But why on earth would you want to sacrifice braking surface area for extra cooling that you don't need?

rom Stoptech:


QUOTE
Which is better, slotted or drilled rotors?

StopTech provides rotors slotted, drilled or plain. For most performance applications slotted is the preferred choice. Slotting helps wipe away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage. Many customers prefer the look of a drilled rotor and for street and occasional light duty track use they will work fine. For more severe applications, we recommend slotted rotors.



That almost sounds like an excuse to use cross drilled rotors, and for your street car which probably is never driven on the track, the drilled rotors are fine, but as Stoptech states, they will crack and are not good for severe applications.

Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it...Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?)

Here is how it works. The friction between the pad and rotor is what causes you to stop. This friction converts your forward energy into heat (remember Einstein: Energy is neither created nor destroyed, it is converted). Now that heat is a bad thing. Yes it is bad for the rotors but it is a lot worse for the pads. A warped rotor will still stop the car - it will just feel like ****. Overheated pads however WILL NOT stop the car. It is here where the rotors secondary responsibility comes in. Its job now is to DISSIPATE the heat away from the pads and DISPERSE it through itself. Notice that DISSIPATE and DISPERSE are interchangeable? Once the heat is removed from the pad/surface area it is then removed. Notice where the removal falls on the list of duties? That'* right - number 3. Here is the list again. Memorize it because I will be using it a lot in this post:

#1 Maintains a coefficient of friction with the pad to slow the forward inertia of the vehicle

#2 DISSIPATE the heat

#3 REMOVE the heat from the brake system

Let'* look more in-depth at each step now shall we? No? Too bad assclown we are doing it anyway.

#1 Maintains a coefficient of friction with the pad to slow the forward inertia of the vehicle:
This one is pretty simple and self-explanatory. The rotor'* surface is where the pads contact and generate friction to slow the vehicle down. Since it is this friction that causes the conversion of forward acceleration into deceleration (negative acceleration if you want) you ideally want as much as possible right? The more friction you have the better your stopping will be. This is reason #1 why BIGGER brakes are the best way to improve a vehicle'* stopping ability. More surface area on the pad and the rotor = more friction = better stopping. Does that make sense? Good. Let'* move on.

#2 DISSIPATE The Heat:
Let'* assume for a second that the vehicle in question is running with Hawk Blue pads on it. The brand doesn't really matter but that is what I am using as my example. They have an operating range of 400 degrees to 1100 degrees. Once they exceed that 1100 degree mark they fade from overheating. The pad material gets too soft to work effectively - glazing occurs. This means that a layer of crude glass forms on the surface of the pad. As we all know glass is very smooth and very hard. It doesn't have a very high coefficient of friction. This is bad - especially when I am coming down the back straight at VIR at 125MPH. Lucky for us the rotor has a job to do here as well. The rotor, by way of thermal tranfer DISSIPATES the heat throughout itself. This DISSIPATION lessens the amount of heat at the contact area because it is diluted throughout the whole rotor. The bigger the rotor the better here as well. The more metal it has the more metal the heat can be diluted into. Make sense? This isn't rocket science here d00d.

#3 REMOVE the heat from the brake system:
Now comes your favorite part of the process. This is what you thought DISSIPATION was. It is ok. I will allow you to be wrong. This is the step where the rotor takes the heat it DISSIPATED from the pads and gets rid of it for good. How does it do this? By radiating it to the surface - either the faces or inside the veins. It is here where cool air interacts with the hot metal to cool it off and remove the heat. Once again there is a reoccuring theme of "the bigger the better" here. The bigger the rotor, the more surface area it will have which means more contact with the cooling air surrounding it. Got it? Good.

Now let'* look at why cross-drilling is a bad idea.

First - as we have already established, cross-drilling was never done to aid in cooling. Its purpose was to remove the worn away pad material so that the surfaces remained clean. As we all know this doesn't have much of a purpose nowadays.

