Q: I am building my second ant spinner and and I was thinking of a bot like hazard it would be an 3/32 aluminum chassy 1.5" tall 4.5" long and 4" wide. I would put a 1/32 titanium wedge hinged on the front it would be at a 40 degree angle. I would use 2 copal 50:1 gear motors with BaneBots 50A 1-3/8" x .4" whells with a half inch hex mount. My battery is a 11.1 volt 850 mah hyperion. I would use the sabertooth 5 dual esc. My weapon motor would either be two Park 370s or two Axi 2208/20s. I would use a steel bar that is 8" long .5" wide and 4mm thick. is this a sturdy design? and what type of motor should I use? and how much energy would I be producing? and what would be my spin up time? thanks a bundle.

A: I don't have nearly enough information to tell you if your design is 'sturdy'. We have a variety of information and design tools that could be of use to you, but we are not an engineering service. Of particular interest to you would be our Team Run Amok Spinner Spreadsheet which allows energy and spinup analysis of proposed spinner weapon designs.

A: Read down thru the end of that post in this archive and you'll find that the answer has already been given. It has nothing to do with that hole - which isn't square.

A: Mark J. here: I'm not a fan of the RioBotz Combat Tutorial. It was written by an engineering professor at a Brazilian university, so it is not surprising that it is written like an engineering text. Engineers have their own style, and most of the formulas are there just to demonstrate that the authors have thought thru their claims and recommendations. The good news is that you don't need to use or even follow their formulas to make use of their design ideas, although wading thru the 367 pages to dig those ideas out isn't easy

Team Run Amok continues to recommend the K.I.S.S. principle. Watch what successful teams are doing. Adapt proven principles to your own design. Keep the construction and maintenance uncomplicated. If something isn't working, change it.

A: 'Verbal Abuse' used the time-honored technique of attacking a spinner before it has a chance to spin-up to full speed. If you can stay on top of a heavy spinner and keep depleting its energy, it won't be able to do a lot of damage to you when it hits. 'Corporal Punishment' (AKA 'Polly Prissy Pants') tried a different tactic and paid the price.

Q: And why Corporal Punishment could absorbs Shrederator's hit well (In Robot Assult 2003)?

A: 'Corporal Punishment' has an I-beam sitting across the front of the robot as a 'bumper'. That's ideal for dealing with a flat-sided 'tuna can' spinner like 'Shrederator', but well less than ideal for an angled spinner like 'Megabyte'.

Q: It looks like Verbal Abuse's lifting plate is pretty hard to control and it always lets its opponents slips off - what caused that happen? Beacuse of that very fast servo motor?

A: I would guess that 'Verbal Abuse' had a poor radio set-up on their weapon channel -- too much emphasis on speed and too little on control. As to slipping off - it is just a flat metal plate. Nothing there to get a grip with.

Q: So... Could I say that Karcas 2 used the same strategy as Verbal Abuse when they were facing to Megabyte?

A: Yes, keep hitting the big spinner to keep it from gaining full speed. The specially designed anti-spinner plow is a big help as well.

A: 'Beta' had a shorter chassis than 'The Judge' and much more power. There comes a point when reaction forces are so great that you have to have some method to keep the robot form simply flipping over when the overhead axe is fired -- which 'Beta' would do without the magnets.

A: Mark J. here: I sympathize with your situation. The guidance from modeling and material properties will help you avoid a number of design mistakes, but there comes a point where the guideance runs out and you just have to build the weapon and try it. I can pass on advice from Team JuggerBot: "Damage is weakness leaving the robot." Make everything strong -- if it breaks, make it stronger.

For what it's worth, your drum design sounds plenty strong to me.

A: There's a saying: "It isn't the wand, it's the magician." In this case: it isn't the wedge, it's the very experienced builder -- not all wedges are created equal.

A: 'Totally Manipulative' (AKA 'Anorexia') was designed to be the thinnest featherweight in existence. The flat top of the robot is less than 2" off the arena floor. Add a very long (~14"), very low angle hinged wedge and you have a target so difficult to engage that many weapons will never even touch it.

The low profile does come with disadvantages. Lacking much offensive 'bite', it becomes nearly as difficult to damage an opponent as it is for an opponent to damage you. 'Totally Manipulative's record of 10 wins and 17 losses reflects this problem.

Q: Is it possible to create a robot with the lowest ground clearence?

A: Well, by definition some robot has the lowest ground clearance. It is entirely possible to make a hyper-low clearance robot, but it would probably create more trouble than benefit. See my comments on 'lowest wedge' in this archive.

A: Internal Combution Engine at Wikipedia.

A: There is a lot to consider when designing a self-righting a robot: possible inverted rest positions, center of gravity, maximum width, mass, chassis geometry, etc. A successful self-righting mechanism must be designed around these factors. Designing a robot with a hammer weapon that also works as a reliable self-righting mechanism is not 'easy'.

A: There have been a great many teams that wished their inverted robots had a Self Righting Mechanism (SRiMech). An invertable robot doesn't need a SRiMech, but an inverted robot that's off its wheels isn't going anywhere without some method of setting itself back upright.

Q: How does a schrmich like Razer's wings work?

A: We have previously described the operation of the 'Razer' SRiMech - search this archive.

A: Beveling the bottom will give greater ability to get under drop skirts and other wedges, provide greater strength, and offer better ability to glide over floor seams. Invertable wedge? Bevel both top and bottom.

A: The 'rolling sphere' robot design is just a small conventional robot placed inside a large ball. The robot drives inside the ball and the ball rolls along in the same direction.

I don't understand the self-righting question -- a rolling sphere doesn't need to self-right.

Q: The destruct-a-bubble wasn't a ball, like the others. It was just shaped like a ball, and had a wheel base, and a retracting spear for weapons. I don't think the ball shape self-righted it, but wasnn't it meant to be a schrimech?

Q: How do the rollbars used on Mega Morg, Spikasaurus, and Major Tom (series 6 version) work? These bars self righted these robots when they were flipped.

A: Both the ball-shape and rollbar designs are intended to be 'passive' righting aids. If you have a hoop, cylinder, or sphere with a 'heavy point', gravity will cause it to roll to put the heavy point on the bottom. If that heavy point happens to be the part of the hoop with your robot's wheels and the diameter of the hoop is large enough to provide sufficient rolling momentum, your inverted robot will roll straight back onto the wheels - with a little luck.

A: Not a SRiMech - see our Beginners guide to combat robot gyros.

Which one do you think is more inventive? Better?

A: Aha! I remember a discussion about precessional propulsion as used on 'Gyrobot' on the RFL forum from a few years back. I didn't know that anyone had built an actual robot. Using precessional forces to 'wobble' toward your opponent has some disadvantages, as the video shows. It's painfully slow and can't back up - those are two very big problems.

Gene Burbeck's 'wackerdrive' as used on 'One Fierce Low Ryda' is different. It uses a domed wheel mounted flat on the end of the weapon live-axle rubbing against the floor to provide thrust -- not gyroscopic forces. The speed and direction are controlled by independently raising and lowering each of the two rear wheels with servos to tilt the axle relative to the floor. Very clever, plenty fast, and moves in any direction. Drawback - the very small drive contact with the arena floor makes it very difficult to control! The same concept was used by an ICE powered heavyweight robot with the descriptive name 'Tip Top' which fought at Robot Wars (fifth series).

Both designs are wildly inventive -- real 'out of the box' thinking. If I had to build a 'bot with one of the drives, I'd pick the 'wackerdrive' and spend a lot of time tweaking the controls.

A: Dominator's shape lets it sit at an angle when inverted. The kick against the arena floor generated when the axe fires will shove it back upright -- with a little luck.

A: You are mistaken - both Panzer Mk. 2 and Panzer Mk. 4 had a pneumatically positionable front wedge/lifter.

Q: And do you think a bigger claw can make Kassinator more competitive?

A: I won't second-guess the builders of lightweight 'Kassinator'. The combined clamp/flipper is an interesting design that will no doubt need some fine tuning.

A: The middleweight 'Zion' was successful - 9 wins/3 losses at BattleBots - but it's design was much more an economy version of 'BioHazard' than 'Complete Control'. The single-pivot pneumatic lifter could get lucky and trap an opponent against its own armor, but it lacked the ability to then raise the opponent off the arena floor. 'Zion' lost to 'Complete Control' at BattleBots 4.0.

A: Have you thought this thru? You can turn any motor into a vibrator by clamping an imbalanced flywheel to the output shaft, but a traditional vibrating shuffler is slow, weak, and uni-directional.

A: There's plenty of room in the pro-series chassis -- that's a weak excuse to have me go take a photo. Still, the Zpatula webpage could use a detailed interior shot. I've added one at the bottom.

A: Mark J. here: the challenges of building an invertible full-body spinner should be obvious. Try laying out a design for one and see for yourself.

In truth, most FBS robots are not successful -- only a very few memorable ones were. I'd say that the small number of invertible FBS robots were no better or worse than the average for the type.

An active SRriMech is generally not required for a FBS, and I know of none that have one. It's nearly impossible to invert a spinning FBS due to the gyroscopic forces generated by the body. An improperly designed FBS may flip itself due to the 'Tippy Top' phenomenon, but this can be corrected thru testing. As a precaution, a passive central pole (as used by 'Megabyte' and 'Ziggo') is all that might be needed.

A: More info needed: Dual ended or hinged wedge? Two wheels or four?

Q: A hinged wedge with 2 wheels.

A: For a 2-wheel wedge you want about 70% of the robot's weight on the drive wheels. Move your batteries and wheel location around to get close to that figure.

A: First, find what weight classes are supported at events you plan on entering. There's no point in building a featherweight (for example) if your local events don't fight featherweights.

Second, check your budget. Combat robots are expensive, and the heavier the robot the more expensive it gets. The heavier weight classes are also filled with very experienced teams who will be more than happy to rip a newbie into small pieces and scatter those pieces around an arena. Make sure you can afford that.

