jerome951

Hi All,

Hoping some of you really experienced track junkies can shed some light on a recent problem with front rotors cracking on my 951S.

Pics of the rotors after only 5 track days are here:
Cracked Rotors

Background:
Over the winter I upgraded the suspension on my car from stock to full coilovers, hard bushings, etc. Shaved at least 4 secs off my lap times, which is a good thing. One of the upgrades I also installed is the OG Racing cool brake kit which consists of scoops mounted in the front dam, a 3" flex hose, and a plate that feeds air directly to the eye of the rotor. I had occasional brake fade in stock form and wanted to eliminate that now that I race occasionally.

I've always used OE (from the dealer) rotors and have had good luck with them. They eventually develop minor surface cracks to the edges, but not after ~15 events. The Zimmermans I tried once had to be tossed after ~4 events due to cracking. I use PFC97 brake pads and am happy with their performance, longevity, and rotor wear.

I started this driving year with a new set of OE rotors and they developed surface heat cracks (almost to the edge) after the first day of driving. I noticed abnormal cracking after the first session. Since it was a cool weekend (temps from 30-50), I partially blocked the brake scoop intake with some tape as I assumed the rotors were getting too much cool air and undergoing rapid heat cycling (I had no brake fade at all so they weren't being overheated). However, after blocking some of the scoop inlet the cracking didn't seem to be slowing down.

During the 2nd day of my next event (VIR), I noticed some serious shuddering under braking. When I checked my rotors, I found that the front right had 2 cracks all the way through the rotor surface and the left rotor had one. It wasn't a real cool weekend so I hadn't blocked off the cooling scoops. The brakes weren't getting overheated as I had no brake fade at all. I've never had a set of OE rotors develop cracks that went all the way through the rotor surface before. All 3 cracks were on the surfaces facing toward the outside of the car.

So, any advice is appreciated.
-- Am I getting too much cooling? (if that is possible).
-- Did I likely get a bad set of rotors?

I've put on my backup pair of new OE rotors for my next event and will try again. My budget can't support a new set of rotors every-other weekend.

Thanks for the input.

ColorChange

Too much cooling is extremely rare and would only reduce your braking efficiency by not having the pad at its optimum temperature. It would have NO incresed cracking effect on the rotor and could only cause less.

It is difficult to judge the quality of the rotors, but that crack looks brutal. My guess would be a defect. The only recommendation I might have is to try a slotted and not a drilled rotor. Drilled rotors notoriously crack due to local stress concentration and this is much less with slotted (not cut all the way through) rotors.

Finally, keep a closer eye on your equipment. That could have exploded on you and that's not good when only three (or fewer) wheels are braking.

hinchcliffe

Wow, I've never seen cracking that deep. Maybe too much cooling. What fluid are you using? Maybe a higher temp fluid without the cooling ducts and see what happens.

Does anybody use the Cryo-rotors, "Frozen" rotors? I have them on my street car and 1 DE event and 10-12k miles street driving and they are just starting to wear past the finishing mark on the original surface. I'm using Hawk HP+ pads. I know, i know, I'm not braking hard enough. Maybe this year.

jerome951

I always use ATE Type 200 (Gold), though this shouldn't have an effect on rotor temps and wear. I'd hate to run w/o cooling and risk brake fade (I would occasionally get brake fade w/ Type 200 fluid and the stock air deflectors on the struts).

ColorChange:
When I started my last session I had some pretty severe brake shudder. Brought the car in and found those cracks. They must have opened up while sitting from my last run (no shudder during that run, though). I'm always keep an eye on my equipment but must have missed the cracks or they were hidden by the pads or wheel spoke. Another brake expert indicated that it's difficult to get too much cooling unless I don't get up to the operating temp of the pad, which is pretty unlikely.

Oddly, I've used about 4 or 5 sets of OE rotors (from a dealer) without this problem. The only things I changed over the winter were the cooling system and new OE rotors (and that I'm a few seconds quicker now).

I'll try another set of rotors and see if I have the same problem. I'd hate to throw an expensive bandaid (e.g. slotted or drilled rotors) at the problem until I figure out the cause.

Thanks for the advice.

hinchcliffe

Cracking is caused by either a quick change in temp or intense overheating. I was thinking maybe if you had high temp fluid and no cooling ducts the brake fade would be cut down. But, since you have the high temp fluid anyway, not sure what to do. Change out the rotors and let us know what happens.

Glen

Impossible to get too much cooling. WHile You are checking things out, when did You rebuild Your calipers last? Check for Pad drag caused by pistons in calipers getiing sticky and not retracting as easily as they should. Many times that can be the cause of premature cracking and warping of rotors that is overlooked. Very easy to rebuild Your own calipers btw.

APKhaos

That's a very nasty crack for a stock rotor!
The light surface crazing [top pic on p2 of your pics doc] is normal. The massive crack is not.

The over cooling theory is not the cause of this cracking.
Glen's point on pad dragging is a good one, and worth checking.

Also take a look at the geometry of the calipers and rotors. You're running with front suspension that has been seriously reworked over the winter. Its possible that the caliper to rotor alignment has been tweaked by some of this work. A small mis-alignment would cause significant forces - enough to cause this problem. Its just something else to check. That cracking is deeper than anything I've seen, so it possible that something other thantypical heat cycle stress is the cause. Best to carefully check the new front suspension first - that's where the major changes happened between last season ans the start of this problem.

kurt M

Did you change pads? Some pads are harsh on rotors and new pads insulate some and keep more heat in the iron.
Are the cooling ducts working? Installed is one thing, doing something is another. If they are not forcing air in then they are blocking it. A pinched hose or the intakes in a spot where the induction is compromised or the like. Get a Raytec or similar IR non contact digital thermo and read the rotor temps when you come in each time to see what you are getting to. Most times a real high overheat makes the iron discolor even in the non swept areas and I did not see this in the images.

