Diagnosing a warped disc

[rufusgti]

Hi
Porsche 944 S2 1990
I believe I must have a warped disc, I can feel and hear the brakes binding on an on/off rhythm while slowing to a stop. Feels awful.
What is the quickest way to find out which disc is warped. They all look fine, and I've tried feeling the surfaces and calipers to see if one is getting hotter after a drive but didn't give any clues.
I could take it to a garage but am trying to do as much as possible myself at the moment.

I'm wondering if there's a quick method I've not heard of.
Thanks

[WinstonWolf]

Wheel off, attach a pointer to a static part of the suspension, temporarily put the bolts back on and rotate the disc. It could also be pad material stuck to the disc, that often feels the same.

[HustleRussell]

rufusgti said:

Hi
Porsche 944 S2 1990
I believe I must have a warped disc, I can feel and hear the brakes binding on an on/off rhythm while slowing to a stop. Feels awful.
What is the quickest way to find out which disc is warped. They all look fine, and I've tried feeling the surfaces and calipers to see if one is getting hotter after a drive but didn't give any clues.
I could take it to a garage but am trying to do as much as possible myself at the moment.

I'm wondering if there's a quick method I've not heard of.
Thanks

Most likely that the root cause is a sticking caliper. Given that the pistons are supposed to float in the calipers, why would a warped disc cause the on/off sensation?

[rufusgti]

HustleRussell said:

Most likely that the root cause is a sticking caliper. Given that the pistons are supposed to float in the calipers, why would a warped disc cause the on/off sensation?

I wondered the same.
So what would I do with that, rebuild or replace. How easy is a rebuild job on a Caliper or are they just not worth trying to fix at home.

[Mignon]

If only there was a way to apply say just the rear brakes and not the fronts you'd be able to pin it down to one end of the car or the other and then be able to put a dial gauge on the discs at just the bad end. Sadly I can't think of one.

[paintman]

WinstonWolf said:

Wheel off, attach a pointer to a static part of the suspension, temporarily put the bolts back on and rotate the disc. It could also be pad material stuck to the disc, that often feels the same.

^^^^This. Will give you an idea of how much/how bad the runout is. Some runout is normal but should be no more than the maximum specified for that vehicle so this is when you would need a dial gauge for accurate measurement.
Common causes for excessive runout with new discs include failure to clean mating surface of disc & hub, foreugn bodies caught between disc & hub, raised metal around studs/wheel bolt holes in hub. Sometimes trying the disc in different positions can produce an improvement.

Edited by paintman on Wednesday 18th January 14:31

[paintman]

Mignon said:

If only there was a way to apply say just the rear brakes and not the fronts you'd be able to pin it down to one end of the car or the other and then be able to put a dial gauge on the discs at just the bad end. Sadly I can't think of one.

There is. Involves clamping the flexi hoses. Use proper clamps & don't do it on the road as you are disabling part of your braking system.
But whilst it might take a little longer removing each wheel & checking each disc for runout is the best way. Also gives you the opportunity to check pads, hoses, pipework & suspension components. And if the wheel will come off in the first place!

[Mignon]

paintman said:

Mignon said:

If only there was a way to apply say just the rear brakes and not the fronts you'd be able to pin it down to one end of the car or the other and then be able to put a dial gauge on the discs at just the bad end. Sadly I can't think of one.

There is. Involves clamping the flexi hoses.

Or it would be nice if the handbrake applied just the rear brakes! Somewhat easier. I think irony is lost in typing.

[GreenV8S]

There was a StopTech article many years ago describing a reaction which can lead to the cast iron rotor transforming to a harder form (Cementite?) after being overheated. I don't know how reliable the article was but it seemed credible to me. Apparently this can be provoked by the pad material transferring unevenly to the rotor, which can happen if the brakes are held on when the vehicle stops after heavy braking. Once it's started, this process tends to accelerate itself because the cementite is harder and wears less than the rest of the rotor so it forms hot spots.

I've had this happen to my cars sometimes after heavy driving, and for me it always seems to be the front brakes that are affected, so unless you have some reason to suspect the rear brakes I would start by servicing the front brakes and see if that fixed it.

[paintman]

Mignon said:

paintman said:

Mignon said:

If only there was a way to apply say just the rear brakes and not the fronts you'd be able to pin it down to one end of the car or the other and then be able to put a dial gauge on the discs at just the bad end. Sadly I can't think of one.

