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4902 Questions and Answers about Combat Robotics
from Team Run Amok
Ten Years of Ask Aaron

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   

Welcome to my website Recent Questions
Not all the questions we receive at Ask Aaron are serious. Some are odd, some misdirected, and a few are incomprehensible. Aaron enjoyed dealing with these questions, and I've collected some of his best responses in a in a special page:
Aaron's Greatest Hits
Aaron's Greatest Hits

Q: Did you and your son ever hope that there would be a third season of Robot Wars: Extreme Warriors to enter? [Unidentified location, North America]

A: [Mark J.] We were looking forward to it! We had two heavyweight robots, twin European frequency R/C radios, a pair of shipping crates, and plans for upgrades. All we were waiting for was the word.

There were all sorts of rumors about a third season. Combat icon Dan Danknick was told by the Robot Wars production crew to get ready to organize the American competitors for a new series. There were also reports of a robot arena being built at Universal Studios for filming the next series. We thought it was a sure thing, but it didn't come to pass.

I've still got our heavyweight lifter The Gap in the workshop. All it needs is a new set of batteries and a full tank of CO2 and it's ready to go -- but it wouldn't be any fun without Aaron.

Q: Hey mark what will happen if i overvolt 24v 300w 2500rpm pankmotor to 30v for drive system? I m bulding a 132lb wedge bot 2 wheel drive with 10" wheel and a gear reduction of 1:30 (too slow)...!

And one weird thing came in my mind that for wedge 600w is nt enough so i was thinking to use two of those motors on one wheel's shaft. [India]

A: [Mark J.] I don't know the 'pankmotor', so I can't comment on how well it will tollerate overvolting. If the motor can survive a 25% overvolt, speed will increase by 25%, torque will increase by 25%, and current consumption will increase by 25%. Since power is the product of speed and torque, the power of the motor will increase by a factor of 1.25 x 1.25 = 1.56 -- so the motor will have a peak power of 1.56 X 300 = 469 watts. Still slow.

Running two of these motors per wheel will generate double the torque for better acceleration, but will supply very little extra speed.

If you have full specs for the 'pankmotor', the Tentacle Torque-Amp Calculator can give you performance parameters for changes in gearing, voltage, wheel size, and number of motors. It's what I use to answer questions like yours.

Q: What is the torque and rpm that provide maxon 148866 with a gearbox of 2:1, of I supply 14.8V [Equador]

A: [Mark J.] OK, let's walk thru it. Here are the specs for the Maxon 148866 motor at it's nominal 12 volt rating -- assuming that you can supply the full stall current :

  • No load speed: 7500 rpm
  • Stall torque: 1.68 Nm
  • Stall current: 105 amps
Raising the supplied voltage from 12 volts to 14.8 volts will raise the speed, stall torque and stall current by a factor of (14.8 / 12) = 1.23:
  • No load speed: 9250 rpm
  • Stall torque: 2.07 Nm
  • Stall current: 129 amps
Then, using a 2:1 gearbox will decrease speed by a factor of 2 and increase the torque by a factor of 2. Stall current is unchanged:
  • No load speed: 4625 rpm
  • Stall torque: 4.14 Nm
  • Stall current: 129 amps
If your power supply cannot provide the full stall current, stall torque will be reduced in proportion to the reduction in current.

Q: when did 2.4ghz radios become popular? [Havertown, Pennsylvania]

A: [Mark J.] February 3rd, 2008 at 10:24 AM pacific standard time.

Q: that's very specific.

A: That's when RoboGames decided that tracking transmitter frequency crystals was too much bother and mandated spread-spectrum radios for their combat events. The 2.4 gHz radios would have taken over eventually, but RoboGames pushed the issue.

About that same time RoboGames also decided that the single most popular large combat class - Hobbyweight robots - was too much bother and dropped it. Superheavys and Feathers followed. Power corrupts.

Q: A builder on one of the forums claims that he spins his featherweight [30 pound class] drum weapon at 20,000 RPM and still gets bite. Is this possible? [Ohio]

A: [Mark J.] Usually, no way -- but I know the bot in question.

