Pat,
I'd be happy to explain:
When the wheel is in the air.
The knock off is attached to the hub and the wheel; the wheel is attached to nothing (air) so the impulse (of the hammer blow) must look for another avenue to be reacted--the hub.
The hub is attached to the bearings it spins on; the bearings are attached to the upright which is attached to the car; the car is attached to the ground through a jack or a jack stand (remember, the car is in the air). Something must react the force of you hitting the knock off, so the bearings take a beating. I have actually seen bearing races imprinted with the roller/ball bearings.
When the wheel is on the ground.
Again, the knock off is attached to the hub and the wheel; the wheel is resting on the ground which reacts the impulse. (The hub is also attached to the wing nut which in turn is attached to the bearings. The bearings still react some of the impulse, but not nearly as much as when the wheel is not sitting on the ground.)
I hope this helps and I didn't get too far off track.
David