It's much less expensive to make rookie mistakes in lighter weight classes than up in the heavy leagues. Start in the lightest event-supported weight class that will hold your interest.

A: Only two:

1. Money (a very large pile), and

2. the world's entire supply of unobtainium.

A: The robot competitions that take place at Dragon Con are run under the Robot Battles ruleset. These rules do allow for robots operated thru control cables as well as thru radio control, but you are not allowed to pull on the cable! Rule 1b says:

If you're pulling on the cable you become a power source. Doing so opens you up to the consequences of Rule 0b - the Frankenstein Rule (I'm not making this up):

You would deserve it.

A: We have discussed the problem with thwackbots many times previously. Current combat robot judging criteria score on only two factors: damage and aggression. A thwackbot design cannot effectively show aggression as it must spin in place. From the guidelines:

This is too big a scoring deficit to overcome. Thwackbots are 'dead'.

A: Mark J. here: We aren't an engineering service and we don't give out specific recommendations on 'how thick' beyond the comments in our armor guide. Medium alloy steel and grade 5 titanium are very different materials. Team Plumb Crazy is fond of steel for their wedges, but I don't know the thickness, alloy, or treatment they use. I'd suggest asking Wendy and Matt at Team Plumb Crazy.

A: Mark J. here: 'Maximus'? Good heavens, no! I have photos of robots with hinged wedges dating back at least as far as the 1996 Robot Wars event. That's five years before 'Maximus' fought.

A: Yes. 'Melty Brain' refers to any system that controls directional motion (translation) on a thwackbot by rapidly changing motor power as the robot spins. 'Death by Translation' uses Rich Olson's custom electronics to measure rotational speed with an accelerometer and pass that information on to a micro controller to do the power changes.

The physics of spinning with a single tangential wheel are difficult to describe. It's a bit like a spinning cowboy lariat - it's only supported on one side, but spinning forces keep it relatively stable. The single-wheel configuration does cause some problems; directional control is not good at higher speeds. It's an interesting experiment, but it has achieved no combat success to date.

A: I don't think you actually need an email at your school to download. Very few sub-college students will have one. Go to the Autodesk Education Community website, register with your regular email address, and see what happens.

A: We were hit by 'Shunt' in 2002 and we weren't impressed by the power of the weapon. It bounced off a rather thin steel top piece and barely made a mark. By contrast, the overhead spike on 'Slam Job' went cleanly thru that same piece of steel in a parking lot match about a year later.

Shunt's axe had a compact 4-bar mechanism powered by a low pressure (about 150 PSI) CO² cylinder with a 250 mm stroke. You can see a few seconds worth of video explaining the weapon linkage here: House Robot test video.

Shunt had the drive wheels close to the center of the robot to place most of its weight upon them. It also weighed a good deal more than the competitor robots; the official weight was 105 kilos, but I suspect it was about 50% greater than that. More weight on the wheels means more pushing power - simple physics.

A: You're reading section 2.2 of the RFL ruleset which lays out the special rules for the 30-lb Sportsman Class. From the ruleset:

A: It's pretty slick. Razer entered the Third Wars with a new set of 'wings' sprouting from the sides of the crushing arm. An inner extension arm on each wing is connected to the body of the robot by a cable running inside the crusher. When the crusher is raised to its highest position, the cable pulls on the wing extension arm and the wings move out to the sides to tip the robot back upright. It's a little slow, but simple and effective.

A: Not very often. See the discussion on 'lowest wedge' a few questions down this page.

A: What exactly do you think that huge 50 pound 3/8" thick titanium scoop with massive 1" thick solid titanium support arms is for?!? 'Breaker Box' was built specifically as a spinner killer and is perhaps most effective spinner killer ever to compete. I suggest you visit the Breaker Box website for details.

Q: Yeah, I checked Breaker's Box's site again and I'm sure it's pretty awesome, but does that mean Jim will give up the idea of Vertical Spining blades?! I really don't expect that happen!

A: Neither do I.

Q: And, do you think Behemoth's big lifting scoop could be a good weapon when up against spinners,too?

A: I don't recall offhand what Behemoth's scoop was made from or what kind of support it had, but the shape was close to correct.

A: Team Death By Monkeys is a 'local' NorthWest combat team and they are friends of Team Run Amok. If I say 'clever' you'll think I'm playing favorites. If I say 'not' I'll insult my friends. I think I'll just say that their logo is awesome.

A: Mark J. here: 'The Great Pumpkin' was a novelty 'bot that fought in one tournament. It had a four-foot diameter fan-inflated pumpkin attached to it's top. It was a joke - move on.

A: Mark J. here: we like simple. Pneumatic flippers are simple, powerful, and can be made from off-the-shelf components. The 'Flip-o-matic' device that Dale Heatherington built is ingenious and beatifully made, but requires a good machinist and extensive design skill. Dale has been very successful with his robots and is well utilizing his considerable talents, but I'm not going to recommend that other builders adapt his methods.

Q: What do you think of Amdroid-A? It doesn't seem very competitive to me by todays standards with spinners and everything. But the control system is pretty cool.

A: Another of Dale's robots - see my comments above. I've seen many homebrew radio systems. Dale's is probably the best, with extensive overload protection and a clever 'boost' function. However, my preference is to allow the robot operator the option of overstressing the electronic components as needed to win a match. I'd hate to be in a pinch and have extra power available but 'locked out' by my own software!

Team Run Amok's motto: "Complex design is easy - simple takes work."

A: Playing the 'lowest wedge' game isn't all that much fun. Many arenas have irregular floors, some with gaps or raised edges that can play havoc with a low wedge. We won a match at the RFL nationals when our opponent's very low wedge penetrated the space between two floor panels and got hopelessly stuck. You have to know a specific arena very well before you can decide how low to go.

Which dragging wedge wins? The sharpest. The front edge of the wedge must be knife-edge sharp and contoured with the leading edge right on the floor. At events with very smooth arena floors you'll see wedge teams honeing the leading edge with files and sandpaper between matches and examining the way the edge sits on the floor very carefully. A material that will hold a good edge is critical here. Like I said, this is not much fun as the sharpest wedge will also be more prone to surface irregularity issues.

With a 4-wheel bot the usual method for a dragging wedge is to mount the wedge on a hinge that allows it to drop by it's own weight to rest on the floor. See the question about the wedge on 'Original Sin' in the archive.

Q: [Chinese Forum] What do you think of 'Verbal Abuse'? I know it did pretty well in NPC Charity event but there are very few information about it.

A: I'm not going to let this turn into a 'Fanboy' site. I don't have much interest in providing opinions on obscure robots, rehashing matches that had no particular impact on the sport, or discussing the merits of every robot that ever competed. Very briefly:

• 'Pyromancer' is successful because it has been well thought out, superbly constructed, and well driven by an experienced team - just like most successful robots. The flame weapon does not substantially contribute to its dominance.

• 'SABotage' was just what you say - a simple and reasonably effective robot. It last fought five years ago and it's design is now outdated.

• 'Verbal Abuse' was an unremarkable 4-wheel invertible wedge with heavy shock-mounted armor that showed up at one tournament five years ago. It did fairly well and was never seen again.

A: The main problem with a dustpan design is the current judging emphasis on damage. They are great at controlling an opponent, but control no longer counts in scoring. The small 'saw on an arm' used by S.O.B / T.D.D. can't do enough damage to really be a threat.

A well-built dustpan can be very tough and resistant to attacks by most weapon designs (they are sitting ducks for an overhead spike) but the lack of a damaging attack is a major drawback.

Q: [Chinese Forum] So, if there's enough space and weight allowance a "Dustpan" with a crusher could make some good damaging effects - am I right?

A: A vertical crusher needs a very solid base or 'anvil' to press against. The floor of a dustpan would be deformed by the force. I guess you could run a horizontal crusher, but then you wouldn't really need the dustpan to contain your opponent.

My first question is How do I measure the belts and pullys to make them fit together for weapons and drive systems? This is the important question I will need this a lot in the future.

A: There is a belt-length calculator on the Timing Belts & Pulleys page at Robot Marketplace. You can play around with pully diameters and center distances and it will tell you how long the belt needs to be.

Q: My second question is what does 25:1 geared mean? How would I determine what ratio it is?

A: A 25:1 gear ratio means that for every 25 revolutions the motor makes the output shaft makes one revolution. You can determine a gear ratio by seeing how many rvolutions the input has to make to turn the output once, or you can count the teeth on each gear reduction stage:

For multiple stage gear reductions you can gount the gear teeth in each stage and multiply the ratios together:

Ratios of planetary gears are a little more difficult, but you can find general information on gear train types and gear ratios at science.howstuffworks.com.

Q: The Banebots P60 series do not have the torque listed for the gearbox. Is there any formula to determine the torque using the motor data and the gearbox ratio?

A: Sure:

So, a motor with 10 inch-ounces of stall torque and a top speed of 15,000 RPM mated to a 15:1 gearbox will produce at the output shaft 150 inch-ounces of stall torque and a top speed of 1000 RPM.

A: Mark J. here: we answer combat robot questions and don't generally consult on class projects. One quick thought: a hinged top cover that flips forward to double the top surface of the vehicle. Thirty millimeters square? That's about the size of my watch!

A: Stephen Felk devoted a great deal of thought to the design and structure, and he did a very fine job of construction - particulary given the limited resources available in his 'workshop'. We have great admiration for builders who do so well with modest budgets.

A: 'Dantomkia' had some unusual features - like the adjustable castor height - but I wouldn't call it 'unique'. It was effective, winning 'Heat C' in both the Sixth and Seventh wars, but it went no further. Several other flippers did equally as well, and some did better.

Q: What make 'New Cruelty' a successful robot? He is neither very fast nor very agile, but it seems it's always well-controlled. Is that a cause? [Chinese Forum]

A: I suspect that eight sticky wheels, a ton of power, and a talented and experienced driver contributed to the success. Dick Stuplich from Team Killerbotics knows how to take advantage of an opponent's weaknesses and he knows how to minimize his own robots shortcomings. Most importantly, he knows how to build a robot that matches his driving style.

