Bill N

Last winter, I replaced the stock brakes on my 1986 951 with S4 calipers. Am still using stock rotor size.

I recently switched to PF 97 pads, and to 032R tires, upgrading from my previous street tires (S03's). A real nice combination on the track, but for one only thing. My rotors cracked. OK, had about three years on them, no problem Replaced with new rotors (OEM), and these, too, are starting to crack after only one event. I am getting my speed up, braking harder, and the 032R's give me that much more traction to do it with.

I will be grateful for any thoughts about how to proceed. Bigger rotors are clearly a good idea, but I am reluctant to go to bigger rims (have two sets of nice 16" Fuchs).

Petevb

You can upgrade to the same rotor dimentions as used on the turbo S and still clear those wheels. These rotors are both a little thicker and slightly larger OD than what you have now. Not sure exactly what part combination you'd want to do this, though- it partly depends on how you've mounted the S4 calipers. Because of the different offset on the '86 cars I'm pretty sure the exact rotor from the 951s/ 928S4 doesn't work, so you might need to go to a aftermaket rotor with adaptor or something similar...

You should also think about changing out your brake bias valve to up the pressure to the rears a little, as the stock 951S had a bit too much front bias.

Stuttgart

How bad are the cracks? You shouldn't worry about replacing them until the cracks connect one hole to another.

Z-man

Bill: Are your rotors solid/vented, or cross-drilled? Also: do you club race or do DE's?

I had the stock 951 / 944S2 rotors on my 944S2. (Solid, vented disks front and rear) This winter when I replaced them, I went with the same Porsche OEM rotors. (Note: NOT the Zimmerman's, but the Porsche OEM stock ones) Why? Last year, even with the rotors just below the recommended minimum thinkness, they weren't cracked, warped, or glazed. (I ran three days at VIR without any problems with these rotors) They worked perfectely. I bought the car with these rotors, and I estimate the previous owner put about 20,000 miles on them (street driving). I then proceeded to put another 20,000 miles on these rotors, and these miles were mostly DE events and autocrosses.

BTW: I use KTP Magnum Gold brake pads, which are considered to be an 'intermediate race' pad. They are also very rotor friendly.

In my experience, I have seen that cross-drilled rotors (even if the holes are cast, not drilled) tend to develop cracks much sooner than solid rotors - it's logical - there are more 'edges' on a cross-drilled rotor where the metal is inherently weaker and most likely to crack.

Another aspect of brake wear that I am learning is brake management - I've begun to learn the proper way to brake to be kind to my brake system. In the past, I used to drag my brakes through the braking zone - this is not good - doing so simply overheats the brakes and tends to cause early warping / cracks / ..etc. There are many others here who can better explain 'brake management,' but in a nutshell, I've adopted the 'brake when you need it, and the pressure you need it' and release and turn.... (Of course, I'm still over-braking too often, but that's another thread!!!)

Hope this helps,
-Zoltan.

GUMBALL

What have you got for brake cooling?

Z-man

Who, me? (Or are you asking Bill?)

I don't have any extra brake ducting, but there's a wind deflector plate behind the caliper that I had removed - didn't do much deflecting, just it did keep the rotor heat buildup from escaping the area.

-Z.

RedlineMan

Hey All;

I will list what I have experienced, and see if I can draw any conclusions that might help here.

My 84 944 has full 86 Turbo brakes. I slot them myself. I had been running Porterfield Enduro pads with great success. I have always been a long & light braker. At one point, I decided that I needed to be a short and heavy breaker to eleviate accumulated heat stress on my brakes.

I did this for the sake of theory because I was not actually suffering and brake problems. Of course I now realize this was dumb. My pads immediately glazed, and before I realized what had happened, the rotors had become effected by being exposed to glazed pads. The surface was burnished, blued, and uneven.

I needed new pads, and theorized that Performance Friction - which had always been a pretty aggressive pad to my knowledge - would machine the surface of my rotors for me. They didn't do that at all, in fact, but I did notice that my rotors began to crack very slowly.

