamil Nadu, India]

A: [Mark J.] On my workbench I have a block of wood that is 9 cm wide and 4 cm thick. Can you tell me how long it is? You can't, because there is no formula to calculate length given only width and thickness.

With proper equipment the stall torque of a motor can be measured, but there is no formula to calculate the torque output of a motor based only on its speed and the applied voltage. If you know the manufacturer and the model number you may be able to find a data sheet for the motor online that includes the stall torque.

Q: How is the length of a servo lifter bar determined? Is it point of rotation on servo to point of attachment or is it the full length of the bar? [Oakdale, PA]

A: [Mark J.] I believe you're refering to a post about lifters in the Ask Aaron Robot Weapon archive:

...The power you'll need from [a servo] will depend on the length of the lifter arm attached to it. For reasonable speed and reliability, select a gearmotor that will stall with no less than 1.67 times the weightclass on the end of the lifter. Here's the formula to calculate the desired torque:

Stall Torque (oz-in) = length of lifter arm (inches) * weight class (ounces) * 1.67

The length of a simple single-bar lifter that plugs into the torque requirement equation is a straight-line measurement from the servo output rotation axis to the tip of the lifter arm.

Q: And is the 1.67 multiplication to provide safety or is the number produced by the equation the actual minimum oz-in?

A: As it says in the referenced post, the formula provides a desired torque that accounts for speed and reliability. To understand where that 1.67 figure comes from you need to know a bit about R/C servos:

  • Servo torque is given as 'stall torque', which is the load at which the servo stops moving.
  • Servo speed is given as transit time under zero load, which effectively never happens.
At stall load you have zero speed and the servo lifter won't actually lift anything. Actual servo speed under load is inversely proportional to the percentage of stall load applied:
  • Loaded to 100% of stall torque, the servo is stalled and has zero speed;
  • Loaded to 90% of stall torque the servo has 10% of its listed speed;
  • Loaded to 80% of stall torque the servo has 20% of its listed speed...
The 1.67 multiplication factor in the equation assures that the servo will not be loaded beyond 60% of its stall torque rating when lifting the assumed maximum load. Under that load the servo will deliver 40% of its listed speed. This provides a good balance of speed and safety from damaging overload.

Q: Hello Mark, JC again.

Thank you for the answer about the RS775s, it was very informative. Banebots also seems to be discontinuing their motor stock, so using something else is more desirable. I have a few more questions if you're willing to answer them.

For the 60lb wedge/lifter my friend and I are planning, do you think two Dustin motors at 24v would work? The veteran builder who is helping us might have some leftover from his heavyweight, and has offered them to us. They would drive the four 5" wheels via chain (we don't know whether they are the high or low speed, or if they can be switched, but we would use a 1.2:1 increase for low speed motors and 1:4 reduction for the high speed motors).

I've seen videos of 60lb robots going about 7MPH (with active weapons) and 12MPH (no active weapon). Using the motor torque calculator, this motor/wheel/voltage combination would give a top speed around 8MPH. Do you think this speed is enough to be competitive (assuming we get lots of driving practice)? [San Diego]

A: [Mark J.] The Dustin motors are easy-to-mount, battle-hardened conversions of the DeWalt 24 volt hammer drill motors and gearboxes. The modifications include improved brush holders and end-caps that provide good reliability up to 36 volts -- more than doubling the power output of the motors at 24 volts. A pair of Dustins at 24 volts would provide adequate performance for your lightweight lifter, but you're missing a bet by not running the motors at higher voltage.

You can certainly try the 'bot at 24 volts and see if it suits you. If you use speed controllers that can handle higher voltages you can bump the voltage later if you decide you need greater drivetrain performance:

  • For a small arena: at 36 volts with 50% greater gear reduction you'll retain the 'around 8 MPH' speed and gain tremendous acceleration with top speed in less than 5 feet (0.6 second).
  • For a large arena: at 36 volts and the same gear reduction you'll have 'around 11 MPH' in 12 feet (one second).
Around 30 volts might be a good compromise -- and yes, do get LOTS of driving practice!

