garrett376

As I need to replace my front rotors, I was curious about something and wanted everyone's advice from experience. My question:

Has anyone had to replace the OEM cross-drilled front rotors before they were worn out solely because of a large amount of cracking? If so, do they more frequently develop cracks necessitating their replacement, or have they made it to where they're truly worn out?

Since I had no problems with the non-cross-drilled rotors at the last race, I don't want to purchase an alternative part that would in fact require replacement before they are truly "worn out" based on the thickness of material left. I am curious if $20/rotor increase in price is worth the perceived benefit of having the rotors cross drilled. If they have to be replaced due to the cracking - my decision is an easy one, in favor of the cheaper non-cross-drilled rotors!

Thanks for your input!

Bill Gregory

You probably know that cross-drilled rotors are lighter, run cooler, and shed rain and brake pad gasses better than a solid-faced vented rotor. Unfortunately, they do tend to crack, and when the cracks connect the holes, the rotor is toast. Rotors that are literally drilled are most succeptible to cracking. Porsche rotors have the holes cast-in during the manufacturing process, which reduces stresses in the metal due to drilling. When I've used rotors with holes, I've had them cryogenically frozen (actually, I only use cryogenically frozen rotors anymore), to maximize their life. The cracking is one reason people use slotted rotors, which retain the metal mass to absorb heat, and have the slots to vent brake pad gasses from the face of the rotor.

Like many things, there are tradeoffs involved. If your brakes are working fine for you now, and your goal is to wear them to the minimum thickness spec, then cross-drilled rotors probably aren't for you.
For our 964's, the Carrera RS rear 299xs24mm rotor has holes, and would fit your C4. To my knowledge there isn't a Porsche rotor with holes that fits the C2/C4 front, although there's one European vendor that makes holey rotors with cast holes.

garrett376

Thanks for the insight as always, Bill. I am shooting for the most economical and functional setup, which appears to place me in the stock setup. I agree with you - and will stick to the non-holey rotors for now. I did notice the holey discs are 2.6 pounds lighter than the non-holey ones - that a lot of metal coming out of there to make 2.6 pounds!

Marc Shaw

I was under the impression that slotted rotors seemed to wear brake pads faster than drilled....is this accurate?

jc964rs

I would keep definitively my oem rotors. Drilled rotors are really more sensitive to temperature changes

On RS, braking is really amazing. But we all experience problems with rotors and, execpt normal use :

- warping
- cracking

These 2 problems are caused by overheat. You must know that the rotors can reach a temperature arround 500-600 degres C

Warping results from excessive use when brand new, or excessive and instant use when cold (the most frequent reason). Overheat or local overheat (for example leaving pads on the rotor when very hot). Drilled rotors ar most sensitive because of the temperature differences they generate.

Cracking is the result of superficial tension due to hard overheat of the rotor. They come quicker on drilled rotors because temperature changes are most important on that kind of rotors. But cracking can be seen on normal rotors (see photo)

Personnaly I would prefer slotted rotors for my RS even if they wear brake pads quicker

SoCalCab

Hi Guys,

I've been reading through the posts related to rotors as I'm having some problems with warping on my 92 C2.

I had the rotors turned shortly after buying the car because I felt some pulsing when applying the brakes at slow speeds. Now, about a year later, the pulsing has returned.

I have to assume the warping is caused by overheating. But, since the car sees street use only, I'm not sure what's causing the overheating. I don't know what kind of pads are on the car, but is it possible that these pads run hotter than others?

I don't think I should need cooling ducts, but obviously the brakes are running too hot. I thought about cross drilled rotors to keep things a little cooler, but I don't see overwhelming support for them here in the forum.

Any suggestions?

Thanks.

jc964rs

In your case this can be caused by rain and too quick change of temperature or an emergency braking with cold rotors on the freeway....But I agree with you, that's amazing

There is one other cause : your rotors are not warped but perhaps you use wrong pads working at a wrong temperature in streets and they leave deposits on the rotors which give you the impression of warping ?

