Our robot was unique this year because it was nothing *but* a drivetrain. Obviously, that helped us keep a low center of gravity.

Our ground clearance was approximately 1/8" - despite that, another robot managed to lift us off our wheels at Nationals :(

The motors were mounted roughly midway between the four drive wheels. Our treads occupied the entire footprint of the robot, practically. They were all driven by a common axle. The battery was actually the highest part of our robot, directly above the motors. It was, however, only 15" or so off the floor.

Yeah, our clearance was was ~ 1/2" and we managed to ride up on a row of balls on the Einstein field during one of our elim. matches at Nationals this year ... at one point during the match, we only had one wheel touching the ground! :(

- Katie

We had a lot of ground clearance with our 10" wheels which was probably a mistake. When I was driving I rode up completely on top of the soccer balls. If I didn't throw it in reverse, the robot would have gone over the side guard rail of the field. (that would have been sooo funny :D lol)

a real drive train
 

Frostbite II
specs 95 lbs
2 drill
2 chipua
4 custom gear boxes only 1" wide, fixed speed
2.5 mph
tank drive
4 6" sold aluminum wheels
bumpers....ha.... a protective rim of 28mm extrusion
2" ground clearance

We were never pushed, this was our second atempt at a four wheel drive or, the second in the frostbite series.

Frostbite I
specs 65 lbs
2 drill
2 fisher price
Custom gearboxes for the drills
Tanks drive
4 specially milled nylon wheels
2.5 mph
28mm extrusion bumper protection
2" ground clearance

the drive trains of the Penguins

Weight distribution is very important for the wheels to have any grab. Trying to concentrate robot weight over the driven wheels will insure you get maximum grip for the matierial you use. We have used a special belting from Small Parts that is brown and has a lot of teeth for gripping carpet. As with any matierial, any drive sytem that causes the wheels to slip on the carpet will eventually wear the surface of the drive wheels. So was the case of the brown belting we used.
As to robot design, I saw many robots in the past that had little ground clearance and a robot frame that extended well outside the driven wheels. Whenever the robot tipped a little, the frame would contact the floor and the wheels would be lifted off the carpet. In this case it doesn't matter very much where the weight is, no contact means no drive power.
Low center of gravity is a must to keep all the wheels on the floor and maximize stability and drive power transfer.