Q: In the archives drum weapons are mentioned numerous times but no where does it actually [show how to] attach the drum to the [live] weapon shaft. Could you explain that to me? [Greenville, Pennsylvania]

A: [Mark J.] Live shafts are not common for drum weapons. A non-rotating dead shaft can be made into a structural member of the chassis to strengthen the weapon mounting area. That's a major plus.

If you have a good reason to use a live shaft, the method of attaching the drum to the shaft is similar to attaching drive pulleys or wheel hubs to shafts:

  • Heavier 'bots with shafts large enough to be broached for a keyway can make good use of that technique to lock the drum and pulley to the shaft.
  • Smaller 'bots can attach the belt pulley to the drum with screws and use the pulley hub to clamp to the shaft. If using a set screw hub, be sure to file a flat on the shaft and use a threadlocking compound to keep the screw tight!
I think that in general you'll do better with a dead shaft.

Q: Second, how in an insect class could you make a weapon with a dead shaft? Every way I can think to configure a dead shaft requires an unusablely large weapon hub/reduction pulley to attach to.

A: What do you consider to be 'unusably large'? Take a look at the beater-bar weapon made for the 'Weta1' beetleweight kit. The weapon rides on small outer diameter needle bearings to reduce the size of the bearing support, and the belt pulley is attached directly to the weapon.

It's also possible to eliminate the belt drive and embed a small outrunner motor into the weapon drum with the entire drum/rotor assembly riding on a dead shaft. I'm not a big fan of direct-driving a spinner, but the new 'Saifu 2' antweight kit powers their compact spinner in this manner. Note that it does require some good machining skills to implement.

Q: How does a fbs [full body spinner] prevent itself from getting fillpped by a wedge in a small arena? [Pennsylvania]

A: [Mark J.] Often, it doesn't. If you look thru the 'Ask Aaron' archives you'll see that we repeatedly warn about big spinners in small arenas. The standard tactic for a wedge vs. spinner is for the wedge to 'box rush' the spinner and trap it against the arena wall before it can spin up to speed. It's quite effective. There are only two practical things a FBS can do to counter this:

  • design around massive weapon motor power to spin-up before your opponent can get to you; or
  • work up enough drive power and control to be able to dodge your opponent's charge while you spin-up.
Another worry: a good hit by a FBS can send it ricocheting off its opponent and into the arena walls, turning itself into a self-propelled runaway pinball. Under the best conditions a FBS is nearly as dangerous to itself as it is to its opponent, and this is greatly magnified in a small arena. Audiences and builders love full body spinners, but they're a real crapshoot to drive.

Q: what is the logic behind the new wedge of original sin? is it beneficial to have such wedge? [Maharashtra, India]

A: [Mark J.] What new wedge? Original Sin has different wedges that it can mount against different opponents, but I don't know of a new one.

Do you mean the yellow spinner-trap? It isn't new -- it dates back to 2012. The spinner trap was developed to control the massive horizontal spinning bar on 'Last Rites' and bring it to a stop. If you're facing similar opponents it's a reasonably effective design, but it doesn't work well against drum spinners.

Bar spinner trap on heavyweight robot 'Original Sin'. Photo credit: Miles Kodama
'Original Sin' with bar spinner trap attached

Q: Could this Festo actuator be used in a beetle if using a 12 grams co2 cartridge? [Connecticut]

A: [Mark J.] Not for a flipper; the gas ports are too small to allow the rapid gas flow required. The Festo actuators are also expensive -- you might expect to pay close to $200 each.

See this post in the Ask Aaron Ants, Beetles, and Fairies archive for a previous discussion on insect class pneumatic flippers that outlines the problems with existing off-the-shelf components.

Q: Hi mark. Servo/fbs beetle guy. Thanks to your site and guided research I have become knowledgeable in the weight class. My tech teacher though also wants me to compete in the 15lb weight class. My question for you is what's different and where do you get parts for a bot of that weight class. I am particularly interested in a gas flipper. Thank you as always. [Pennsylvania]

A: [Mark J.] First, the 15 pound weight class is not one of the classes commonly contested at open combat tournaments. It is reserved for 'educational' robot tournaments put on by organizations like Bots IQ and the National Robotics League that have their own rules. Your first steps will be to determine which tournament your instructor wants you to enter and obtain a full set rules and competition info.