A conventional drumbot has the drum well out in front of the rest of the robot and rams the weapon directly into the opponent. In this particular robot the drum is set fairly well back and rather tall 'wedgelets' lead the opponent up and into the drum at a height close to the drum axis. This gives the drum a good chance to impact the sharp leading edge of the opponent and provides a favorable attack angle.

This isn't 'bite' in the conventional sense, but it does allow a very fast drum to connect and release a decent hit IF the driver can coax their opponent up the wedge.

Two in one day. Must be a phase of the moon -- are the grunion running?

Q: how many people in iraq [Philadelphia, Pennsylvania]

A: [Mark J.] combat robots? Not any, that I know of.

Q: 5. In what sense did the "Sixties" begin before 1960 and continue beyond 1970? What events might one choose to define the beginning and the end of a "long Sixties," and why? [San Francisco, California]

A: [Mark J.] I've got an uncle who took a bad fall on an icy sidewalk. He believes to this day that Nixon is still the President. It'll be 1969 for him until our robot overlords arrive to set us free. Does that help?

Q: Hey mark.. I have seen Robo Bacon in antweight matches in robogames 13 and I have decided to do that robot weapon for 35 kg bot ... What kind of weapon is that? How it differs from double teethed drum? Which produces more impact on opponent? please prefer some material to do that weapon.... [India]

A: [Mark J.] How can you decide on a robot weapon before you even know if it has any advantages?

Team Uai!rrior's antweight 'Bacon' has a single-tooth spiral drum weapon, similar to hall-of-fame middleweight 'Professor Chaos'. Go read section 6.3 of the RioBotz Combat Tutorial for an explanation of the advantages of an asymmetric single-tooth spinner.

See this post in the Weapon archive for some notes and a diagram on spiral drum construction. The machining is difficult, and balancing the drum is a real challenge. A spiral drum is typically made of aluminum with steel inserts for balance and a hardened steel impactor.

Antweight drum spinner 'Bacon'.

Q: Hi..mark..what is the range of hardening H13 and D2 material for tooth of drum.. [Pune, India]

A: [Mark J.] Six posts down this page is the post from a couple days ago discussing of D2, A2, and H13 tool steels. Read that.

General notes:

  • H13 is a 'hot working' steel designed to retain useable properties at VERY high temperatures -- not the conditions encountered in a drum impactor tooth. Room temperature hardness and toughness are poor. There are much better drum impactor materials.

  • D2 is a steel designed for high resistance to wear in prolonged use in dies and shearing blades -- not the primary property of interest for impact teeth. At HRC 60 hardness it has an acceptable blend of hardness and toughness. I would rate D2 steel as a fair impactor material.

  • A2 is a general purpose tool steel. At HRC 60 hardness it has much greater toughness than D2. It makes a good impactor material.
The preferred impactor material is S7 shock-resistant tool steel, but I'm told that it is not available fron Indian sources. Tempered to HRC values around HRC 58 it has several times the toughness of D2 or A2. It may be overkill, but if you have access to S7 you'll probably want to use it.

Q: hello i am preparing for techfest i need urgent guidance about how to use 2.4 ghz transmitter to move weapon and locomotion [India]

A: [Mark J.] If you're in need of urgent guidance you would be well advised to search the FAQ and the Ask Aaron archives to see if your question has been previously answered rather than wait for me to get back to you. In this case, it has been discussed and answered in great detail.

  • Start by reading thru the FAQ. Pay particular attention to question #19, but read the whole thing -- if you don't know how to rig your R/C transmitter to control your motors there's a whole lot more you're gonna need to learn about.

  • Many Indian robot builders use solenoids to control their robot motors, so you'll want to read the special Solenoid FAQ to see if that option will work for you.

  • There are many, many, many questions on this topic answered in the Ask Aaron archives.

    • The Motors & Controllers archive has answers to many questions on speed controller selection for drivetrains.

    • The Robot Weapons archive has answers about weapon motor controller selection.

    • The Radio & Electronics archive will supply details on programming your transmitter for intuitive control of the drive and weapon motors, as well as help on battery selection.
That should get you a start.

