So I've posted recently how I'v been getting a very distinct annoying vibration at highway speeds in my 944. It is definatly coming from the rear end, no steering wheel vibration. With bald tires in the rear i figured that could be it, maybe off balance or alignment. So i got a full alignment and new tires balanced. It helped the problem, the car used to vibrate at about 60 mph steadily getting worse. But it didn't resolve the problem completely... the car still vibrates pretty violently at 70+. What could this be?

also forgot to mention i checked the rear suspension when the car was on the lift, I'm not too familiar with suspensions etc.. but nothing felt loose or anything or out of the ordinary.

 

Bad shocks would be my guess. How old are they?

 

bad shocks, bad CV joints, wheel bearings shot, loose axle nut (the big one), your dog still tied to the bumper...?

 

Most tire shops balance the wheels using a centering cone
in the hole in the middle of the wheel: THIS DOES NOT WORK most of the time.
The wheel must be mounted on the balancer using the lug mounting holes!
You may have to pay extra to have this done as it takes more time. It is worth it.
FWIW, bad shocks will not cause a vibration.

 

Thats news to me, I use centering cones every time and no issues yet.

What kind of tires are you running? You could have a bent rim? You need to have the rear wheels road force balenced.

 

+1 for typical tire shop balancing.
They usually just balance ok and don't really tell you if they think there is a hop in the wheel and im not sure their machines balance to high speeds. They will basically tell you that 70 is the speed limit so they wont guarantee a balance above that speed anyway

 

I kind of agree with this, you dont check for out of round(bent rim) when balencing tires unless you stand there and watch every tire, I am normally mounting and balencing at the same time so i cant, if the complaint is a vibration then i will check them.

Mark

 

what should i start with, cheapest on

 

Shock are easy to determine if they are bad. If they are original I'd probably assume they are not what they should be.

Push down on the rear bumper. If it rebounds more than once they are most likely bad.

CV joints can be checked out by visual inspection.

 

The rear shocks are fine, I had new ones put on a few months ago. I'll take a closer look at the CV joints when i get a chance, what exactly should I be looking for..The boots seemed to be fine and I couldn't feel any play.

 

Bad CV joints will cause a vibration under power but will go away when coasting.

 

It still vibrates at highway speeds even when I take it out of gear, or just depress the clutch. So I guess thats scratched... One thing I am worried about is a bent rim, I hit some nasty potholes on my cross country drive a few months ago, and this problem started near the tail end of the trip.

 

A bent rim was causing vibration in my car at about 70 mph. It's cheap to have it checked at a tire shop. If that checks out then have them check the balance on the rear wheels.

 

I would guess shocks as well, What brand shocks do you use?
John_AZ

 

The vibration is speed dependent, and the drivetrain has been ruled out because he said when he pressess in the clutch its still there. (unless it is in the diff-unlikely) that inlcudes drive shafts. I havent ever seen a wheel bearing cause a vibration like this. It will end up being a bent wheel or a tire out of balence. Shocks would not give a vibration, they would just give a bouncy ride.

My .02

Mark