9-5 Aero vibration under braking

[LongQ]

I noticed recently that under moderately heavy braking I could detect to a slight vibration with steering wheel shimmy - but not always.

Out on track today for some gentle exercise and under heavy braking, as you do, it was very noticable - almost worse than my old Omega which I thought was impossible! The disks and pads have done about 16k miles I think and the visible pads seem to have plenty of material left.

It could be wheel balance - new front tyres will be required in about 2 to 4k miles I estimate - but there are no other tyre problem like symptoms I can think of.

I'm also wondering if if could be a brake system problem but I suspect it might be suspension - a bush or something - or a joint in the steering. It has clonked sometimes ever since I have had it but checks have not revealed anything to date.

Maybe wheel bearing or something? If it was anything at the front I don't think it would be audible as bearing usually are - not much sound from the front seems to get in the cabin.

Any other thoughts?

[davidy]

LongQ

I have something similar to this with a 2.3LPT 9-5. It has been happening for the last 15k miles or so (my car has done 136k miles), only really detectable under really heavy braking, the whole of the front of the car seems to shimmer slightly. I changed the discs/pads about 6k miles ago, this made no difference, but I did change the tyres about 2k miles ago and the vibration has got less. The car was serviced today and I had the dealer check over all the front suspension, because at my mileage a bit of wear is allowed, but no faults found.

Because the tyre change made a difference, I wonder if I have a slightly non-round wheel, combined with possibly an out of balance tyre (not as bad as now though) which causes an effect only when under heavy braking.

I have a set of steel wheels with winter tyres and I may pop those on shortly to see if the effect stays, or could it be a slight fault with the ABS unit? Like you I'm scratching my head a bit on this one, as otherwise the car drives faultlessly.

davidy

[LongQ]

Now that's interesting. Mine has just clocked up 111k. I've had it since about 96k (2 years though, my mileage is well down on what I anticipated) and I think the dealer fitted new disks and pads pre-delivery. I didn't ask for them but they did look very smooth and bright for a while. Remarkable performance as well but not quite so much 'bite' now.

I had the tyre pressures up for the track use today which does make the low profile tyres very sensitive to the road surface but this seems to be more than just pressures and seemed to continue somehow when steering but off the brakes but only of left hand bends. That said it could be an attribute of the ESP system and the LH bend thing might simply be that most of the bends were right handed of course, none of the lefts being similar to the worst feeling rights!

It could be related to tyres but they Michelin which usally are built quite consistently and the wear patterns a very even right across the tread.

I was expecting to trash the fronts today on track but it hasn't happened so I can persue investigations without making the change which may hide the problem. I did have the front brakes smoking at one point but that never repeated and the problem pre-existed the smoking pads.

Thanks for the input. I'll let you know if I find anything.