A: It all depends on the arena.

• If your wedge doesn't drag, a wedge that does is gonna get under it.

• If your wedge does drag, it's gonna hit every irregularity in the arena surface. I've seen dragging wedges get stuck in floor seams and lose matches.

A: There is a 'secret': Beginners Guide to Gyros.

A: Both of these robots from Team Coolrobots were 'reaction hammer' designs (see discussion in the archive) that are simple, but which do not deliver much weapon power. 'Overkill' was probably more successful than it should have been based on it's combat capability. Judges really liked the huge shiny blade. It was simply more visually impressive than the simple pick on 'Toe Crusher'

A: Sometimes the 'only thing you can afford' ends up costing you way too much.

• The selected ESC interfaces to a computer serial port, not an R/C receiver. It also has a 1-amp rating -- that's less than your small Johnson motors pull free-running. It is useless for your application.

• The gearboxes you've selected will not bolt-up to the small Johnson motors -- the gearboxes are designed for a much smaller motor, and they do not specify the gear reduction they have.

• If you're short on cash, don't try to go long on power. The small Johnson motors can pump out almost 1/3 horsepower each at 11.1 volts and will require a very serious (expensive) ESC to handle them. That would be a lot of power even for a hobbyweight.

Q: How about this Buehler gearmotor? Its faster than the other one.

A: Better - at least the motor and gearbox fit together. It's specs appear to be kinda similar to the ML-30 motor, but you're still a long way from 'good'. The quality of the gearbox is unknown, the shaft is an unusual diameter, the motor is used, and 200 RPM is only going to give you 1.78 MPH with the 3" diameter Colson wheels. That's not even walking speed.

There are good reasons why popular gearmotors are popular: they work. If you go for an unknown and untested motor you're probably going to find out rather quickly why nobody else uses it.

You want affordable gearmotors for a beetle? Try the BaneBots MS250-20-180. At 4.8 volts with 3" wheels they will push a beetle close to 6 MPH in an 8 foot arena with pushing power to spare. They are the same price as your 'surplus' gearmotors, will operate from a pair of low-priced 5-amp insect speed controllers, require only a small NiMH battery pack, and are simple to mount in your chassis.

The BaneBots 2 7/8" wheels and 4mm hubs would be a good match for these gearmotors.

A: There's a reason that it didn't sell: Team Whyachi builds quality robots, but 'Y-Pout' was an experimental design that just didn't work. See a discussion of Y-Pout in the archive.

If you're looking for a 'pit pass' just to go to the next BattleBots and hang out, Y-pout should get you in the door. I'm assuming that it comes without radio gear, support equipment, and spares -- be prepared to spend some money on those items. I'd rather build a 'bot than buy one.

Mark J. here: I'm not certain that 'Y-Pout' even qualifies for competition under the current BattleBots rules. Section 9.1.2b requires an active weapon which operates 'separately from the Mobility System'. I'd get an opinion from the BattleBots technical crew before committing to purchase.

I also wouldn't count too heavily on the November BattleBots happening. Just a hunch.

A: Nice group of guys. At Robot Wars Extreme Warriors 2 they walked thru the pit area and handed out autographed posters of 'Razer' to every U.S. team. I was off someplace and didn't get to talk with them, but I still have the poster hanging in my room.

Razer itself completely rocks! It has one very clever feature (plainly visible in many photos) that significantly adds to the success in deploying the piercing weapon, yet which I've never heard anyone mention. It is an unusual use of an uncommon combat robot component. Bonus points to anyone who can identify the component and how Team Razer makes use of it.

Q: Just a guess, are the barbed teeth Razer's distinctive feature????

A: No, the jagged teeth on Razer's weapon don't help with deploying the weapon.

Q: Is the component on Razer the wings? Team Razer used them to self-right.

A: No - the powered wings were unusual, but SRiMechs were very common at Robot Wars.

The uncommon components in question replaced very common components on Razer about the time of the Third Wars. Razer used two, and they were powered. All the other robots I've seen that had powered ones used three or four. 'Killer B' had two, but they were unpowered and mounted sideways at the other end of the robot! What are they?

Q: Is the component on Razer the omniwheels?

A: Yes! A pair of omniwheels replaced the conventional wheels at the rear of Razer. Conventional wheels remain in place at the front. It is most unusual to have two powered omniwheels in parallel at one end of a robot. Now - tell me why Team Razor did this.

Q: Team Razer did this so they could maneuver around the opponent's weapon and attack them?

A: A pair of parallel-mounted omniwheels does not give powered side-slip control (omnidirectional motion). You'd need Mecanum wheels to do that, and four of them. There is a discussion of omniwheel use in the archive -- search for 'omnidirectional'.

So, if the omniwheels on the rear of Razer don't give it controlled side-slip motion, what specific advantage do they provide for a four-wheel skid-steered robot? Hint: Team Razor installed a peizo gyro at the same time as the omniwheels.

Q: Did Razer use the omniwheels so it could spin its weapon around quickly to face its openents?

A: You're on the right track; I'll give it to you. I wondered if anyone had noticed the unusual application of omniwheels at the rear of Razor and had figured out what advantage they gave.

A conventional 'tank steer' four-wheel robot has to skid tires sideways in order to turn because all of the wheels are pointing straight ahead. This takes a fair amount of power and can result in difficult and unpredictable steering results. Omniwheels have no resistance to sideways motion because their tread surface is composed of rollers. The rear omniwheels don't have to 'chatter' in a turn -- they just passively sideslip which makes the robot as maneuverable as a two-wheeled robot but with the stability and pushing power of a four-wheel robot. Very slick!

Team Razer added a peizo gyro to keep the rear end from slipping more than needed and 'spinning out' in a turn. Added advantage to the design: if you try to push Razer from the side, the rear wheels slide around effortlessly and the robot pivots around the grabbing front wheels to put you right in the mouth of the weapon.

A: Robot Arena is a pretty fair physics simulation, but even small differences between the simulation and the 'real world' can allow unworkable design elements to appear successful. If you're fighting in the real world, look to proven designs from real robot combat for your inspiration.

A: I don't predict fantasy matches, and I don't evaluate fantasy robots. I will say that the weapon appears to store very little kinetic energy and applies it to the opponent only under specific circumstances. I'd build something else.

A: Mark J. here: we selectively use rubber or spring mounts only for special cases. Armor that is inherently springy (UHMW polyethylene, polycarbonate, titanium) does not benefit much from shock mounting, although we often do use rubber grommets when mounting polycarbonate to relieve stress at the mounting points. We believe the best candidates for shock mounting are stiff plates of relatively small size: battery covers, rear panels, wheel covers.

Panels that are shock mounted do not fully contribute to the structural rigidity and strength of the chassis. Shock mounting should be used only in designs that do not rely on integrated stressed armor panels for strength.

We did have the front wedge on 'Run Amok' spring mounted at Robotica and Robot Wars Extreme Warriors 1, but it was more trouble than it was worth. We reverted to solid mounting for RWEW2.

A: All at the back?? The benefit from 4-wheel drive comes in getting all the robot's weight on powered wheels. If the drive wheels aren't carrying all the weight, you aren't going to gain much benefit. If you're thinking of using more wheels to get it to track better in a straight line, a gyro is a very worthwhile alternative.

A: Mark J. here: sorry, I've never done any aluminum brazing. I cannot comment except to say that I don't know of anyone using the technique in combat robot construction -- there may be a reason.

A: Two-wheeled robots tend to be difficult to keep on a straight path, but a small weight offset isn't going to be noticeable.

A: That's a great idea... if the robots you plan to fight are armed with guns.

How about tests that more realistically model the type of abuse your robot will actually face? Your whole robot has to put up with serious punishment, not just the armor. Drive it into a wall at top speed - forward and backward. Pick it up and drop it a few times onto a hard surface from about four feet. Kick it across the room. Drop a bowling ball on it. Hit it with a hammer. If something breaks, make it stronger.

A: Mark J. here: heavyweight 'Y-Pout' and middleweight 'Why Not' were experiments by Team Whyachi in a mechanical solution to the problem of obtaining controlled movement in a rotating 'thwackbot' spinner. Several mad scientist combat robot teams have worked on electronic solutions to this 'translational drifting' problem, calling the result 'Melty Brain' or 'Tornado Drive' propulsion. There is a good explanation at the SpamButcher website.

The Team Whyachi solution has a small conventional robot in the center of the whirling larger robot that steers the three main propulsion wheels via cam actuated rods based on the position of the small 'NavBot' relative to the larger spinning mass. Two article reprints from "Real Robots" magazine attempt to explain this with photos and diagrams: Y-Pout and NavBot.

Y-Pout's record: zero wins, two losses. Why Not's record: one win, two losses.

For best overall pushing performance, a gear ratio and wheel diameter combination should be chosen to break traction and spin the wheels at about half the stall amperage of the motor. For a 30 pound robot with a pair of RS-550 motors at 12 volts and 4 inch diameter wheels, a 16:1 gear ratio is close to optimal. You can check motor performance with different gear ratios and wheel diameters with the Team Tentacle Torque & Amp-Hour Calculator.

A: Mark J. here: good practice for an end tap calls for the material thickness to be no less than twice the major diameter of the machine screw. An 8-32 screw has a diameter of .164" which calls for a minimum material thickness of .328. I don't know what your application is so I can't even guess at how much further you could shave that.

A: I can't recall any real jumping robots, but there was a net-dropping flying robot at the 1995 Robot Wars: J.D. Streett's 'S.P.S. #2'.

The current RFL Standard Extensible Rule Set allows for 'jumping and hopping' as a means of controlled mobility. Also allowed are rolling, shuffling, and air-cushon hovering. Sustained flight is not currently allowed, although the event organizer has the final call on both jumping and flying.