There is. Involves clamping the flexi hoses.

Or it would be nice if the handbrake applied just the rear brakes! Somewhat easier. I think irony is lost in typing.

Providing the handbrake works by applying the pads to the disc & not the set of small brake shoes to the drum part (bell) of the rear discs on many vehicles including the 944......
http://www.pistonheads.com/gassing/topic.asp?t=791...
http://www.pelicanparts.com/techarticles/Porsche_9...

Edited by paintman on Wednesday 18th January 18:29

[rufusgti]

paintman said:

Providing the handbrake works by applying the pads to the disc & not the set of small brake shoes to the drum part (bell) of the rear discs on many vehicles including the 944......
http://www.pistonheads.com/gassing/topic.asp?t=791...
http://www.pelicanparts.com/techarticles/Porsche_9...

Edited by paintman on Wednesday 18th January 18:29

Fantastic find, thank you. Actually the handbrake isn't set right so this is another job I can do while checking the brakes. Although this can't be connected to my warp/seized brakes it's certainly a job that will benefit having that knowledge to work with.

[tapkaJohnD]

GreenV8S said:

There was a StopTech article many years ago describing a reaction which can lead to the cast iron rotor transforming to a harder form (Cementite?) after being overheated. I don't know how reliable the article was but it seemed credible to me. Apparently this can be provoked by the pad material transferring unevenly to the rotor, which can happen if the brakes are held on when the vehicle stops after heavy braking. Once it's started, this process tends to accelerate itself because the cementite is harder and wears less than the rest of the rotor so it forms hot spots.

And, Lo! http://www.stoptech.com/technical-support/technica...

Note the Author's name. Carroll Smith, no less.

John

[Little Pete]

You could try your local MOT station. If they use a rolling road brake tester they could easily check which disc/discs are causing the problem. A small contribution to the tea and coffee fund usually does the trick!

[anonymous-user]

rufusgti said:

HustleRussell said:

Most likely that the root cause is a sticking caliper. Given that the pistons are supposed to float in the calipers, why would a warped disc cause the on/off sensation?

I wondered the same.
So what would I do with that, rebuild or replace. How easy is a rebuild job on a Caliper or are they just not worth trying to fix at home.

I've had warped front discs. Under braking the front of the car Huddersfield. Under heavy braking it judders so hard it feels like it will shake itself to pieces. You want to step off the brakes as soon as you can to avoid damaging the car.

A binding on/off sensation sounds different to me.

[PositronicRay]

Greg66 said:

I've had warped front discs. Under braking the front of the carHuddersfieldUnder heavy braking it judders so hard it feels like it will shake itself to pieces. You want to step off the brakes as soon as you can to avoid damaging the car.

A binding on/off sensation sounds different to me.

Not just Huddersfield, I had this problem in Bromley once.

[anonymous-user]

PositronicRay said:

Greg66 said:

I've had warped front discs. Under braking the front of the carHuddersfieldUnder heavy braking it judders so hard it feels like it will shake itself to pieces. You want to step off the brakes as soon as you can to avoid damaging the car.

A binding on/off sensation sounds different to me.

Not just Huddersfield, I had this problem in Bromley once.

LOL!

Judders, not Huddersfield. iPhone auto"correct" at work (just tried it again, the cheeky little fker).

[The Wookie]

It's DTV, Disc Thickness variation OP. Happens if you have a disc with excessive runout from overheating then do lots of light or motorway driving, the slight contact in certain places wears the high spots off the disc and you end up with a torque variation, nodding at low speed and judder at high speed. If you have a dragging caliper it will happen more readily through overheating the disc and creating the runout and/or closing up the clearance to create the light contact.

Forgive me if I'm stating the fecking obvious but the easiest way to diagnose both is to put it on a ramp and spin the wheels, if one is noticeably stiff compared to the opposite side then you have a winner. You can also confirm excessive runout or DTV by seeing if the effort to turn the dragging wheel varies significantly with rotation

[Mr2Mike]

The Wookie said:

It's DTV, Disc Thickness variation OP. Happens if you have a disc with excessive runout from overheating then do lots of light or motorway driving, the slight contact in certain places wears the high spots off the disc and you end up with a torque variation, nodding at low speed and judder at high speed.

Also happens if a car is left standing outside for prolonged periods as the bit of the disc sitting under the pads corrodes more than the rest.