Once we know what set of rules you will be using we can further discuss the details of design and components. Very generally:

  • Robots in this weight range generally use modified cordless drill motors or BaneBots gearmotors.
  • Pneumatic flippers in this size range are often based around paintball gun tanks and regulators, but suitably sized valves and actuators can be difficult to find.
Pneumatic flippers are discussed in depth in the Ask Aaron Robot Weapons archive.

Q: Hello. Yes, the class is the BotsIQ. I also found the rules pdf. In this weight class what are some popular designs and what are commonly used materials/thicknesses? (I know vague different designs require different materials but I just want a rough idea because I doubt all use 3/8 lexan and 1/4 aluminum.) Would a flipper be competitive in this class or should a spinning weapon be on my agenda instead?

A: Go search 'YouTube' for 'BotsIQ'. It's all about eggbeater and drum spinners. There's very little room around the edges of most of the arenas for a flipper to score an 'Out of the Arena' (OOTA) knockout. Even if there was room, the eggbeaters are doing at least as good a job of tossing their opponents into the air as a flipper might.

I think you can see enough of the bots in the videos to get a good idea of the materials used. The 'bots take a lot of abuse, and chassis rails of 1/2" aircraft alloy aluminum are not uncommon.

Q: What are the advantages of a weapon system without using belts but rather attaching a wheel to the brushless and spinning it that way? [Pennsylvania]

A: [Mark J.] There are many more disadvantages than advantages to mounting a weapon directly on the motor shaft. The only real advantage is simplicity of design. I'm a great supporter of simple design, but in this case I really can't recommend it. The disadvantages are too great:

  • A larger and more powerful motor is needed to spin the weapon quickly to speed.
  • Although slower to spin up to speed, the direct-drive weapon will spin too fast for good 'bite' at full speed.
  • Weapon impacts will be transferred directly to the small and fragile motor shaft, bearings, and case -- unless "extensive modifications are made.
You'll break a lot of weapon motors by trying to keep things a little too simple. It is possible to modify a brushless motor to take the extra strain of direct-drive weapon duty, but it takes quite a bit of work. Search the Ask Aaron Robot Weapons archive for 'Mike Jeffries' to see how it's done.

Q: what is the strategy to be used against anti wedge skids and what should be the angle of the wedge for 60kg robowar bot? [Tamil Nadu, India]

A: [Mark J.] A 'strategy' is the goal you want to accomplish; in this case that would be successfully using a wedge against a robot with anti-wedge skids. A 'tactic' is the method you employ to accomplish that goal. I think you're looking for a tactic rather than a strategy, and there are several available:
  • High-speed ramming attack: Many Indian robot battles seem to consist of two robots cautiously approaching each other and then surging a few inches forward to engage weapons. Bah! Those anti-wedge skids make a fine place to ram and shove your opponent back into the arena wall. Repeat until they break.

  • Side attack: With good speed and maneuverability it is fairly easy to circle around to the side or back of a sluggish opponent. Use your wedge against unprotected targets.

  • Redesign the wedge: Anti-wedge skids will catch on a standard blade-style wedge, but a wedge of long teeth may slip around the skids and get under the chassis. See photo of a possible rake-wedge design at right.
Don't get stuck thinking about your robot in conventional ways. Sometimes there's a good reason why other competitors do things, and sometimes they just do them because that's the way everybody else does it. Think different.

The topic of wedge angle has been discussed many times here at Ask Aaron. As a start, try a search of the Ask Aaron Design and Construction Archive for "best angle".

Q: excuse me what weight is the fingertech viper combat robot? : / [St. Louis]

A: [Mark J.] From the FingerTech 'Viper' webpage:

[The Viper kit] base weight is only 313grams (11.0oz) leaving:

  • 141grams (4.97oz) for additional parts if entering the Antweight (1lb) weight class

  • 1049g (37.0oz) if entering the Beetleweight (3lb) weight class.

