So what you're saying, if I understand it correctly, is that the mating surface of the inside of the wheel that contacts the hub must be the same size as the hub mating surface, or you get shimmy. Bloody L. I'd never have thought such a thing could make a difference, and in fact it's not even something I'd ever considered that could affect...er....anything!

I'm trying to work out why this would happen, but I'm coming up blank. But it must be a known issue if you can get something to eliminate it! Congrats to you both for sussing it! - although it seems to have taken you 7 years