jerome951

Thanks for the info guys.

One thing I forgot to mention (nomay be relevant) was when the rotors were new on the first, cold day, I was tuning my new suspension. This meant in the morning, I would drive 3 or 4 laps to see how the shocks felt, do a cool-down lap, bring the car in and make an adjustment, run 3 or 4 more laps, cool down lap, pit and adjust, etc. Did this probably 5 or 6 times before I started running full sessions. While I was near the wheels and brakes making my adjustments, they never felt overly hot.

Renn 951, glad to hear this shouldn't be a normal occurrence w/ the OG kit. I used to get the normal crazing after a day or 2, but it took about 15 more events before my rotors started to look like Mike's. Mike, my rotors looked like that after the first day.

Redlineman, your comment about the smooth, new surface is interesting. I'm sure you've seen out-of-the-box OE rotors. Is this why they come w/ a finish that feels like a file? They are far from smooth on the surface...

Now that I have an IR thermometer I'll do some experimentation to verify if the calipers are sticking or not. I'll check the inner and outer surface temps of the rotors after some street driving and see if there is any pattern which may point to a dragging pad. If I don't see any problem here, I'm leaning toward either a bad batch of rotors or an improper break-in process.

Thanks again for everyone's assistance.

ed devinney

Mike B - the checking looks *ok* to me (not that I'm an expert) but I think the rotors are near the end of their life. In addition to the checks, from the pic the ridge near the hub looks looks pretty significant. You'll need to change them out sooner rather than later, and it's early in the season - what better time than now?

Bill935K3

I use 928S rears on my car and had a set of factory drilled rotors crack bad enough that we junked them after 4 days using Pagid orange pads even though we went through the whole drill RE brake-in. We changed to Alcon solids (which we were already running in the front but slotted) and have had no problem since. FYI After road Atlanta we have decided to give Pagid blacks a try as I could use more braking. We got some interesting info from data acquisition, at higher speeds 120+ I was applying 1350 PSI on the fronts with no lock up. Note going to solid rears really changed balance IE locking rears (more than could be adjusted with bias control) we ended up changing rear master cylinder to get balance back.

RedlineMan

OK;

Let me rephrase. A rotor that is broken in has a different surface than a new one out of the box. A fresh rotor - be it new or resurfaced - will not stop the same as one broken in. I might guess that your problem might stem from that. I have noted new rotors do not stop well at first.

Again, this is a theory.

kurt M

Shock failure, single or short duration overload. Fatigue is more a fail mode of repeat load or repeat non failure overload. Rotor metal alloys are most often chosen for thermal and casting attributes. The metal is designed to accept thermal insults and imbalances but can still be overloaded short or long term. One way to crack a real hot rotor is to splash it with water. Water has 600 times more heat conductivity that air and to that add the increased heat absorbtion from change of state (evaporation) you can create extreme imbalances in temp.


(Sorry Jerome, Little hijack)
“Note going to solid rears really changed balance IE locking rears (more than could be adjusted with bias control) we ended up changing rear master cylinder to get balance back.”

This is just the kind of data I am presently looking for. Did you change anything else other than the rotor, IE pads? Do you know of any good internet source for this kind of data?

Matt Marks

Jerome - are you using a lot of ABS?

Not sure if this makes a difference, but there was an interesting sidebar in GRM this month from the GT2 team (or whatever the call it) at Daytona that runs a monthly diary for GRM. One of the drivers was having consistent brake problems with one side of the outside wheel (in the case of daytona, this was the passenger side) brake pad being worn away to nothing, while the inside pad had plenty of life. They eventually figured out that their ongoing brake problems only occurred with one of their 4 drivers, and traced it back to where the PSM - even though disabled, was still cycling - it's still on in the background. It apparently only used the outside pad of the caliper to provide the occassional stability change, but that ended up wearing out that particular pad and put him in the wall.

I'm assuming that we was pitching the car in on a regular basis, so the PSM squeezed the outside brake with just the outer pad just a hair to try to induce a touch of understeer tokeep the car neutral.

