Calculating Wheel Size

[Paul Copioli]

Pick the smallest wheel diameter the obstacles allow you to get away with. Smaller wheels allow for less gear ratio in the gearbox (to get the same speed/force characteristics). There are a few white papers on this. Ken Patton and I also did an FRC conference in Atlanta and we had some content on this very question.

With respect to wheel width, I have used 1/2", 1", and 1 1/2" for reasons other than traction (packaging, for the most part). We have used the same two tread types and our frictional force never changed between the wheel widths. For the most part, the coefficient of friction is not dependent on width of tires. For many other reasons tires on cars (or race cars) are wide, but coefficient of friction is not the primary. Drag race tires are really wide, why? The thickness of the tire walls is a major contributor, temperature rise is another. The other major contributor is torque transfer between the hub and the rubber part of the tire. In this case the tire pressure and the surface area are major contributors (as well as some other tricks) to decreasing the likelihood of the rubber separating from the hub.

For FIRST robots, the phenomena listed above just do not apply.

-Paul