A: Curt Meyers' superheavyweight 'Jaws of Death' first fought at BattleBots 3.0 in 2001. Its most recent appearance was at RoboGames 2008. There have been a number of design revisions over the years, but it has always had big hydraulic pincers. The hydraulic system is powered by an internal combustion engine. I cannot find specific details on the hydraulic system, but 15 tons of force from the hydraulic cylinder is entirely possible. Force at the pincer tips would be much less.

The robot has been modestly effective. Overall record for 'Jaws of Death' is 5 wins, 8 losses.

A: I suspect that their initial designs were both based on the more general theme of a bulldozer, but 'Behemoth' did evolve over time to more closely resemble the house robot. When 'Behemoth' first appeared at the Second Wars its only weapon was the bulldozer-like lifting blade. By the Fifth Wars it had added an overhead axe positioned to work with the lifting blade, just like 'Shunt'.

A: Mark J. here: most likely not. When you change the scale on a design the mass changes faster than the individual length/width/height dimensions and you change the need for strength in structural components. If you made an ant the size of an elephant it's skinny little legs would collapse under the weight. Conversely, an ant-sized elephant would have much thicker and heavier legs than it would need.

A: Yes, although the rules in force at the time did not refer to 'walking'. The 1997 Robot Wars rules allowed 'Legged' heavyweight robots to weigh up to 300 pounds, and the BattleBots 2.0 rules allowed the redesigned and beefed-up 'Snake' to compete as a 'Non-Wheeled' superheavyweight at a weight up to 488 pounds.

'Snake' would still qualify for a 100% 'non-wheeled' weight bonus under the current RFL ruleset, as it's motion is not dependent on rolling or cam operated mechanisms. Seems you don't need legs to walk.

A: BioHazard had no side shirts in it's debut appearance at Robot Wars 1996. The drop skirts appeared the next year. With 'BioHazard' on it's back, the hinged side skirts lay down flat against the ground. When it tries to roll upright, a skirt will fold up against the chassis and form a 'stop' that makes the rolling action too difficult to complete.

A: It takes nothing away from the classic combat robots to say that they are not a match for robots that have the advantage of seven additional years of design evolution. It's the same at the highest level of any sport: you don't race a seven year-old car, you don't play with a seven year-old racket, and you don't run in seven year-old shoes. Technology changes and you either update or become obsolete.

A: Building a hobbyweight ot featherweight robot is a lot like building a beetleweight -- just bigger.

• Look over the probable competition in your weight class;
• Decide on a design that would be competitive and within your budget and skill level;
• Examine possible drive motor configurations for your design using the Team Tentacle Torque & Amp-Hour Calculator;
• Select a speed controller based on the 'peak amps' provided by the Tentacle Calculator;
• Sketch up your component layout and estimate weights and budget;
• Go for it.

A: Why do people write to a combat robot site with mousetrap car questions?

Most mousetrap cars are designed for distance, not speed. The best distance moustrap cars creep forward very slowly -- speed is inefficient. For distance you want large, skinny wheels like CDs or even old LP records.

If you're building for speed you have other considerations. If it's a drag race style event for lowest time over a fixed distance you'll want the entire energy of the moustrap to expend itself in about the first 3/4ths of the course and then coast the rest of the way. Size the wheels accordingly. The tires will need enough traction to avoid power-wasting wheelspin, so some type of rubber tire may be needed. Experiment!

You can find more help for mousetrap cars of all types over at the Doc Fizzix website.

Love the site. [BDsquint- FOBOT]

A: Mark J. here: thanks for the love, BD.

We both understand that the integrity of an arena depends on more than the thickness of the Lexan. The type of framing and fastening is critical in determining how much abuse an arena can put up with. That said, a well-constructed arena with 1/4" Lexan walls should keep you well ahead of beetleweight spinners for some time to come.

I can't give you a specific number of Joules beyond which I'd start to worry, but it's unlikely that your containment will fail abruptly and catastrophically. Lexan will deform and absorb a really enormous impact. You'll notice severe marring and obvious damage to the polycarbonate well before there is danger of a breach. As long as your framing keeps the edges of the Lexan from parting, you'll have plenty of warning when spinners get close to dangerous energy levels.

Many arenas have a 'bumper strip' of material set in just a bit from the polycarbonate walls about spinner-high. This dissipates a good amount of the energy from a spinning weapon before it can reach the outer wall. Good idea!

Update: sorry, you can scrap that question. I looked back through the archive and after reading some other answers in which you said "Make it as thick as you can and still make weight", I'm just going to do that. Thanks.

A: Mark J. here: thanks for checking the archive. The 'make it as thick as you can' rule is generally a good model to follow. I don't think that .0625 aluminum would be thick enough. Fiasco uses .375" 7075 aluminum. For a beetle you won't need to go that thick. The application of some very sketchy engineering formulas leads me to believe that a high-strength aluminum alloy at least 0.125" would be in the right ballpark. Thicker would be better if you have the weight to spare.

A: The S28-400 Magmotors are only 3" in diameter, and the rest of the drive train is built to be no taller than the motors -- small wheels, compact chain drive, tiny sprockets. The 4-bar lifter weapon folds down flat to nest inside it's own forward control arms. The linear actuators that power the lifter are very compact. No magic involved, just first rate design.

Every section of the Team BioHazard website is required reading for anyone building a combat robot. Motor tips, parts selection, electronics, and materials are all well and concisely covered.

Get reading!

A: Generally, yes.

A: First, check to find out what weight classes are supported at your local events and how many competitors are in each class. The events page at The Builders Database can give you this information. You don't want to build for a weight class that has no competitors!

Next, consider your budget. It's better to build a light robot with good components that can be re-used on future projects than stretching for a heavier 'bot made out of pieces that you'll want to upgrade for your next robot. Buying good components will save you money in the long run.

A: You can go buy a 1" hole saw, but for a small job like this I'd score the outline of the hole onto the aluminum and then drill a series of small holes just inside of the scored line and use a small file to break thru from one hole to the next. A curved file can then smooth out the hole to the scored line.

A: Mark J. here: a few thoughts:

• You haven't calculated weight. Titanium is lighter than steel, but multi-inch thick pieces are still very heavy. I figure your front scoop will weigh 150 pounds by itself.
• With the current damage/aggression scoring, you're gonna have to do better than push opponents around to win matches.
• That's way too much horsepower for a robot that only goes 8 mph. You need horsepower for acceleration and speed. The amount of pushing power is limited by the weight on the drive wheels and the available traction. Horsepower excess to that requirement just spins the wheels. Half that much horsepower would push just as hard.

A: All common questions that we have previously answered in detail. Click those green buttons near the top of the page to access the archives and get reading, pilgrim. Start with the .

Some short answers to get you started:

• all the money you can spare, and then some;
• yes -- check our book reviews and section 629.892 at your local library;
• if you don't have a rich uncle you're not gonna get a sponsor;
• one thing at a time, with a lot of mistakes along the way.

A: I don't wander thru the pits with a caliper in my hand measuring armor, but as a guess: 'Shovelhead'.

A: The rear chassis uprights on 'Ice Cube 3' are bolted in place rather than welded. They can be removed along with the rear armor to make it easy to change the motors and gearboxes. Sometimes it is worthwhile to sacrifice a little strength to make between-match repairs go more smoothly.

A: You'll need a gearbox! A gearbox reduces the speed of the motor and increases the torque. Without a gearbox, the motor would not have enough torque to give the robot any pushing power at all, it would use way too much amperage, and it would melt very quickly from the stress.

The heavier your robot is and the larger the wheels are, the more gear reduction your motors will need. The Team Tentacle Torque & Amp-Hour Calculator shows that a pair of the 'Small Johnson' motors in a 12 pound robot with 3" wheels would do well with a gear reduction somewhere around 12:1.

A: Mark J. here: I've seen many beautifully crafted, titanium armored, mega-powered combat robots that could not fight their way out of a paper bag. Why not? Because getting the basic design principles correct is way more important than all the exotic materials and CNC machining in the world.

Team Tiki got the basics very right with 'The Brown Note'. The robot was low, powerful, and very controllable. The steel wedge was so nasty to start with that it didn't really show any additional damage. Once 'K2's nasty spinner got hold of the plywood 'The Brown Note' was just so many splinters, but the lesson to be remembered is that you must spend the time and energy to design your robot around the functions it must perform well to be successful before you get to the less important aspects.

You may wish to examine the career of heavyweight 'Evelyn, a Modified Dog' from Team K.I.S.S. for additional support of this theory. As the builder of another plywood covered combat robot once said, "Complex design is easy -- simple takes work."

A: We've answered questions just like this before, so dig thru the archives, starting with the . Building a robot to 'just move around' is very different than building one for competition -- meeting the competition rules takes planning and added expense, so decide before you start to build. Read thru the archives, see our recommendations on books, and dig in!

A: There are advantages to both designs, but I'm a believer in the chains and sprockets approach.

• With a motor on each wheel, if one end of the 'bot is lifted off the ground you lose the power available from the motors on that end.

• With chains and sprockets, all the motor power on each side remains available to any wheel in contact with the ground.

A: Mark J. here: If you want similar performance to Vladiator you'll need a similar power to weight ratio. Vladiator is a 340 pound superheavyweight powered by twin Etek motors producing a combined 30 horsepower: 340 / 30 = 11.33 pounds per horsepower. To get that same power ratio in a 120 pound middleweight you'd need: 120 / 11.33 = 10.6 horsepower.

The NPC-02446 motor that comes with the 'build your own gearbox' kit puts out about 0.75 horsepower. You'd need 14 of them to get close to the power you're looking for!

Three horsepower from four of the NPC motors will give more than adequate performance in a middleweight -- the Team Tentacle Torque & Amp-Hour Calculator shows a top speed across a 36 foot arena of 14 MPH in 2.7 seconds. You just aren't going to get the rubber-burning-mad-gerbil-in-a-popcorn-popper action that the high-end powerbots can display.

