ever since i bought the car there is a light shimmy at about 80. when i bought it it pulled (and still does) to the right a little.

I have not been able to get the front end aligned, as all the shops here no matter what i tell them lift the front end as soon as they get it...

On the way back from SITM the slight shimmy turned into a worse vibration, and now felt like it may have been coming from behind me...

My question is what could this be? It seems to remain constant in relation to speed, not RPM. I slid the car in neutral and let the rpm drop, then revved it, and the vibration remained constant.. this leads me to believe that it is NOT the torque tube, as it should change with rpm. Correct?

 

tires, need to get alignment corectly done and then replace the worn tires

 

Could be a bent wheel, or simply an out of balance tire. Minor bends don't show vibrations until higher speeds, and will actually sometimes go away when you go above the speed range (i.e. you will feel the vibration at 70 mph, but not 60 or 80 mph).

 

Steve, take a minute and check your wheel bearings for play. I imagine that would show up before 80, but it is so easy to check, why not rule it out?

 

thanks.

just for reference, making sure my thinking is correct....
the torque tube would change vibration with engine rpm since it is directly coupled to the motor

 

:...the torque tube would change vibration with engine rpm since it is directly coupled to the motor"

Yes, in an automatic car the driveshaft is fixed to the crankshaft and turns at engine speed.

In a five-speed, the driveshaft will be at engine speed with the clutch pedal out (clutch engaged); and will be proportional to road speed with the transmission in gear.

Your vibration is tire/wheel (front or rear), or half-shaft, or (less likely) transaxle related.

 

thanks guys

just found what may be some front right wheel bearing play.

 

Steven, locally... try to find a shop with a Hunter 4-wheel alignment machine (the higher model #, the better). The wheels don't come off the rack. That will be one of your best friends for 928 ownership... and hitting potholes. IIRC, most Firestone service centers have them these days.

Otherwise I agree with what has been mentioned, vibration at vehicle speed would/could be wheel balance (or bent); half-shaft/CV assemblies; wheel bearings; 4-wheel alignment. If you want an autocrosser or canyon carver -corner balancing as well, but very specialized shops do that.

Best wishes on the diagnosis & fix!

BTW, great meeting you at SITM! Keep your eyes peeled for the monthly meets!

 

While it is possible that loose wheel bearings are the problem, it isn't too likely.

Check the bearings by lifting that side of the car, then holding the wheel at top and bottom while checking for play. Holding the front and rear of the wheel checks the steering more than the bearings.

 

Then have an assistant hold the brake pedal down while you repeat the above. Any play that now disappears is due to wheel bearings.

 

A good addition, Jon!

 

Two other things that can cause vibration has to do with the tires themselves...if a car has sat with a tire flat for a period of time the tire may vibrate and does not recover IME. Also, along the same line, many of the tires used are directional, if they are run in the wrong direction they may vibrate, and are ceertainly unsafe.....one of my cars had 2 tires reversed, and another had sat flat on a couple of tires.....they both vibrated at about 65-70.....new tires running the right direction resolved both....

 

Dr. Bob,
I will recheck the trans mounts but they were replaced 5 years ago, 37 k miles driven since. Everything else seems to be in good order with proper torque.
The shudder is much more prevelant when at slow/low rpms and torque applied through acceleration. High rpm, acceleration shows no vibration that I can feel
Thanks for your input, it's always appreciated .
Lon

 

this sounds like a half shaft /CV joint issue,
swap the shafts side to side see if anything changes,
if it does,
replace the half shafts with fresh units

 

Hey Stan,
I've had a similar problem (around 78 mph)....just a slight wiggle that disappears more or less as I continue to accelerate. I might be wrong, but I really think I don't feel it sometimes on an uphill or downhill trajectory.
Anyway, I'm curious about what can go wrong with a half shaft that would cause such an issue....assuming it someone didn't get bent hitting a curb or something of that nature.
If the cv bearings are serviced, and seem to move cleanly with no "slack" before reinstalling, it seems to me that the issue couldn't be related to the half shaft. If the cv joints are worn enough to have some slack in them, I would assume they can wiggle around while driving and cause some vibration. But, I'm guessing. I'm wondering if someone that knows what's up can fill me as to what it means when someone says a half shaft is bad?