Q: Hi Mark...How we can use ampflow F30-400 motor effectively ?? Whether i can go for increase in voltage or increase in current ? What will happen if I use it with 24v and 150 amp suplly ? [India]

A: [Mark J.] There seems to be some considerable confusion about electric motor power requirements amongst Indian robot builders. The relationships between voltage, current, and motor performance have been discussed in many previous posts to Ask Aaron, but let me summarize them here.

About current: an electric motor will consume current based on the voltage supplied and the work load placed on the motor.

    PMDC motor torque/speed/current chart
  • The AmpFlow F30-400 motor operating at a supplied 24 volts under no-load will spin at 4800 RPM and consume 2 amps of current.

  • Any work load placed on the motor will decrease the RPM and increase the current consuption in proportion to the load as the required torque increases. Double the load = double the current consumption.

  • If the load placed on the motor is so great as to stop the rotation of the motor completely, the motor is 'stalled'. When stalled, a Permanent Magnet Direct Current (PMDC) motor like the AmpFlow will consume the maximum current it is capable of drawing (stall amps) and produce the greatest torque it is capable of producing (stall torque). Note that it is inadviseable to design a drivetrain or weapon that will risk stalling its drive motor.

  • If the power supply is incapable of providing the full stall current to the motor, the maximum torque available from the motor will be reduced. At 24 volt stall, the AmpFlow F30-400 motor can draw a bit more than 370 amps of current and produce 2500 ounce-inches of torque. If your power supply can provide only 150 amps of current, the maximum torque of the motor will be reduced to: (150/372) * 2500 = 1014 ounce-inches of torque.

  • Using a power supply that can provide more than the stall current of a motor will provide no increase in motor performance.
About voltage: changes to the voltage supplied to the motor will impact both the speed and the current draw.
  • Doubling the supply voltage will double the motor speed. The AmpFlow F30-400 motor has a no-load speed of 2400 RPM at 12 volts, and 4800 RPM at 24 volts.

  • Doubling the supply voltage also doubles the torque and doubles the current demand. The AmpFlow F30-400 motor has a stall torque of 1250 inch-ounces at 186 amps at 12 volts, and 2500 inch-ounces of torque at 372 amps at 24 volts.

  • Power is the product of torque and speed. Since doubling the voltage doubles both the torque and speed of the motor, doubling the voltage increases motor power by a factor of four (twice the RPM * twice the torque = four times the power). The AmpFlow F30-400 motor has a peak power output of 0.75 horsepower at 12 volts, and a peak power output of 3.0 horsepower at 24 volts.

  • At 48 volts the motor would have a theoretical peak output of 12 horsepower -- but it would also have a VERY short life expectancy. Overvolting motors will reduce their longevity and should be done with care and moderation. Search the Motors & Controllers archive for 'overvolt' for additional information.

Q: Hi Mark, Sage here. I was thinking of making the bot such that the plow could be interchanged for a wedge in the pits (I'm thinking about a hinged plow and wedge) because I kinda have a bad feeling that a wedge could get underneath my plow. I'm really not sure which would get underneath which, so right now I'm just guessing!

I saw the Combots V Breaker Box vs Sewer Snake fight and noticed that the plow could lift the forks easily. Also in the fight with Great Pumpkin it could get underneath easily but I think that may be partially due to the fact that when GP accelerates it's nose lifts up a bit making it easier to get under.

What do you say would a low wedge (somewhat like one on Original Sin) get under a plow or not?

Should I try interchangeable weapons or stick with the hinged plow?

Are there any specific advantages for the wedge over the plow (like flipping vertical spinners over, for eg. Terminal Velocity vs Pipe Wrench, which I think couldn't have been possible if PW used a plow instead of a wedge) and for the plow over the wedge so I can change my strategy against different bots?
Thanks a lot! [India]

A: [Mark J.] There is no advantage in design between a wedge and a scoop (plow) in a contest of who gets under who. The difference comes in construction skill and attention to detail. Lowest edge wins, whether it's on a wedge or a scoop.

The advantage of a scoop is that the gentle curve allows it to better fit the radius of a vertical spinner/drum and prevent the weapon from obtaining 'bite' on the scoop surface. The curve is also useful in preventing an opponent from driving 'up and over' your 'bot.

