All three 928s I have owned so far had a vibration in the body at speeds around 85mph.

Since this vibration seems purely speed dependent and is not in the steering wheel, I strongly believe it must have to do with wheels, brake discs or axles - the main rotating parts in the back of the car.

I kind of rule out the torque tube, since the vibration is not engine speed related.

I've tried rebalancing the tires - even on the car - and in one case had the rear rotors replaced, but never had any real and lasting success.

Someone once told me this might have to do with the grease inside the rear axle boots becoming settled when the car is parked for prolonged times, but nobody else has ever confirmed this. If this was true, why wouldn't the grease redistribute in the boot when the car is driven over a couple hundred miles?

Or could the old shocks have in any way contributed to this? New ones were just installed - with no effect on the vibration, though. The rear tires are 17" RE730s with approximately 2500 miles on them.

I would like to know, if you guys have experienced similar vibrations, and if you found a cure that lasted. Thanks!

 

Nicole,

Bridgestone's are nice quality rubber, but unless they have been "shaved" before leaving the tire plant, tires CAN (not all the time, but can) be less than 100% round. This would not be felt at say 55mph, but north of 75mph or so, a slight vibration can be induced. (yeah guys, it is a bit of a stretch. but it can happen.) And one of the premiums you pay for on a P-Zero (and the like) is an extremely round tire, that stays extremely round at speed so as not to bounce and shred at 200mph...

(you have ABS, so I will rule out the obvious flat-spotting of tires as you come to a screeching halt from 80mph on the 280 curve at Serramonte...;-)

Also, as tires are used there can be issues with how the belts "bed in". (once again, a bit odd, but we're looking for minutae here)

Also, all wheel bearings being properly seated and in good shape can be an issue as well.

If you have had the wheels balanced "on car", this should correct for imbalances in the wheels, tires, and rotors.

If you have addressed the shocks recently, that is also off the list of things to look at.

You may also have failed transmission rubber mounts, though I'm not sure if this would produce only a high speed symptomology.

After that, worn suspension rubber and other parts in the suspension would be my line of attack. But look for something loose first - always go with the cheap and simple before replacing everything in sight.

HTH,

Greg

 

Another potential source may be weakened transmission mounts, allowing a resonance through the drive train at the speed you indicated.

 

Have you done a wheel alignment?

Also, It there any excessive wear indication on the inside tread of the front tires?

 

The car got a new set of shocks and was realigned this past Monday. The rear tires have 2500 miles on them.

The last alignment had been done 7k miles ago.

 

have you also checked for CV joint wear and excessive play in the half shafts.?

 

John:

No, I have never had the axles checked. How long should these rear axles usually last? All three cars were between 65k and 90k miles. Is there an easy way to tell how good or bad the rear axles are?

 

John or Anyone:

Do you repack grease into rear wheel bearings, and/or make any adjustments?

 

Nicole,
Years ago...
When the huge,heavy, battle cruisers of the late 50's through the very early 70's ruled Eisenhower's superhighways the speed limits were about 75mph. I can remember a vibration that was common to damn near everything that rolled. Depending on the car, it would start out as a sort of light, humming, vibration then work its way through an actual shimmy to include a noticeable -tactile & visual- shaking of the steering wheel. It usually, appeared at 65mph and above, got worse, then would disappear at around the high 70mph range. It would reappear as you slowed down thru that speed range, again.
Back then before the Big speed balancing machines of today they would do an alignment,jack the car up, then put what I call the spyder on -wheel clamps on each end of the spyder leg, 4 legs- stick an electric, dual roller tire spinner-up thing-a-ma-bob under the tire. The wheel/tire assyembly would then be spun-up to an outrageous speed and was balanced with a center of the spyder, adjustment ****.
Weights were determined and applied both inside and outside the wheel. Sometimes you would see a wheel with six or seven weight of varing size -something wrong-.
This type of spin balancing almost always cured the vibe/shimmy, was kind of neat because, wether by intent or not, the tire was mounted to the wheel and then the whole thing was torqued to the hub so EVERYTHING that spun on that wheel assembly was together just as it would be rolling down the highway.
Unfortunately there were two MAJOR drawbacks: 1.) Time intensive. 2.) On occasion the spyder would come loose at 70/80/90... and get launched across the shop trashing itself and anything that it met during the journey.
Failing resolution using any of the above posts Snow posted a while back some oddities in the tie rod/camber adjustment. I believe he recounted his observations in the recent tie-rod posts.
Good luck, HTH
Keep em' rolling
John S. & Pattycakes

 

Nicole,
I know you said your vibration was not engine dependant but lets make sure.

Engine: running transmission in neutral or park rev the engine to 2500 RPMS. If there is no vibration we can for certain eliminate the engine, front flex plate, torque tube drive shaft, rear flex plate, and torque converter.

Potential source of vibration: transmission and mounts, differential, axles, rotors, tires, rims, or exhaust system. If the rotors are out of balance you will feel a pulsation in the brake pedal. Is their any noise or clang associated with the vibration when you drive the car? Or when you engage drive or reverse from a dead stop? If not we can rule out the transmission and differential. Maybe even the axles. The reason why I say maybe is because the axles could be worn and not produce any noise.

To eliminate the rims and tires as a source of vibration, switch the rear tires to the front.

Check the exhaust system and see if any parts are rubbing against the body of the car.

Check the transmission mounts. The distance between the differential and the top of the car is not great.

As to the grease in the axles settling in one side, no way. The spinning action of the axle alone would distribute the grease. But don’t rule them out, they can still be a source of your vibration. Re-torque your axle nuts. Three 928s with the same vibration, maybe its just a characteristic of a 928, or maybe its just exhaust resonance throughout the car.
Does acceleration and deceleration change the pitch of the vibration?

