Bill Ball

For over a year I have had a vibration that can shake the whole car. Characteristics:

1. Starts almost always at 76 MPH. Below 76, smooth as glass. Peaks at about 80-90 MPH.
2. Erratic and can smooth out at 120 or so. Yeah, I know - just stay above 120 and be happy.
3. Can be subtle or rather pronounced shaking.
4. High frequency, like a fast flutter.
5. Seen with 3 different sets of wheels and tires.
6. Not felt through the steering wheel - Tie rods and balljoints renewed a couple of years ago. Upper A-arms replaced a few months ago with rebuits.
7. Replaced the torque converter bearings, although the old ones were tight - no change.
9. Examined TT - bearings snug, right location, no noise, left TT alone.
8. Checked all suspension bushings and wheel bearings.
9. Swapped rear axles side-to-side - no change. Axles are rebuilts put on 5 years ago. Seem fine.
10. I don't feel any handling issues related to the vibration.

This vibration seemed to appear soon after I renewed the transmission mounts. The mounts are properly positioned relative to the crossmember per the WSM. [LATE EDIT: Actually I goofed this up - not sure that it matters. When I find out I'll edit again. See post 103 for details. Adjusting the tranny mount clearance correctly made no difference.]

It feels a lot like a rear wheel balance issues I have seen before. I did get new tires about the same time. I went back to the shop to have them check the balance and they found one rear wheel was off 3/4 ounce. I thought that was it, but there was no change and, as I note above, other sets of wheels and tires did nothing to change the vibration.

It does not appear to have gotten worse over time. I've driven probably 20K miles since it appeared.

I'm puzzled and not sure what to do or check next.

See post #113 for an update with the solution. [EDIT: Ends up, teh problem is still there, now over a year since this thread was started. I have NO IDEA what the cause is.

Landseer

Any chance its misfire? Spark breakdown on a cyl, or over/under fueling?

supercedar

Bill,

When I had my new wheels and tires mounted a couple months ago I felt a rumble in my seat. I went back to the shop the same day and had them check the rears and found one to be out of balance. This has happened on a couple occasions at different tire shops. They do not balance as well as they could it seems. One time they said it was the machine that must need to be recalulated.
Hope you find it.
Thomas

White Lightnin'

New CVJs or complete half-shafts versus rebuilds may help (?)

Reminder: just look at the burnout clip in your sig to see what they have been through.

IcemanG17

my out there thought.....did it do it with the supercharger installed? Or only with the stock setup? Why...I dunno.....

namasgt

Are you saying it could be the rear wheels because the shaking starts from the back?
Have you checked to see if one or both of the new mounts have come loose?
Could it be possible that one of the mounts installed is defective?

Nicole

I have that, too, BTW. But I did not realize I can make it go away above 120 mph...

They say the 928 is very sensitive to any kind of imbalance in the rear. But nobody has been able to tell me, yet, what it is that allows this vibration to happen.

I have a feeling this is started by something that rotates at a certain rpm, or bounces from uneven roads at a certain frequency, and then resonates somewhere in the suspension system.

But I'm probably crazy...

JG928S4

I agree w/Nicole, has to be something that is spinning. Rotor, Axle, Rim, Tire ?

--------
Resonance, in layperson's terms:

The off-centre mass in the tire can be imagined as a pendulum. Left to its own devices after an initial push, it'll swing back and forth at some frequency. If you give it a push at the right time each swing, it'll swing higher each time -- much like pushing someone on a swing. If you push at the wrong time you'll probably damp out the swinging. The natural swinging frequency of the pendulum is called the harmonic frequency.

Like the pendulum, the off-centre mass of the rotating object has a harmonic frequency. Driving at certain speeds will cause the wheel to rotate at a frequency close to the harmonic frequency, causing a resonant vibration. At other speeds the resonance does not occur because the harmonic frequency is not reinforced.

Bill Ball

Both. Supercharger is gone now, but I had it when the supercharger was on the car.

Bill Ball

Although it feels like wheel balance, I have no evidence of wheel or tires issues. I've tried 3 completely different sets, including an older set that were fine when taken off. Rotors spin true.

Axles could be it, but I can't demonstrate any looseness. I'm tired of throwing new parts at this when the old parts are good. I wasted my time doing the torque converter bearings. The old ones were literally like new.

I'll take the axles off again and take them to someone who should know if they are good or bad.

Bill Ball

If it were wheel balance or out of roundness, which it certainly feels like, it points to it being the rear wheels because I can't feel the shaking in the steering wheel.

What do you mean by "mounts"?

Bill Ball

This car runs like a bat out of hell, even though it no longer has the supercharger, just an x-pipe. I recently changed the plugs and fuel injectors when I removed and sold the supercharger. No change.

JG928S4

get on a dyno and get the wheels spinning at 76 ? During sharks in the parks get a few of us on the road on either side of you and watch the wheels at 76? Is there a way to safely get the car on jackstands and watch the rear when you feel it shaking w/or w/out wheels on (sounds dangerous).

Bill Ball

Good suggestion about the dyno...a dyno run is in the plans for later in the month. I've thought about running the car at high speed with the rear wheels elevated, but I wasn't real comfortable with the thought it might vibrate off the stand. We've already had one car go through the back wall of our garage....
http://www.billsworkshop.com/P928S4/...full_story.htm

Lizard928

I like the dyno suggestion. However I have seen this with slightly loose front wheel bearings.
However you state that you have checked them. The dyno will tell you if it is rear or front though.