You can get into trouble with a scoop if its radius is smaller than the radius of the vertical spinner weapon you face -- such as the large radius weapon on 'Terminal Velocity'. A change from a direct frontal attack to an angled attack in such situations can help to overcome that disadvantage.

In general, I'm not a big fan of interchangeable weaponry -- particularly if it's just swapping out a scoop for a wedge. The pits can get hectic between matches, and trying to squeeze in time for a weapon swap may result in some critical item being overlooked. Also, your 'quick change' fittings are unlikely to be as strong as more permanent mounts. I'd build a good, strong, shallow scoop and run with it.

Wedge and Scoop

Q: Hi Aaron, excuse my writing mistakes because I do not speak English but want to build the robot battle Ziggy. What materials do I need ?, How I can build it? [Ecuador]

A: [Mark J.] Where do you plan on fighting a superheavyweight combat robot in Equador? See Frequently Asked Questions #2, and read thru the rest of the FAQ while you're there.

Superheavyweight 'Ziggy' is a very complex combat robot with a great many custom made parts. You might as well ask me how to build an automobile. It also uses high-pressure pneumatic components that are far too dangerous for a beginning robot constructor. Build something simpler.

There are many posts about 'Ziggy' in the Weapon archive that may give you some insight into its design and construction.

Ziggy's flipper.
Click for larger image

Q: hey Mark., what are the merits of using chain drive and belt drive in LAST RITES ?? what would happen if we use belt drive instead of chain drive ., [India]

A: [Mark J.] The 'Last Rites' chain drive has been previously discussed. See this post in the archives.

Note: 'Last Rites' still has the chain drive, but now has a mechanical clutch mechanism to provide a little 'slip'.

Q: i am using EN24 metal for my weapon teeth ,what is maximum hardness mark ? please tell in HRC . 50 hrc is good for great impact [Tamil Nadu, India]

A: [Mark J.] EN24 is the European designation for SAE 4340 chromoly steel. This alloy is a general purpose low alloy steel, but it is not commonly used for weapon impactors. Weapon teeth require a combination of hardness and impact resistance, but EN24 looses impact resistance rapidly with increasing hardness.

EN24 can be hardened to HRC50, but at that hardness it would be VERY brittle and have a very high risk of fracturing on high-energy impact. For comparison:

  • At HRC 45 hardness, EN45 steel has a Charpy impact score of 21 -- quite poor.

  • At HRC 55-58 hardness, S7 tool steel has a Charpy impact score of 171 -- excellent.
That's why tool steels are the preferred impactor materials. If I had to use EN45 for impact teeth I wouldn't harden beyond HRC 40 (Charpy impact 45), and I'd make the teeth extra wide and deep.

Q: hello mark . can u suggest drum teeth metal for us . here s7 tool steel is not available and its imported price is high , can u suggest some other steel for drum teeth impact , if any other tool steel can replace s7 tool steel means please mention all listed the above with hrc rating ?

A: I'm not going to look up the properties for all the possible alloys since I don't know which are available in India. Commonly available impact-resistant tool steels include O1, A2, D2, S7 and W1. Find out what your suppliers offer and look up the properties for those steels. Don't assume that the greatest HRC is best -- you want an alloy that is both tough and hard.

Q: gud eve ,in India available H13 , A2, D2 tool steel alloy , from this three which is good impact for drum weapon ?

A: From may answer above -

  1. "Find out what your suppliers offer..." Well done.

  2. "...and look up the properties for those steels." FAIL.
Any of the three available alloys would be acceptable for impactor use. There are differences in expense, workability, ease of heat treating, and hardness/toughness blend. I know which I'd choose, but your priorites may be different. To make your choice I again suggest that you look up the material properties of the alloys.

Here's a link to a site with data sheets for all three of your alloy options: Simply Tool Steel.

Q: hi mark is there any tutorial or blog which could guide us about driving of our bot means any tips or guidelines that can we follow or from u as well? [Maharashtra, India]

A: [Mark J.] Aaron listed his favored driving exercises in this archived post.

