What Will Crack Drilled Rotors??

North Coast Cab · North Coast Cab's Garage

As I work through my brake issues I'm looking to replace the front rotors and pads. I'vead about cracking cross drilled rotors. So...I drive the car on the street, 3-4 autocrosses and 1-2 DE's per year. Should I go with solid, drilled or slotted rotors? I know the slotted ones seem the best but they are very expensive. Are the cross dirlled rotors a much better performer, or should I just stick with solid?

JG

Jack Olsen · 09-07-2003, 09:43 AM

I'd go with solid or slotted. The drilled ones may look cool, but they crack. I don't know of any significant advantage the drilled ones have over slotted.

Randy Webb · 09-07-2003, 09:48 AM

THey might run a bit cooler, and might weigh a bit less. I think PAG has ones with the holes cast in, so they won't have stress cracking like drilled ones. If PAG is still making holed ones, then you can bet there is some advantage, tho it might only be manf. cost.

I would definitely avoid the drilled ones -- failure could be catastrophic.

350HP930 · 09-07-2003, 09:54 AM

The danger of the small superficial stress cracks that develop on the surface of cross drilled rotors is highly exaggerated.

masraum · 09-07-2003, 10:00 AM

I'd go with stock solid rotors.

There are two trains of thought.

1 By drilling you remove mass, by removing mass you remove the capability to absorb heat which is the primary purpose of rotors

or

2 By drilling you create more surface area thereby aiding in cooling which is necessary to dissipate all of the heat that the rotors absorb. Also it may make the unsprung weight lower.

Personally I think the solid rotors will perform as well and will last longer.

island911 · 09-07-2003, 11:28 AM

Quote:

Originally posted by 350HP930
The danger of the small superficial stress cracks that develop on the surface of cross drilled rotors is highly exaggerated.

I totally agree!

It's sad that this "convetional wisdom" of "cast" being better than "drilled" is so wide-spread. (there have been so many poorly drilled versions, I suppose.)

In the mid-80's, after much testing, Porsche found that drilled, and chamfered (or radiused) holes out performed the cast holes.

The drilled have the big advantage, over the cast, in that they don't collect and hold the brake dust, like the cast holes.

Also, even solid rotors can develop cracks.

kellzey · 09-07-2003, 11:31 AM

My Boxster nas drilled rotors in the Front.

ChrisBennet · ChrisBennet's Garage

The "problem" with drilling rotors is often one of misapplication. When you have a big beefy rotor and you cross drill it, you get little cracks - like "crows feet". When you take say an SC rotor (a relatively thin rotor) and you drill it, you get cracks that propagate much quicker - right out to the edge of the rotor. I must admit I haven't heard of this happening recently though. Well not since say last week.

A friend tried some drilled (Brembo? ATE?) rotors on his '69 track car. He was surprised to see a crack propagate from one of the holes right out to the edge of the rotor.
I haven't tested this personally but I was under the impression that pad outgassing was not as big a problem with todays racing pads and thus the holes are solving a problem that doesn't exist.
(Of course, if you are a rally driver on a wet stage, the holes do serve a purpose.)
-Chris

89911 · 09-07-2003, 01:46 PM

I just got my custom rotors back from Brembo and they didn't give me any choice. They came that way.

Bill Verburg · Bill Verburg's Garage

There are no oem rotors supplied w/ holes smaller than the 299x28 and 304x32 Brembos supplied on some Porsche models. Note that these are much larger than the 282x20 or 24 used on a 911 oem.

Porsche has not used drilled rotors since the '70's, all oem rotors w/ holes have them cast in. In addition the internal structure of the rotor is not the plain internal structure used in the smaller rotors. It is designed to complement and reinforce the cast in holes. Here is a pic of the interior cross section of a drilled Brembo

one of these is ok to drill and good for track use one is junk, can you tell which is which?

Hint there was a vendor here telling us how good the poor ones are.

CB hit the nail on the head, the smaller 911 rotors are just not beefy enough to properly support the holes in extreme use.

Take a look at the thermographs in this thread where all of this was discussed back in Feb.

Bottom line; plain or slotted is the way to go

350HP930 · 09-07-2003, 02:16 PM

BTW - The new porsche ceramic brakes are also cross drilled.

How bad or outdated can it be if porsche's new $10K brake option is cross drilled.

I have seen a couple low mileage units in the shop with the same superficial stress cracks that you can find on steel cross drilled rotors.

dd74 · 09-07-2003, 02:20 PM

I've heard higher pedal pressure is required for drilled rotors, and overall, for the street, they aren't nearly as good as the solid rotors. My wrench does not recommend them for street-driven cars.

Bill Verburg · Bill Verburg's Garage

Drilling does not affect pedal pressure in any meaning ful way. It was originally used for lightening, wet bite and to help the outgassing from pads. The first 2 are stall viable the last is no longer an issue.

For street use it is a toss up, Most race cars do not use drilled rotors.

Slots do help keep crud from accumulating between the pad and rotor thus enhancing the initial bite and reducing noise.

dd74 · 09-07-2003, 03:28 PM

Can someone, if they felt so inclined, just drill out their solid rotors themselves? Is there some special process or technique to the drilled rotor?

350HP930 · 09-07-2003, 03:39 PM

Yes, you could cross drill your own rotors but if you did so it would be beneficial to either apply either a heat or cryo stress relief procedure afterwards.

Most metals are strange beasts and do not appreciate holes in the middle of highly stressed pieces.

You may sometimes hear them referred to as stress risers by us engineering types.

North Coast Cab · North Coast Cab's Garage

Thanks for all the input.

I will be purchasing new solid rotors.

JG

jazzbass · 09-07-2003, 06:10 PM

Ask APKhoas about cracking drilled Carrera rotors. I saw the drilled rotors (Zimmermans IIRC) he just changed out last month (said he put them on in March) and both of the fronts had cracks that ran from the hub to the edge of the disk. It wasn't "superficial surface cracks" around the holes. Convinced me to stick with solids.