after a drive on the freeway, the car shakes heavily at 75 mph all the way to 100. is this a bad balance on the tires? or am i looking at ball joints????

 

Your problem is most likely from the tires being out of round or out of balance. Before you have them balanced put the car up on jack stands and check the tires for being out of round. The tires may have flat spots from sitting or from hard braking. Let the car sit overnight and check them with a dial indicator. If they are out by .040" (1mm) or more then just balancing will not solve the problem. A real good tire shop that deals with fixing tires for racing cars can true up the tires. I first try to get rid of the flat spots by driving the car. I set the pressure at 20 psi and drive the car for 30 to 50 miles and then put the pressure back to 36 psi and then put it on the jack stands over night and then measure. Now be careful if you try the 20 psi. trick. Remember all the upside down Ford Explorers.

 

Ern, the front tires were just RECENTLY replaced by the dealer i bought the car from 2 days ago. Is this a balancing issue still????

 

If your steering wheel shakes, the front tires are out of balance. If the car (body, seats, etc.) shakes, it comes from the rear. I've had these problems to a more or lesser extend on all three 928s I've owned so far. Recently somebody recommended looking for a tire shop that has a "Hunter's GSP9700 Vibration Control System" - a sophisticated tire balancing machine. Go to the following website to find someone in your area: <a href="http://www.gsp9700.com/" target="_blank">http://www.gsp9700.com/</a>.

I have not tried this, yet, as I am waiting for my front tires to wear off completely. I also found that I experience less vibrations if I keep the tire pressure up to spec, or (better) a little above.

Still, you can't completely rule out other moving parts. Hopefully Dr. Bob reads this post - I'm sure he can give you more details. Or look in the archives for my thread on this issue.

 

Nicole you just exactly described the problem i have!!!! so it is definitely tires then i take it?????

I appologize if I seem like I'm trippin' but I like to see my cars in perfection and well maintained and i just wanna know what is wrong before i go about spending the money to repair and replace.

 

I had just recently looked up something from a past article. It is said that all 928s with AFTERMARKET rims have this problem in time. I have 17" C2 Turbos on my 928 S4. The article on rennlist is from your previous post Nicole. And it looks like it says that bent rims are the culprit. I think I may have a bent rim. The wheels are definitely not original Porsche. They are by FRH!!!! Which is probably why i have the same bad shaking at 85 just like you have!

 

EuroSpec -

I have had irritating wheel-balance and vibration issues on my S4 since I bought it. It has the "manhole cover" wheels still (not for long), and whenever I put new tires on, or even take off the wheels, it changes the vibration for better or worse. I understand that you MAY have a bent rim, but do the jack-up and role test, buut ALSO, maybe just take the wheel off and put it back on in a different way - you know - turn it a bit, and put different holes over different studs. It irritating, and normal balance machines only dirve the wheel at somehwhere around 60mph or less? I've never had a vibration below 80!
I was going to look for an "on the car" wheel balance in SoCal, but I never have been successfull. There is supposed to be one in LA though.

 

First you have to be sure the tires are round. Rims bend pretty easily on the inside. They are strong on the outside but the wide rims do not have much strength on the inside. You can run the dial indicator on the inside of the rim with the tires on to get an idea if the rims are bent. If you don't have a dial indicator, one with magnetic stand can be purchased rather cheaply from Harbor Fright. The last time I had new tires put on the car, I brought the rims home and checked them before the new tires were installed. Two wheels were way out of spec. Porsche suspension is not very forgiving for hiding shake induced from the wheels. Put you two roundest tires on the front until you are sure what the problem is. You can get an idea if they are round by just spinning them and setting something heavy up near the wheel and looking closely with a bright light, but nothing beats having the right tool for the job. Most tire shops do not pay much attention to the rim being bent or the tire being round. They balance and send you on your way. The Porsche spec is "Max permissible lateral aand radial runout on aluminum rims = 1.0mm (.040")".

A number of comments have been made about Germany does it better. My answer to that is you get what you pay for. In Germany the speeds on the Autobahn are 10 to 20 mph faster that our freeway speeds. The surface is incredibly smooth and any problem in the tires is very noticeable. The mechanics are good and people pay the price. You can run a car in the US for about half the price of Germany. We kind of have a throwaway mentality and are not willing to spend so much but quickly discard and move on. It looks like the world follows our trends. I got a kick out of the front load washer discussion. Bendix invented the automatic washing machine in the
good Old USA and it was a front load machine. Boy, now I am showing my age! Front load machines have always been available in the USA but the consumer choice has been a top loader. Once again, you get what you pay for, but an American made washer is an unbelievable bargain considering the performance at the price point.

 

I was told that every car's suspension has a point where it tends to shake the most. That has to do with the rotation speed of the wheels matching the resonance frequency of the springs. On the 928 that speed seems to be around 85mph, but maybe the progressive springs make it a much wider range? Any physics experts here? Dr. Bob?

 

Yes, the problem is most likely out of balance, or , out of round condition on your tires and/or wheels.

That having been said, there are several other pieces that can cause vibrational issues...

If your front rotors are warped, they will eventually cause vibration at speed even without application of the brakes.

If your wheel bearings are out of adjustment or worn out, you can get a vibration that gets worse with speed.

If your shocks are worn/blown, you will develop a vibration that gets worse with speed.

If your steering rack bushings are worn/failed, you will develop a vibration at speed.

Any of the above items being out of order will take a minor imperfection in wheel balance/tire roundness/the road, and turn it into a vibration that you notice.

Always start with the cheap and easy, then go to the harder and more expensive.

Greg

 

I would suggest that you put the front tires on the back and the back tires on the front and see IF it changes...Then go from there.....

 

NO!!!! DON'T DO WHAT OLIAS SUGGESTS!

You have an S4, with larger rear than front tires - PLUS you have upgraded to larger than standard wheels.

The tire shop mixed up front and rear on my first 928 S4 (which had standard tires), and I did not realize it right away. I hit a bump on the way home, and the wheel hit the lip of the front fender. It scratched off all paint and underbody protection down to the metal. Worse could have happened...

 

EXCUSE ME

 

Olias: Don't take it personal, please! I just wanted to make sure what happened to me won't happen to him. It would be no problem on your car, if it has the standard wheels and tires.

 

Nicole I took a look under the rear today....

GET THIS!

The driver's side rear tire is blistered and SCALING on the inside of it. The tire pressure is low and there is a major and critical flat spot on it where it sits. The car is getting 2 new rear tires immediately tomorrow, Kuhmo Ecsta Supras, like those that are on the front.

Thank you everyone for your replies! They have been most helpful!