At last I have got round to doing it. Some of this is by Mat Irvine who was the technical consultant for Robot Wars , with other stuff from my first hand experience, observations and complaints from other teams.
First things to consider (red being what I consider most important):
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1) Decide what weight class to enter, bearing in mind that most of the action is middle/heavy. Also what background in RC/engineering you have had, as obviously Rolls Royce would do well (ok bad example), but Dartford Girls School (Detonator) I think chose a bad weight class to be in. When you have decided what weight class your aiming for take 10% off (and some more money than allotted) for 'building allowance' as numerous bits and pieces will have to be included that aren't included in your inventory such as nuts and bolts which can add up to a substantial amount.
2) Get the biggest most powerful motors you can, overkill is not an option and then get speed controllers to drive them. This may sound arrogant but after talking to people they all said "I wish I had bigger motors and better speed controllers". If anything is going to break it will be the speed controllers and motors, and I can mention lots of cases such as Shogun and Nemesis with speedo's and Dreadnaut with motors.
Featherweight robots usually employ one of two options, either RC car chassis (Demolisher, SAT'arn, Krayzee Tokyo) or a specifically built chassis (Cunning Plan, VOA, Bugs). All of these used electric's but an IC (Internal Combustion) could be used, but I will mention IC's later.
The problem with RC chassis are that they aren't designed for big heavy bodyshell's and that was why SAT'arn broke.
Lightweight
Featherweights with weapons! The more weight can be put into bigger motors, larger motors, better armour and weaponry.
Middleweights
At least a big weapon by this weight with big motors and more power. Normal RC stuff is almost useless at this stage as servo's need to be big and speed controllers expensive. To be honest I wouldn't build a robot this heavy or bigger if your on your own or have limited engineering knowledge.
Heavyweights
Big boys toys. 72.3Kg (170lbs) of metal hitting other robots at 10mph+. More weapons…
Wheels are the first option and are the easiest to use and easily brought from the use of RC car/aircraft wheels in F to go-cart/caravan wheels in H. Tracks are another option and provide more grip but they can be hit sideways and slip off, therefore I would make a mock-up and check first. As for legged entries (+other) I would avoid them as the maneuverability problem is a major one.
Everything is bolted to this so it has to be strong, yet light (straight of the Stella Artois? advert). The featherweights could use RC car chassis but due to experience I would avoid these. Plywood was used for F+L as it provides some degree of impact absorption and is fairly light. For M+H you are looking at steel/aluminium welded frames. Stability is important as the closer you can get the weight to the ground the better, although your robot might be able to work upside down like TRACIE. The bodyshell can be an integral part of the chassis rather than a solid take off part.
If your competing in the F category next year plasticine is a very good impact material. It was worth a try wasn't it?
As just mentioned the bodyshell can be part of the chassis like Barry and Reali-t or separate like Bodyhammer and Cunning Plan. Each has it's advantages but make sure you can get to the bits in side easily, as when it starts to smoke you will want the cover off asap.
Materials are easily available from impact resistant, saw loving (in order of preference) wood, glass fiber, carbon fiber and kevlar. But for saws and grinders metal is preferable from the light aluminium to steel to the expensive titanium, also alloys. Plastics are also good from the impact resistant polypropylene to the bulletproof lexan. (They are also clear and very good if you want to pose).
The stating place of many robots. "Mum I want a chainsaw for my robot", "Ask your father when he gets home".
I am a bit fan of using spikes but that's probably due to my 3rd year engineering project being based on impacting metals with pointy objects. If your robot has enough speed and maneuverability it should easily be able to put a good dent in an opposing robot like Reali-t against Bodyhammer or a spike on an arm such as Killertron.
Rotary weapons are always a favorite and look good when sparks are flying everywhere. Make sure the disc is rotating so the enemy are forced up, otherwise your robot is lifted off the ground. Angle grinders are deadly if used right with a thick disc, and not like the BBC's thin ones that shattered every time they hit something. All rotary weapons are good against fibrous materials and metal, you can even get metal cutting chainsaws!
A less spectacular but viable alternative is a lifting device such as used on Biohazard in America.
If anything will go wrong it will be in this section, and 99/100 it will be motors or speed controllers.
Firstly choose a motor. Wheelchair, C5 and industrial motors are all viable for the bigger robots, with drill motors used on the F+L. I would stay away from the 'hobby' motors.
After you have a motor it might include a specs sheet saying it needs 50A continuous and 100A start. Say two 12V motors starting and stopping all the time at 70A, and an electric saw at 50A all the time. That's 190A, which is quite a lot and needs very hefty wiring and a very big speed controller. Be warned a speed controller rated at 100A continuous would blow up, always go for the best one you can afford and have a spare, the same goes for a motor. 190A for 5 mins (length of a round) suggests multiple gel cell batteries, but again be warned as a 10Ah battery doesn't give 10A for a hour, more like 42 mins. Guestimating that means your looking at a 23Ah battery (190/(42/5)), which is 2*15Ah batteries in series (to be safe). But this much current is a serious problem so if you can keep one battery to each wheel if it's independently driven.
Voltage is also a problem as there is a limit for both dc and ac, but most big motors are either 12 or 24V which are within the limits.
To drive the message home try and buy overrated speed controllers and don't build your own as they will break!.
Not strictly electronic but pneumatic and hydraulic motors are available.
I have briefly touched on this but the first decision is car steering or independently driven wheels which provides a more expensive maneuverable robot.
I would personally chose the independently driven wheels/tracks which provide unrivaled control and when a mixer is used it can be driven like a car rather than a tank. A problem is that two motors and speed controllers are used instead of one which raises the cost.
Since motors spin at 10,000rpm a gearbox is needed which in the case of F+L is ideal as electric drill motor gearboxes are ideal . For bigger robots wheelchair gearboxes or simple chain reduction techniques can be used. Toothed belts can also be used as a good one would be very impact resistant, but can be prone to tearing if too much torque is applied.
Suspension could also be used in both a suspension orientated role or a getting hit with something role.
Try to make your robot as maneuverable as possible as when something's attacking the back of your robot your in a bit of trouble. Speed is also important but too fast and or in more trouble than if it's too slow.
RC gear is an expense you can't get round. Avoid AM like the plague as in a TV studio there is a LOT of interference. FM is preferable and if you have loads of money go for PCM (Pulse Code Modulation). Also have at least 2 crystal sets as everyone else will be using your frequency, it happened to most people. 2CH Tx (transmitters) can be used but a 4CH is preferable for upgrade (and posing). More expensive controllers even have inbuilt mixing functions.