I had new discs and pads fitted late 2012 and within a couple of thousand miles, went to Spa (March 2013 in the snow!) did a relatively slow track day (though I imagine the temperature differential of brakes to ambient was quite large) and noticed a distinct judder arrive in the braking during the day. It turns out the discs had warped. In road use subsequently it's not been too bad but having done another track day the judder is back with vengeance.

I am sure someone has posted that 964's had an inherent problem with discs warping when the cars were new... something to do with the metals used in the hubs and discs expanding at different rates. I also believe the post suggested Porsche got fed up with keep replacing discs so went to skimming the discs on the car to overcome the issue. Over the years, I understand this issue becomes less until at 20 years old it is now uncommon. Maybe I just bought duff discs (Sebro so should be OK but anyone can have an issue)

I guess my questions are:
1. Is this bit of history true and is there anything else to add (for interest)
2. Has anyone had brake discs skimmed on the car and if so, was it successful?
3. If this is a route to go, does anyone have a UK recommendation of where to get it done (Herts / Essex ideally)

Thanks as always.
Mick

 

Discs do not warp, they may wear unevenly due to pad/caliper problems

 

^^^^ What he said. 'warped rotors' is the feeling you describe but extremely unlikely to be the real cause. Check the surfaces of both the disc and pads. Also check the wheel bearing.

Is this the front or rear? If it's the rear, do you come back into the pits after a hot session and apply the hand brake?

 

There's a white paper floating about (read it first years ago via the Miata community) which states that judder is generally not from brakes warping, but rather poor/uneven transfer of friction material to the disc. Further, I used to work with a guy who was a test driver for Girling years ago and he told me that skimming rotors is hardly ever warranted.

What pads are you using? Perhaps fitting something else would help; you could consider getting a set of more track-worthy pads and using them to scour off the rotors.

Also, continuing robt964's line of reasoning, have you replaced the control arm bushings in the front?

 

Hi Mick , discs can warp . You can check the 'runout' on yours by simply setting a static probe eg screwdriver to just touch the disc and then rotate the wheel - will be visible wobble or audible scrape . Normally caused by overheating just one part of the disc - eg holding brakes on while hot - I did mine at Spa some years ago when red flag forced cars off track and then had to queue in ( downhill) old pit lane on run down to Eau Rouge.
You can try getting them skimmed by a specialist eg Parr - I used Tech9 Motorsport near Liverpool .However this is not always successful . The metal has a distortion memory and resets itself when gets very hot again so warp returns . The skimming of course is done cold.

NB if you don't see any runout when checking then the problem may be something else eg old brake pad deposit on the disc and you can remove that either with clean aggressive pads or skimming.

 

I had a set of front rotors that developed severe judder after only 3 trackdays. I changed the pads for Hawk blues and the judder disappeared within a few miles and never came back. Did quite a few more trackdays with them without any problems. The hawks are very aggressive pads and had cleaned off the uneven deposits from the previous pads that was causing the judder. I guess skimming would have the same effect.

Unfortunately, the down side is that the dust from Hawk Blues is very corrosive and was not a long term solution. So I then decided to have the brake cooling fitted. This has allowed me to use less aggressive pads because they're no longer overheating and therefore are less liable to leave deposits on the rotors.

 

Carroll Smith “The Warped Brake Disc and Other Myths of the Braking System,”

http://www.mossmotors.com/SiteGraphi...ake_discs.html
"Abrasive Friction: According to Carroll Smith, author of “The Warped Brake Disc and Other Myths of the Braking System,” abrasive friction involves breaking the crystalline bonds of both the pad material and the cast iron of the disc. Breaking these bonds generates the heat of friction. In abrasive friction, the bonds between the crystals of the pad material (and to a lesser extent, the disc material) are permanently broken.

The harder material wears the softer away, meaning the disc wears the pad. When we see the word friction, it is abrasive friction that comes to mind."

 

Exactly, you cannot warp a disc. How hot does it need to be to heat up an iron disc 28mm thick and then distort it? Red if not white hot!

 

Have you ever seen a warped cast iron cylinder head ?
Do you know about pickled cast iron blocks ?

 

Interesting stuff. The number of times I have heard talk of discs warping and they don't actually do it after all

Anyway, for those that have asked some questions:
1. Full suspension check and in fact disc replacement by Center Gravity in the UK who have an excellent reputation. No issues found (other than thin discs which were replaced!). I therefore believe the whole steering / suspension / bushes etc to be in decent condition.
2. Brake pads are Textar and discs Sebro so pretty standard
3. It is the front discs and fully accept the point ref not using the handbrake coming off the track and I haven't
4. The front brake pads do carry signs of the pads resting against them. I kind of hoped a further 3000 miles and another track day might clean this off.
5. My local mechanic ( decent guy but Alfa Romeo trained, not Porsche) suggested the discs were warped using the method Oldtimer highlighted. It is he that has suggested maybe skimming the discs ref pad marks and warping

I fully accept the point ref 28mm discs warping and would not have thought it possible given the sheer size of them but I suspect it is temperature differential as much as sheer heat that could cause an issue I.e. Heat build up where the calipers are coming off track compared to the cooler parts of the disc.

I appreciate the comments, please add any you see fit. The debate regarding whether discs actually do warp will be interesting

Cheers
Mick

 

Stick some agressive pads on there, 90% chance that will fix it.

 

Hi Mike,

I agree with Oldtimer, it is possible to warp a disc when you come back in after several hot laps, and then keep the brake pedal applied when you come back in...which keeps the heat in the pad area as the rest of the disc cools down. However, it's hard to warp a 964 disc in normal use.

As Boxsey says the Hawks will clean up a disc surface and he's also right about the dust...well certainly as far as the amount of it...it gets everywhere!

I now run Pagid RS29s, which along with ducts results in no heat build up issues that I've noticed. Dust levels aren't too bad with the Pagid pads.

I hope you find a solution.

When I can afford it I'd like to go for floating discs, but my 996t has today thrown a strop and it seems the power steering pump has failed. D'oh! So it'll be a while before the disc upgrade happens!

 

My post took too long to type, so it's old news!

Agree that maybe you've got pads which aren't retracting fully...this could lead to them sticking and this will probably be caused by the metal plates lifting due to brake dust getting underneath, again possible due to different expansion rate to the caliper metal!

I have to shave my pads to get them in and to enable them to move freely!

Really must spend the cash to get the calipers refurbished.

 

I had this problem with my 993 but only noticeable during high speed hard braking. I never got to the bottom of it but I ruled out discs warping given that the style of driving would not create the temperature plus the discs/pads had been replaced less than 4k miles ago. I suspected plate lift in the caliper but sold the car before I got to the bottom of it.

You can have the discs skimmed. I had my 964 skimmed not long after I bought it. Mainly to get rid of surface corrosion, so it can be done.

 

I've had similar issues where the vibration was so bad that I didn't want to use the brakes. This is not fun on the track. I was also using the stock Textars. I took some sandpaper to the discs to remove the buildup. The brakes worked fine after that. If you see residue on the disc, it is probably the problem. You should be able to clean up the slides with fine sandpaper if that looks like a problem.