Q: I am from chennai, India. I am interested in combat robots. I am 15 years old. Can you please tell me what is the difference between ampflow and mag motors? [India]

A: [Mark J.] Magmotor Technologies Inc. is a manufacturer of industrial motors and related components. They make a wide range of industrial motors covering a broad range of specifications. Their motors are designed to provide reliable service over a long service life under continuous operation.

About 15 years ago, combat robot pioneer and champion builder Carlo Bertocchini worked with Magmotor to produce a special series of motors designed specifically for combat robots. These new motors were designed to provide much greater power output than standard Magmotors over shorter operating cycles. Carlo initially sold these high-performance motors as a variety of Magmotor, but for several years now he has marketed motors, motor controllers and gearboxes exclusively under the name AmpFlow.

The AmpFlow motors resemble Magmotors because they use the same motor casings, but the performance of the motors is much different. Don't let anyone sell you a Magmotor and tell you that it's the same as an AmpFlow.

Q: Can you also let me know as to what is the best way for a drum bot to defeat an well armored, ramp or wedge, invertible bot?

A: Speed and maneuverability are important in drumbot attacks. The last thing you want to do is slowly move forward and grind uselessly on their wedge-- that's playing to their strength. You must get into a position where you can use your weapon against a part of their 'bot that has hard edges for your impactor to grab. Attack the outer edge of their wedge at an angle, or rush in against unprotected wheels. Keep hitting them 'til they break. Watch videos of 'Tauro' to see how it's done.

Q: Is there a way to calculate about how thick your weapon shafts should be? This is for a beetle fbs about 3" tall. Bowl plus teeth weighs at 1.25 lbs. It is a live shaft and I was planning on using a 6mm shaft but am worried it is too thin and will bend or snap despite the 2 spaced apart ball bearings. It is being spun at about 4000 rpm.

Thanks. [Pennsylvania]

A: [Mark J.] From an theoretical engineering standpoint -- if you know the exact force vectors that will be applied to a shaft you can absolutely calculate the required diameter for a shaft of specific material and length. All it requires is a degree in mechanical engineering and a few hours of your time.

From a practical standpoint -- figuring out the force vectors for a shaft in a combat robot that may be impacted from any angle and with unknown amounts of force is effectively impossible. As we recommend in Frequently Asked Questions #17:

Look to see what other builders with similar designs are using and learn from their experience.
If it breaks, make it stronger.

Q: I loved it back in the day when robots were made resourcefully out of junkyard stock, or parts salvaged from things you wouldn't expect. Are there any successful present-day robots made from scraps & salvage, or have custom-tailored parts for combat robots completely taken over? [Waltham, Massachusetts]

Indian Robowar competitors A: [Mark J.] I'm with you, Waltham. Scrapbots are pretty much extinct in the US and Europe, but amazing resourcefulness is alive and well in India.

'Robowar' is a very popular activity at technology festivals around India, and the level of adaptability and imagination that goes into these 'bots is refreshing and wonderful!

Try video and image searches for 'robowar india'.

Indian Robowar machine

Q: Hi, beetle lifter guy again. I'm trying to get a final parts list together and need to know the following:

1. Can I hook up 2 motors on one side inorder to get them to spin in sync?

A: [Mark J.] Yes -- multiple brushed motors can be connected to a single channel of an ESC as long as their combined current draw is within the current capacity of that channel.

2. Do I need a servo booster?

A: No.

3. Do I need a servo controller or can I simply hook it up into my Sabertooth 2 x 12 RC Dual Motor Speed Controller?

A: Neither. The servo plugs directly into the R/C receiver (see next question).

4. What do I need to make my servo (HS-7954SH) work on the left stick?

A: On most transmitters used in North America the up/down axis of the left stick is channel 3 -- it may be marked 'Throttle' or just 'THRO'. Plug the servo into that port on the receiver.

