[Pat Roche]

One thing to take into consideration when looking at shaft selection is the support of the shaft. If your shaft-wheel is out there with no protection you'd better beef it right up. When you use a "protected" shaft(has two bearings/bushings/etc. on the two opposite sides of the wheel) you have to consider two things, the lenght of the shaft and the weight of the robot. The shorter the shaft the less flexx theyre will be and the smaller diameter you can get away. The longer that shaft gets the larger diameter you need to use to prevent the shaft from flexing/bending.

Another thing to consider is the material that the shaft is made out of. Aluminum would need to be a considerably thicker shaft than a steel shaft.

-Hope thats helpful and good luck
Pat