I have a hard time believing it is the PF-97 pads that are doing this. I am more inclined to think that it was the change in braking style that caused it. As Carroll Smith says, these components need to be broken in carefully, and then used consistently. Changes in style or components will effect them, most likely negatively.

It is my theory that a change in components or utilization will cause this cracking to happen in instances where it normally wouldn't. I am about ready to change these cracking rotors out with an indentical new set. I will start them off with fresh PF-97 pads as well. I will find out soon enough what the story is.

GUMBALL

Redline Man:
First, brake pads from one manufacturer usually do not work on rotors bedded or run with pads from another manufacturer. To change pads, the rotors must be cleaned with sandpaper or scotchbrite first, then re-bedded....and follow Caroll Smith - his books are the Gospel for car preperation....
Second, using an agressive pad will allow you to brake harder, which means that you are on the brakes less, which means less heat.....
Try to find a pad that is agressive enough, but does not eat the rotors. If in doubt about which pad to run, call PFC (800-521-8874) or Hawk (800-542-0972) As with most cars, you might need to run different pad compounds front and rear.

Z-man:
The deflectors were there to do just that - deflect air into the the rotors. Regardless, if you are doing track events, you will need to run cooling hose - ducting air to the center of the rotors, (4 inch dia.preferably) The spinning rotor acts like a centrifugal pump - pumping air.

ColorChange

Guys, I disagree on the braking/heat issues. The energy is the same if you brake longer with less pressure or harder for shorter time. The only effective way to "save the brakes" is to go slower and terefore brake less. I can show the calculations if necessary. The brakes turn energy into heat. Your car only depends upon the speed change. Whether it takes 1 sec to brake from 120 to 60, or 3 seconds, the total energy (heat) is the same. The first method will peak the temps higher, but total BTU's, heat generated is the same.

Patrick

The peak temperature is what's causing the cracking, not the heat in BTU's. And while you are right that in a car that is purely decelerating because of the brakes, the total heat is the same from one initial speed to another, that is only true in the ideal. In reality there are two other major contibutors to the equation. First, there is the engine still adding some energy (probably applies to left foot braking and some in heel & toe). Then, there is the speed reduction from wind resistance, rolling friction, mechanical friction (including engine braking) etc. These forces are related to speed, but are fairly constant for a given speed. Therefore, braking from 120 to 60 in one second (is that possible?) puts considerably more energy into the braking system than does a 3 second braking.

Bill N

Hi, everyone, thanks so much for all your thoughts. In response to questions and issues so far, and to note an error in my original post:

1) NOT OEM rotors...they are Zimmermans...Zoltan, do you know of some issues here? They are not drileed or slotted.

2) Bedding the brakes...I could probably have done a better job. Picked the car up, drove right to the track (175 mi), did a little work on the way, and tried to do a little more on warm-up laps.

3) Cooling...I just have deflectors. The plate behind the rotor is in place.

4) Pads. Actually, I switched from the KFP Magnum, which gave me no problems, and seemed to leave the rotors much smoother than the PF's. On the other hand, are more expensive, and I am not sure the braking power was as good as the PF's, though I switched to the S4 calipers at the same time, and it's hard to tell. Maybe I should switch back?

4) Braking style. I am not a real late braker, but neither to I do a "limosine" style. Am probably now tending to brake later (more confidence with better brakes). While it's probably true that it takes a constant amount of energy to slow the car a given amount, I wonder if there's an effect for how the energy is dissipated....perhaps later, harder braking concentrates heat in the pads and rotors, and longer braking gives more time for it to dissipate up the system, compromising, eg, brake fluid instead? I think I heard this argued somewhere.

5) Pretty sure I don't have room for bigger rotors. There is presently 1.5mm clearance between the caliper and the rim!

