I've been thinking about "upgrading" my brake rotors on the next brake job. Recently, I've seen slotted and cross-drilled rotors available on Ebay. They seem to be more form than function. Perhaps I'm misinformed (why should this be any different?), but I thought that slotted rotors were notorious for eating pads. Most OEMs that ventilate their rotors (Porsche, Ferrari, modern sportbikes, etc.)in this way don't slot, they only cross-drill. Is pad life compromised by slotted rotors? Also, do you have an opinion on cryogenics for rotors? Thanks...
04-12-2004, 05:23 PM
Drilling is primarily for weight reduction, and isn't something you want on a street car. Makes the rotor much more likely to fail, and reduces mass, which we want to absorb heat.
Slotting does tend to increase pad wear, but also helps get ablating brake material away from the pad-rotor interface, which is A Good Thing. I'm using slotted, which I think is good, but I still really want a brake upgrade.
No comment on cryogenics, other than some of the serious long-range shooter types swear by it for barrels.
04-12-2004, 05:30 PM
You would never go back, would you?
04-12-2004, 05:39 PM
Porsche now only admits to cross drilling for one reason, and that is water evacuation from the rotor surface.
Their website is being "retarded" at the moment, otherwise I'd direct you to it (it's under the safety - braking system option).
04-12-2004, 06:42 PM
I'm loving it. It's going to cost me $200 in premature tire wear, I can tell already. And worth every penny.
04-12-2004, 08:15 PM
When used in conjuntion with better brake pads, can increase braking power, especially the initial bite. When I had my Q45, I changed to cross drilled rotors with Repco pads and it braked so much better than stock. When I got my A8Q, the brakes were the only thing about this car that was disappointing. Don't get me wrong, they are quiet and smooth but take more effort than I like. JMHO
04-12-2004, 09:57 PM
slotting can help vent the pad, but drilling (or chould that be using cast holes?) is purely cosmetic...
04-13-2004, 02:56 AM
04-13-2004, 06:31 AM
I wouldn't have said notorious, but both slotted and drilled rotors do increase pad wear. There are benefits however:
1. The edges of the slots or holes help keep the pad free of glazing. The natural result is increased pad wear, but I never felt that it was excessive.
2. These biting edges also slightly increase the effective cf of the pads, which may improve pedal feel.
3. The holes or slots give steam a place to go for that first application of the brakes with cold wet pads and rotors.
The disadvantage of drilled rotors:
1. The holes must be chamfered to eliminate areas of localized stress, potential sites that could initiate cracks. Unfortunately, it's difficult (i.e. $$$) to chamfer the inside of a hole on a vented rotor. As a result, it's not done; and you can see what happens in the pic of a Brembo on a Porsche. Cracking normally is not a problem if you don't track your car.
2. Brake dust gets sucked into the holes, coats the cooling vanes, and reduces heat transfer to the air.
IMHO, when it comes to cyrogenics and brake rotors, we're talking snake oil. I can't find the link, but I did see one track test where a cyro treated rotor was mounted on one front corner of a racecar, and an untreated rotor on the other. Afterwards both rotors were compared by a metallurgist -- no difference in wear or metallurgy. This makes sense to me: How can cast iron treated at liquid N2 temps be unaffected by repeated and rapid heating and cooling from 1000 oF to ambient on the track?
"On all 911 models, the front and rear discs are cross-drilled to improve braking in the wet. The brakes respond faster because the water vapor generated under braking can be released more efficiently."<ul><li><a href="http://content2.us.porsche.com/prod/911/carrera_models.nsf/usaenglish/safety_aerodynamicsbraking_system">Porsche.com</a></li></ul>
04-13-2004, 07:02 AM
I'm short of time so I will keep this brief:
1. holes in rotors
No one, NO ONE, should EVER drill holes in brake rotors. Well, if you are racing you can, but at the end of the race you throw the rotors in the garbage. Drilling a hole creates a stress riser, and the rotor will quickly fail.
Porsche (and M-B, etc.) CAST the holes in the rotors. Even so, the rotors with holes have a shorter life.
2. Braking Performance
Holes or slots have very little performance improvement. The holes do increase performane in wet conditions, are a little lighter, and do help evacuate gas created by vaporized pad material. However, the performance increase is so slight that I doubt anyone would notice it.
3. No rotor change is going to significantly change your braking performance. If you feel you have a braking problem, try changing pad types first. If that doesn't work for you, you need to change the brake system (rotors, calipers, pads).
4. About the only reason you would need to change the brakes would be for heavy track use.
04-13-2004, 07:57 AM
There's a VERY long <A HREF="http://corner-carvers.com/forums/showthread.php?s=&threadid=14827&perpage=40&pagenumber=1">Corner-Carvers Thread</a> where the general conclusion is that no one casts holes into cast iron rotors, they're all drilled. That's also my conclusion from cuatroloop's link; Porsche would have said "cast" holes rather than "cross-drilled". Finally, I once looked closely at a brand new Porsche rotor, and it sure looked drilled to me.
04-13-2004, 02:21 PM
along with whether they are cast or drilled.
the only benefit i've ever heard is to do with water dispersal - and this makes no sense to me from the perspective of physics (the hole is very small relative to the wet pad and rotor).
one thing that drilled rotors will do is crack. my porsche 993tt ones did...