Anyone knowledgable on ABS on a 951S that might have some insight on whether or not the ABS only pumps one side (the outside perhaps?) of the caliper which may have contributed to overheating the outside of the rotors?

jerome951

Yeah, that's it. It must be my PSM kicking in. ;-)

The ABS controls pressure to the caliper (only 1 hose going to the caliper) so it can't differentiate between outer and inner pistons.

I've been pretty gentle on the ABS this year because I'm not pushing the car to 100% as I learn how to drive a new suspension. At Summit I frequently trigger the ABS going into 5 and occasionally going into 1, but that's it.

Alan Herod

I agree with Ed. The cracked rotors appear to be more then two days old. I cracked a rotor like (the big crack that went through) a few years ago when the rotor was near the wear limit. I have relatively new rotors on the SC that I installed last year and they have about 8 days on them. The non friction surfaces do not look near as aged as your cracked rotor. I suggest that you 'mic' the rotor and see where you are with regard to the wear limit. When I cracked the rotor at Summit Point, braking became more interesting. I brought the car in and visually checked everything without removing the wheels, but the crack was hiding behind one of the spokes. Of course all the rotors that I have in the garage have similar cracks to the smaller superficial ones shown in the picture. Also I agree with others that have posted and don't believe the cooling air can provide significant enough quenching to crack the rotors. I am not sure what the effects of driving through a big puddle would be with super-heated brakes. Call Paul Weston, he is one of your fellow region members who exercises a 951 similar to yours. He may have some insights.

Edited -- Don't know why I got that wrong and sent you a PM with more info.

dave morris

Paul Weston. I think that's what Alan meant.

jerome951

Thanks all.

Paul Weston is helping me and based on some additional pictures, I think he identified the problem (hopefully).

When I routed by brake hose, I had to make a severe bend to clear my new sway bar, greatly reducing possible airflow to the rotor eye (likely less than I was getting from the factory setup). I am going to reroute the hose so it is more straight and see if that makes a difference (which it should).

I thought that the force of the air at track speeds would create enough pressure and flow enough air volume through the hose kink, but methinks that was an incorrect assumption.

smokey

I had the OG Racing Cool Brake kit, but switched to the 968 M030 scoops: simpler,with fewer problems like kinked and worn hoses.

a4944

Smokey, are the 968 M030 scoops much better than the stock 951 deflecters? I can't touch the lug nuts after a run and wheel weights keep falling off, even with aluminum duct tape. My brakes are getting very hot. I took the splash guards off last time I installed rotors. I think this may have been a mistake.

Mark

runnin' on empty

I note that the two major 'thru' cracks are 180' opposite each other; could this be the the result of a very slight, but significant 'folding' stress on the rotor due to hub 'crowning' or uneven torqueing of lug nuts? Or could a 'shock' load force such as the striking of a 'gatorback' curb induce such fracture? Perhaps a number of small factors adding up to the complete result? Sorry, more questions than answers; I know.

smokey

a4944: the 968 M030 deflectors are much better than the stock 951 deflectors. The 968 M030 ones have a scoop on the A-arm and another one on the strut, while the stock 951 deflector is fed by the scoops and ducts in the grille. The 951 duct on the passenger side is partially blocked by the oil cooler. At one point, I removed the OG Racing Cool Brake kit, and went back to the stock deflectors, and there was a noticeable deterioration. The Cool Brake kit and the 968 M030 scoops are both good, and probably about equal when the Cool Brake hoses are in one piece and unobstructed. However, the hoses wear and kink. With the 968 scoops, you have to be careful about damaging the ones under the car, but I've had no problems. I can't imagine using them in the snow, however. They're standard Porsche parts, and Diversified Cryogenics (if I recall correctly) sells a kit with a special bracket to mount the scoop on a standard control arm. That's what I used. The 968 control arms have little bosses cast in to mount the scoops. (Sorry to tangent the thread.)