6) Cracks are not too bad yet, and I think I will likely run another event on them. The brakes are, however, squealing like hell.

Cheers
Bill

Bill Gregory

Performance Friction wouldn't agree with that, although I recall a Pagid rep saying that Pagids should only be used on new rotors or rotors previously using Pagid pads.

PFC says, "If the disc has been bedded with 'other' manufacturers pad materials, the [brake bedding] process will take slightly longer"...."This is because the "other" friction material must be cleaned from the disc, and the Performance Friction "transfer layer" must be imparted to the disc. This procedure [bedding on the track] is the best way to accomplish the removal of the competing friction material from the disk. Sanding/machine grinding the discs will not typically decrease bedding time, and will harm the performance of the disc" (italics are PFC's).

The PFC bedding procedure is: "On the vehicles "out lap", perform several stops, with progressively higher pedal pressure and braking force, and from higher speeds. You will feel the effectiveness of the brakes increase with each successive stop. You should take 6-8 "snubs" per lap and is typically completed in 1-2 laps. If "other" friction materials have been run on the discs, this procedure could take as many as 3-5 laps."

SundayDriver

ColorChange is correct. The amount of heat (which is NOT the same as temperature) is virtually identical. The wind resistance and other factors are almost identical - whatever difference there is, is trivial when compared to total energy. Therefore we can say that the energy is the same, regardless of braking technique. Now, assuming we are talking about the same change is speed, the difference between 1 second of braking and 3 seconds is that the rate of heat generation is higher for the short braking application. This means shorter braking will generate higher peak temps. However, those peaks are not that much different from each other (e.g. if you peak at 1100 degress with hard braking, then doubling the time is not going to drop the peak to 500 degrees. I would guess it is good for only ~100 degrees or so.) If your brakes are marginal, then it will make a difference. If your brakes are well matched to the car, then lengthening the braking won't make much difference. The exception would be a very long enduro where that might make the difference between a needed pad change and not.

Z-man

Bill:
1) Porsche OEM rotors are more expensive than the Zimmermans, but I have heard that Zimmerman makes both of the rotors. Again, from what I've heard, there are a couple of differences between how the two rotors are made - one of them is how the rotor is balanced. I also wonder if Porsche has a higher standard than Zimmerman, and thus the 'Porsche' rotor needs to be made with tighter tolerances. Again, this is only my speculation.
I have only heard that the Porsche OEM rotors seem to work better than the Zimmermans, but have not heard any specific issues anyone has with the Zimmermans.
When I had my rotors replaced this winter, I was given the option of going with either Porsche OEM or Zimmerman. Since the Porsche OEM rotors worked so well on my car, I chose to stick with the more expensive rotor.
2) Regarding bedding the brake pads - IMHO, doing this properly is essential. With the KFP's, I know the bedding procedure is done when the brakes start squealling like a stuck pig in a butcher's shop. That coincides with the better stopping power that is evident.
There is a notable difference between the two: the time the pads are not in contact with the rotors - which is essentially cooling time. Towards the end of a 25 minute DE session, the extra cooling time allowed in a 'harder - shorter' braking style could be the difference between brakes that are holding fine and brakes that are starting to over-cook and fade. When I stopped dragging out the braking zone by braking a little later and a bit harder, I also noticed that my brakes were able to last longer into the session without any fading. Again, this has been my personal experience - NOW, I need to learn to brake lighter and less and trust the car will handle the turns.

There are definately two ways of approaching the braking zone. I personally have found that by adopting a later / harder braking technique works best for me. For now...
-Z-man.

GUMBALL

In regard to heat imparted to the brake system:
-people who are "light breakers" tend to drag the brakes - rather than "on and off", thereby imparting more heat to the whole system.

In regard to changing pads:
If there is any build-up on the rotors from whatever pads that were run previously, it must be removed before new pads are installed, otherwise you will get buildup on top of buildup, reducing braking efficiency, and giving the feel of a warped or cracked rotor.