If everything checks out then change the axles.
Good luck,
Steve C

 

Use this with caution!

One possible approach:

1) Put the nose of the car against something immovable.

2) Jack the car using a good floor jack on the center crossmember under the transaxle. Put two good quality, heavy duty jackstands under the jackpoints in front of the rear wheelwells. Lower the car until the weight is shared among the two jackstands and the jack. The rear wheels should be at least six inches off the floor, and eight is better.

3) Cautiously run it up to the speed where you normally get the vibrations. Back off if the vibrations get severe. Try to determine what is causing the vibrations.

You can remove the rear wheels to eliminate them - I would put the lug nuts back on to ensure that the brake rotors stay tight. Don't break the ends out of the lug nuts!

 

Hey Wally!

I was just searching for an old post about a tip Joe Rausa gave me back in april when i was trying to isolate my rear end vibrations at speed. You pretty much summed it up.

I jacked the car up just like you mentioned and I had my wife work the throttle while i checked for up/down wobbling axle shafts. (i had none)

Joe also said check for side/side and up/down play (when car is turned off, again i had none) and that a smidge of in/out play is ok.

My vibrations turned out to be bad tire balance (1/2 to 3/4 oz off all wheel)

That elminated alot of vibrations. I still have some, but i think in my old bugger its normal since i got the solid motor mounts and the vibration feels like a big ole v8 doing its thing.

 

Hi Nicole:

I've danced this dance on a few hi performance cars in my past, and in each case but one it came down to wheel & tire balancing. The quality of the balancing equipment is very important, and the quality and skill of the operator is even more important.

At my last wheel balance, I found a tech who was willing to discuss his machine settings, and how they relate to using stick-on weights inside the spokes on my slotted S-4 wheels. The modern machines allow you to specify the width and offset so it can calculate the correct weight to add, all based on placing hammer-on weights at the perimeter of the wheel lip. On the 16" S-4 wheels, the weight location on the inner face is more closely related to the diameter of a 14" wheel at the lip face. Further, the width and offset need to be re-calc'd to get it all to work on the machine. The width you plug in needs to be about 3" less than the actual wheel lip spacing. The 7" fronts have aout 7.5" center-to-center spacing on the lips, and you will be moving the inner weights in about an inch, and the outers in about 2". You can add pencil lines on the inner surfaces with the machine turned by hand, and measure to get the exact spacing between lines that will indicate the center of the weights when installed. Plug that value in, and then calculate the actual offset from the midpoint between those lines and the mounting face. Plug that in the machine too. Now, it will give you the correct weights and positions for the stick-ons.

Remember that fronts and rears are different. And because the vibration feels like it is coming from the front or rear only does not mean that much. Do all four wheels at once and cut down on possible problems.


After that, have the wheel balanced dynamically. Pull the wheel off and immediately drop it onto a static balancer as a final check. A static balancer is the one with the bubble level in the middle of the centering cone. It should be perfect. If not, it goes back on the dynamic machine for another try.


The auto shop at my local community college has a pretty fancy new balancer. It is supposedly available for trouble cars, but I haven't had the need for it yet.


By the way, that last/best wheel balance was done by a guy at the local Costco (!) tire center. I brought the wheels in to them, hadn't purchased the tires from them, but they did them for me for $4/wheel and listened to my ranting about the weight geometry. This was a hurry-up deal just before a road trip, intended to get rid of some of the same vibrations you are reporting. Car felt like a different beast after this treatment, on my well-used BFG's (24k on them at that point.) I did buy lunch for the balance guy and the manager the next time I was in, and get stellar service now on all my cars.


CV Joints hold a lot less grease than it would take to make a difference inside the boot.


CV joint wear is typically in the forward direction, and goes away when you let off the throttle at your critical speed. Some folks cheat the CV joint gods by swapping left and right joints so the thrust surfaces are on the back-up side. Anyway, to check all you need to do is run the car 'til it vibrates, add a few MPH to be sure, then back off the throttle and see if it smooths out. This was a problem on the one front-drive car I've owned, a Saab Turbo. The inners on that were the problem in the end.


Another thing to consider: If the balance is OK right after you have it done, but slowly deteriorates after that, look for tire creep on the rim. I add a spot of nail polish on the rubber by the valve, and check that once in a while to see how much creep there is. A heavy foot on either pedal risks this, and there isn't a good cure that I'm aware of. A lifetime balance deal might be the best bet if you find the tires are moving a lot.


Hope this helps!


dr bob

 

Tag team input from Wally P and Dr. Bob (welcome Bob) - their combined insight should have you vibration-free in no time!

Let us know what progress you're making Nicole.

 

Thanks so much for all this great information!

I have to tell you, though , that Wally's approach scares the hell out of me. Don't get me wrong - It does make perfect sense! But I really don't have the facilities to try that at home.

If things went wrong in the garage, I'd find the shark in the kitchen, probably topped by the water heater. And I have to admit that's not exactly my idea of a gourmet dinner... ;-)

Based on your input I will systematically check or recheck all the things you suggested, when the roads get dry again in the Bay Area.

Next will be to have the wheels rebalanced. I probably need new front tires in the next two or three months, so it would be a good idea to do it then. I will definitely take Dr. Bob's writeup to the tire dealer when I go (I had no idea wheel balancing is so sophisticated!).

If all this does not help, I will have to leave it to the mechanics. My limit in tinkering is usually whenever major mechanical or safety relevant components are involved - or if it gets seriously greasy... (yuck!) ;-)

In any case, I learned a lot of new stuff from you guys. I will let you know once we have figured it out!