5. How is this wired together -- 1 2c lipo (for servo), 1 3c lipo(for drive motors), 4 drive motors, servo, rc receiver, speed controller, switch? Thank you as always. [Pittsburg, PA]

Wiring diagram for four-motor robot with servo lifter weapon A: See the diagram at right.

  • If you buy a two-cell 'receiver pack' battery -- it will come with the correct connector to plug directly into any unused port in your receiver. You have verified that your chosen receiver will operate on 7.4 volts, right? You may put a switch into the power line or simply unplug the battery to power-down your receiver and servo. Check the event rules to see if they require a switch on this power line.

  • On the Sabertooth motor controller, clip and tape back the red wires on the leads that run to the receiver. These wires carry 5 volt power from the ESC to the receiver, but since you are powering the receiver with a higher voltage/amperage battery you don't want the ESC trying to power it as well. If you are using 'EleVon mixing' to control the robot throttle and turning from the right transmitter stick you will plug the two leads from the ESC into channel ports 1 and 2 (ELV and AIL) in the receiver.

  • Your Sabertooth ESC will come with instructions on connecting the three-cell battery and motor leads. The diagram shows a 'Dean's plug' used to connect the battery to leads from the ESC for rapid disconnection and charging. A second plug is used as a 'removable link' to power/un-power the drive. Again, check the event rules to see how they want the main power disconnect to function.

When you get it all wired up, follow the instructions down near the bottom of our Guide to Transmitter Programming for Combat Robots to get the robot to correctly respond to transmitter commands.

Q: Hi sir, for a four wheeled bot with dimensions as 600*600mm is it advantageous to keep the centreline of front wheels in some offset of about 20mm with rear wheels centre line both driven by sprocket chain reduction powered by E30-150 [A builder in Maharashtra, India]

A: [Mark J.] Wheel centerlines for four-wheeled robots are typically kept aligned with no offset. I see no particular advantage or disadvantage to a small offset, provided that the chains are kept in perfect alignment.

Q: Respected sir we are developing a wedge bot in 40kg segment. The design is similar to original sin. The weight of the robot is 25 kg without wedge. I am confused with the weight of the wedge. Is there any formulation for this? Can you suggest weight of the wedge. Thank you. [A different builder in Maharashtra, India]

A: [Mark J.] Wedges aren't built to a specific percentage of robot weight -- they're built to be strong enough to survive the abuse they will receive.

  • Differences in the width and height of the wedge to suit robots of different designs will call for heavier or lighter wedges accordingly.

  • Changes to the material from which the wedge is constructed will also cause differences in the weight; tempered high-strength steel wedge can be thinner and weigh less than a non-hardened mild steel wedge, and a titanium wedge can be lighter still.

Take a look at successful wedge designs on robots of comparable weight and layout. Examine the material, thickness, design, and mounting technique. Use that as a basis for your own wedge and calculate weight accordingly.

As an example: a plate of mild steel 500 mm by 250 mm by 6 mm thick might be formed into a reasonable wedge for a 40 Kg Indian robot. That plate would weigh about 3.5 Kg. Add perhaps another 2.5 Kg for supports and mounts and you could have a durable wedge.

P.S. - From now on I think I want every question to start with "Respected sir". That's a lot better than what they call me on the forums.

Q: How do you calculate what servo you need in oz-in as well as speed for this: [upper design at right]

and this design: [lower design at right]

I have [the T.i. Combat Robotics 4-Bar Simulator] but don't think it would be very helpful considering they are not traditional designs. [Pennsylvania]

A: [Mark J.] The upper design at right is Team Run Amok's inverted 4-bar lifter. The T.i Simulator cannot handle the calculation of torque requirements for this inverted design, and a force analysis of a 4-bar system is very complex. Estimation of torque required for that specific lifter has been recently discussed -- see this post in the Ask Aaron Robot Weapons Archive.

The lower design is not a 4-bar lifter, it is a simple lever arm directly attached to a servo. This is an entirely traditional design. Torque calculation for a simple lever arm has been discussed multiple times at Ask Aaron -- see this post in the Ask Aaron Robot Weapons Archive.

