Quattro handling problem - looking for ideas!

[MikkiJayne]

Hi all,

My 2002 Avant Quattro has developed a really horrible handling characteristic where it feels like the backend is steering itself when going over rough surfaces, white lines, drain covers etc. It pretty much has a mind of its own when it does this and sometimes I need to be really on the ball to catch it at freeway speeds. Actual mechanical grip is fine though.

It's as if the alignment on the rear end is screwed up, but it isn't. I've had the car aligned three times using two different racks and currently every setting on the car is within 3 minuts of spec. I don't think its possible to get it any closer tbh.

Its not the tires - I've tried three different sets in three different brands & sizes - all do the same thing.

I'm 99% sure its not the dampers as in all other respects it drives fine. Corners well, isn't bouncy over speed bumps etc.

I'm also 99% sure its not a worn bush in the suspension as those have also just been checked and all seem to be tight an intact.

No unpleasant noises, and it tracks in a straight line on a flat road. Tyre wear on the rear is flat and even. The rear is at stock height with stock suspension. The front has stock suspension too but a combination of spring platforms to make it an inch lower. I haven't changed anything other than wheels in the time this has been happening / since it started.

So now I'm stumped

Has anyone seen this before? Got any suggestions?

Thanks

[TurnipLicker]

Have you looked at the front setup?

I thought Quattro had a FWD bias, therefore if you feel like your being driven from the rear, theres going to be something wrong at the front, pushing the power to the rear wheels?

[a4edwin]

mine does it if the road gets a little bumpy it feels like the rear kicks out to to the left or right. or if i go around a bend and the road isn't smooth it's as if the rear jerks left and right. i noticed it after i put in coilovers and 19's on i did both at the same time. and i did the alignment right after c/o's and wheels can't really push the car with confidence without getting nervous

[MikkiJayne]

a4edwin - thats exactly what it's like. Very unsettling when hooning and its really spoiling the car. Interesting that yours does it too though. How many miles are on yours?

Turnip - I've checked everything front and rear and I can't find anything wrong / worn / broken anywhere. Thats why its so puzzling. A friend suggested I may have a bad centre diff though, which I'm still investigating. That seems to tally with your thought that the power is getting pushed around wrong. I'm sure a Torsen either works or it doesn't but I'm contemplating sourcing another one to see what it does...

[MikkiJayne]

Ok so here's a wierd thing - this seems to be directly related to the stiffness of the tire sidewall

Snow tyres 205 55 16 (jelly) - don't do it at all
Michelin Pilots 225 45 17 (quite hard) - do it badly
Pirelli PZeros 245 40 18 (rock hard) - nigh on undriveable
Yokohama Advan wet slicks 240 610 17 (soft for what they are) - barely detectable
BFG Profiler 235 40 18 (soft) - don't do it at all either

Yes, I've had all five sets on in the last 4 days My neighbours think I'm nuts.

I've discovered that the upper rear control arm bushes are a bit soggy. I'm wondering whether the hard sidewall is making these work too hard and screwing up the geometry. I'm gonna buy a new one to see how hard it is compared to whats on the car and go from there.....

[a4edwin]

car has 103k and wheels and tires were brand new. only thing that i have thought is toe in.
it's like a common not so common problem. others have had the problem but haven't heard of a solution