A: Mark J. here: there is more to being a good weapon motor than power. A weapon motor must be reliable and must have enough information available to allow the needed weapon calculations. Without a full set of motor specifications you're just guessing.

As is typical for inexpensive Chinese brushless motors, the motors you reference come without the specifications needed to calculate their output power. If the specifications did exist, there would still be the question of their reliability in combat. The motors are made for model aircaft -- an application which places much different stress on the motor than spinning a robot weapon. My recommendation is to use a motor that has a proven track record in robot combat -- so, of the motors you list, I'd have to go with the Speed 300.

A: Megabyte's weapon is powered by a Briggs & Stratton Etek motor producing about 15 horsepower at 48 volts. Stall current is about 700 amps with motor stall torque close to 90 N-m. The motor is geared 4:1 to the shell. A pit photo of 'Megabyte' shows four 24 volt 3.6 amp-hour BattlePack batteries supplying power to both drive and weapon. Each pack weighs 4 pounds.

A: Sure. I should warn you that you'll have no speed or direction control over the motor, but you can definitely hook it up.

In other words: no. A motor has no capacity to interpret the signal coming out of your receiver.

A: Always happy to hear we were of some help. ; ^ )

A: 'Potter's Wheel' was one of a great many robots that fought once, lost, and were never seen again. In this case it lost its only fight to 'Son of Bob' in the middleweight division at BattleBots 4.0. Its spinning weapon appears too small to be an actual potters wheel. I have no other information. If there was anything interesting or educational about 'Potter's Wheel' you would not have trouble finding it.

Q: are you sure the disk on Potter's wheel, is not actually a mounting disk? 'cause if you get the DSL mod for Robot Arena 2, there is a replica and it is spinning a potters wheel, and you can use the wheel, to :S and do you know what happed to the LW flame thrower bot, Bob?

A: No, I'm not sure. Like I said, it looks small for a pottery disk - but I don't have a good enough picture to tell for sure. Either way, it was not successful.

With 30 wins, 10 losses, and five class championships in the MechWars and WBX series', 'Bob' was a very successful robot! 'Bob' was from a Minnesota-based team and has not competed since the Minnesota MechWars series folded in 2007.

A: Mark J. here: I can't isolate one piece of the weapon and tell you how well your whole weapon system will work. Each piece must be in balance with the other elements of the weapon system, and the weapon system itself must be in balance with the rest of the robot. I suggest you start with the Team Run Amok Spinner Excel spreadsheet to examine the interaction of the blade, drive, and motor.

A: Mark J. here: we are not an engineering service, but I can tell you that steel and aluminum have different strengths and weaknesses which makes a simple 'resilience' comparison difficult. The thicker 6061 T6 aluminum has greater tensile and yield strength than the 4130 steel -- but the thinner steel is stiffer, more shear-resistant, and has greater surface hardness. Additional differences will depend on the heat treatment used on the 4130 steel.

A: The Robot Dojo hack page was written before LiPo battery packs were widely available. It was difficult to build a NiCad pack of sufficient voltage and capacity to fit in the original 4x AA battery compartment. If your LiPo battery will fit into the battery compartment it would be a good idea to put it there.

A: 'Thor' had a hydraulic overhead club weapon powered by an on-board internal combustion engine.

A: We don't compete in sumo, but I do know that the sumo guys use custom molded urethane traction compounds that really push the limits of traction. The IFI wheels also look too heavy/rugged for sumo use -- the smallest size weighs more than half a pound.

Q: then is there anyway to get one of those treads used on the beast build by carlo ?

A: As I recall, Carlo Bertocchini made the treads on his sumo robot 'Beast' out of pure gum rubber; one-off custom items. They may not be legal under current sumo rules on allowable traction compounds.

Q: IS the wheels sold by team whyachi offer good traction on wooden surface covered by 3mm thick of hard rubber?

A: We have no experience with either the Team Whyachi wheels or hard rubber surfaces. Suggest you check your local library for a copy of Robot Sumo: The Official Guide by Pete Miles for general information on sumo traction solutions.

A: We have no experience in welding titanium, so cannot offer advice on how your repairs would best be done. You might consider taking this opportunity to convert to bolt-on steel impact teeth. Spinners in heavier weighclasses generally have bolt-on impactors that can be replaced easily.

A: Neither option is particularly good. A re-bent tooth has been weakened and is likely to fail, while a missing tooth is just asking for trouble. I can't recommend going into combat with a weakened gearbox -- the safe thing to do is replace it.

