Vibration under braking - where next?

[944Nick]

I have a difficult to resolve problem with a 911 993 C2. If you fit new disks, set them up correctly with a dial test indicator so that the overall hub+disk runout is less than 0.05mm (allowable limit is 0.08mm IIRC), and run them in carefully, after a couple of thousand miles I am still getting vibration felt through the steering when the brakes are applied. It's about the same whether the brakes are hot or cold, maybe a little worse when hot.

So far I have:

- swapped out two complete sets of disks and pads, the last set being from Porsche (so hopefully good)
- checked over the calipers - none of the pistons are sticking and the pads slide freely in the calipers
- I am careful not to let the pads sit on hot disks (and they haven't got that hot anyway) to avoid pad transfer
- pulled hair out

I have also had the local Porsche specialist run the car up on the MOT brake testing machine and they report that all of the disks are causing the vibration to a greater or lesser extent.

What would you do next?

[one eyed mick]

Try turning the disc on the hub by one wheel stud hole ,if it reduces vibs go on another hole do each wheel separatly test after each movement ,not an old wives tale !!! it has worked for me numerous times

[LarJammer]

Your problem may be caused by something other than the brakes. get the wheels rebalanced first, check for suspension play. it may be worth getting the brakes skimmed on the car, you can virtually eliminate runout.

[944Nick]

I see the logic of moving the disks one stud at a time, but I really want to understand what's causing the problem in the first instance, otherwise I'm doomed to a lifetime of disk jiggling. Appreciate the suggestion though.

I appreciate that the cause could be elsewhere (bushes, bearings etc) but I am only getting the vibration under braking and it doesn't occur with new disks. I would expect suspension components and things such as wheel balance to be problematic irrespective of the condition of the disks. All wheels and tyres have been balanced in the last couple of thousand miles as well.

My strong feeling is that if it only occurs once disks have worn in and been able to develop some disk thickness variation, that it is very likely to be a disk, caliper, wheel or hub problem but I'm not sure which it is at this point.

One possibility, suggested by my good local-ish BMW specialist, is that the wheels - which have been refinished - have uneven paint on the area that mates to the disk bell. Sounded a bit unlikely on first acquaintance, but it would only take a couple of thousandths of an inch of paint in a high spot to allow quite a lot of movement in the disk bell when heated. The wheel faces did have paint on and this has now been removed though I don't suppose this will fix the disk problem.

Has anyone else had problems with paint on the wheel mating surface?

[oakdale]

During many years in the motor trade, I have found this sort of problem to be nearly always caused by corrosion between the hub and the disc.

You may or may not need to replace the discs again, but either way make sure the hub and disc mating surface are scrupulousy clean using emery paper and a wire brush, then reasemble with a very thin coat of copper grease on the hub to ptevent corrosion.

[944Nick]

Thanks for the input Oakdale. Unfortunately in this case the hubs and disk bells were cleaned very carefully using a tool made for the purpose and there was no appreciable runout on the assembled disk/hub (which is what you're trying to avoid by having clean hubs).

[oakdale]

944Nick said:

Thanks for the input Oakdale. Unfortunately in this case the hubs and disk bells were cleaned very carefully using a tool made for the purpose and there was no appreciable runout on the assembled disk/hub (which is what you're trying to avoid by having clean hubs).

Yes but the point I'm making, is that although there was no runout when first fitted, unless something like copperslip is used on the mating surface, corrosion will inevitably start causing the disc to be pushed away from the hub.

The joint between the disc and hub is not sealed and road water gets in resulting in corrosion that doesn't occur evenly.

The force generated by this corrosion can easily push an alloy wheel away against the wheel nut and cause the disc to runout.

[944Nick]

Point well made. I will check and report back.

[theshrew]

New discs can be faulty ive had it before

Lie /\ make sure the hub is clean this can make a huge difference

Also check the hub see if thats running as it should. If thats moving about it will tranfer to the disc

[theshrew]

New discs can be faulty ive had it before

Lie /\ make sure the hub is clean this can make a huge difference

Also check the hub see if thats running as it should. If thats moving about it will tranfer to the disc

[George111]

I'd also check the wheel bearings and all bushes, really push hard to see if they move.

[9xxNick]

Having gone through all of the possible causes and corrected them where possible, it seems very likely that the problem was the paint on the mating faces of the wheels. This has been removed and the brake vibration problem has diminished significantly.

I haven't removed the disks to de-rust them but it's not likely that they've become less rusty over the course of a couple of weeks, so the wheel paint theory seems to be holding water at this stage. That's not to say that the disk/hub interface can't corrode and cause problems but it seems not to have been the issue in this case.

I'm posting this since I would never have come up with this as a cause of the problem and I hope it may save someone else some effort somewhere down the line.

regards
Nick

[andygtt]

A slightly worn or non round hub will hold the wheel bearing fine on static inspection but after a short period will cause brake judder.

On our nobles we have this a lot and its regularly a poorly machined hub, and they can also wear when the nut comes slightly loose ( another common failure and ironically fitting a Porsche nut is solution lol).... The bearings used on a noble are actually porsche / BMW items.

[9xxNick]

Interesting thought Andy. On the Nobles, do you tend to get this on multiple wheels simultaneously, or is it typically confined to just one? Three of mine are causing this issue on the 993.