993 Drilled Disc Cracks

[993rsr]

After ditching my discs each time the cracks get over 7mm, thought i'd investigate how potentially dangerous these were.

Put the disc on a surface grinder and ground the disc surface on the worst affected area until no cracks were evident.

Took off 0.40mm and as you will see from the attached pictures, cracks gone. This means less than 5% of the disc is affected as they are 8.20mm thick.

Make your own minds up, but I will be getting more days out of my discs in the future.

http://i6.photobucket.com/albums/y211/993rsr/Disc2.jpg
http://i6.photobucket.com/albums/y211/993rsr/Disc1.jpg

[MOD500]

I get mine skimmed when the cracks get too big, though I believe you can only skim them once?

[993rsr]

No idea, would'nt bother personally, unless they are warped.

[Mark_H]

I only used to replace mine once chunks of disc started to fly off!

[Merritt]

I personally consider it too much of a risk to run on cracked discs. For the sake of a set of discs that don't crack, are you really prepared to put your life and you P&J on the line?

I changed the drilled discs on my GT3 to the Brembo floating discs (I know lots of others that have also done this) and they don't crack. Infact they take one hell of a beating. Surely you guys must have similar alternatives?

Steve

[993rsr]

No, on the shelf there are no simple alternatives. Do not consider there is a risk in using these disks that are still usable.

[kiko]

Hey Merit, where did you get your Brembo discs and how much did they set you back? I'm in the need of new rotors on the Turbo and this seems to be a good idea for both track and road.
Cheers
Francisco

[Merritt]

993rsr said:

Do not consider there is a risk in using these disks that are still usable.

As I said, personally I disagree. It would also appear that Porsche disagree as I have a copy of the Porsche Technical Bulletin for checking front discs. The main point in this bulletin is that the maximum permissible crack length is 5mm and they must be replaced if the cracks exceed this length. I will try and scan a copy and post it on here.

Kiko - I got the discs from Parr motorsport & they were very helpful in answering all my questions. I fitted them myself (very straight forward) & I have kept the old discs as evidence should the need arise (the cracks were well over 5mm by the time I replaced them)


Cheers

Steve

>> Edited by Merritt on Tuesday 26th July 08:26

[dds1]

Merritt said:

993rsr said: Do not consider there is a risk in using these disks that are still usable. As I said, personally I disagree. It would also appear that Porsche disagree as I have a copy of the Porsche Technical Bulletin for checking front discs. The main point in this bulletin is that the maximum permissible crack length is 5mm and they must be replaced if the cracks exceed this length. I will try and scan a copy and post it on here. Kiko - I got the discs from Parr motorsport & they were very helpful in answering all my questions. I fitted them myself (very straight forward) & I have kept the old discs as evidence should the need arise (the cracks were well over 5mm by the time I replaced them) Cheers Steve >> Edited by Merritt on Tuesday 26th July 08:26

not to be cynical here, but i'd say porsche have a somewhat vested interest in amking sure you replace disks as often as possible.

I wouldnt advocate waiting until big chunks fall off, but a carrera cup racer was telling me recently how (before they used ceramics) they changed them after every race by which time the cracks were rather a lot longer than 5-7mm and they had never had any problems with the disks crumbling.

I don't say this to encourage people to skimp on safety, more that I would say 5-7mm of cracking doens't warrant disk replacement, I would take it more as a wear indicator that they will need replacing reasonably soon.

[Merritt]

dds1 said:

not to be cynical here, but i'd say porsche have a somewhat vested interest in amking sure you replace disks as often as possible.

I know what you are saying but the other way of looking at it is that they don't want legal action against them for discs failing in use.. I don't disagree that there probably is a margin here but do you want to run it that close?

Steve

[dds1]

I also see what you are saying. I try and replace them when I feel they need replacing, which is typically when cracks look like they might start to join up given another trackday!.

