There is no real regulation on the number of wheels we can use. All it says is we can only use the rover wheels. FIRST gives us 4 of them in the Kit, but you can easily purchase more from AndyMark. I'd say you can definitly use more than 4 wheels (At least I'd HOPE! thats what my team's doing!
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As for if anything else can touch the floor, the rules are QUITE clear... NO! That's SO agrivating!! the wheels are the only thing from your robot that can come in contact with the game floor... so no other wheels/ arms or anything can touch.