Mike:

Great thread,...:)

There are a LOT of variables at work here that affect how brake rotors behave:

1) Peak temperatures. These can be measured with heat-indicating paints made specifically for this task.

2) Rotor material. Although cast iron looks pretty much the same, there are major differences between OEM grade cast iron and the metallurgy found in professional racing rotors made by Brembo, AP, and Alcon. I don't have room to go into all the specifics, but a conversation with race engineers from these manufacturers will provide that information. From a user POV, real racing rotors are FAR more durable and crack resistant.

3) Rotor design. Several variants here; slotted, drilled, AP's "J-Hooks, and many kinds of vane designs made to control thermal expansion. Nextel Cup cars have contributed to huge inroads in rotor designs due their rules requiring 15" wheels and rotor size contraints. Try stopping an 800+ HP 3400 lb car on 15" wheels for 3 hours,...:)

When I see a Porsche OEM plain rotor with surface cracks, I know from experience that its seen major temperatures and certain brands of race pads exacerbate the propensity for these to crack. Improved cooling can help but I don't get overly concerned unless the cracks grow and reach the inner annulus or outer edge.

In most cases, controlling fluid & pad temperatures are usually more immediate concerns,...:) :)

I'd conclude by mentioning that many years ago, a very qualified engineer who was very active with PCA and worked at Weissach for awhile witnessed an on-track accident where an improperly slotted set of rotors on a 911 cracked across a slot and split in half. This triggered a locked wheel at high speed and the car went off course and flipped several times. I have no issues with slotted rotors as long as they are properly done.

Naturally, I've just touched on a very complex subject but there are LOTS of good sources for information out there and I'd offer that one should look at other venues of racing to learn a thing or two.

I do hope this helps a bit.

Steve, what constitutes a "properly done" slotted rotor?

BTW, great thread. First I'd heard of slotted rotors cracking. And of course, I have slotted rotors in front on my 73E. :D

Hi Jim:

The slots should never extend to the inner annulus or the outer edge and should be no deeper than 2mm.

How long do you think it will take a 5mm crack to grow into a 10mm crack? Given that the progression will be nonlinear wrt time.

Yes, the cracking is heat related, cooler will always be more durable in this regard. You need to do everything you can to cool things down.

If I noticed them w/ 1 more run, I might take a calculated risk and finish the day, if I noticed them w/ a week of preparation ahead I would replace them.

If you can't cool them, I would think about upgrading

For the street you can use them for a long time that way, they will never get hot enough to matter.

My opinion - and I run slotted rotors now - a crack in a drilled rotor - follow the recommendations = no more than 5mm - no more than adjoining 2 holes - I'm a pansy though and would probably replace WAY before then... but plenty of folks have run this way - there is a history.

If I saw one of my slotted crack - they would be replaced before I got on the racetrack. As Bill said - these are nonlinear. running something you have the opportunity to replace is a pure crap shoot. Doing on a closed course, you being the only driver is one thing - doing it where there are other cars/drivers at a DE is wrong - because now you are taking a risk with other people's cars/lives, etc.

Bill,

In my experience TMF cracks start out very quickly because the thermally induced compressive stress yields the metal, and the cool down is when the crack forms due to TENSILE overload. Then....they go slow.

To a large extent a TMF crack is a stress relief crack. So the formation might occur in one really hard run, and then subsequent runs to the same or lower level will not cause the crack to grow. I.e., the metal yielded once, it will not exceed the yield strength again until harsher conditions arise.

The fact that the crack is open, and plopped right in the middle of uncracked metal (when the part is cold) is clear evidence the metal yielded in compression. I have seen startlingly huge TMF cracks that were "benign"!

What will cause cracks to grow in a increasing rate is the underlying stress in the rotor. If there is high tensile stress, then the presence of a crack can reduce the capability significantly. But if the stress is moderate, then the crack may never grow.

Oddly, the TMF cracks are not easily visible this evening. Seems a leisurely 75 mile round trip to work was sufficient to smear over the cracks. When I got to work this morning, they were hard to find. This evening, hard to see. And would be impossible to see if you did not know they were there. I must have caused them (or opened them up a little) during my "spirited" drive Saturday night. Picture a Interstate clover-leaf interchange, and light traffic. Several opportunities to turn right. My Saturday night drive was *Just* like that :)

But I have to admit, getting data is proving to be difficult, and yes time is of the essence. I'm getting close to buying rotors....and pads.... Cooling ducts will have to wait till late this year.

Thank you everyone for the input, although your opinions are not "Data" they are importance when trying to make a judgement call.

I'll keep looking, and until I get rotors, I'll keep pushing them hard (data collection, you know)

I can't really add a lot to the excellent points made here...but here are a few:

- people talk about crack propogation with holes...and advise to go with slots to alleviate the problem. Well...if the "slot" is ( IMHO) "cut" as a channel, with very sharp corners forming the "trough" of the slot...then you're not much better off as to the possibility of starting a crack. I would propose a "U" shaped or even hemispherical (lower half of a circle) cut to lower the local stress concentration.