Before you ask -- check the Ask Aaron Archives
for an existing answer to your question!
4-Bar Servo Lifter

Simple Servo Lever Lifter

Q: Hi Mark,
A friend and I are looking to build a 60lb wedge/lifter robot. We've received some parts from a veteran builder, some of which are 1:10 and 1:20 speed reducers. According to the builder, they're very sturdy, but heavy (about 3lbs each) and inefficient (worm gear drive). If possible, we'd like to use these for our robot for their durability and cost (free). Additionally we are looking to use four 5" wheels powered via chain and 18v batteries, with a desired speed of 15 MPH. Using the various calculators you've provided, we've calculated the required minimum torque of 2.8 ft-lbs and recommended 960W of drive train power.

My questions are:

  • Is using these speed reducers feasible for a lightweight wedge/lifter robot?

  • Would a Banebots RS-775 be suitable to use with a 1:10 reduction? Each motor outputs just over 600W (at 50% max speed and 50% stall torque), and the reduction would provide double the minimum torque and give a theoretical top speed of 25MPH. With estimated 75% gearbox losses [should be 'efficiency' -- only 25% loss] that drops the output power to 450W per motor and a top speed of 18 MPH, which suggests to me that it may be suitable, assuming the motor does not break.

  • Do you have a suggestion for a motor which would be better suited for use with these speed reducers?

Thank you, JC [San Diego]

A: [Mark J.] Your calculations are a bit off. The power output from the gearbox will be down by perhaps 25%, but since power is the product of torque and RPM, each of those values is reduced by only 13.4% (1 - square root of 0.75). I make calculated top speed with 5" wheels and a 10:1 reduction @ 18 volts to be right at 25 MPH.

The bad news that the 10:1 reduction ratio gives really awful acceleration. It would take nearly 9 seconds and 45 feet to reach your desired 15 MPH, badly bogging the motors and pulling a combined 100+ amps for a good part of that time. That stinks!

The picture isn't much better for the 20:1 gearboxes. Top speed there is only 12.5 MPH for the RS-775s with the 5" wheels. Acceleration still lacks: in a 24 foot arena you'll only get 11 MPH and it will take nearly 3 seconds to cross the arena. That still stinks. A pair of RS-775 motors @ 18 volts simply don't put out enough power to give the performance you're looking for in a 60 pound robot.

If you really want to use those gearboxes to get close to 15 MPH with reasonable acceleration you're going to need to substantially upgrade your motor selection. A pair of AmpFlow F30-150 motors @ 24 volts with 8" wheels mated to your 10:1 gearboxes will push a 60 pound 'bot to 14 MPH in a 24 foot arena and your 'bot will cross that arena in 1.6 seconds. That's pretty good, and the current consumption peaks at a very reasonable 37 amps per motor.

Those gearboxes may not turn out to be a bargain, even if they were free.

Q: To comment on the number of teeth of beetle fbs, would it not be better to have a lesser bite given the size of the arena and that you will most certainly be box rushed? Less teeth would make it easier to balance and when impacted the shell rpm would not drop as severely. If the impactors would be sharpened it would increase the bite into plastic or thin sheet metal. [Pennsylvania]

A: [Mark J.] I don't think you have a good understanding of 'bite'. As defined in section 6.3 of the RioBotz Combat Tutorial:

"The tooth bite is a distance that measures how much the tips/teeth of the spinner weapon will get into the opponent before hitting it."

There are numerous posts about 'bite' in the Ask Aaron Robot Weapons archive. Some highlights:

  • Fewer teeth give better 'bite'.
  • Lower rotational speed gives better 'bite'.
  • Higher closing speed on your opponent gives better 'bite'.
  • A single-tooth spinner has the best 'bite' but can be a challenge to balance.
  • The ability of sharpened impactors to 'dig in' to a soft or deformable surface is not the same thing as 'bite'.
  • If you have poor 'bite' at low RPM you'll have really awful 'bite' at high RPM and you never will get a good hit.
  • A weapon with good 'bite' will be effective at transferring destructive force to its opponent and will lose nearly all of its speed on impact. Less speed drop = less energy transfer = less damage to opponent = less desirable outcome.
  • If you're being 'box rushed' you're going to have great 'bite' due to the low rotation speed but little stored energy. You should gear and power the spinner to provide effective energy storage quickly while retaining good 'bite'. Evasive maneuvers while your spinner gets up to speed is a viable strategy.