A: Sounds very good -- might even be overkill. Armor material and thickness choice depends on the design of your robot, how the armor is supported, and the combat tactics you plan to use. You're in the right ballpark.

A: I'm sure your design concept is very clear to you, but I'm going to need more information before I can make recommendations on motors. For a start:

• What weight class are you designing for?
• What type of weapon will you be powering?
• Tell me more about rotating the weapon around the robot.
• What balance do you want between mobility and weapon power?

Q: I'm thinking of making it a middleweight, and the weapon will be a pneumatic arm similar to that of the Judge's, and it will have a vertical spinner attachment, or else it can be swapped out for a flipper or a hammer. It will be mounted on a small housing that is held by and arm that will rotate it around the robot's body. Since the weapon will take up a a lot of weight, along with the armor the drive doesn't need to be fast or have a lot of pushing power. Also since the weapon will have some big stresses, what do you recommend for the arm, and the arm that moves the weapon around the body. Thank You.

A: A couple of suggestions:

• Robots with multiple weapons do less well than robots with a single weapon. Multi-weapon robots were fairly common at Robot Wars, but none of them did well. I'd suggest sticking with a single powerful weapon.

• Have you ever played around with a gyroscope? They are very difficult to point in a different direction, exerting plenty of force at a right-angle to the direction you try to turn them. A vertical spinner acts like a gyroscope that resists being re-pointed. Putting one on the end of an arm and trying to move it quickly in an arc would flip your robot over! No matter how powerful and heavy the arm holding it, you couldn't move it any faster than you could if it were just mounted to the robot.

A: Seems unlikely. Once flipped over it would be pretty stable resting on the rotor -- the 'bot body would just spin freely. You'd need a fairly complicated SRiMech to put it back upright. However, gyroscopic forces make a 'bot with a big spinning rotor difficult to flip over in the first place.

A: Mark J. here: clever thinking, but weight and mass are different properties. Mass is a measure of an object's resistance to change in velocity, while weight is a measure of force exerted on an object by gravity. The formula for kinetic energy is:

Magnetic downforce only increases the apparent weight of a robot, not the mass. The kinetic energy of your example robot can only be increased by increasing its velocity.

A: My best advice about tracks on combat robots is "don't do it". Tracks are way more trouble than they are worth on a smooth surface.

Most tracked 'bots are about 'square': the tracks are nearly the same length as the width of the robot. It helps maneuverability to have a tread support sprocket near the center of the track that is just a little lower than the front and rear support sprockets -- similar to the layout for a six-wheeled robot. Avoid the pain and go with wheels.

A: Set screws suck. They come loose at absolutely the worst times. You can bolt thru the pulley and use the radial prop mount holes on the motor case, or drill all the way thru the pulley hub and the motor shaft and drive in a small hardened pin.

If you have to use setscrews, file a deep flat spot on the motor shaft, use Loctite, and check it for tightness before every match.

Q: To attach a pulley to my Axi, could I just screw down the prop adapter around it really tight, and not use any set screws?

A: I don't have an Axi 5330 here to look at, but isn't the prop adapter held on with set screws? Set screws are not a good method of securing a mechanical linkage to a shaft. What is adequate for a propeller spinning in air is not adequate for a pulley that will encounter much greater forces.

You want a mounting method that relies on something other than friction to prevent rotation of the pulley and which will not fail if a threaded connection loosens a bit -- and a threaded connection being directly stressed will loosen!

A: A wide wheel track results in a stable and easily controlled 'bot with a slow spin rate when turning. This is desireable in ramming 'bots that must be carefully positioned for an attack.

A 'bot with a narrow track is more difficult to control in turns and has a higher spin rate. The higher spin rate is desireable in thwackbots that rely on a high spin rate for offense.

A: Since when are moustrap cars combat robots?

Larger wheels have lower rolling resistance than smaller wheels. They also make it easier to obtain a high ratio between the action of the trap spring and forward movement without efficiency-robbing gears. A really good mousetrap car will creep forward very, very slowly -- speed is not efficient!

I've seen mousetrap cars use old LP record albums (search your local thrift shop) as wheels. They work great!

A: Mark J. here: Beats me. Chassis strength depends on design, triangulation of members, gusseting, armor type, method of armor attachment, size, and construction technique as well as the amount and type of material used. Without knowing a whole lot more about your design and building skill level, I can't begin to answer your question.

A: No limit in the rules, but the larger the 'bot the thinner the armor has to be to make weight. Most builders keep them compact.

Q: How heavy is a typical featherweight?

A: The typical 'bot in any weight class is very close to the weight limit. The RFL featherweight limit (North America) is 30 pounds. UK featherweights have a 12 kilogram limit.

Q: What is the size of a normal featherweight?

A: A typical featherweight might be 16" square and about 4" high. There is a lot of variation and there are no size limits -- as long as it can fit into the arena.

Q: How light can a featherweight be?

A: There is no minimum weight specified in the RFL rules, but if you're at or under 12 pounds you qualify as a hobbyweight. Lighter 'bots are at a disadvantage in combat.

If you're serious about building a combat robot, you really must make the effort to attend a combat tournament and see a real competition. You'll get answers to questions that you didn't even know you should be asking!

Q: Can a DeWalt14.4v old style drill motor be good to power a wheel for a featherweight?

A: A pair of DeWalt 14.4 volt motors/gearboxes would be a good choice for a featherweight. With 3 inch wheels and the gearbox locked in 'high', the 'bot would have a theoretical top speed of 12 MPH at 18 volts. The motors could spin the wheels while drawing only 22 amps. All very good!

You can 'test drive' a selection of motor/gearing/wheel/weight combinations at the Team Tentacle Torque & Amp-Hour Calculator.

A: Mark J. here: I have no idea what you're talking about. Perhaps if you explained your question in detail and included links I could help.

A: According to the archived Stinger website, their top speed is only 9 mph -- not at all fast for Robot Wars. Power came from twin Bosch GBA 24 volt motors -- about 1.25 horsepower each. The motors were geared for acceleration and pushing power. I think it's their quickness you remember, not their speed.

A: Mark J. here: torque isn't the issue -- total power is the issue. Clamping force and speed will depend on the geometry of the clamp as much as the torque of the actuator. You usually aim for a minimum three times as much clamping force as the weightclass you compete in, so about 200 pounds of force at the clamping point for a lightweight.

A: If the arena you compete in has a smooth enough floor for drag wedges to be sucessful, you can probably get away with a hinge on your wedge that will allow it to drag as well. Put a limit on how far the wedge hinge can move -- you don't want it to fold back under the 'bot, or flip upward.

A: Read down about seven questions for information on 'hubs'. Attaching a wheel directly to a motor is a very poor idea -- without gear reduction, performance will be awful.

A: Different arenas have different problems with floor seams. Even the same arena can have tight seams one time and problems the next because of minor changes in how and where it gets assembled.

I can't recommend a robot design that depends on the skill of the arena assembly crew. I also don't like the idea of a wedge that can fold back under pressure and lift the front of the 'bot up off the arena floor.

A: Possible, yes. Good, no. An overhead thwackbot like Toe-Crusher needs to have the mass of the robot nearly balanced on the drive axle in order for the acceleration/deceleration torque of the drive to be able to throw the weapon 'over the top'. The overhang behind the drive axle has to be able to clear the ground as the 'bot swings over for weapon impact. With six-inch wheels, you would have less than three inches of rear overhang -- that's probably not enough to pack batteries and other heavy components to counterbalance an effective weapon hanging off the other end.

Q: What's the best way to distribute weight in a thwackbot?

A: As mentioned above, an overhead (torque reaction) thwacker needs weight positioned out behind the drive axle to help balance the weight of the weapon out on the end of the boom. Place your batteries, electronics, motors, everything you have space for out in back of the axle centerline. I'd aim for about 85% of the total robot weight to be on the drive wheels with the remaining 15% resting on the weapon. If you get too much weight on the weapon it will become difficult to swing the weapon over for impact. If you have too much weight on the drive axle the weapon impact force will be reduced. Some 'trial and error' tuning will be needed.

The approximate weight distribution applies to traditional non-overhead thwackers as well, but you'll have more space behind the axle for weapon weight offset.

A: Mark J. here: I don't know what your chassis design looks like, but I'd drill and tap multiple mounting holes across the top and bottom of each the two aluminum plates and both thru the top and bottom plates of the robot chassis. The plates will need cutouts for the wheels. Additionally, I would use the existing mounting holes on the plates to bolt thru chassis tubes or into bracing blocks or bulkheads perpendicular to the plates. You do not want the plates to distort in relation to each other or the gears will die!

A: Ramiro Mallari built 'Punjar' from exercise treadmill parts. The very wide wheels are converted treadmill rollers. Perhaps he believed that super-wide tires would give super traction?

Punjar had a long and successful career. Active from 1996 to 2001, Punjar racked up a 14 win / 8 loss record. That places Punjar 24^th in the all-time heavyweight rankings -- 10^th among those with 20 or more fights.

A: Not even close. Here's what the 2007 RFL rules say:

Pressure Drop's walking mechanism is cam actuated via a linkage from a continuous rotary source -- no weight bonus for that.

A: Go look around the Mechanical and Drive Components page at Robot Marketplace. While you're there, browse the rest of the site as well -- it will answer a lot of basic questions.

A: A wood baseplate! You don't see much of that anymore -- but I like it. A good quality plywood ('marine grade' is best) is really quite strong for its weight. Don't use woodscrews to fasten down your components; drill thru the baseplate and use a nut/washer/bolt to securely anchor everything.

There are a lot of choices for the rest of the chassis. Square steel tube can be cut with a hacksaw, drilled, and bolted together. Use nylock nuts or a liquid threadlocker to keep everything from vibrating loose. You'll want to gusset all the joints for strength. Armor can then be fastened directly to the square tube chassis.