Q: can I get a new gear or gear case or should I just buy the whole thing?

A: You could ask around on one of the forums to see who has a Copal gearbox left over from a blown motor, or you can buy a new 'whole thing' and have a spare motor ready in your parts drawer.

I'm the one trying to do an upgrade for power soccer, and I haven't been able to find a suitable gearbox for the magmotors anywhere. Team Whyachi emailed me back saying that 16:1 was the maximum they could do.

A: Mark J. here: first, I think you mean 'higher' torque constant. Lower armature resistance from less and larger wire will result in greater current flow at a given voltage and greater stall torque. The torque constant equation is:

Sorry to hear that Team Whyachi cannot provide a suitable custom gearbox. I have no other contacts.

A: Mark J. here: you aren't interested in torque or power consumption? The Speed 280 and 300 motors are generally used for model aircraft or combat robot weapons where neutral timing is not required. I don't believe that any of the Speed 300 motors are neutrally timed, and the one Speed 280 motor that I know of that has neutral timing does not meet your speed requirement. If I knew what you plan to use the motor for I could be more helpful.

Background for readers: motor timing relates to the alignment of the commutator and brushes to the magnetic field of the motor. A neutrally timed motor will operate in either direction with equal speed and efficiency. A motor with 'advanced' timing will gain speed and efficiency in one direction and sacrifice both in the other.

Q: I am looking to replace the stock motors on the B-16 gearmotors with something more powerful and since I plan on using ~1.5" wheels, I need a motor that has pretty high rpm in order to have decent speed.

A: For wheels that small I think it would be both easier and better to switch to a different gearmotor. Consider the BaneBots 9:1 24mm RS-370. With 1.5" wheels at 7.4 volts, a pair will give about 8 MPH top speed and break the wheels loose when pushing at just over 3 amps apiece.

A: No need to go high-tech here. I'd suggest a simple arch of 1/16" steel 'piano wire', available in 36" lengths at hobby shops. The trick will be getting the right shape and position to allow the lifter to operate as a righting mechanism.

A: There are two versions of the Park 370 outrunner. Using the brushless stall torque calculator from the Team Run Amok Spinner Excel spreadsheet with the following data from the Hobby Zone website for the 1360 Kv model EFLM1205:

• 11.1 volts
• 1360 rpm/volt
• 100 m-ohms resistance

A: You can match the three wires from the ESC to the three wires from the motor any way you like. If the motor turns the wrong direction for your application, swap any two of the wires and the direction will reverse.

A: Voting for the Combat Robot Hall of Fame is open to all members of the combat robot community. The 2011 vote will be announced in prominent combat robot forums in the US and UK sometime in August of that year.

A: We've said this before: the weapon is perhaps the least important system on a combat robot. Drivetrain, radio set-up, general construction practice, and balance are all much more important. Too much emphasis on the weapon will result in a poor robot. 'Metroid' has a weapon well balanced with the rest of the design.

A: Brushed motors should be 'broken in' to allow the countour of the brushes to wear down to match the curve of the commutator. This will provide maximum contact area between the brushes and commutator which will minimize heat build-up, arcing, and potential damage when run at full voltage under load. The basic procedure is the same for all brushed motors:

• Place a small drop of light lubricating oil on the exposed motor and gearbox bushings. You can apply it with a toothpick. Make sure the motor shafts are turning freely.

• Hook the motor(s) up to a variable power supply (your electronic speed controller will do nicely) and run them unloaded at a steady, moderate speed for five minutes.

• Turn the speed up a little and run for another five minutes.

A: Most of the top ranked antweights at BotRank.com are two-wheel drive, but they all have a skid or castor rather than a second set of unpowered wheels. Trying to drag unpowered rubber tires sideways to turn will severely limit the maneuverability of a robot. A two-wheel drive robot may not have the pushing power of a four-wheel drive machine, but if you've got a big weapon you don't need pushing power.

A: I have not used the Pololu TRex Jr ESC but specifications are quite similar to the discontinued Barello Ant 150 -- both have a third channel for weapon control that the Ant 100 lacked. It appears to be suitable for insect class robots.

Somebody out there must be using one -- write in a review.

A: There is more than RPM in the equation:

• a 2" drum spinning at 10,000 RPM has an impactor velocity of about 90 feet per second;

• a 6" disc spinning at 3700 RPM has an impactor velocity of almost 100 feet per second, has better 'bite', and most likely much greater potential energy than the small drum.