Q: What is the best substance to clean your tires with? Riobots suggests WD-40 because it leaves a sticky residue but you suggest lighter fluid for the opposite reason. Is there an even better product like Windex or something? [Pennsylvania]

A: [Mark J.] Save your Windex for cleaning the arena Lexan. Both WD-40 and lighter fluid are solvents that are effective in dissolving and removing oily deposits from tires. Lighter fluid evaporates and leaves only clean rubber behind -- it is my preferred cleaner for solid tires. I've never been able to find anything 'sticky' about the residue RioBots claims to have discovered from cleaning tires with WD-40, but feel free to experiment.

For foam rubber tires like 'Lite Flites' it's possible to gain a bit of traction by using fluids that chemically decompose the rubber a bit and leave it a little sticky. You can do this with expensive traction compounds available thru R/C racing sources, or you can visit your local pharmacy and purchase a small bottle of 'oil of wintergreen' (methyl salicylate) that works quite well. Wipe it on, let it set for a couple minutes, then wipe off any excess.

Note that a 'sticky' tire surface will very quickly pick up dirt and crud from the arena surface and loose it's traction much more quickly than a simple clean tire. Plan accordingly.

Q: will ar400 plate acts a barrier for an s7 tool steel single teethed drum bot [Tamil Nadu, India]

A: [Mark J.] AR400 steel is a fine armor material generally suitable for defense against spinning weapons, but effective armor depends on more than the material from which it is made. The ability of any armor to withstand a specific attack depends on thickness, temper, design, and mounting -- as well as the material.

Likewise, the effectiveness of a weapon depends on more than the material from which it's impactor is made. You're asking the wrong questions.

Q: What was the earliest use of an asymmetrical spinning weapon? [Philadelphia]

A: [Mark J.] Well, there was that fight where 'South Bay Mauler' lost one of it's impactors and went all wobbly but kept spinning. Does that count?

1. Can you put a 'D' shaft in a linear or ball bearing?

A: [Mark J.] Ball bearing - generally yes, but the manufacturer may recommend a specific shaft diameter tollerance.

Linear bearing - depends on specific bearing design. Consult the bearing manufacturer.

Assorted servo horns 2. What is the purpose of shaft collars?

A: A shaft collar locks onto the shaft to prevent it from sliding thru a bearing/bushing. Set-screw collars suck -- use a clamping collar if available.

3. How do you connect a servo to the sheet metal you want to lift? Thank you. [Seattle, Washington]

A: Servos come with one or more 'servo horns' that mate to the splines on the servo output shaft. Select a suitable horn and run sheet metal screws thru the metal and into a few horn holes. Yes, I said horn holes -- you're welcome.

Q: Hello, beetle fbs guy again. Not a real question but rather and explanation. If you recall you suggested I use an aluminum spacer and make the bolts the impactors. What I proposed to do was cut the beam in half making a square c. Then I would run two bolts trough the roof and through the base along with about 5 bolts through the impactors along the wall. With the bowl the design will most likely finalize into an undercuter. Would I be wise to make tall impactors like that design but not mount through the roof?

Thanks as always. [Washington]

A: [Mark J.] Aha! I was picturing a much smaller cross-section beam in a different configuration and it wasn't making sense. Now I've got it.

Steel cooking pots Take your stainless steel mixing bowls back and get yourself a straight-sided steel cooking pot of suitable size. I'd leave off the angle over the top for the roof mount because spinner shells with a tall 'bracket shape' to their mass can run into stability problems -- see this previous post on spinner instability.

I'm not sure what shell dimensions you're contemplating, but to insure stability I'd restrict the height of each vertical impactor to no more than one-third the shell diameter. Example: 9" shell diameter => 3" max impactor height. That's a pure eyeball guess based on unstable spinners I've seen. You can try a taller impactor and trim it down if it gets funky at speed. You probably aren't going to hit anything more than a couple inches off the floor anyhow.

