shimmy on smooth pavement -- driving me nuts

Here's a riddle for you: My car is a '03 C2 with the Pilot Sports. ~5k miles. I'm experiencing a strange vibration from ~50-60mph. It's worse on smooth/new pavement, and nearly absent on rough pavement. It feels almost like it's a side/side shake, rather than "up/down". Hard to describe. I feel it in my cheeks and eyeballs - same shake with clutch in/out - same shake on/off power - I'm pretty convinced it's coming from the rear of the car, since I can't feel it in the steering wheel. - I've tried rear tire pressures from 36-44. Shake seems slightly worse at lower tire pressures. - tires "look round" -- no obvious signs of belt problems. - The problem got worse when I switched from SportDesigns to Carreras (same tires, swapped between rims). But, the problem was there, to a lesser extent w/the OE wheels. Here's what I've done - I've had a very good local tire shop balance the wheels twice, with no difference. - I put the spare on each of the rear corners, with little difference - the steering wheel doesn't shake No, I haven't taken it to the dealer yet. I view them as a sometimes necessary evil Any ideas? As a last resort, I'm going to throw some WinterSport M2s on my SportDesigns this week (my upcomign winter setup) to see if it makes a difference. My guesses are: - somehow both rear tires have belt damage that itn't visible to the eye? - the rear tires are cupped? Thanks in advance, Scott.

Had the same symptoms, which only got worse, until it got noisy. At that point I got pissed and curious and took the rear wheels off to do a close inspection. The rear alignment was off and by the time I looked at it the tires were ruined. I replaced the tires, got the alignment done and all has been well since then. BTE the conti's get very noisy toward the end of life.

The Carrera wheels are lighter than the Sport Designs, so I guess that's why you are feeling it more with the Carreras. My guess is that although invisible to the naked eye, your tires are damaged and need replacing. Until then, I would not speed or drive "spiritedly".

Gretch, when you say your rear alignment was off, and you could tell this by inspection (with the tire off), what exactly did you see? How was your alignment off? Too much toe? Thanks, Scott.

Leon, doesn't it seem strange that both rear tires would be "damaged"? (I could still feel it, with the spare in either rear corner).

What I found upon inspection was the inside shoulders of both tires were gone................right down to the steel belts (in a very short period of time). Bad cupping from the inside edge to the middle of BOTH tires. So bad in fact I had to swap the wheels off of my 928 to get it (my C4) to the dealer. BTW, I could not observe this just by looking at the car from the rear. I was astounded at what I found when I pulled the tires.....and of course the factory alignment is NOT warranteed, so I had to eat the tires and the alignment.