[ubermeister]

The formula for force of friction is sum(mu*F_n), where mu is the coefficient of friction for each wheel (i hear 1.3 for the FIRST sticky wheels) and F_n is the normal force (weight) on each wheel. Surface area (i.e. number of wheels) does not come into play. If all your wheels are identical, mu=1.3, and your robot weighs 150 lbs, the force of friction is 195 lbs. This means you can accelerate at up to 195 pounds force / 150 pounds mass = 41 ft/sec^2 before your wheels start to slip.