A: Let's start by getting the numbers right. The Pololu 20D44L weighs 1.7 ounces, just about 3/4 the weight of the B16) at 2.3 ounces -- not half.

There are a lot of factors that go into gearmotor design: design voltage, expected longevity, efficiency, required strength, etc. Let's compare the two at their best voltage. At 12 volts the B16 produces 12% more stall torque, 35% more RPM, and more than twice the total output power of the Pololu at 6 volts. I haven't used the Pololu motor, so cannot comment on its durability.

A: I have no idea. Automotive starter motors are generally inappropriate for use in combat robots. They are designed for high loads for very short time periods. Starters are usually 'series wound' motors that have a different torque curve than permanent magnet DC motors, which makes weapon design computation more difficult. Builders have tried to use starter motors for weapon power, but I don't recall any that were particularly successful. I recommend against their use.

A: Mark J. here: your approach is well thought out and will work -- if your ESC is actually limiting current the way you think it is. Many 'current limiting' ESCs don't directly limit current, they just dial back the PWM in response to the heat level being generated by the FETs. The 'programmable' current limit may just be setting the response to a thermistor on the heat sink. When cool, these ESCs will provide more than the limit setting; when good and hot, less. Check with the manufacturer to see how the current limit works with your ESC.

A: The VDD gearbox is too large to fit 'in' the Inertia Labs chassis, but it could be mounted on top. However, I wouldn't recommend a large vertical blade for that chassis -- it's too narrow to be stable with the forces the blade woud produce, and the blade would have to set too far forward in the chassis for good balance. A horizontal blade could work, but the VDD gearbox is too tall for that, I think.

Which design would you rather have?

A: The servo-powered lifter on 'Warpig' is simple to build, easy to drive, and reasonably effective. I would recommend that design for builders new to building and driving lifters.

Personally, I would rather have a design like 'Wizard Beard'. Their lifter wedge itself could use a makeover into something more like the scoop on Team Nightmare's 'Breaker Box' to avoid being such spinner bait, but the flexibility of lifter action makes for a versatile and tenacious robot.

Q: If you made a much wider tip on the lifter of Warpig, would it increase its effectiveness?

A: A wider tip would change the characteristics of the lifter, but I can't gaurantee that it would be more effective. A narrow tip can follow the contour of the arena floor more closely and may be better at getting under low wedges, while a wide tip might offer more lifting opportunity and less chance of the opponent slipping off. Changing to a 4-bar lifter might be the best option.

A: Mark J. here: on the Team Run Amok page I announced that there would be no further mention of BattleBots due to the long history of faulty and misleading information from the company. That prohibition applies to the activities of BattleBots Inc. and not to the efforts of participants in their events. The participants have my sympathies but they really should know better than to associate themselves with BattleBots, IMHO.

If a show airs, it will be open for discussion here.

A: Any standard SRiMech will do nicely -- just make sure you don't activate it when the blade is spinning! Type and placement will depend on the design details of your robot.

Technically, if your robot is 'invertible' it wouldn't need a SRiMech -- it means that the robot will operate when inverted.

A: Mark J. here: Team Whyachi would have been my recommendation, and Odyssey would have been my backup suggestion to NiCads. You're on the right track -- best luck.

A: Mark J. here: there is no magic number for spinner RPM, but the faster it spins the less bite it's going to have. At 4000 RPM, one of your two bar tips will pass by every 7.5 milliseconds -- it's going to be difficult to insert much of your opponent's 'bot into the 'damage zone' with any higher speed!.

As mentioned below (read down three questions), it is possible to increase the time between impactor passes by decreasing the number of impactors. In the case of a bar spinner, you'd need to shorten one leg of the bar (half an inch should do) and add weight to that shortened leg to keep the bar balanced. With a single impactor you can double the RPM of the weapon and still get the same 'bite' as a weapon with two impactors.

Q: Then what about the mantisweight Chaos Theory? It has a [96 toothed] 14 inch saw blade, and apparently it can catch the other robot very easily. [Watch Chaos Theory vs Zillion] Why is this?

A: If you are fortunate enough to fight a robot with exposed sharp edges sticking out you don't need much 'bite' to catch that edge. Watch Chaos Theory vs Mystery Box and see the big multi-toothed blade skitter off the flat wedge because it has so little bite. With fewer teeth it would have done much better.

A: You've got to control the drive motors somehow -- you certainly can't just plug them into the receiver! The FingerTech tinyESC weighs only 3 grams and should work well in a fairyweight robot.

