[John Neun]

We are testing with a single shaft and two parallel Skyway wheelchair wheels 2.5 inches apart. The wheels squeeze the ball against a plate which can be moved to vary compression. You bring up a great point with friction, normal force, and slip. Our contact point is very short, so the ball has to be accelerated very quickly. We also improved performance by improving frictional characteristics of the wheel, but the performance change did not correspond with the normal force change, or the torque input versus ball range. I bring up the viscoelastic effect because the ball seems to return more of the energy used to compress it than I would have otherwise expected.

Thanks for all the comments!