[andreyf]

Wheels with an odd number have less variance in how much weight they can support along their edge. An even number of spokes makes a wheel with "weaker" points midway between the spokes, where the wheel won't hold as much weight.

[jonp]

Thanks. For some reason I was thinking of old-fashioned wooden wheels rather than modern car wheels. I guess someone figured out the physics at some point and there was no looking back.