Vibration at a specific speed is a harmonic related problem. Doubtful your thrust arm bushings are going to fail just when you change rims. Bigger rims are not going to give you a way smoother ride, just the opposite, so if your ride is now way smoother, then your other wheels must have been in really bad shape.

You can balance a square tire, it's just not going to run smoothly. I'd try a Hunter 9700 balance. Just ask others on this board what a 9700 balance did for their shakes (and this after they had high speed balances done as well). The Hunter may even find that one of your new tires has belt separation which can also cause the problem.

Don

i just bought a pair of 18x8.5's new ///M5 rims for my 92- 525i and its a way smoother ride and all but for some reason it shakes at 55 exactly, any more or less it dont shake. what i mean by shakin is the complete steering wheel and you can also feel it on floor, i bought the rims and tires brand new there high speed balanced, i balanced them 2 times due to the fact i thought that was the problem but i guess not, my wheel alignment is good, any answers to this problem?? i got them a week ago and the more i drive it the less it shakes maybe they need to be broken in? i really dont know, answers if any please.
(Will post picture as soon as it developes)
Thank You:
Alex