The 2-ball (snicker) system is way better than our original idea. More stability, and just as maneuverable. Here's what Andy said was the main disadvantage of the drive:
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The main disadvantage is what I will try to describe as a "second level of traction loss". Consider a wheeled-driven robot. If the robot is pushing against an immovable object, then the wheels will most likely spin (the wheel torque results in a tangental wheel force that is lower than the robot weight x coefficient of friction, but that is a lesson for another day). When these wheels spin, the robot has lost its ability to move because of a loss of wheel traction with the carpet. In a ball drive system, there are two points of contact and EITHER could lose their traction. The small wheel can lose its traction with the ball, and the ball could lose its traction with the floor. The opportunity for traction loss in two places results increased efficiency losses in this style of drive system.
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I think the wheel-to-ball traction loss can probably be remedied by using larger rubber wheels, which would provide a bigger contact area between the wheel and the ball, while still only giving the ball reasonably small resistance while rotating around the point of contact (the other wheel turning would rotate the wheel along this axis). The ball-to-ground problem could be fixed by using a tread pattern on the ball, which is simple: it only involves epoxying mountain-bike-like-tread-patterned rubber sections instead of smooth ones. Comments?