Next - In terms of cooling: Yes - x-drilling does create more areas for air to go through but remember - this is step 3 on the list of tasks. Let'* look at how this affects steps 1 and 2. The drilling of the rotor removes material from the unit. This removal means less surface area for generating surface friction as well as less material to accept the DISSIPATED heat that was generated by the friction. Now because of this I want to optimize step one and 2 since those are the immediate needs. If it takes longer for the rotor to get rid of the heat it is ok. You will have a straight at some point where you can rest the brakes and let your cooling ducts do their job. My PRIMARY concern is making sure that my car slows down at the end of the straight. This means that the rotor needs to have as much surface as possible to generate as much friction as possible and it needs to DISSIPATE the resulting heat AWAY from the pads as quick as possible so they continue to work. In both cases x-drilling does nothing to help the cause.

Now let'* talk about strength - and how x-drilled rotors lack it. This one is simple. Explain again just how drilling away material/structure from a CAST product DOES NOT weaken it? Since you are obviously a man of great knowledge and experience surely you have seen what can happen to a x-drilled rotor on track right? Yes it can happen to a non-drilled rotor as well but the odds are in your favor when pimpin' bling-bling drilled y0! Since you are also an expert on thermodynamics why not explain to the group what happens to a cast iron molecule when it is overheated. I will give you a little hint - the covalence bonds weaken. These bonds are what hold the molecules together boys and girls. You do the math - it adds up to fractures.

So why don't race teams use them if they are so much better? Consistency? Hmmmm . . . no. I am gonna go with the real reason her chodeboy. It is because of several factors actually. They are as follows but in no particular order:

- Less usable surface area for generating friction
- Less material to DISSIPATE the heat away from the pads
- Less reliable and they are a safety risk because of fatigue and stress resulting from the reduced material

Cross Drilled Rotors
The most popular way of removing the built up gas from the surface of the pad is by drilling holes in the surface of the rotor. There are many advantages in this technique, as many small holes can be arranged to scrape the pad continuously and also provide more surface area for heat transfer to occur.

The drawbacks of this technique are that too many holes destroy the structural integrity of the disk, as well as cause problems with crack propagation as the brake rotor fatigues. If you change your rotors every weekend, or inspect and replace often, then it works out OK. If you wait till something fails, well, eventually something will. These holes are also excellent places for cracks to start, due to their inherent heat loading cycles and the physical fatigue properties of a hole versus an unbroken surface.

Slotting rotors
When looking for a method to reduce the gas between the pad and rotor without the drawbacks of drilling, slotting comes to mind. A slot, milled such that the pad is scraped from inside to outside as the disk rotates, allows for the gas to be removed without disturbing the structural properties of the disk as badly or creating such physical stresses on the disk.

Drawbacks to slotting are that you are again reducing the frictional area of the disk. While these losses are overcome by the amount of gas removed and the amount of friction gained, this has to be kept in mind such that the slots milled are just large enough to do their job- and no more.


And some more information I found:


Nearly everyone on this list has seen a cross-drilled rotor. They can be seen on most street-roaming exotica such as Porsches and Ferraris. They can be purchased from several aftermarket vendors found in Car and Driver, Motor Trend, Auto Week, etc. They are almost always on anything that is raced professionally, so they must be good for your street car, right?

WRONG.

('scuse me while I put on my Nomex undies...)

OK, wrong, maybe, application- and a whole lot of other things-dependent.

Here'* the breakdown of "whole lot of other things-dependent". You be the judge.

Why are rotors cross-drilled? Primarily to reduce the chance of brake fade during heavy brake use, such as while circle track or road racing. Cross drilling will also improve brake cooling, provided there is a steady supply of cool air forced to the center of the rotors. This means brake cooling ducts.
Why do brakes fade? Brakes fade because the brake pads get so hot that the organic binders holding the pad'* friction materials together begin to boil and liberate large amounts of gaseous by-products. This gas forms a layer between the brake pad and the rotor that drastically reduces the coefficient of friction between the two surfaces. Cross-drilled rotors give this gas layer an easier escape path than just past the edges of the pad. The holes allow the gas to vent through the internal cooling slots of the rotor.
What about grooving rotors? Grooving rotors accomplishes much the same thing as cross-drilling; it'* just not as efficient at allowing gas removal. Grooving also provides a slight scraping effect to the pad which can break a glaze buildup. Glazing is a hard, glass-like layer that forms on the surface of a brake pad that has gotten WAY too hot, molten actually. Needless to say, glazed brake pads have a rather poor coefficient of friction and should be replaced.