I'd strongly recommend getting a copy of Grant Imahara's book, Kickin' Bot. It covers everything you could possibly need to know about chassis building, as well as every other aspect of combat robotics. It will save many times it's cost in time, materials, and performance.

A: It's usually a 'trial and error' process. Design and build your 4-bar lifter for its primary function as an effective weapon. Once the robot is working you can try extensions to the height and width of the top lifter arm until you hit a combination that will tip your inverted 'bot back onto it's wheels -- like the top 'claw' on the 'Pack Raptors' (pictured). Designs with greater lift will require smaller extensions.

General robot design can affect the ability to self-right as well. The original (1996) version of Biohazard was able to reliably self-right, but lost that ability when defensive side-skirts were added. A narrow or short 'bot is easier to self-right than a wide, long 'bot.

A: Robot Sumo! Great stuff. I'm guessing that this will be remote control sumo. Check the previous sumo tips in the Ask Aaron archives. A few specific suggestions:

• Go to your public or school library and find a copy of Robot Sumo: The Official Guide by Pete Miles. You'll get enough tips from Pete to stomp your opposition.

• Keep your design simple. Sumo matches are won by looking after the details, not with complicated design or exotic weapons.

• Yes, use a wedge -- they are useful for more than defense. Once one or more of your opponent's wheels are off the arena surface you have a strong pushing advantage. Your wedge should be razor-thin at the front, make even contact all the way across, and scrape the arena surface.

• If you absolutely must have a weapon other than your wedge, consider a simple servo-powered clamping arm on top of your 'bot to grab and hold your opponent after they climb your wedge. That should be simple, inexpensive, and effective -- but I'd just stick with the wedge.

• You've got a pretty big 'bot on a really small arena -- gear for pushing torque, not speed. Speed is not your friend on such a small surface.

• Superior traction beats superior power. Coat your tires with a thin layer of pure silicone rubber sealant and let cure completely (about a day). Rub the tires between matches with alcohol on a clean rag to remove any dust and contaminants.

• There's usually a weight limit in sumo. If there is no weight limit in your challenge, make the 'bot heavy! A heavier 'bot has more traction, can deliver greater pushing power, and is harder for a light 'bot to push.

• Get the 'bot built early enough that you can become very comfortable driving it in an area as small as the arena. Practice your driving skills!

A: We don't use Computer Aided Design software for our robots so I can't make a recommendation, but if you type 'free CAD' into Google, you'll find links to a ton of them.

A: Your problem isn't the weight of your armor, it's how the weight of your 'bot is distributed. I'm guessing that this is a two-wheel drive robot and that your drive wheels are too far away from the center of mass to get good traction. You need to either redistribute the heavy components of your 'bot to put more weight on the drive wheels, or move the drive wheels closer to the center of mass.

Q: That's a good answer, but I didn't tell you enough about my 'bot. I made it cheap -- it's a remote controlled car (worth £10 or $20) and the armor is what once looked like a dustpan but with a cover. It's covered in decorative foil and stickers. Is there anything I can do to improve the turning on the car?

A: I like your use of available parts! You've built a 'bot and you're out there having fun.

Your front tires are probably soft plastic and may not have enough grip to turn the 'bot. Their grip can be greatly improved by coating them with a thin layer of silicone rubber sealant, available at a hardware or auto supply store. Use the 'pure silicone', not 'siliconized' caulking or another type of sealant. Any color will do. Clean the tires with alcohol or another solvent and let dry completely before applying the silicone to get the best bond. The sealant is pretty thick and sticky to apply, but it doesn't need to be perfect. Let the silicone cure for a full day and give it a try.

Another possibility is that the weight on the front wheels is too great for the small steering servo to overcome. You can't replace it -- your radio isn't compatible with hobby-grade servos. If that's the problem, you're back to shifting weight toward the rear drive wheels to lighten the load on the front wheels. Best luck!

Q: I'm writing back to thank you for your help. The robot's steering has greatly improved and I owe all my thanks to you.

A: Glad I could help!

A: Rounding the edge armor of your 'bot could help resist hits from vertical spinners, but angled armor has to face all sorts of weapon attacks. Sharply sloped armor can be effective against many weapon types: vertical and horizontal spinners, bricks, and rammers. If you add drop skirts you have good protection against wedges, lifters, and flippers too. Rounding the armor as you suggest could retain some additional protection from vertical spinners when inverted, but would leave you very vulnerable to wedges and lifters. Are there really so many vertical spinners fighting at your local tournaments that you have to design specifically against them? Also consider how often you will get inverted if you have effective drop skirts.

If you are really worried about inverted protection, the best solution could be a V-profile plus a hinged drop skirt. The armor profile remains the same when inverted and you maintain protection from lifters and wedges. See the diagram.

A: The motors must be securely fastened to the chassis by mechanical means:

I don't know which 'Beetle Gearmotor' you're using, but they likely have some mounting holes that could be put to good use with some simple angled metal brackets. A pair of automotive steel hose clamps fastened around each motor and thru the chassis can be used in a pinch. Machined aluminum or UHMW polyethylene clamp mounts surrounding each motor and gearbox would be better. Look around the 'net and see how other builders do it.

A: The Robot Marketplace antweight battery packs are compact, light, and well made -- but they have only a 370 mAh capacity. Fine for an antweight, not nearly enough for a hobbyweight powered by RS-540s.

Pay attention to the 'estimated battery capacity required' that the Team Tentacle Torque Calculator provides. A hobbyweight powered by RS-540s plus a weapon motor is gonna want more than 1000 mAh.

A: See previous post on battery capacity.

A: I think you're asking about drop-skirts. They are armor panels hinged along the top edge that drop down at an angle and slide along the arena floor. Small 'bots can use strong tape to make the hinge, but larger 'bots need full-length mechanical hinges.

A: You rubber-mount armor by running the mounting bolts thru a large rubber grommet in the chassis. Be sure to use a 'fender washer' on the back side of the grommet, and self-locking nuts. Don't tighten the mounting nuts down very tight -- leave room for the rubber to flex and absorb impact. Rubber mounting is particularly useful for polycarbonate armor, which tends to crack at high-stress mounting points.

A: Mark J. here: A speed controller is one of the last components to consider when building a 'bot -- not the first. Your speed controller specs will be determined by the amperage requirements of the drivetrain. The process goes like this:

1. Decide what you want your 'bot to do in the way of fighting strategy.
2. Outline a design that will accomplish that strategy.
3. Find components that will meet the design requirements.

A: Robot sumo competitions have highly variable rules, and careful study of the rules is needed to figure out what designs will work well. Are you entering the autonomous or R/C category? What weight/size class? What material is the arena surface made of? Do the rules allow magnet traction aids? Do the rules allow vacuum traction aids? Are there rules about how 'sharp' the edge of your wedge can be? Are 'sticky' tires allowed, or do tires have to pass the 'paper drop' test?

Write back with details about your competition, and tell me more about your motors. In the mean time, try to find a copy of Robot Sumo: The Official Guide by Pete Miles in your local library.

Q: I am entering a R/C category. The maximum weight is 2.5 kg and the robot must fit within a 50cm X 50cm box with no height restriction. The arena floor is made of vinyl. The rules do not specify anything about traction aids (nor do I know what they are). There are no rules about how sharp the wedge can be. And, tires may not leave residue on the playing field. Thanks!

A: It's details that win R/C sumo competitions. A wedge is a winner only if it's better than your opponent's wedge. You need to make sure that it sets perfectly level all the way across it's width and that the leading edge is sharp and flush with the arena surface. Read thru those rules again -- there's usually some restriction on dangerous sharp edges. If not, make it razor sharp, but fit a cover to it between matches for safety. Side and rear drop-skirts are uncommon in sumo competitions and may be more trouble than they are worth.

Tires are another critical detail area. Super-tacky reusable lint rollers are very effective as sumo tires, and they leave no residue. See Dave Chu's Sumo Project for an example of their use. Clean and dry them before each match. Other tire compounds (polyurethane, silicone rubber) are useable if you're not willing to custom make your hubs and tires. See Pete Miles' book referenced above for details.

Traction aids are devices that make the apparent weight of the 'bot greater in order to get more traction. If the arena has a steel base, magnets can be used to pull the 'bot toward the surface with great force. A magnet-bot can climb right up the vertical face of a steel cabinet and even run upside-down on a metal ceiling! A similar effect can be achieved on non-magnetic surfaces with a vacuum fan system and sliding surface seals.

I'd suggest sticking with a conventional design for your first build. Finish it early enough to get plenty of driving practice. Keep things simple and sweat the details.

A: Mark J. here: Is that total stall torque from all motors at the axles, or stall torque available per driven wheel? I also need to know the wheel radius, the number of driven wheels, and the weight on the drive wheels to run the calculations.

'Punjar' was a scoop-fronted rambot with a lot of speed and pushing power. To duplicate that performance, you'll want to design for thrust available at each driven wheel to be about two times the weight normally on that wheel. Much more than that just smokes the tires and wastes weight (and money) on heavy drivetrains and batteries.

I'll guess that 788 inch-pounds is the total torque available per drive wheel for a two-wheel drive 'bot. Assuming a 3" wheel radius, that gives you (788/3) = 263 pounds of thrust and no more than 50 pounds of weight on the wheel. If my assumptions right, that's more than five times the weight in thrust -- way overkill. If that 788 in-pounds is the total of all your drive axles, you're pretty close to right.

A: It's simple to build a 'bot with a high spin rate, but it's very difficult to get it to spin and stop just where you want it. Your skill as a driver will be the determining factor. You also have to plan for unexpected situations where you aren't free to maneuver. Most builders armor-up all the way around.

A: Spinner killers are all about the scoop. For a great example, take a look at Jim Smentowski's middleweight actuated-scoop spinner killer Breaker Box. The scoop must start out at a very shallow angle - nearly horizontal - and curve up to around 60 degrees. Length and height are about equal, with a gentle and uniform curve. Jim's using 3/8" titanium for the scoop, with very heavy support arms. His scoop makes up almost half the weight of the 'bot! The simplest way to figure the surface area of a curved surface is to make a thin cardboard mock-up, then flatten it out and measure.

