[Al Skierkiewicz]

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.