Equally important to getting your driving practice is to tweak the transmitter settings to make the radio interface comfortable for the driver. In combat your nerves will be on edge and a too-responsive robot will become impossible to control. See our transmitter set-up tips in the Run Amok R/C Transmitter Programming Guide".

Q: Hey Mark,
I was wondering if this solenoid valve [Ebay link removed] could be of any use in pneumatics?
Thanks [Washington]

A: [Mark J.] The only description given is:

micro mini pneumatic air solenoid valve
three ways mini solenoid valve for air exhaust DC3V/4.5V/6V
Tube diameter: 3.0mm(metal)/4.5mm plastic
No flow rate specs, no max pressure, no flow diagram. For $9 it might be interesting to play with, but I can't tell if it's of any insect class combat use.
Ebay micro pneumatic valve'

Q: Hi..I making a drumbot..using 2 wheel drive and confusing about CG and wheel placement.. [India]

A: [Mark J.] I'm not sure exactly what your question is.

A two-wheel drive drumbot will typically have at least 70% of the total weight of the robot supported by the drive wheels. More weight on the drive wheels equals more traction, but also risks stability problems on acceleration. See comments on robot design and center of gravity at

The RioBotz guys favor puttting almost all the weight on the drive wheels and stabilizing the 'bot with castors -- see sections 2.7.4 and 2.7.7 of the RioBotz Combat Tutorial.

You can get more weight on the drive wheels by moving robot components (batteries, electronics, weapon motor...) toward the rear of the robot, or by moving the drive wheels forward.

Riobotz heavyweight robot 'Touro Maximus'

Q: hello mark,could you suggest me a motor to hit a badminton smash for robocon.Since we are starting from scrach we have poor knowledge about motors.hope you respond me positively. [India]

A: [Mark J.] A few problems:

  1. This isn't about combat robotics;
  2. The hamburger is bad;
  3. The challenge of building badminton playing robots is HUGE!
Best I can tell there is exactly one badminton playing robot in the world (Video). It travels left/right on a beam (no forward/back motion), it has a room full of computers and at least two technicians controlling it, and it can return shots within only a very limited range of trajectories. It can't serve, it can't play drop shots, it can't cooperate with a robot partner, and it can't play in a court the size proposed by the Robocon challenge.

I really can't see the competition being a success. I recommend spending your time on a more reasonable challenge.

Badminton racquet and bird

Q: hello, please i need to know what kind of gearbox is necesary to transmit 75Nm of torque, and 500rpm, using two 11.1 V lipo batterys and this motors: Maxon RE series motors. [Equador]

A: [Mark J.] The link you provided takes me to a Maxon motor catalog with 131 pages of RE class motors -- I can't tell which motor you're asking about. If you could supply the specific Maxon part number I could offer some comment.

In general, Maxon 'coreless' motors capable of the output you're looking for are VERY expensive and have robot combat use only in high-end sumo robots. Maxon does make planetary gearboxes (also VERY expensive) capable of transmitting 75 Nm of torque, but the reduction ratios of those gearboxes cover a limited range and cannot provide the 500 RPM output you seek. The mounting face of the larger Maxon motors does not match the faces of any commonly used robot gearboxes I know, and commonly used robot gearboxes are rated for no more than 50 Nm of torque. You may be looking at a custom (EXPENSIVE) gearbox solution.

Q: hey mark .. fork tooth guy again .... sorry I am troubling a lot .... We have a lot of drum bots in India , so my primary concern would be getting under a drum bot .... now these things have the skates which support the heavy load of the drum .. so with a wide wedge it would be difficult to get under head on ... ( getting the wedge form the side would be tough manoeuvring ) .. can you suggest something to easily get under a drum if you go head on ... OR what should be the strategy against a drum bot? Can I have some other attachments against horizontal spinners using the same bot ? [India]

A: [Mark J.] Aha! Thank you for the explanation. I better understand what you're trying to accomplish. The 'spinner killers' in the US all use wide scoops or wedges, and these work well against drums too. Here's why:

  • If the low wedge runs into the drum before it hits the front skids, the low contact angle hit will 'pop' the drum weapon upward and lift the skids to allow the wedge to penetrate beneath them.

