new to me 2003 stock suspension, super sport tires with 50% tread left - had wheels rebalanced but still have a fair amount of vibration on the steering wheel and I can feel in in the seat as well. It it varies with speed and pavement quality. On smooth asphalt it is not noticible but on groved concrete it's pretty bad but goes away over 90 mph
any suggestion on what to look for that could cause this?
thanks in advance

 

may I suggest you get new tires all around.
had the same experience with my 996TT. Visually the tire treads appeared fine, even measured
~5/32 tread depth.

Put new tires on........problem disappeared !

Tony

 

grooved concrete is about the lousiest surface to drive on and attempt to determine the source of wheel vibration. its awful stuff. find some fresh blacktop.

i think you said it best when you said: "On smooth asphalt it is not noticeable"

 

Thanks - I will swap out the tires and hope that does it
tjeu are really loud but still have really good traction

 

Make sure the tires are properly road force balanced. My car is very susceptible to poor balance, so I have to make sure whoever is balancing gets the tires to under 15Lbs on the road force balancing machine. I had to go through 3 shops until I found somebody who did it right. Sigh...

 

Be sure the tire pressures are correct. Over inflated tires can produce a vibration that is indistinguishable from that arising from tire imbalance.

Do the tread surfaces show any signs of scrubbing/cupping or other abnormal wear signs? If so an alignment could be called for.

I run the tires on my 996 Turbo (and Boxster too) right down to the wear bars without real issues other than the tires get noisy as heck and the rears are more inclined to pick something up and get flat.

Note as new tires wear they can go out of balance. This is unfortunately "normal" for big old wide tires as the tread wears down heavy or heavier areas wear away and the tire's balance changes. More than once when I replace the rear tires on either of my cars -- after around 20K miles of service -- the fronts need rebalancing to address the out of balance condition that has developed as the front tires have worn down some.

Might add that often the rear tires develop a bit of vibration too as they wear down but this doesn't seem to be as much a problem maybe because the weight over the rear tires. However, my day to day usage seldom has the car at a speed that makes the rear tire imbalance a real problem. But if I am venturing out on a long road trip where the car will spend hours a day at high speed I generally do this on fresh and balanced tires.

 

What are your tire sizes? You get to far out of spec and they will cause front diff lockup and shutter.

A sure fire way to test this is if you drive in the rain and its worse then in the dry.

 

Update
I replaced all 4 of the tires and it is much improved but not perfect - I will take it to get road forced balanced and 4 wheel alignment.
I would now say that the vibration is very minor but I don't want any at all.
Over all the car handles really, really good and the suspension feels great.

 

i had tru line do the road force balancing and four wheel alignment and it now drives very smooth and all of the vibration is gone - it feels great