Small surface cracks on front rotor
 

Preface: I searched the forum, but could not find an answer to this specific type of question:

I just noticed some small (5mm) surface cracks on my left front rotor. They are radial cracks, typically one large one (5mm) with a few, very small ones near by, ~1-2mm long. They look to be surface cracks. Pretty much in the middle of the wear surface, and in the middle between slots. The rotors have about 34,000 miles on them, and ~10-12 track days on them. The right-front looks fine. The slots are still visible, but the rotors have only about 25% of remaining wear allowance.

The cracks look to be Thermal-Mechanical Fatigue cracks. I.e., initiated during periods of high compressive stress, induced by high thermal gradients through the thickness of the rotor. Cracks open up when thermal gradient subsides. They do not look like fatigue created by high circumferential (hoop) stress.

The rotors are slotted, cyro-treated. Hawk HPS + Track brake pads. No brake cooling ducts. ATE Super Blue fluid.

The question is: How much cracking is OK for use on street and Driver Ed track days? Should I replace them at the first sign of surface cracks? That seems overly conservative. But I just don't know.

I have seen many drilled rotors with significant number of cracked holes and they were used on the track.

I will eventually replace them, but I need to know if I should consider this:

A) "urgent", as in, before my next track day in May, or,
B) "soon" meaning this summer before my two-day event in August, or,
C) "monitor" meaning the small surface cracks are OK, but watch them for growth, and plan to replace them, sometime this year.
D) "Don't worry" as in, they all get surface cracks, and that is not a big deal. Replace when rotors reach servicable limit.

I would replace them asap. I wouldn't want to be in front of you when it comes apart ! But my brakes are very important to me, as I am not that good with bodywork.

What brand of rotors were these if you don't mind my asking.

I'm not 100% sure, the previous owner put them on.

I wrote down the name "Sublet" in my service history book. But I am not sure if that is a name or not. Does that make sense?

Given that your rotors are almost at the wear limits anyway, I'd keep an eye on it during your first track day and in the meantime order some new rotors.

That said, I've had really good luck with the OE Zimmerman rotors. No cracking at all. I have used them for about 10 years, both with and without brake cooling. I use performance friction pads. I suggest that you stick with non-drilled, non-slotted OE rotors for the replacements.

Here's the cooling kit that I have:

http://www.ogracing.com/eshop/home.asp?categ=1

The time to install a cooling kit is when you are replacing the rotors. This one requires that you drill and tap a few holes on the back of the rotors for the block-off plate.

Cheers,

Jeff

Jeff,

I'm curious. Why recommend plain rotors vs. slotted?

BTW, I won't buy drilled, because they tend to crack in the hole during repeated track use.

My personal experience with slotted rotors (ATE Power Disc) was not good (cracking and pad deposits). These problems were completely eliminated by switching back to the Zimmerman rotors with the same pads. It could have been those particular rotors or that particular brand, I don't know. Also, I simply don't think they are necessary.

Cheers,

Jeff

mechanic's usually write 'sublet" where they farm out work to be done, such as machining, or make a purchase for something not in stock.

If you do a search on Jack Olsen, I believe he recently replied to a post, stating the replacement criteria for rotor cracking.
Here's what Jack wrote:
"You need to replace them when the cracks are longer than 7mm, or reach the edge of the disk, or connect any three of the venting holes."

This is like doing "science by consensus" -- it doesn't work that way.

I think Bill Verburg has posted on this. Search up his opinion, Jack's (as per above) and others like Steve W., TRE, etc. I don't know the background of the all the folks posting above, but just running a poll where anybody can vote doesn't give good or safe answers to factual questions.

What I was looking for was a criteria for allowable crack length. I will search some more. But "Jack Olsen" was not a search criteria I would have thought of ;-) Thank you.

The poll was to gauge peoples opinion/passion for the answer.

I saw Jack's response. But I could not find any other thread that discussed specifics. Is there a official Porsche position on this?

As long as you're doing rotors consider getting the 24mm Carrera rotors and calipers to match.

They are a nice upgrade in thermal capacity and no other changes to master cylinder or rear brakes are required.

PCA Technical Q&A Section does mention the hole-to-hole crack limit. The response indicated cracks are OK provided they don't link hole-to-hole.

But my rotors are slotted....

My opinion, and its just that, is that rotors are consumables.

I race, and cooling is very important, of course. Get some cooling, no matter what you do upgrade-wise.

That said, get new ones now. Why screw around. As parts go, they are cheap, and yours are worn out. And you have plenty of time.

Ever see a rotor fail? That's the non cheap way to do it. Besides, rotors with 75 more (wear) material will cool better.

guys, are we talking about slotted or drilled rotors? Haven't heard about slots cracking. Drilled rotors certainly exhibit cracking. Surface cracks at each hole not extending all the way from one hole to the next are OK. If the holes are connected by the cracks, then it's time to replace them.

The real question is this: Do you feel at ease when you step on the brake pedal hard? If you have questions, then replace them. The last thing you want to be thinking about is "will my brakes work?"
I'm not an expert and I didn't stay at a Holiday Inn last night but I would think that you have some life left in them for street but would definitely not take them back to the DE until they were replaced.

The rotors in question are slotted, not drilled. Hence the oddity of the surface cracks. But a good point was mentioned about the thickness. Certainly these rotors can no longer absorb as much as before.

I am trying to understand the balance between acceptable and not. For instance, wear limits exists for a reason. Porsche is a high performance car, and there are Porsche recommended limits for wear. So one could choose to be overly conservative and replace at a much shorter interval, and based on the relative cost, maybe that's OK. But the removed parts are still good for a long time. Do we extend that same logic to really expensive stuff like a engine rebuild? No.

Arguably, brakes are vastly more important than the engine. Meaning, I value stopping far more that accelerating. And being a little conservative on brakes is fine.

Also, if these were drilled, would everyone run out and replace the rotors at the first crack? No. You would not get very much life out of them, and the cost would become significant.

So, there must be a reasonable limit, that provides for a significant life margin so that catastrophic failure is very remote. Crack growth rate is the key. Usage playing a heavy role in the rate.

Why am I being so persistent? As an engineer, I can't justify throwing out acceptable parts, just because they are inexpensive to replace. Because that is an arbitrary measure. I.e., fix cheap things because I can afford to do it. You replace part only when they are worn out, damaged/unrepairable, or have indications of potential problems (reliability).

What I can do is argue for being cautious based on potential risk, and the lack of good data supporting continued use. Some personal experience with TMF suggests this is not an immediate problem. But what I don't have is experience with TMF on disk brakes.

The existance of limits for drilled rotors, makes me think there is a limit for all rotors, plain or slotted. Everyone would benefit if we found out what that limit was, so people like myself (or worse, someone at a track event who suddenly notices the cracks) knew what to do immediately and near-future.

Here is the real life question: If I was one session into a DE when I first saw these cracks, would I have to park it for the day??

Good analysis - the problem will be getting the data...

Well, my thoughts were that with time being a precious commodity, and the results of a failure being rather catastrophic, the actual liklihood was less important, and the analysis was less likely to be done, as time is money.

As an excercise, it would yield interesting results...

But in the meantime, I'd get some new ones on there and duct them properly in time for the next DE.