  • If the wedge hits the skids before receiving impact from the drum, your four-wheel drive should give better traction than the two-wheel drive on the drumbot and you can shove them wherever you like.
You're in control either way. If you're having trouble getting under the skids with a full-frontal attack, try easing the edge of the wedge in toward the middle of the drum at an angle a little off head-on.

Q:something about tyres ...... they say friction does not actually depend upon surface area ... assuming that coefficient of friction remains same throughout the arena.... how would it actually make a difference using [wide] and [narrow] tyres . theoretically it should not ... but what happens in real? And how [wide a tire] should I use for my 4WD wedge bot ? one thing I noticed is that the skid drive is difficult in [wider] tyres

A: You are correct. Wider tires - in theory - do not generate greater traction than narrow tires bearing the same load.

Race cars use wide tires to better control heat build-up under racing loads, not directly for increased traction. Race tires also get physically 'sticky' when hot and can generate coefficients of friction greater than 1.0 at the expense of greatly increased rolling resistance. Neither of these situations apply to combat robots. Some early combat robots used go-cart race tires with very poor results because the robots just don't generate the high-energy loads needed to bring the race rubber 'up to temperature'.

So, why do some robots have wide tires? There is that little caveat you mentioned in your question, "...assuming the coefficient of friction remains the same throughout the arena". Combat arenas are dirty, dusty, oily, greasy, messy places. A wider tire covers a larger 'contact patch' and will be less effected by small slick spots.

I can't supply you with a formula to calculate the optimum width for your tires. It would depend on robot weight, power, rubber compound, and arena conditions that you really aren't going to know in advance. Your observation that wider tires do make skid-steer robots less maneuverable is a good one. It would be nice to be able to arrive at the arena with an assortment of tire widths and compounds to try before the event, but that isn't terribly practical.

My advice is to stick to moderate tire widths and to clean them with a solvent before each match. I use 'lighter fluid' (caution: flammable), but alcohol (caution: flammable) works well too.

Q: Hi aaron, excuse me I have a question... Is this battery enough for 2 motors banebots Rs550 for build a sumo robot? This robot weigh 3kg without force magnet. [link to cheap Hong Kong 5000 mAh 4S Lipo battery removed] [Equador]

A: [Mark J.] Scroll down the page about nine posts for a previous answer to battery capacity required for a 3kg sumo robot powered by RS-550 motors. The 5000mah battery you found has at least five times the capacity you need for a sumo match. To give a more precise answer I would need to know the motor gearing, wheel diameter, and the amount of magnetic downforce you're using.

Advice: since you don't need a battery this large, spend some of the money you're saving to buy a better quality Lipo -- something from a different distributor.

'Newton' beetleweight combat robot Q: hi mark
Newton - Weapon test - Beetleweight combat robot from TeamExpat
impressed in seeing their weapon. i want to know the construction of that weapon and they have used 1200kv brushless motor for their weapon. can brushless motor be used for running weapons in combact robotics [India]

A: [Mark J.] The full build log for 'Newton' is in a thread on the FRA website: Team Expat Beetleweight Adventures.

Nearly all current insect-class robots use brushless motors for their spinning weapons - see FAQ #23.

Q: hi mark ... I have decided the general strategy for playing in 60 kg category . As I have a descent control on manoeuvring the bot so as to put the wedge under the opponent from the side . I have decided to make a 4WD wedge design with only a single long teeth like solid fork coming out. this will be non adjustable and only the tip of the wedge will be touching ,scraping the ground .there will be a similar wedge on the back side having inclination upwards so as to play even if the bot is inverted ( the lateral view will look somewhat like a parallelogram ) . the bot is vulnerable to undercutter types ... but i plan to ram them before they catch speed .... because the arena is very small ... can you please tell if there is something i can improve in the design . the body will be box type .... so how can i avoid spinners because they take the corners . also can you help me out in choosing the correct material is the wedge is solid ... long teeth like protruding .... has this kind of design implemented before [India]

'Mini Maxbot' combat robot A: [Mark J.] Relying on a strategy of attacking a vulnerable exposed side is not terribly workable. It's a rather long path to maneuver around to the side of your opponent, and all they have to do to counter is pivot. You might get lucky against your first opponent, but your second opponent will be waiting for that move.