Another question about my spinner killer: do you think I should go with .25" S7 tool steel or 3/8" titanium 6Al-4V? I thought titanium would be better, but S7 tool steel is what impact teeth are made of, so it should be able to stop the spinners I guess. The tool steel would weigh about 3 pounds more for the size of my scoop, but would cost much, much less. What do you think?

A: Mark J. here: S7 tool steel is both hard and unyielding, which makes it perfect for transmitting energy from a rotary weapon to the target via small teeth. To resist the impact of spinning weapons you're looking for a material that can spread the force by deforming and then snapping back into position -- a property known as 'toughness'.

Although tough for a tool steel, S7 alloy is more brittle than titanium and in large sheet form may crack on heavy impact. You run the risk of a big weapon hit shattering the large plate like glass! If you want to use steel, consider a tough spring steel that can flex on impact, like EN47. Remember that you'll have to heat treat either metal after the scoop is assembled to obtain optimum performance, and this will add to your costs.

6Al-4V titanium is extremely tough and resilient. It will take enormous impact and come back for more. It's perfect for absorbing spinner attacks. It is expensive, but the experienced builders that use it wouldn't be spending their money if it weren't worth it.

A: Mark J. here: the conventional way to build a combat robot is to decide what the robot should do to give you a tactical advantage and then draw up a design for a robot that will do what you need. You've drawn up a design and now want to figure out what it will be able to do. I smell a train wreck.

• Three-wheeled omnibots are inefficient in utilization of available power: you'll never be able to use more than 2/3 of the total motor capability in linear motion.

• Omniwheels have a low contact area and poor traction. They were designed for low-speed, low-power applications. They are not suitable for putting a lot of power to good use.

• The only combat omnibot I can think of is Alcoholic Stepfather. It uses four mecanum wheels -- not omniwheels.

Q: Ok, I did a little thinking and the way you stressed mecanum wheels, I think I should use those. I had planned on making a Why-Not / Y-Pout style thwackbot with these wheels. What if I came up with a way to keep the impact away from the robot itself, maybe a hammer on a steel cord connected to each point on the triangle? In my mind, I think that would work. But then again, I don't have the experience you have, so what do you think?

A: I'm glad you're giving this project some thought. I have respect for builders that are willing to try something different. I did some research and found a lightweight 3-wheeled omnibot with a spinning undercutter blade weapon called Y-chromosome that fought at Battle Beach (video) and BotBash (video). You might want to talk to the builders at Team Radicus.

Here are a few more things to consider:

• Your omnimixer will not allow you to spin at high speed and simultaneously move in a controlled manner under radio control. The only omnibots that I've seen move while spinning did so at a very low spin rate along a pre-determined course under full computer control. The technology for a high spin rate while moving is called 'Melty Brain' or 'Tornado Drive' and has been custom implemented on only a very few 2-wheeled thwackbots.

• A mecanum wheeled omnibot requires four motors and wheels. A three-omniwheel 'bot would work better for a thwackbot, but I really don't see much advantage over a two-wheeled conventional thwacker.

• The current RFL judging guidelines are stacked against thwackbots. All points come from damage and aggression. If you can't move while in 'attack mode' you aren't going to get many aggression points. Sit and spin never wins.

A: The only active team I can think of is Zwolfpack Engineering. Their lightweight "1st Abe Lincoln on the Moon" has a successful implementation of Melty Brain. Note that the system got its name because thinking hard about how to get it to work causes such mental overload that it may actually melt your brain. Zwolfpack has obviously suffered significant cerebral damage as evidenced by the names they choose for their `bots.

The Zwolfpack website (www.zwolfpack.net) is not terribly helpful, and I don't have other contact info for them. You might try asking around at the RFL forum. My advice is to stay away from Melty Brain.

A: Search the Ask Aaron archive for 'hobbyweight', Peter. You'll find more than a dozen posts about motors, weaponry, radios, and battery capacity.

A: That depends on what you call a 'walker'. Shufflebots can be very fast -- Dave Hall's 'Drillzilla' claimed a top speed in excess of 30 MPH, but shufflebots are no longer considered to be 'true walkers' in the rules. In the British TV series 'Technogames' the walking sprint race winner 'Scuttle' covered the 25 meter course in just over 7 seconds, about a 7 MPH average speed. Scuttle doesn't qualify as a walker under the current rules either.

Building a true walking robot is very challenging. You might get some tips from looking at designs in Servo magazine.

A: I don't know of anyone currently building true walkers for combat. I'd suggest getting a copy of Servo magazine and looking thru the ads for walker kits. The makers of these kits may be able to supply parts and design ideas for your combat walker.

A: Instead of wheels, shufflebots have two or more long 'feet' on each side of the 'bot that are operated by crankshafts. Each 'foot' is lifted and set down again as the crankshaft turns to move it a little forward or back. The motion is bouncy and makes maneuvering tricky. Shufflebots once got a weight bonus as 'walkers', but a few years ago they changed the rules and shufflebots now don't get that extra weight.

A shufflebot requires a complex drivetrain and custom made parts to make a slower and less maneuverable 'bot than you'd have if you used wheels. Don't bother!

Note: Dear Aaron, the 2006 draft of RFL rules offer a weight bonus to non-wheeled robots including shufflers. [Ted J.]

Not true, Ted. Section 2 of the RFL 2006 rules says, "There is no weight bonus for shufflers or other forms of locomotion which are predicated on rolling." Shufflebots are dead.

A: Mark J. here: I could use a little more info on what you're building! The selection of a proper bearing or bushing depends on how long the support needs to last, how precise the alignment needs to be, the type and magnitude of the force that will be applied, the speed of operation, and the depth of your wallet.

Since you're powering this mystery device with a servo, I'm guessing that the force and speed are both pretty low. Since it's in a combat robot, I'll assume it doesn't need to last forever and that extremely precise alignment is not an issue.

You can likely get away with just drilling an appropriately sized hole in a block of strong, slick plastic -- like UHMW polyethylene, polycarbonate, or nylon. If you need to absorb high shock loading, you might want to go with a bronze 'oilite' bushing. Oilite bushings are cheap, durable, and widely available in a variety of sizes.

A: Sounds like somebody wants me to do their homework for them.

An automobile gets better gas mileage when it travels slower, right? That's because friction and drag increase quickly with greater speed. It's much more efficient to move along at a slow and steady pace than to sprint forward and coast to a stop. A well designed mousetrap car can travel dozens of yards if it keeps speed under control.

Hard, large diameter wheels reduce rolling resistance, and when combined with a low gear ratio will reduce the torque available to accelerate the car and will keep speed down. A typical mousetrap car has one end of a thread wrapped around the drive axle, with the other end pulled by an extension bar from the mousetrap. The number of turns the rear axle makes while the mousetrap unwinds is the equivalent of a 'gear ratio'. A longer extension bar on the trap lowers the effective gear ratio.

Your goal is to keep the car just barely moving along for the entire time the trap unwinds. Tinker with the length of your trap bar extension to get a steady forward crawl -- lengthen it if you're going too fast, and shorten it if the car stalls.

Hey, this question isn't even about combat robots! Why am I answering it?

A: See the Wikipedia differential article.

A: Mark J. here: some machine shops just don't accept small one-off custom jobs, but a clear explanation of what you want and a friendly smile will improve your chances. Machine shop time isn't cheap -- do as much prep work as you can and listen to their questions and suggestions. Let them know that you appreciate their taking time to work with you.

A: Mark J. here: There are a variety of large-hole making devices that will fit in standard-size drill chucks. Here are some options:

• The proper way to do this is with a Hole Saw. A 3/4" hole saw with arbor and pilot drill bit should run about $15 at your local tool supply shop. Get one with a central 'pilot drill' to guide the saw. You can also buy a hole saw set with several diameters for around $30 - they come in handy!

• You might be tempted to use a wood boring Spade Bit. This is not recommended. Although they are inexpensive (about $3) and will crudely bore thru Lexan, a spade bit is difficult to position and control precisely with a hand drill. If you insist on using one, drill a small (1/8") pilot hole, securely clamp the plastic to scrap wood, and securely clamp the wood to your workbench. Be prepared to ruin a few pieces.

• With a soft material like Lexan, you can always scratch a 3/4" circle in the surface (a penny is 3/4" diameter), drill several small holes to 'rough out' the interior, and finish with a curved hand file. This will get you 'reasonably round' if you have the patience.

A: You make something better by standing on the shoulders of giants.

A: Mark J. here: simple answer first: most hobbyweights I see are built around a hypothetical top speed of around 12 to 15 miles per hour. Hypothetical top speed is calculated by the formula:

Example: a 6000 RPM motor with a 6:1 gear reduction and a 12" circumference tire gives:

For help with motor, gearing, tire, and battery selection, check the Team Tentacle Torque Calculator. Be sure to investigate the 'Acceleration" button.

A: Mark J. here: in designing a combat robot, you have to think about how all the components of the machine will work together. A chassis design that would work well for one design could be a disaster in another design. Since I know nothing about your design, I can't comment on your frame.

You have a number of choices for a heavyweight 'bot. You can use welded tubular steel, bolted steel angle, interlocking aluminum flat panels, a stiffened composite pan, or bonded polycarbonate. You can integrate the armor into the chassis as a stressed component, or have a separate armor shell. Be sure to consider your skill level and experience in working with the materials involved, as well as your budget.

A: Mark J. here: That's over two pounds worth of motors in a six pound 'bot! How do you plan to use that much power in a small arena? By the time you add in batteries (lots of 'em for those motors), gearboxes, wheels, and a chassis you're not gonna have much weight left over for armor or a weapon. Keep your design elements in balance - go with smaller motors.

A: Mark J. here: Taps are available in diameters up to 4 inches, but they are expensive. You may also consider thread milling for large diameter holes. Unless you're going to be doing a lot of large diameter tapping, give the job to a machine shop.

