How does one tell if shocks are to blame for bad vibrations at speed above 40 mph in

[tshih]

My wife's 993 was driving fine and developed a severe vibration at speeds over 60 mph after returning from a DE at the Poconos 2 weeks ago. The history of this car was that it came with 18" after market wheels and tires which were replaced with OEM 17" Cup wheels and tires. The ride improved and the car had 45000 miles on the 18" wheels and 3000 miles on the 17" wheels.

The ride up to the DE and during the DE was normal but on the way back the vibration got pretty severe (car felt like it was pitching front to back and side to side). Do shocks that are worn behave like that and is there a way to check their condition while in the car? Do they go bad all of a sudden like in this case or is there another explanation?

The steering wheel is tracking straight at speed on a flat road surface but the vibration is present.

I rebalanced the fronts but the symptoms got worse (car vibrates at lower speeds (40+mph). Was told to check the front control arms' bushings with a large prybar. (anyone can give me detailed instructions on how to pry to look at the bushing condiitions?

[tshih]

UPDATE !

Prroblem with vibration was actually due to bad rear left tire. My very talented new Porsche technician came up with the seat of the pants diagnosis in less than 500 ft of test drive. The tire (sumitomo HTRZ 255/40/17 threw a belt which was deforming the tire near the center of the thread area. I didn't see the bad lump when I was focused on the front tires and wheels.

[Jbabic]

Yes, thats what I was gonna say, just from reading your post.

[aigel]

Well, glad you like your mechanic, but that's a pretty simple thing to diagnose! It is easy to say, but I would have pointed you to a rotating object (tire or wheel) as well.

George