A: My question is: 'effective for what purpose?' High-angle wedges can be used as part of a 'rambot' strategy, low-angle wedges can be used defensively, a curved 'scoop' design is common for nullifying the attack of a spinner. The wedge angle is only one piece of the weapon design, and the weapon must be integrated into the design for the rest of the robot.

If you're interested in an analysis of wedge design, have a look at the wedge section of the RioBotz Combat Tutorial. I don't think they have it quite right, but it's a good start.

A: Mark J. here: Robotic Death Company has not released enough specific information for me to calculate the stored kinetic energy of Megabyte's weapon. A rough guess at speed (1000 RPM), mass (45 kilos), dimensions, and mass distribution puts the energy around 50,000 joules.

A spinning cylindrical body with the same mass and diameter of a spinning bar will store more energy, which makes it a potentially more effective weapon. A simple spinning bar the same weight and diameter as Megabyte's shell would store only about 40% of the energy at the same speed. The actual weapon effectiveness is dependent on a variety of design factors.

'Bite' is independent of the shape of the energy storing body. It is primarily a function of the time interval between passage of the weapon impactors, which is dependent on the weapon RPM and the number of impactors. A weapon with a lot of 'bite' has a longer interval between impactor passage, which allows an opponent approaching at a given speed to penetrate farther into the weapon's destructive radius. You may have noticed some spinner weapon designs that are counterweighted to allow a single impactor point for greater bite.

Robotic Death Company bolts the impactor blades onto their spinners.

A: Mark J. here: the greater the spin speed of a rotary weapon, the faster it must approach the target in order to get a chunk of the opponent deep enough into the weapon for good 'bite'. The only video I've seen of 'Sting' has a single 'weapon-to-weapon' impact against an eggbeater. The relative closing rate between Sting's drum and the opponent's spinning eggbeater would be very fast, which would allow enough bite for a powerful impact. This is a special case.

If you listen carefully to the video you can hear the weapon speed on 'Sting' change throughout the fight. I assume the operator knows that hitting an armor panel at full speed would not be effective and he is moderating weapon speed to match the target opportunity.

A: We've said before that no matter what type of robot you're building it makes very little sense to purchase a 'cheap' radio system. If you continue to build robots you will soon want to upgrade your radio and will find that the resale value of that cheap radio is small. Buy a good radio system and your robot will benefit from the improved performance and your wallet will benefit from better resale if you do need to sell.

I'd recommend a full-featured DSM 2.4 Ghz system with a display screen and full failsafes. Such a radio meets all performance requirements for all weight classes. The popular Spektrum DX6i transmitter with BR6000 receiver is an example.

A: See #5.

A: I assume you're using a gearmotor with an ESC to control your lifter, and not a servo? There is a good explanation of limit switches at www.techno-stuff.com/limit.htm. The third circuit example (the one with two switches and two diodes) is a bi-directional circuit -- your ESC would substitute for the battery. Position the contact switches at the minimum and maximum lift points and you're good.

Relay boards used to control linear actuators often have built-in limit switch inputs. I use the Team Delta R/C dual ended switch to control the lifter on Zpatula to keep things simple.

A: Hard to say much without at least a photo -- I don't even know what weight class the robot is. I do understand the basic problem: the force vectors of an impact with an overhead spinner are all above the center of gravity of the robot. A weapon impact tries to flip the robot over backward. The farther above the center of gravity the impact site is, the greater the flipping force. Hitting a wedge simply aggravates the problem. Is this a bar spinner? You could try bending the ends of the bar downward a couple of inches to move the impact point lower and closer to the level of your robot's CG.

A: That's exactly how the failsafes on the BR6000 receiver function - all channels are driven to a pre-programmed position on loss of transmitter signal. You can set the failsafe position wherever you like: see Spektrum's BR6000 failsafe position instructions.

The AR500 receiver would only failsafe channel 3 to its programmed position and would send no position information to the other channels. Other manufacturers have different options on failsafe positioning, but your Spektrum is working as designed.

A: I disassembled the lifter gearbox and located the slipper mechanism: one of the gears had a special sawtooth-ended hub held by spring pressure against matching teeth on the base of the driven gear. When torque reached a specific level the hub would be forced away from the gear and 'click click click' until the load dropped enough to re-engage. I drilled two holes thru the hub and into the gear, then fitted steel pins in the holes to lock the assembly together. An alternate solution would have been replacement of the spring with a rigid spacer.