- Porsche publishes very specific recommendations as to what constitutes an "acceptable" crack and goes on to show pics of "unacceptable" cracks too. This is in the workshop manuals..probably under a "Turbo" supplement. I did a quick scan of my manuals and couldn't find it. It mimics what Jack Olsen is quoted as having said here...but I couldn't verify the specifics. I do recall it was influenced by the number of cracks, are they in a radial direction, and do they connect "X" number of holes. If I can find and quote it... I will in a later post.

Wil

Interesting web sites:

Science Good summary of brakes and it does mention cracking. I quote Larry Carley, "Some minor surface cracking is tolerable and can often be removed by resurfacing, but large cracks or deep cracks weaken the rotor and increase the risk of catastrophic failure"

DOT Rules

exerpt: Appendix G to Subchapter B — Minimum Periodic Inspection Standards

Brake Drum or Rotors. (cause for rejection)

(1) With any external crack or cracks that open upon brake application (do not confuse short hairline heat check cracks with flexural cracks).

So the Federal DOT is letting surface cracks go, provided they don't open when the brakes are applied. I never thought the DOT would be big risk takers.


Track Use PDF DBA Talks about heat checking, and etching. Tough to say if they allow some cracking or not. Do you remove and inspect your rotors after every track day?? :)


Based on this limited web search, I think the small surface cracks are OK.

Wil,

Great!! I would love to hear what Porsche says. One picture worth a thousand posts....

I agree the slot weaken the rotor, but done correctly the remaining thickness holds it together. My cracks are between slots, so the extra thickness there helps.

If I had a visible crack in the slot iself!! Wow! I would not have started this post! I'd be ordering rotors right now.

Throw on some solid rotors, the crack thing becomes moot.

I have considered drilled or slotted rotors but good advise steered me away from them.

Mike,

Please replace your rotors before the next event. I don't want you running into me at the track. Besides, there will only be room on the trailer for one car. ;-)

ajt

AJT,

I'm working on it!!!!

Of course I'll be behind you, you'll be holding me up. LRP NOOB! :)


Those are words to choke on, aren't they? As the 944 sails by... Bet you won't pass me under braking :D

Mike

P.S. Did you intend on editing your post at 9:44 PM?

I have polled three people who race competitively. One in GT3S (911), another in GT2S (928), and the third had been racing in SCCA ITA. They basically are in agreement with Steve@Rennsport:

All three say the small cracks are OK. All three say you must replace when the cracks get close/reach the edge.

So, by people who really care, their experience is: small surface cracks in the rotor surface are OK. Pay attention to them. But don't feel like they are not track worthy.

For me, that is good experiential data, is consistent with many sources (including Federal DOT Inspection Standards)

IMHO, here is my personal inspection criteria for slotted and plain rotors:

1) Small surface cracks are acceptable in the middle 1/3 of the rotor surface. :)

2) Cracks that begin to extend into the inner 1/3 and/or outer 1/3, or cracks that appear to have grown significantly since last inspection, are cause to replace the rotor, and should be changed as soon as possible. , Stop DE, and drive home very carefully.:(

3) Surface cracks that reach either inner edge and/or outer edge, are unacceptable and rotor should be replaced immediately. Stop DE, and drive home very carefully if a short distance, otherwise trailer it.:mad:

4) Deep/through cracks are unservicable, do not drive. Trailer car home. Count your blessings:eek:

5) Any crack in or adjacent to a machined slot is the same as #4.:eek:

This is just my criteria. Your's is up to you. Thank you everyone for your input.

Mike,

Glad you changed your brakes. It was a fun event. Hope I didn't "hold you up".

BTW, you were going really fast when I passed you. ;-)

ajt

Yes Andy, you passed me in the wet....

For those of you who participated in this thread, or who looked on and read all the posts, here is what REALLY happened....

I ended up changing the rotors. But not because of the surface cracks we talked about:

1) The rear rotors were just about at the limit. I think they were the originals with 104K. So it was going to happen anyway this season.

2) While carefully inspecting the fronts, I did find one surface crack at the ID next to one of the slots. Based on my criteria, this is unacceptable. So it became a moot question. They had ~30K on them.

BTW, my slots were the "bad" type. Slots ran all the way from the ID to the OD, breaking the extremes of the wear surface.

It was a good thing I changed them, because along the way I found the LF hub inner bearing seal was severly torn. So new bearings and seals all around. The rain would have destroyed the bearings in short order if I had not replaced the seal.

A little bit of a panic job since I had to order parts and have them FedExed.

Thank you Wayne and Co. for getting me new bearings and seals ASAP!!!

AS you might have surmised, Lime Rock was a real puddle. Rain all day long. The 911 was sliding around a lot. Andy was able to come up to speed within 4 sessions, and was a little quicker than me at the end. Ego, and the car, remained intact :)