I'm assuming that you want two impactors. Using three impactors greatly reduces (eliminates?) any possible instability, but also reduces the impactor 'bite'. I think you'll be OK with two impactors.

Don't forget to put washers under the nuts securing the bolts on the inside!

Q: How do 3 impactors make a fbs more stable than 2 impactors? In my mind this makes it less balanced and therefore less stable. For a 10" fbs about 3" tall would you recommend 1,2,3, or 4 teeth. (Spinning at about 5000 rpm). Thanks.

A: I messed up the link to the previous post on spinner instability, but it's fixed now. Give that a read. The math gets really deep really fast.

A radially symmetric object with three primary masses can be perfectly balanced to spin in its axis of symmetry. Most small motor armatures have three iron poles. The trick here is that an object with three primary masses has identical moments of inertia for the two axis at right angles to the axis of symmetry. That effectively guarantees that rotation in the axis of rotation has either the greatest or smallest moment of inertia of the three possible spin axis and is therefore stable. The same cannot be said of a rotating mass with only two primary masses -- it may be stable or it may not.

Have you calculated the energy storage capacity for your shell? I know you likely don't have performance numbers for that motor that you had lying around, but you may find that it takes a very long time to spin a 10" diameter shell to 5000 RPM -- maybe too long for a small insect arena.

How many teeth?

  • One tooth: best bite, but can be a real problem to get balanced and may or may not be stable.
  • Two teeth: fair bite, easy to balance, may or may not be stable.
  • Three teeth: bite decreasing, easy to balance, almost certainly stable.
  • Four teeth: poor bite, easy to balance, probably stable.

Yes, there are tricks to increase bite for multiple impactors but I recommend staying simple, it's getting late, and this answer is long enough already. I'd go for two impactors. If it isn't stable we can fix it.

Q: Hi Mark. Beetle fbs guy again. So I took your advice and purchased a few stainless steel mixing bowls (keeping the receipts until the design is finalized). The great part is now the overall weight is down quite a bit. The bad is I no longer know how to mount the teeth I'd like to use considering that a bowl does not have a flat side to easily put bolts through or a place to mount easily on top or underneath. I was thinking about making some wood blocks and sanding them down for a easy mount for bolts but then this takes up interior space requiring a larger bowl. Any thoughts? [Washington]

A: [Mark J.] As I recall, you have hollow square steel beam that you want to use for impactors. You told me you planned to 'cut it in half' but you gave me no dimensions except the 1/4" wall thickness. I can't really picture what you're trying to do.

Bowl FBS spinner combat robots

Angle-sided 'bowl' spinners typically have fairly small impactors mounted along the lower lip of the bowl. They are long enough to allow for several mounting bolts thru the impactor and the lip. 'Sandwiching' the lip between the impactor and a fabricated metal reinforcing strip helps to strengthen the mounting.

Of course, if you want tall impactors you can always go with a straight-sided steel cooking pot...

Q: As you may remember I also am building a beetle lifter. A few weeks ago we discussed a servo lifter and you gave me an approximate lift time of 0.6 seconds. If my strategy is to box rush the opponent and back them into a wall then use the lifter this time is completely acceptable. However, I was wondering if a faster or more powerful lift was possible if I went with a traditional 4 bar design? If yes, then in four bar what bars would be extended/shortened to do this?

4-Bar Servo Lifter A: The balance between lift force and lift speed can be changed, but the product of the two will remain constant:

  • Double the lift speed by lengthening the 'H' bar, and your lift force will be cut in half.

  • Double the lift force by lengthening the 'F' bar, and your lift speed will be cut in half.

I'd suggest sticking to the approximate bar ratios I gave you in the earlier post.

Q: Your help has been greatly appreciated over this entire design process.

P.S. Would you mind if I named the fbs 'Bad Hamburger'?

A: Aaron was sure that somebody would eventually name a robot 'Bad Hamburger' as a reference to his unanswerable question analogy. Go for it.

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.

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