Hey,

I'm looking for some help with a slight brake shimmy I have on my S2. It's a '90 model, with the Brembo calipers. The shimmy is slight, but definite, and it's annoying. It is most noticable when braking gently (and often at low speeds). If you brake hard it seems to go away.

It was very bad, so I changed the front disks and pads (which were worn out anyway) and front wheel bearings and this helped a little. I removed and re-built all the calipers (replacing a piston in one of the front calipers as it was jammed and I damaged it getting it out) and this helped a lot, but didn't quite cure the problem.

As far as I am aware the rest of the mounting points on the suspension are OK. The ball joints on the wishbones are in good condition and the tie rods were changed when I changed the suspension about 3 years ago.

My thoughts now are whether one disk isn't on quite straight, or is warped. I have a Dial Test Indicator (DTI) but am unclear about how to use it on the brake disks; when the wheel is fitted the hub assembly (with wheel, disk and everything else) is clamped up and hence will sit differently to when the wheel isn't on, which is how you test it with the DTI. Therefore I am not entirely sure how accurate a reading you can get with a DTI without the wheel on and everything torqued up.

All suggestions welcomed - thank you.


Oli.

 

Oli,

I could ask if you used ceramic pads? or maybe did you have the rotors professionally machined?
You might have to take the rotors to the shop instead of using your DTI.

Here is a better list of causes by Bendix.
Maybe in the "Pedal Pulsation" section?

http://www.bendixbrakes.com/techCorner/faq.php

A lot of reasons for "Slight brake shimmy"
Rebuilding the calipers was the right first choice of repairs.

GL
J_AZ

 

John,

Thanks for your answer. The pads are standard (Pagid, I think) so I doubt they are ceramic. The rotors weren't machined at all.

The 'Pedal Pulsation' document is helpful - thanks. I think it's a case for me to take the disks off again and clean carefully behind them, and clean the front face of the hubs, and possibly clean the inside mating face of the wheel as well. Ho humm, fun and games for a warm day ...

Thanks for your help.


Oli.

 

May be out of alignment or your tires aren't balanced. Had this on my S2 for a while, took forever to get sorted out. Alignments never did a thing for it until I took it to a Porsche specialty shop and now it's smooth.

 

If you take the discs off, I'd recommend taking them to a shop to have them turned and checked for warpage before putting them back on. Sanding or burnishing may not be sufficient enough.

 

Thanks. Given that the front disks are new I'm hoping they are straight and true.

The rears may be a different matter tho!


Oli.

 

If it's out of alignment or a tire is worn oddly or a little out of round or out of balance it can produce results only under braking like that as well.

 

Oh, for the dial indicator, the only way I can think of to use it would be to come up with some spacers and do up the lug nuts so that it's all clamped down as though the wheel were installed, then measure rotor relative to the caliper as it turns.

 

BP,

Thanks - lots more things I can get worried about then!

Yes, spacers for the wheelnuts is a logical idea. I suppose the ideal would be centre of an old wheel, with everything from the spokes outwards removed. Hmmm. It's all a bit more than I have the capability to knock up at home. Maybe I need to go and see what I can buy from my local scrapyard.

Thanks for the input.


Oli.

 

Chaps,

An update on this one; one of the rear disks was slightly warped. (It was also very old - the car was running on the same rear disks it had when I bought it, 60,000 miles and 7 years ago). I managed to stick the DTI onto the suspension components with the wheels still on and diagnose it that way.

New disks and all is sweet again. Very sweet actually ... it's amazing how annoying a warped disk and vibrating brake pedal is!

Thanks for your help.


Oli.