Something bears further explanation here: two characteristics that I've mentioned so far, pad outgassing and glazing, are normal characteristics of the older style, high-organic content brake pads, the kind with lots of asbestos in them, the kind that aren't generally available anymore. Why would anyone want to use a high-organic, asbestos brake pad when the new generation of metallic, carbon, and kevlar-based linings have higher coefficients of friction that are more consistant versus temperature, lower organic binder content, and substantially longer life than their asbestos counterparts? I'm not saying that the newer pad materials won't outgas and fade. They're far more resistant to this behaviour than the older materials because they are higher temperature materials to start with and they have much lower organic content. I won't go any further with this now, wait for Part 4: Sorting out the different brake pad materials.

OK, so you're still salivating over that cross-drilled rotor ad in the latest issue of Motorhead Fanatic magazine. Go ahead, order 'em. In fact, while you're at it, order several pairs; you're going to need some spares. These guys are just taking standard rotors and drilling them full of holes. The structure is weakened and the mass of the rotor is reduced, therefore limiting its ability to deal with heat buildup. I can almost guarantee that a set of these drilled rotors will be warped and dangerously cracked within 12 months, 2 or 3 months the way I drive. (I road race my SHO, that'* why I have the 13" Baer front brakes, with undrilled rotors.)

But Porsche and Ferrari do it! Yep, and their rotors are designed at the casting stage to be cross-drilled. They have thicker rotor surface sections and more material in the cooling slots. They are stronger to begin with so they will hold together longer despite the cross-drilling.

Next time you're at the race track, get a garage or pit pass and spend some time talking to the mechanics. Ask them how often they replace the drilled rotors on their cars. The answer you're likely to hear is "every couple of races", assuming that they can afford it. Ask to see a used-up rotor. It will have radial cracks around nearly every hole. On holes near the outside edge of the rotor, the cracks may extend to the edge of the casting. Get more than a couple of these major edge cracks and the rotor is likely to self-destruct by throwing large hunks of itself at the insides of your wheels. Stopping is difficult when this happens...

At my last visit to the SHO Shop, I spied a pair of SHO front rotors that had been drilled. They were so badly cracked and heat checked that they would have shattered from a drop to the concrete floor. According to Vadim, they had been on a car for about a year.

Let'* sum up the advantages and disadvantages:

For cross-drilling:
1) Better removal of gas buildup during brake fade.
2) Better cooling than a solid rotor, but only when force-cooled (brake ducts).
3) Looks cool.
4) Still looks cool when pieces of shattered, drilled rotor are embedded in your nice alloy wheels.


Against cross-drilling:
1) Shortens rotor useful life.
2) Drilled rotor will likely warp faster than a solid one.
3) Shortens brake pad life.
4) Only marginal improvement in fade resistance with modern brake pad materials.
5) Can result in catastrophic rotor failure.

OK, so you still want drilled rotors. ( I want to be locked in a closet with Gillian Anderson of the X-Files but that hasn't happened yet...) May I suggest the following:
1) Upgrade to larger rotors so they can take the heat buildup.
2) If you're into road racing, ditch your fog lamps (I have) and use the empty holes to start some 2" diameter silicone flex hose. Using great patience and a [lot] of cable ties, direct the duct along the subframe and onto the lower control arm so it discharges near the center of the rotor. You now have brake cooling ducts.

Let me say a bit about grooved rotors before I conclude. Grooving is not nearly as hard on the rotor'* structural integrity as cross drilling. If you must do something visually appealing to your rotors, get them grooved. Grooving will accelerate brake pad wear, be prepared for this. Also, get in the habit of inspecting your rotors periodically, especially if they are drilled or slotted.

Or, get some brake pads with gas grooves in them. Gas grooved pads look like they have a saw kerf across their narrow dimension. Typically, the kerf depth is about 2/3rds the total depth of the friction material. Racers have been known to groove pads with a hacksaw blade, although I don't recommend that you try this at home; "standard disclaimer applies".