good read on slotted/drilled rotors

[zxtwou2]

found this on another forum, rings true for our cars as well (was found on a Honda forum, coppied there from a Celica forum)

Cliffnotes: Slotted/drilled = for looks except for a few rare cases which shouldn't apply if you're on this forum.

QUOTE(Taken from a sticky at Celicatech)
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First, lets get some physics. Tell me how a heatsink with less mass will cool better? You do realize that a brake rotor acts as a large heatsink to transfer heat from the brake pads to the rotor. The heat generated from pads has to go somewhere and so it transfers to the rotor and caliper.

Porsche claims: "Discs are cross-drilled to enhance braking in the wet. The brakes respond faster because the water vapour pressure that builds up during braking can be released more easily."

They have said nothing about enhancing normal braking circumstances and the larger diameter rotors probably make up for the lack of material present in a smaller cross drilled rotor.

From Wilwood's website:

Q: Why are some rotors drilled or slotted?
A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity.

Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications.

As for the porsche rotors, a few notes from a forum I frequent:


1) The holes are cast in giving a dense boundary layer-type crystalline grain structure around the hole at the microscopic level as opposed to drilling which cuts holes in the existing grain pattern leaving open endgrains, etc, just begging for cracks.

2) The holes are only 1/2 the diameter of the holes in most drilled rotors. This reduces the stress concentration factor due to hole interaction which is a function (not linear) of hole diameters and the distance between them.

3) Since the holes are only 1/2 as big they remove only 1/4 as much surface area and mass from the rotor faces as a larger hole. This does a couple of things:

It increases effective pad area compared with larger holes. The larger the pad area the cooler they will run, all else being equal. If the same amount of heat is generated over a larger surface area it will result in a lower temperature for both surfaces.

It increases the mass the rotor has to absorb heat with. If the same amount of heat is put into a rotor with a larger mass, it will result in a lower temperature.

3) The holes are placed along the vanes, actually cutting into them giving the vane a "half moon" cut along its width. You can see that here:

This does a couple of things:

First, it greatly increases the surface area of the vanes which allows the entire rotors to run cooler which helps prevent cracks by itself.

Second, it effectively stops cracking on that side of the hole which makes it very difficult to get "hole to hole" cracks that go all the way through the face rotor (you'll get tiny surface "spider cracks" on any rotor, blank included if you look hard enough).

That's why Porsche rotors are the only "crossdrilled" rotors I would ever consider putting on my car.

BTW, many of the above features are not present in older Porsche brakes. The above is for "Big Reds" and newer.

This is quite different from the standard drilled rotors you get from brembo/kvr/powerslot/"insert random ricer parts brand name here" brake rotors.

Further proof of the uselessness of cross drilled rotors are found here:

Those Poor Rotors


Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the 40

[powder]

But... they look pretty...

[1981gMachine]

Not new news for me. I've known slotted and drilled were not the best for most uses. Why butt heads with a forum full of people who swear by them...

[zxtwou2]

on a side note...lots of people who experience brake fade on their street driven cars, is because they have old fluid, or low boiling point fluid (like DOT3). the fluid soaks up moisture over time or can become boiled, and then the vaporized water causes the pedal to become very soft becuause the system now has air in it. you can't even go by the "dry boiling point" of brake fluids, because even as you pour the fluid into the system, it soaks up moisture. brake fluid is an incredible moisture absorber. always look at the "wet" boiling point instead. for daily use, DOT3 is fine, for track use, DOT4 is best. DOT5 is not acceptable for brakes not designed for it, as it's a silicone base.

[powder]

Not new news for me. I've known slotted and drilled were not the best for most uses. Why butt heads with a forum full of people who swear by them...

Yeah but do your techs at the dealership know it is the real question.

[1981gMachine]

Yeah but do your techs at the dealership know it is the real question.

They put ford parts back on ford cars. So there is no issue. I'm sure some techs will know it but not all. I learned about it in my chassis fabrication and high performance engine course. I'd say maybe .5% of the cars that come through the shop have mods done. And nothing major at that.

[powder]

It was a joke. I really didn't care.

Have a nice day.

[1981gMachine]

It was a joke. I really didn't care.

Have a nice day.

yea, I know. You were just digging stuff up from another thread.

[Beodude123]

Pretty good read. I had read that on here a while back... I have slotted rotors. I think they look neat. I know they don't help...

[el sid]

Good read, no doubt

[mechtech]

Porsche Racing used to take the factory street drilled and slotted rotors off and used plain ones for track use.

[pinkshinyalan]

One thing I haven't heard discussed specifically is the weird C-slots I've seen in some BMW rotors. Maybe that's an aftermarket thing, I don't know. They don't look like the placement has a pattern, and they're C-shaped holes either cut or cast in. I can't find pictures of 'em though.

I'm sure it has all the same caveats as drilled rotors, I've just never seen it mentioned. *shrug*

[Beodude123]

Never seen those before... But I would imagine it wouldn't be any different than other forms of brake mods.

[zxtwou2]

Formula One brakes

[iceracer]

The information seems familiar.