Brake wibble

[Sam]

My 964 has brake wobble.

I changed the discs, pads and bearings and put poly bushes in the lower arms, which cured it for maybe 1000 miles. Then it came back.

So it wasn't any of those.

The internet suggests I get the discs machined whilst they're on the car to true everything up. Is the internet right?

Might it be a dicky piston in the calipers?

Any other ideas?

[butzsi]

Sam, the poor quality of materials currently used in most manufacturers brake discs almost ensures run-out developing after a few hundred miles.

For this reason many manufacturers insist that on car disc brake lathes are installed at all their main dealers.

Porsche GB, and many independents like myself, use the Pro-cut machine with excellent results.

http://www.procutusa.com/

You can check to see if run-out is present on your discs by using a dial gauge in a suitable holder.

Give me a call if you want to know more.

Chris

[Sam]

Thanks Chris.

I have a dial gauge with a magnetic base, so should be able to get a decent measurement.

What sort of run out is acceptable?

[butzsi]

Depends on the diameter of the brake disc but anything above 0.002" may start to give symptoms like pulsing of the brake pedal.

If you take your car to your nearest friendly mot station and get them to put it on the brake testing rollers, with light but constant pedal pressure if the needle on the meter is fluctuating up and down on a particular wheel then that would be the source of the run-out.

Chris

[Sam]

Thanks again.

[Sam]

> Depends on the diameter of the brake disc but anything above 0.002" may start to give symptoms like pulsing of the brake pedal.

I've finally got round to checking this.

I make 0.002" 0.05mm.

At the outside edge of the disc I've got 0.11 one side and more like 0.18 the other.

For both to have gone wonky I guess it's poor materials. Maybe I should have paid the £150 for Porsche bits rather than £60 for Pagid.

[Gary71]

You also need to check the mounting surfaces are completely clean and rust/contamination free before fitting the new discs.

I'd check this again before investing. Although I agree on car machining fixes it it really shouldn't be necessary as no one does it on new cars!


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[hot66]

I have the same sam on my 2.4.... new set of discs and it went. Its now back again ... gets real bad when hot. Might have to look into the machining or just buy some genuine discs

[Gary71]

I've bought Brembo discs for mine and had no issues, despite trying really hard to warp them


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[graemesse]

I have seen this before with pattern parts. I suggest you bite the bullet and fit OE discs from Porsche

[Nige]

Stick a set of Brembo, Zimmermann, Textar or similar quality discs on.

[jb]

Maybe check one of the pistons is not stuck

[smallspeed]

I've never had any issues with discs warping over the last 10-15yr on any of the cars I've owned (Porsche, Mercedes, BMW, MG, Mini, Peugeot, Lancia, Mazda, etc.) regardless of application (ie road, track, off-road). I always fit brembo or OEM discs so perhaps that's the key? But any vibration has either been the dreaded lower arm bushes on BMW's (v.common issue) or more commonly pad deposits on the discs..

Have you tried going back through the disc/pad break in procedure to see if the discs clean up in case its this? Seems odd that they start out good (ie discs are square) and then go "bad" so quickly after fitting, unless of course the brakes are getting a massive hammering and warping due to heat? If you have a sticky piston this could be the case.. ..if so, you should be able to feel this after a 5-10min drive without too much brake application as a big temp differential between left/right discs (I say feel - don't make the mistake a friend of mine did and actually touch the disc just incase! He now has a nice AP "J-Hook" pattern on his index finger..)

[Sam]

Hmmm.

Clear as mud then.

I've changed the lower bushes for poly ones, so it's not that.

It seems the possibilities are:

Sticky calipers (both sides? Seems coincidental)
Shitty discs
Wonky hubs

I think I'll go for new OE discs first, then if that doesn't cure it at least I'm machining good discs.

[Gary71]

You won't go wrong with the OE discs. For a car with measurable performance, i.e. a 964 not my old T, then you need to fit the best.

We (OEM of heavy fast cars) spend an inordinate amount of time and effort on disc and pad compositions to avoid distortion, cracks, and as a priority for many of our customers noise!

Balancing the requirements of low speed noise, 200 mph stops and durability is a challenge I'm glad isn't mine!


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