A: Inconel is a specialized corrosion and heat resistant alloy designed for extreme environments. If you were fighting a robot armed with a plasma torch and an acid spray it would be your material of choice. Its physical properties are similar to a high-strength aluminum alloy, but it weighs three times as much. Verdict: too heavy to be good robot armor.

You can see the robot video on youtube. The bot who wasn't flipped o'er is ours. It's urgent pls. Any help will be appreciated greatly. Thanx in advance.

A: Mark J. here: we've seen this type of instability before. It pops up in full-body spinners and designs like yours. A spinning object - such as your rotor - will attempt to orient its axis in a manner which will achieve the greatest rotational stability. If you spin a hard boiled egg on its side, it will change the axis of rotation and spin on its end. See the explanation of the Tippe Top for the related physics.

In your particular case, I believe the rotational mass is unstable and is attempting to reorient its axis to correct this -- turning your robot over in the process. I would suggest reducing mass in the upper bar of your weapon and adding mass to the lower tips. A counterintuitive alternative might be to raise the center of gravity of the robot chassis. [see revised solution below]

P.S. - About the video - are you insane? Standing two feet away from an operational spinner in combat, holding a control cable up out of the way??? Somebody is going to get a chunk of robot embedded in their head! Cut it out!!!

Q: hi Mark sir. i asked a question regarding stability of our robot. u suggested that it's a sort of tippe-top phenomenon. i wanted to ask whether decreasing the rotor speed from 2000RPM TO 1000RPM will increase the stability of robot?

also did Son Of whyachi had rotor speed of only 950RPM? is that sufficient to damage the other robot? our rotor has dia. of 75cm. thank you.

A: I've given some additional thought to your stability problem and then did a little experimenting. Of the three natural axis around which a rigid body may be rotated, only two are stable. Your weapon is spinning along the third, unstable axis! Search the web for 'rotational stability' for more detail on this phenomenon. The three-armed rotor on 'Son of Whyachi' was stable in this particular rotational axis but the model I made of your two-legged weapon is not.

Why does your rotor become unstable at relatively low RPM when others like it do not? Check for any 'flex' in the weapon rotor -- although not responsible for the instability, flexing may create trouble at a lower speed than would otherwise be seen. That leaf spring you're using may be distorting and causing the onset of the problem.

Decreasing the speed of the rotor would not remove the instability, but would reduce the magnitude of the forces generated and make it more manageable. Unfortunately, reducing the speed of the rotor by half will decrease the rotational energy of the weapon by 75%. Not good!

The rotor on 'Son of Whyachi' did spin at 950 RPM, but there is more to the energy equation than RPM. The rotor was five feet in diameter which translates to a tip velocity of 175 MPH and way more than 50,000 joules of stored energy. A very rough estimate of the energy of your weapon spinning at 1000 RPM: 2750 joules. I'd suggest correcting the rotor instability and keeping the RPM up for maximum impact.

A: Only if the motor is spending time at stall, which a brushless motor shouldn't do! Hobby brushless motors are designed for aircraft use where they never stall and are rarely bogged down at low RPM. They generally come with specifications for maximum amperage and how long they can survive at that current level. For longevity you should limit the current to that maximum recommended level, either thru the ESC or thru the design and use of the weapon.

A: I would recommend not going cheap on critical robot elements. You know how well the Axi works, you know it is an exact replacement, and you don't know anything about the off-brand motors except that they are cheap. Save yourself frustration and disappointment -- buy a new Axi.

Q: Hi Aaron, I have done some more research about direct replacements for the Axi 2217/9D and found this one. I know you said that going cheap is bad, but I've already killed 3 Axi's, and they are just not worth it for 90 bucks apiece.

This one doesn't show the internal resistance, so how can I figure out the stall torque? Would it be better than the Axi? BTW, the kv and length are better than the Axi for my purposes Thanks.

A: Mark J. here: if a part is failing, going to a cheaper part is NOT the solution. Why do you believe that a $14 chinese knock-off is going to be better than an Axi? It isn't. The Axi can handle 34 amps for a full minute. The knock-off dies after 15 seconds at 25 amps. If you've blown three Axis you're going to eat these like popcorn. The knock-off is also heavier, spins slower, and is less current efficient. Without an internal resistance number you can't calculate stall torque, but I'd bet that the Axi is better.

First, take a look at your design to see what is contributing to the weapon motor failures. Consider going to a less radical wire count -- the Axi 2217/16 or /12 would be more reliable, but would provide somewhat less power. Power comes at the price of reduced longevity.