A: Mark J. here: pocketing removes material from low-stress areas of a panel or component by milling away some of the thickness while leaving a border of thicker material. If properly done this results in reduced weight while retaining the majority of the strength of the original panel. The location and depth of pocketing requires extensive stress analysis of the component.

The exterior panels of combat robots don't really have any 'low-stress' areas. They can be exposed large forces from any angle and at any point. I do not recommend pocketing external 'bot panels -- although it looks really cool.

A: Building a successful combat robot requires experience in mechanical design, construction techniques, material properties, and control systems. See my FAQ post on where to get help in these areas.

Robot builders use a variety of methods to design their robots. Some use computer aided design tools, some build detailed cardboard models, and some just design as they build. We like to sketch out the overall design, make a complete parts list - with prices and weights, and jump right in to the build. Our sketches wouldn't be much use to anyone else building a combat robot, and copying someone's exact design isn't as rewarding as designing your own 'bot, anyhow.

Keep your first 'bot simple and make sure the mechanical basics are well covered.

A: Combat robots don't have axle hooks. See web articles on mousetrap cars.

A: Technical question - Mark J. here: that depends on how many and what type of motors you use, the gearing, tire diameter, and driving style as well as the weapon motor type, how much you use it, and how heavy the spinning mass is.

Look at 'bots that have a similar set-up to the one you plan and use their experience to help you decide on a battery pack. You can also get some help from the Team Tentacle Torque Calculator which provides an estimate of required battery capacity based on motors and various design factors.

A: Hi, Nick! Since you're asking about windshield wiper motors, I'm guessing you're thinking about a hobbyweight or featherweight class robot? We built a hobbyweight with windshield wiper motors almost five years ago. Even though we were running the 12 volt motors at 24 volts, the gear reduction was so large that they were really slow! They were also very heavy for the power they provided. I'd stay away from them.

Keep your first 'bot simple. Wedges win more matches than 'bots with active weapons, so get some experience before you start showing off with fancy weapons. Cordless drill motors are fast, powerful, simple to mount, and pretty cheap. Fasten them down to a simple flat panel base, armor up, and bolt on a sturdy wedge. Spend some money to buy good speed controllers -- that's the one place you shouldn't scrimp. Search the archive for some other tips and book references. Best luck!

A: Try: www.shender4.com/thread_chart.htm for pilot and clearance hole sizes.

Once you have the hole, the tool used to put threads on the inside is called a 'tap'. The tool for putting threads on the outside of a rod is called a 'die'. You can purchase an inexpensive Tap and Die set, or you can purchase individual taps and dies at your local tool store. A brief guide to tapping holes with hand tools can be found here.

A: Mark J. here: Countersinking forms an angled cut-away around a hole for a flat-top fastener that leaves the fastener top flush with the surface to provide a smooth surface.

Counter boring forms a flat-bottomed recess around a hole to provide fastener clearance and/or offer a flat surface for the fastener on a curved, sloped, or irregular surface.

A: If you don't want to buy hubs, then spend the money for a mask so nobody will know who that guy was that had his wheels fall off. Read my post about hubs in the Ants, Beetles, and Fairies section.

A: Mark J. here: There are three broad categories of screws: wood, self-tapping, and machine. A 4-40 screw is a machine screw that is should be threaded into a pre-tapped hole. Drill a pilot hole with a #43 drill bit at low speed, then use a 4-40 tap to create the threads. Alternately, you may use self-tapping screws with Lexan. They will also require pilot holes, but will not require pre-tapping the threads. Cramming a machine screw into an unthreaded hole is a weak bodge - don't do it.

A: Mark J. here: Motor mount design depends on powertrain layout. If a large motor is being held in critical alignment for an open gear or chain reduction, it must be held very precisely with no wobble or the gear/chain system may fail. Such designs should secure both ends of the motor. Mounts for small motors with attached gearboxes (gearmotors) that do not rely on the mount for alignment of the gear reduction may allow or even encourage a little shock-absorbing 'give' in the mounting design and material.

A popular design for insect-class robots with small gearmotors is the wide circular clamp: a machined hole on a block of aluminum or plastic (UHMW polyethylene or Lexan) with a gap along one side that can be closed down with screws/bolts to clamp the gearmotor in place. The wider the clamp, the more secure the mounting. Remember, 'bots with exposed wheels put more strain on the mount.

I've also seen low-budget mounts made from wood blocks and steel automotive hose clamps that were surprisingly functional. You don't need a full machine shop to build a workable 'bot.

Mark J. here: They aren't thwackbots -- triangular 'bots with an omni wheel at each apex are called 'omnibots'. Yes, they can rotate, but the really cool thing is that by differentially powering the three wheels, they have the ability to move in any direction without turning -- that's called 'holonomic motion'. There's a cool video of a four-wheel holonomic omnibot (same principle as a 3-wheel version): holonomic at www.charmedlabs.com. An R/C omnibot requires a computerized transmitter with programmable mixing to properly do it's tricks.

Autonomous omnibots can be programmed to move (slowly) while spinning (slowly): Frisbee at www.charmedlabs.com, but doing it at variable high spin rates and variable directions under radio control is a REAL challenge. It's called 'Melty Brain', 'Tornado Drive' or 'Cyclone Drive' and many builders have spent years trying to get it to work. It requires on-board processors, motion sensors, sophisticated programming, and it still doesn't work reliably. Don't even try to cram all that into an antweight!

You need a 'hub' to connect a thin weapon to a small diameter shaft -- a precision machined connector that will hold the weapon in correct alignment and provide sufficient depth to allow a stable and snug fit onto the motor shaft. This hub will be the most highly stressed part of your weapon system, so don't try to bodge this.

Note that it's generally not a good idea to connect a weapon directly to a small motor. It is difficult to attach a hub securely enough to a small shaft to be able to transmit large weapon loads. Search the Ask Aaron archive for tips on belt drives that can give better weapon performance. Hubs and belt drives smaller than 3mm bore are hard to find.

A: Thanks!

Omnibots are way cool to watch! The can move forward and back and turn like a regular 'bot, but they can also move sideways without turning and even rotate while moving in a straight line! Check out the videos of the flame throwing superheavyweight omnibot Alcoholic Stepfather.

Omnibots use some variant of an omniwheel or mecanum wheel with built-in rollers that allow it to provide thrust in one axis and slip without resistance in another direction. By applying power selectively to different wheels you can get the 'bot to perform its tricks. You can view a detailed description of a robot using mecanum wheels here.

Omnibots may have three or four wheels. Each wheel needs it's own speed controller and dedicated radio channel. Check the earlier post on omnibots for more videos, and search the web for 'mecanum'. I think you'll pick up the idea after you see a few examples.

You actually have two challenges in running an omnibot: building the drivetrain, and programming the R/C transmitter to make the 'bot controllable. You'll need a computerized R/C system with multiple user programmable mixes or some double-fancy helicopter swashplate settings. If you get the 'bot built and need help with the programming, write back and we'll put our heads together!

Q: Could I make an omni-bot in the 'Robot Arena' simulation game?

A: Hmmm... There aren't any omniwheels or macanum wheels available in the Robot Arena parts box, and the control options don't offer channel mixing for proper omnibot control. If you did manage to cobble one together, it would be a poor representation of the real thing.

A: Small 'bots usually have a skid of slick plastic -- like polyethylene. Bigger 'bots may have a ball caster or an omni wheel. Our heavyweight 'bot 'The Gap' has a wide roller machined out of Teflon in the front so it can slide sideways and roll forward.

A: Tech question, Mark J. here: the smaller timing belts (MXL series) are rated up to 20,000 RPM, and can transmit as much as 400 watts of power. They're much more effective at power transmission than other belt types. For a full engineering summary of timing belt selection and performance, see: www.roymech.co.uk/Useful_Tables/Drive/Timing_belts.html.

A: Tech question, Mark J. here: The wider the belt, the more power it can transmit. Wider belts also stay in place a little better.

A: There are plenty of websites that give detailed information on autonomous sumo 'bots -- try Dave's Sumo Robot Project Page for a start.

A: A tank-steer 'bot steers the same way no matter how many wheels it has: all the drive wheels on one side of the 'bot turn at a different speed and/or direction than the wheels on the other side. Robots with four or more wheels drag their wheels sideways a bit as they turn. That takes a little more power, but it works just fine.

A: If tracks were a real advantage, lots of winning robots would use them. They don't! I'd go with wheels.

Wheel/tire selection depends on many design factors with which the wheels will have to fit. If your wheels will be exposed to attack, use a solid or foam-filled tire that can't go flat.

For general use, I like the Colson wheels. They have been used on combat robots for many years. They're pretty light, durable, have good traction on arena floors, and are inexpensive to replace when damaged. Team Delta sells Colson wheels in different sizes and will even make wheel hubs to fit your drive shaft.

A: Thanks! How long it takes to build a robot depends on how complicated the design is and how much experience you have working with the tools and materials you need. Getting some friends who have skills you don't have will help a lot! I've seen experienced teams build a heavyweight 'bot in a week, but our larger 'bots take us about two months to put together. That includes time to design, get parts, build, and test.

A: Thanks, Coleman! Before you start building a 'bot there are a few things to take care of. First, read the rules for the competition you plan to enter very carefully. You don't want to build your 'bot only to find out you aren't legal for the competition! Next, take time to find as many websites from competitors who have been to competitions like the one you want to enter. Read everything they have to say. Finally, draw up a design that you can actually build. The coolest design won't do you any good if you don't have the skills (or money) to build it.

The words 'best' and 'cheap' don't go together in robot building -- especially with speed controllers. You don't want cheap parts that will put you out of the competition if they fail. See what other builders that have been successful use in their 'bots. It won't be cheap speed controllers! For reliable parts, try Team Delta (www.teamdelta.com), and the marketplace at www.robotcombat.com.