The old S8 is having a bit of a Wobble
 

Good morning all,

One that I hope you may be able to help with.

The S8 has a strange Wobble!

At 70 to 80mph I can feel a vibration/ wobble coming from the underside of the car that you can feel through the floor/ seat. It feels like a mount/ rotation/ driveshaft/ engine is wobbling a little when going along.

Its odd, noticeable and constant as I drive so I was wondering if anyone has had similar and can point me in a direction here.

To cover a few questions off, this is not brakes or suspension and I do not get anything through the steering. Road surface and condition do not make any difference. All the wheels have also been balanced to try and resolve this problem as it was my first thought. Tyre pressures also checked and the tyres are all good, reasonably new and the same brand all round. Recently passed the MOT with no advisories and all the suspension arms and bushes are OK too. Finally no clunks or bangs, no drivetrain shunt, nothing excessive or noticeable when I just rev the engine and the gearbox has been fully rebuilt a year or so ago.

The car has now done over 200k so I suspect something worn/ old as the problem was not sudden but has come on over the recent month. Not sure if this could be the propshaft or its bearing/ mount getting a little old?

That sounds like a rear tyre out of round to me - possibly a flat spot or a slight bulge in the tread surface. You can easily overlook that on a balancer as the wheel is covered by the guard, and it can easily be balanced out.

I would jack each rear corner up and rotate the wheel by hand, feeling for any deviation in the shape. With one hand turning the wheel and the other palm on the tread surface you can generally feel if the surface changes. Or, swap the rear wheels to the front and see if it moves to the steering.

Another possibility is a bit of rust or grit between either the wheel hub and the disc, or the disc and the bearing hub, which is causing run-out on the wheel once its mounted on the car. This is generally too subtle to see without a dial gauge so if moving the wheels end to end didn't help I would look at pulling the rear discs and making sure all the mating surfaces are spotless.

The propshaft spins too fast to be described as a wobble. If thats out of balance it tends to show as a vibration around 40-60. By 70-80 its too fast to feel in my experience. The prop will tend to straighten itself due to rotational inertia so a duff centre bearing is usually felt as a thump through the floor on over-run.

From your description, definitely something to do with the rear wheels or how they attach to the car +++

Could also be a buckled wheel. Don't which wheels you have but the FE wheels seem particularly easy to buckle. I've buckled my wheels a couple of times on pot holes. Had very similar vibration symptoms.

Having waited for MJ to post ..... I can say that I'm with her on this. +++:ROFL:

I have had 2 instances of similar wobble at 70ish. It was noticeable vibration which, in particular, caused the front passenger seat to vibrate back and forth. On one occasion it was caused by non-hubcentric wheel spacers which are really hard to get to sit exactly central so when removed the vibration disappeared. The other was newly - but badly - balanced wheels (in fact, i think one of the weights had come unstuck as there was a sticky patch but with no weight.)

Well Ladies and Gents, you were spot on.

Spent a few hours today looking at the wheels, cleaning up the wheel mounting surfaces and rotating them all to see where the problem was. Also to confirm these are 20" FE alloys on the car.

And indeed it was one of the rear wheels +++

Car has the spare on the rear drivers side now and is a smooth driving car again as I tested it up and down the M5.

The wheel in question is not buckled, "looks" OK and has been recently "Balanced" but none the less it was the reason behind the wobble.

Mine have never been visibly buckled either. The inside edge just gets slightly off-round so they no longer run 'true'. A good alloy wheel refurb place should be able to check it and straighten it for you, if it is slightly buckled. You might be able to tell if you measure the diameter of the inside of the wheel at various points and look for any slight differences in measurements.

I had this on a rear wheel and the only way to see it was by rotating the wheel on a balancing machine, while holding a screwdriver by either the inner or outer rim so you can see if it is running out of true