[s-cars] Strange front end vibration or shaking - 94 S4

[Paul Gailus]

I believe what could be causing this vibration is:
1.) normally, the control arm has tension on it because the
center plane of the wheel is outboard of the downward
force applied to the top of the spring by the weight of
the car.

2.) when you turn left at speed you reach a point where the
inward cornering force at the tire overcomes this tension.
If there is any lateral play in the ball joint, bushings etc., 
the bottom of the strut will then move inward.

3.) The end of the steering arm in front of the strut is 
held from moving laterally by the tie rod. Therefore,
the inward movement of the strut bottom will also cause
the strut assembly and wheel to turn right. This then reduces
the cornering force so that it no longer overcomes the
tension due to the outboard position of the wheel,
causing the strut bottom to move back outward. This makes
the wheel turn back left again, and the oscillation cycle
continues.

So I would look for anything that could allow lateral play
at the strut bottom (like bushings, ball joints, etc.).
Play may be more difficult to find with just one wheel off the
ground because of the upward force of the sway bar on the
control arm. It might be best to have the car on a rack so you
can safely apply some more substantial forces to check for play.

-Paul

----- Original Message ----- 
From: Tim Sexton <vader at qwest.net>
To: <s-car-list at audifans.com>
Sent: Friday, February 25, 2005 3:15 PM
Subject: [s-cars] Strange front end vibration or shaking - 94 S4


> Hello All,
>     When making left hand turns at speeds of greater than 75 mph, the 
> front end on my 94 S4 begins to vibrate/shake fairly violently.  It 
> feels like it originates somewhere in the front right suspension.  The 
> car has new tie rods and control arms and I've lifted the wheel off the 
> ground and can see no visible play anywhere in the suspension when 
> physically moving the tire in various directions.  I've also rotated the 
> tires and still get the same bad vibration/shaking.
>     I was searching through the archives and saw a few other posts with 
> the same symptoms, but didn't ever find if anyone had come up with a 
> solution.   I'm wondering if it's a bad wheel bearing, but I'm guessing 
> that if the wheel bearing was that bad, the noise would be pretty loud 
> and I'd be able to detect it when moving the tire with the suspension 
> off the ground.  Any ideas, or did any of the other guys with the same 
> issue ever find the culprit?
> Thanks for the help,
> Tim Sexton
> 
> Colorado Springs, Colorado
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