I've seen a number of combat robots employing a wide range of fork-like wedge teeth. The design has been fairly common when used with an integrated lifter weapon ('Pack Raptors', 'Tazbot', 'Pipe Wench', 'Vlad the Impaler', 'Wendingo', 'Takatakatak'...). Individual long wedge fingers are not as strong as a solid wedge might be and are susceptible to impact damage. Solid plate wedges have proven more durable and more useful against a wide range of offensive weapons -- particularly spinners.

Dual-ended parallelogram-profile robots were also fairly popular back in the BattleBots era ('Bad Attitude') but are no longer seen in robot combat. The added weight and awkward design is not worth the protection against being inverted. If being inverted is a real concern, a single hinged wedge that can drop down into a new position if inverted is more effective. Do you really expect your low wedgebot to be flipped very often in a small arena?

If you're worried about sharp corners on your robot attracting spinner attack, round the corners off! Even a little work with a hand file can round off a sharp edge enough to be useful, but a gently curved surface is ideal. Very hard surfaces are also useful in preventing a spinner from digging into the surface and getting 'bite'. Best option is probably keeping your wedge between you and the spinner.

There is a lot of discussion about wedge material choice in the Materials & Components archive -- suggest you look there for ideas. A great deal depends on what materials are available to you in India. A tough titanium alloy is ideal, but resilient heat-treated steel alloys can be a good solution as well.

'Origninal Sin' and 'Sewer Snake' combat robots Q: hey mark ... the fork tooth guy ..... Now , if I make a hinged wedge which is invertible .... and I bend the edges of the wedge downwards ( for preventing side attacks ) like woloop or original sin.... then when the bot gets upside down .. then the portion bent down will be upwards ... will that be a good wedge?

A: I really wonder why you're so worried about being invertible. Small arena + low wedge robot = low chance of being flipped. The wedges on 'Wallop' and 'Original Sin' would be next to useless if flipped over. I'd suggest worrying more about building an effective robot and less about being flipped.

Q: Or else how about forks like skewer snake? are they invertible? are they vulnerable to spinners?

A: 'Sewer Snake' can self-right if inverted, so it doesn't worry about inverted operation. It also has an assortment of front attachments. When fighting horizontal spinners it has been known to replace the forks with a smooth conventional wedge -- a much better choice.

Q: if we want to give maximum impact of drum teeth to the oposition,then what should be the position of the skid,it should be closer to the drum or should be away. [India]

A: [Mark J.] Suggest you read section 6.6 of the RioBotz Combat Tutorial for a full treatment of weapon support and maximizing impact. Read the rest of the RioBotz tutorial while you're at it.

In general, your skid should be placed as close to the drum axle as possible and be as stiff as possible.

Q: hi mark.
i got two ampflow geared motors e30-150
its rated rpm is 700rpm i need to reduce it to 300rpm
what are the possible ways to reduce the rpm of the motors [India]

A: [Mark J.] I could give you the full list of ways to reduce the output RPM, but I'd rather give you a single method that makes sense for your application. Offhand, I can't think of a reason why you'd want a specific 300 RPM output for a combat robot. You paid good money for the power and speed of the AmpFlow motors. My question to you: why do think you 'need' to reduce the RPM?

Q: reply to mark: in india arena is so small so 700 rpm is difficult to control im not using speed controller (out of my fund) in such cases i would like to run those ampflow motors at 300rpm.

A: Overall, it would have been better to select gearmotors more closely suited to your requirements -- but you do have some options:

  • Running the motors at 12 volts instead of their rated 24 volts will cut the RPM in half. It will also cut torque in half, which will reduce acceleration. If your primary concern is controllability, this may be an effective and simple option.

  • A chain drive running from the gearmotor output to a new wheel carrier stub axle with sprockets in a 3:7 ratio (like 12 teeth on the motor, 28 teeth on the stub axle) with give you 300 RPM. This is heavy, adds new failure points, and wastes the very well constructed output shaft on the MagMotor gearbox. This reduces the RPM but increases torque, which will increase the responsiveness of the robot and may add to your controllability issues.
You didn't mention the construction details of your robot, like weight and wheel size:
  • If your main concern is a reduced top speed, you can reduce speed without changing RPM by reducing the wheel diameter. The MagMotor gearbox can be used with wheels as small as 4" diameter. This has the same torque-increasing issue as the chain and sprocket solution but is simpler, more reliable, and actually reduces weight.

