Simplest, I imagine, would be to have some sort of friction system ( e . g . a small, conventional axle / hub system that pressed down on the top of the spherical tire, supporting the weight of the car and applying torque to the wheel in a wide range of directions ) . for something more science fictionish, I imagine you could invent a mag-lev system ( maybe a spherical hub inside the spherical tire that contained a superconducting magnet ? ) . propulsion in this case would be trickier-I can visualize a system for rotating the wheels based on things like maglev trains or rail guns, but it would basically involve turning the entire wheel-well into a complex electromagnetic control system, and I can't imagine it would be practical or efficient.
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