You just might have gotten a bad batch of iron too. I saw a set of rotors with the same size cracks on early 911 in GT-3 class. The rotors would heat up and made a thumping sound and feel but felt fine when cooled off. The car never had this failure prior and did not do it again after the rotors were replaced.

Adam Richman

About 4 years ago (think there is a thread on here about this), we had something like 7 cars (BMWs, Porchies) all significantly cracking the OUTSIDE side of the rotors on day one. Temps never got above 40° and everyone of the 7 that had this issue had brake ducting. We supposed it was extreme cold air coming in against the outside causing the cracks (VIR has some long straights), everyone was removing or completely blocking the duct intakes on day two. A few more folks that weren't part of this discussion of closing off the ducts had the big cracks on day 2, no one that covered up the ducts did however.

This was a NASA or Speedtrial event IIRC - it was either Dec/Jan at VIR Full.

Bryan H

I too have had rotor cracking to the point of peices comming off on my F class 911. This was when I ran PF 97's. I also had seal failures in the callipers. I had 3 seal failures in one weekend at TMS. The theory was those pads passed too much heat to the callipers. In any case a change to Hawk Blues fixed the whole issue. I now replace my front rotors once a year and the rears every other year from wear instead of half that time due to warping.

Bryan

jerome951

Thanks for the info guys.

The cracking was on both rotors. One rotor had 1 crack, the other 2. All cracks were on the faces toward the outside of the car.

Tony, my front suspension work was only struts and sway bars. A-arms and spindles have been in place for 2 years or so. I'll check everything at the tech session next weekend when it's on the lift. I did pull off the hub to install the cooling backing plate. Is it possible that an over-tighted wheel bearing could cause a rotor misalignment?

Kurt, I used a set of old pads from last year at SP in March then switched to a set of new pads before Zone 2 VIR. Didn't run the old set lower than normal. I have had rotors discolor a little when overheated (bluish tint) but I didn't see it on these. I think I was getting decent cooling as when I came off the track and did the 'warm the hands near the wheel' test at SP they weren't as hot as before. There was a nice line of dirt on the inside of the wheel near the vanes so it is blowing something through. I did pick up an IR thermometer (only the 500 degree model; gotta love Harbor Freight) for further troubleshooting.

Good thought on the brakes dragging. I thought that a possibility as well. Pistons seem to retract as before and pads are wearing evenly. Would seem odd that both calipers would begin dragging the outer pad at the same time, but it is possible.

I'll try another set of rotors at the upcoming Mid-Ohio event and hope for the best if I don't see anything in the suspension.

jerome951

It sounds plausible that the rapid heating and cooling could cause cracks. Odd though that the cracks are on the outside surface when my ducts feed air into the eye of the rotor (equally cooling both surfaces), but I guess it is possible. Maybe the surface cracking after my first day was so bad the rotors were permanently damaged and then slowly degraded over the next 4 days.

I've been using PF97s for about 3 years and have never seen this problem even with lesser brake cooling. The first few days my pads were thinnist (e.g. <1/2 pad thickness left) but I've run them down to the width of the backing plate every time without this issue.

Renn 951

Jerome:

I've had the OG Racing cooling kit for 3 years now with no problems. Last year I upgraded the front brakes to the S4 brakes (same as your Turbo S) and started using PF97 pads, with the same OG cooling kit. I got the light surface crazing early, but no other cracking on those rotors after a lot of use over the last year. While running at Summit Point in March with the cold temps I never bothered to close off the cooling ducts either.

When I took the car in for VA safety inspection in December, the first place I went to told me my rotors had "dangerous cracks" and they would fail me. He didn't buy my argument that it was just surface crazing and the car had been repeatedly inspected by Porsche mechanics over the last year with no cause for concern. Next place I took it to never raised an issue; he just wanted to know why my "parking brake" light on the dash wasn't working (wire connector by the handbrake lever had come loose!)

Mike Buck

That is a dangerous looking crack Jerome. Below is my left front rotor as of today. 24 track days on it. First 20 on a one set of PF97's, last 4 on a new set. I added the 968 cooling deflectors late last year, but before that I had no special brake cooling. My struts didn't have the factory deflectors either. Does this look more normal to everyone?

I was thinking about replacing these before the next DE.

RedlineMan

Hey;

Metal fails because of shock. You will not be able to cool them fast enough or severely enough to cause this problem. It is my opinion that something is causing a severe overheating situation.

It's my guess you did not break the rotors in properly. Slow and steady buildup to full braking is key. When a rotor is new, it is far too smooth on the surface. This smoothness actually can create extreme heat because the pad is not gripping the rotor efficiently, but sliding accrost it. It is similar to if you glaze pads.

The ONLY time I have had cracking was when I failed to note glazed pads soon enough. The rotors turned blue and crazed on the surface. This glazing resulted from a conscious decision to change my braking style, in this case going from long & light braking to max threshold style. Components broken in with one particular style do not like to be "shocked" by being forced to do something they are not accustomed to.

In your case, since your rotors are new, they are not used to ANYTHING. You must acclimitize many things in life to their surroundings before they will work properly.

Just a guess.