  • If your main concern is controllable acceleration, you can reduce acceleration by increasing wheel diameter. A larger wheel will make the robot more controllable with simple on/off/reverse switches. Top speed will be limited by the slowed acceleration in the small arena. If you chose this option, make sure the larger wheel diameter does not permit stalling of the motors under heavy pushing loads. A 60 kg robot can effectively use up to an 8" diameter wheel with the MagMotor E30-150 gearbox.

UnMakerBot - lightweight combat robot Q: Hi..Mark..what is the dimensions,motors and details informarion of UnMaker bot.. [Maharashtra,India]

A: [Mark J.] I have no details on the 2014 Stem Tech Olympiad lightweight champion. Best I can do is the photo at left of the 'bot with its cover removed. The motors look like Castle Creations products, but I can't say which ones. If you're trying to duplicate the 'bot you'd better have serious machine shop skills and deep pockets.

Weapon shaft clamping collar Q: Hello, I am designing a 15lb horizontal spinner robot, with an uncanny resemblance to Hazard. I was trying to figure out a way to securely mount my 5.16lb, 24in bar across the top. I have visited Team Delta's website, and they seem to be using shaft collars both on the top and bottom. I am currently running a dual bearing support system, and a v-belt pulley driven by a beefy brushless motor. Any help would be appreciated! Thanks -- Erik [Winchester, Massachusetts]

A: [Mark J.] I wrote one answer to this question, briefly posted it, thought a little more, took the original post down, and started again. Let's see if this makes sense.

Hazard's 22 pound spinning blade is not directly fixed to the 1.5" diameter steel weapon shaft. It is squashed firmly between two shaft collars (see picture) that are tightened onto the shaft by machine screws that reduce the diameter of the collars for a clamping fit -- not held in place by set screws! Custom phenolic washers on either side of the blade form a mechanical clutch that allows the blade to slip on impact and limit the shock transmission back to the drivetrain (Hazard Build Report).

I'm not very keen on squishing a weapon blade between two collars and relying on friction to spin it up, but it worked very well for 'Hazard'. If you do decide to use shaft collars, I would strongly recommend clamping collars over set-screw collars, but I think I'd axially fix the blade by broaching the shaft and blade and fitting a key to lock rotation -- let your v-belt act as the slippy-clutch.

Q: I've found an inexpensive ESC [link removed] that looks like it will work for my robot, but it doesn't have any type of overcurrent protection. Is it practical to go into combat without some current protection? [Deep 13]

A: [Mark J.] You've found a very simple ESC with no reverse current protection, no current limiting, no thermal protection, no LiPo voltage shut-down, no mixing, no midpoint throttle sensing, no DIP switches, no jumpers, no cooling fan, no mounting bracket - no frills at all. I rather like that. There's no complex manual to read thru and no way to get the set-up wrong for your combat application. Just make sure you wire it correctly or you'll fry the ESC.

Most ESC 'protection' circuits fuction very simply: when they sense a problem they shut down the robot. 3 malfunctioning robots

  • Getting too hot? Shut down.
  • Too much current? Shut down.
  • LiPoly battery voltage low? Shut down.
This can be handy in set-up and testing sessions, but THE LAST THING you want in actual combat is for the robot to decide on its own to shut down. I'd much rather squeeze the last few seconds of performance out of an ESC and be mobile at a critical point in the match than have the controller decide it's had enough and quit. An extra few seconds of power can be the difference between winning the match and being ripped to shreds.

A combat robot should turn off ALL 'protection' in combat: no fuses, no LiPo voltage, no thermal, no overcurrent - NOTHING that lets the machine decide it should shut down on it's own except radio fail-safe. It's fine to have sensors trigger audio or visual warnings about overloads, but let the driver decide if it's a good time to ease up or shut down.

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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