[Merritt]

Here is the document.. Sorry its in three separate links but my scanner is RUBBISH!!!

http://homepage.ntlworld.com/steveb.merritt/doc_p1.bmp

http://homepage.ntlworld.com/steveb.merritt/doc_p2.bmp

http://homepage.ntlworld.com/steveb.merritt/doc_p3.bmp

right click on link and 'save as' to local drive then open normally (otherwise the quality in the web browser is poor)

Cheers

Steve

>> Edited by Merritt on Tuesday 26th July 08:49

[CGF993]

Being a relatively new owner or Porsche can you advise if cracking discs is a common condition to look out for.
My 993 C4S had new discs fitted 5000 miles ago prior to my purchase and to date I have only used it on the road.
Is cracking more prone to brakes being used on track days where more heat is generated and more extreme braking is applied than on fast road use ?

Cheers

CGF993

[johnny senna]

Thanks to 993rsr for this useful information.

This cracked discs business is something that interests me as well as regards my 993 RS. I have exactly the same problem as 993rsr. There is no simple off the shelf non-crossdrilled alternative to the OE Porsche cross drilled discs, unlike if you have a GT3. Pity.

I went tp my local independent Porsche specialist to show him the cracks in my discs the other day, they are anywhere between 2 and 7mm and he told me NOT to replace the discs. He said they are very superficial and they don't affect the strength of the disc at all. They run race cars with discs like this and it doesn't bother them. They tend to measure disc degeneration by looking at the thickness of the disc (as you normally would on most other cars!!).

The cracks seem to appear after 4 track days, especially at Bedford!! I am on my 3rd set of discs this year. I tell you what, I'm not going to change them every 5 seconds due to superficial cracks. I do quite a few track days and this would be expensive and overkill.

Also 993rsr...I suppose you are going to keep grinding your discs as soon as cracks appear? I don't suppose there is any other way of seeing how deep the cracks are? Or can you put an instrument into the cracks to see how deep they are? If I could do this myself at home using such an instrument I would be happier.

[911nutter]

993rsr said:

Took off 0.40mm and as you will see from the attached pictures, cracks gone. This means less than 5% of the disc is affected as they are 8.20mm thick.

that is the most intersting part. It really does put the crack problem (or not problem) into context.

with the cracks being so shallow, does it really then matter if the cracks join hole to hole? you still effectively have well over 7mm of thickness left. why should long cracks be detrimental if there is so much disc thickness left.... are we just changing discs because that is what everybody else does?

why can't one grind 0.4mm off the disc and hey presto you effectively have a new disc?

anyone with any understanding of material composition and heat transfer out there?

[kiko]

When you gring a perfurated rotor don't you have to smooth out (I miss the word) the edge between the hole and the disc surface? Does anyone knows if there is a hi-performance replacement for the 996 Turbo rotors?

[t urbo]

Why do they need to be ground because of cracking anyway? My discs cracked on the first track day and ive used them on 3 further track days and over 6k street miles with no problem. The pads are the biggest problem. I think at bedford the tt's and GT2'2 suffer most because of their weight and the additional speed they gain down the 1km straight.

[lali]

I've just had a set of Brembo cross drilled disks fitted to my 944S2 along with some Mintex 1155 pads.. now im not getting anywhere near as much speed down the back straight at bedford as you 911 boys but this issue around cracks has played on my mind a bit but i took the plunge based on the superior knowledge of my garage who run a brace of very successful racing 944's. My question has to be though... Why do the disks crack? is it a symptom of the massive and sudden changes in heat from cold to stopping something travelling 150mph? and if so then surely it just means that the discs need to be "up to temp" as well as the pads??
apologies if its a dumb question...

[verysideways]

good point - but i suspect it's because the discs are so thick, and have a lot of cooling through the curved vanes in the middle, so the hard pads with our heavy/fast cars means that the disc surface temperature probably goes through the roof going from 150mph to 50mph in a short distance, but due to the thickness of the disc it doesn't heat through that quickly, so the result is that the top most surface gets cracked.

(this is all just an educated guess, i could be entirely wrong)

this would also explain why the cracks don't go deeper.

VS

does that make any sense?