Hey Bob, you are showing your age... I believe that post you are referring to was closer to two years ago! My calculation did show that 3/8" diameter steel would be the minimum "safe" design for a full size wheel. However most of the ones I've come across have been around 1/2" to 3/4" diameter. Good memory, BTW.

Eightbit - A couple of reasons, which go hand-in-hand based on my experience with arcade steering wheels.
1. The shaft diameter would need to be larger, thus not lending itself to a very good free spin unless you had a much larger diameter flywheel.
2. Arcade steering wheels didn't have separate flywheels. The steering wheel itself was the flywheel, with a good portion of the weight on the circumference. If you take the steering wheel off, the unit itself spun very poorly. Once I put a spinner knob on a steering wheel just to see how it would work (I do lots of crazy stuff like this...), and the spinner "action" just wasn't there.
In short, large shaft diameters (greater than 1/4", I mean) combined with lack of a flywheel do not make good spinners. They make great steering wheels, though!