You may want to have a look at the following thread started by my good friend Allen Gregory of Team 3847, especially post #16 where he describes how to make a 1:1 VersaPlanetary gearbox.
www.chiefdelphi.com/forums/showthread.php?t=111360&highlight=small+wheel+shoo ter
The gearbox will take the side loads off the motor shaft preventing premature wear/failure. We originally had a single wheel mounted directly on the shaft of a Mini-CIM. After about 20-30 shots (some with extreme compression), the rear bearing of the motor was worn into an oval and the motor armature dragged on the inside of the magnets. Since we did not want to wait for another motor, we ended up changing to RS550's with a CIM-ULATOR gearbox that I had left over from the previous year. The CIM-ULATOR also took the side loads off the motors.