Marc Shaw

SoCalCab - is it possible that the pads are binding a little all the time so generate heat as you drive, even without the brakes being applied?

Marc

Bill Gregory

Mark,

Some things to check. First, the rubber brake hoses should be replaced around every 10 years. Over time, they can swell internally which can affect the hydraulic action of the brakes. Unless you know whether they've been replaced, I would recommend that as one item to do.

You're correct that for street usage, you shouldn't be seeing symptoms of warping, whether induced by pad deposits or true warping. I'm not found of turning rotors as that removes valuable metal which reduces the heat sink capability of the rotor. If it were me, I would go for a drive, come back and take temperature readings on each rotor, to test if there is any binding of brake pads against the rotor. If they are all the same temps, I would get some factory pads, or Axxis or Metalmaster, or something you know is geared to street usage. This should eliminate the possibility that your pad is depositing on the rotor. If one rotor is significantly hotter than the others, you have brake pad binding, which could be caused by a hydraulic line that needs replacing, or a caliper that needs rebuilding. The caliper binding could be due to corrosion/crud under the spring plate or the pistons could be binding.

An expensive option, would be to replace your rotors with cryogenically processed rotors (I use Diversified Cryogenics,www.frozenrotors.com, no affiliation). There's debate on how well they work, or not, however, in my experience, I've found they do make a difference in longevity (that's with track usage, btw). And most 964 racers I know use them. I think the underlying rotor makes a difference too. Rotors supplied by Porsche 'seem' to be better/last longer than OEM.

Unless you have a reason to switch, I would stick with stock rotors for street usage.

SoCalCab

Thanks to all for the feedback. I appreciate the suggestions. Now I have a few things to try to help pin it down.

It sounds like a binding pad could be the problem. Unfortunately, I don't have records from the PO. So, I'm never sure what's been done or what type of part was used. My approach has been, when in doubt, replace it.

Using that approach, I'll take Bill's suggestion and replace the brake hoses and the fluid while I'm at it. I've seen some discussions about rubber hoses vs. stainless steel. Any preferences?

My brake fluid is about a year old. Is there a chance that old fluid could affect the hydraulic action enough to cause a caliper to hang up?

I'm not fond of turning rotors either. And since, I've already done it once. I'll probably go with new stock rotors and street pads once I figure out the source of the overheating.

Thanks again for the help.

Bill Gregory

For street usage, it's hard to beat the Factory rubber hoses, which, fwiw, are now 993 part numbers. No maintenance and replace every 10 years - not many parts like that! Even the current factory racers are using rubber hoses. I also use the Factory rubber hoses.

The stainless covered kevlar/other material lines are popular as they don't have the slight bit of give that rubber hoses have under pressure. Some can tell a difference, others can't. One concern with stainless covered hoses is that they need to be inspected frequently, and certainly replaced more often than every 10 years, in fact some racers replace them each year.

SoCalCab

Thanks, Bill. It sounds like the factory brake components should work fine for me.

alan911sc

Bill, I have cross drilled front rotors on my 1990 C4. I picked them up on Ebay or from the Rennlist classifieds, I forget which, about a year and a half ago. I'm certain they had Porsche part numbers on them. Are you saying that Porsche no longer makes factory cross drilled rotors for the fronts from your post above.

Bill Gregory

I'm not aware that Porsche makes cross drilled rotors that will fit the narrow-bodied (C2/C4) 964's with stock brakes - I know they don't list any in the Porsche parts catalog. However, I'd be very interested in a Porsche part number if you have one.

JoeW

Sorry to be late to the party -- my 93 C2 (RSA) has the 95 993 drilled fronts (also found on the 928S4, I'm told). Now this was done by the PO, and I've never checked vs. a stock car so I don't know if more was changed out than the rotors and calipers.

Just an fyi, if useful.

Joe