Newbie question about brakes.

[Crystallized]

Be gentle, since I really don't know the names for basic stuff. If I get it wrong, don't laugh =)

I wanted to know what exactly the holes in the discs do for braking? Does it just cool it down? Enhance braking? I usually see it on expensive cars, or those AMG Mercedes Mods or most Porsches or BMW M roadsters.

What do they do?

[shiyan]

I'm not sure myself, but I think they:

reduce weight of a larger size disk, improve cooling, help remove the build up of brake dust which reduces brake fade.

[Dammmittt]

all but the brake dust and brake fade.

Depends on the disk, but ventlitating the discs either by drilling or slots helps greatly with cooling, performance and wear. Also helps reduce warping on the rotors. Brake dust is dependant on the pads...nothing to do with the disc itself.

[pbdm4k]

Slotting is stronger than cross drilling.

Drilled rotors tend to crack under extreme use. It's also one reason you will never (well, almost never) see cross drilled rotors used in professional motorsports. Dimpling and/or slotting is better in the sense that it offers the same cooling effect as cross drilling but maintains the strength and rigity of the rotor.

[AP Racing - Chris_B]

Originally Posted by pbdm4k

Slotting is stronger than cross drilling.

Drilled rotors tend to crack under extreme use. It's also one reason you will never (well, almost never) see cross drilled rotors used in professional motorsports. Dimpling and/or slotting is better in the sense that it offers the same cooling effect as cross drilling but maintains the strength and rigity of the rotor.

Very late to the game here, but in case anyone stumbles across this thread, some things need to be clarified. Drilling rotors give the following advantages (proven in lab tests and lots of track testing, not theorized):

1. Increased initial (cold) pad bite as pads get up to operating temp quicker. The multiple leading edges are also believed to contribute to the performance gain.
2. Better cooling at very high temperatures (with a negligible benefit at lower temperatures). This appeared to vary a bit with hole size and hole density.
3. Much quicker recovery when rotors are wet as the water vapor has somewhere to go.
4. Trivial reduction in weight, usually less than 3/4 pound in the front and 1/4 pound in the rear.

Disadvantages:
1. Will crack sooner under severe usage (heavy track use or canyon bombing). All iron rotors will start cracking eventually from repeated and severe heating and cooling from elevated temperatures. This starts with the tiny spider web-looking cracks called "crazing". Cross-drilling accelerates this occurrence depending on how they were drilled, size of holes, number of holes, where the holes were placed and the type of chamfering (if any). This is not an issue at all on the street, even for spirited driving (unless inferior quality iron is used and/or they are improperly machined -- which lots of companies do, unfortunately).
2. In some cases, and it seemed to depend somewhat on pad material, a slight increase in pad wear may occur.
3. If holes are not placed in a precisely engineered pattern, severe rotor and pad grooving will occur.
4. Sometimes light groaning can be heard while braking (more with heavier braking) or air noises (sounds like clicking) when not braking. Seems to generate the occasional comment or complaint from convertible owners after driving through a tunnel. Impossible to hear at full throttle, so it has never bothered me. Of course, years at the race track might have had some negative effect on my hearing (not to mention sanity....).

Slotted rotors --
1. Good initial pad bite, but not as good as cross-drilled.
2. Helps to clean the pad face and clear dust by virtue of their sweeping action.
3. More resistant to cracking under severe usage than cross-drilled rotors.
4. Pads wear a little faster than plain face or cross-drilled.

Bottom line -- not all cross-drilled or slotted (or combo) rotors are created equal. Lots of piss poor ones give the industry a bad name. The most important thing to do is talk with a knowledgeable supplier and truthfully let them know how you intend to use them. The right pad/rotor combination will make your day. The wrong setup for the way you drive can be extremely disappointing.