Odd wheel issue

[bmwmike]

Not sure if this is technically drivetrain or not, sorry if it's in the wrong place.

Am trying to figure out what's wrong with my rear wheels. There is an bad vibration occasonally present ever since a BMW dealer had my car last September. They've had it back several times and are now saying it's the engine.

Vibration is whole car - especially floor and seats - and not steering wheel which suggests rear wheels. Odd thing is, whilst it usually comes on at around 74mph is doesn't always do so and can be hard to recreate. Some days it might be as low as 60mph.

Managed to get a set of spare alloys on the back and sure enough vibration is gone.

Now the odd bit is, the dealer has balanced the wheels twice and I've also had a trusted local garage redo them and they both say they are straight and balanced.

As the vibration comes and goes I'm thinking it could be an internal fault with the tyres. No obvious buldges and outwardly the tyres look in good condition. Or, both wheels are slightly buckled and they come in and out of alignment as the relative position of the wheels changes?

I'll get another garage to check them and take the tyres off. Tyres are RFT and have about 4mm of tread left so no harm replacing them. I'm just loathed to put new rubber on if it's the wheels at fault.

Thanks
Mike

Edit no dents cracks or dings on the wheels either

[bmwmike]

Thanks sunbeam. Interesting. So an internal tyre fault could be a possibility.

Think I'll get another local to take the tyres off and spin the rims up and check balance without tyres and go from there.

Checked the mating surfaces already and are clean etc.

Hope it's not the alloys at fault. It was fine till BMW dealer had it, and I've basically sat on my hands since so it's probably a bit late to go back at this stage. Live and learn.

Be nice just to have the car back to normal.

[bmwmike]

Good idea but they are staggered. Stupid BMW thing.

I just had a brainwave and cut a piece of dowel to fit exactly inside the rim and then checked fit at various points. Looks like one of the rears is most definitely oval.

Guess I shouldn't be surprised but the BMW dealer and the local independent both said the wheels were straight.

Guess it's just a bog standard buckled wheel thread after all..

[bmwmike]

Thanks Ian. Really appreciate your reply.

If you saw my other thread I probably mentioned about how great the car was before the dealer got it, was smooth at any speed and on all surfaces. Was a delight.

The two spare wheels I've tried that I mentioned in the first post are actually an 8J and a 9J wheel, one is an RFT and the other a nRFT and, and they have different profiles. In other words, completely mismatched on same axle! Even with that, it's great and reminds me what I was missing since this all started. It's back to how it was. Even at low speeds it feels better and I can't believe I didn't try that sooner.

I can't leave it like that of course, but is OK for short trips till I sort out the two rear 9J rims. This whole thing has stopped me selling the car as I didn't want to punt it on with a fault.

Lessons learned, if it feels like wheels it probably is, no matter what the garage says, and, I'm dropping the mondial warranty (pity) as it ties me into BMW dealers too much. Not going near one again if I can help it.

Cheers

Edit thanks for the tip on Centre Gravity - noted for future ref.