:idisagree:Not 100% correct. You guys want the truth from an engineers point of view.
First and foremost, the purpose of the brake rotor is as a surface for the friction compound (brake pad to act upon) and as a heatsink. The brakes slow down/stop the car simply by converting kinetic energy into heat energy by means of friction. The reason rotors are big heavy things is because they need to be able to absorb the energy in the form of heat. The main reason for Big Brake Kits is not really to increase braking torque (how much force the brakes can retard wheel rotation with) but simply to install bigger rotors that can handle more heat. If you wanted to increase braking torque ONLY, you could do that simply by using higher friction coefficient (known as Mu) brake pads. On street tires, all that would do is make it easier to lock up your wheels when braking.
How well a brake system (not looking at the brake pads yet) handles heat is affected by how much heat the rotors can absorb and also how quickly they dissipate the heat they do absorb. The more mass, the more heat they can absorb, the better the airflow over and through the rotor, the more effectively the rotor can transfer that heat to the ambient air flowing over it.
Vented rotors make use of this by increasing the surface area and airflow and directionally vented rotors are even more effective. A vented or directionally vented rotor acts in a similar fashion to an impeller blade/wheel (like a turbo compressor or hairdryer). The spinning action forces air outwards from the center and it induces airflow through the rotor with cold air being sucked in at the center and the hot air being flung out the edges. Vented rotors dissipate heat much better than non-vented rotors and directionally vented rotors improve the effectiveness.
Crossdrilling on the other hand does NOT show appreciable cooling differences when in use. They will help cool the rotor qucker when the car is stopped and sitting but they actually REDUCE the heat capacity of the braking system when the car is being driven hard maing the brakes actually run a little hotter. So you're thinking, "Wait a minute,... say what? If increasing surface area means better heat dissipation, then more holes means more surface area, so it should help at least a bit right?" Wrong.
The crossdrilled holes do not act the same as the vented passages in the rotor. The venting is radial to the rotation and draws air through the vanes. the cross drilled holes are axial to rotation. The difference in airflow when you're blowing through a straw (the vented vanes) as opposed to blowing across the hole of the straw is huge. Now you're thinking, yeah, but they use devices like paint guns that work by drawing paint out of the bottle by blowing compressed air over the thin tube. Well, you're talking about Bernoulli's principle here.
Fast moving air has lower pressure than slow moving or stagnant air. The fast moving air over the tube is at lower pressure than the air in the spray gun bottle so the higher pressure pushes paint out and Voila, you're apinting your car/house/whatever. The principle doesn't work when the air is moving fast over both ends of the tube (in this case, the hole drilled through the rotor. There is no significant airflow through those holes when the rotor is spinning and any airflow is FAR less tha the airflow through the vanes in a vented rotor. Ah... now you see,... but wait, wasn't there a mention of higher temps due to cross-drilling? Even if the airflow is minimal in the drilled holes, why would it cause higher temps? Because it reduces the mass of the rotor, thus reducing it's capacity for absorbing heat.
What happens when you use your brakes hard? Well the brakes get hot obviously. But what would a brake rotor temperature chart look like? Well, upon first application of the brakes, the temp rise is very rapid and increase as long as the brakes are applied and the wheels are still turning (friction still producing more heat). then when you let off the brakes, the temps level out and start to drop. They drop faster if the car is stil moving since there's more air flowing over the rotors, but they don't completely cool off that rapidly. if you're driving hard and braking a lot, the rotors never cool off, but each subsequent braking event adds more heat to the rotor potentially before it can dissipate it. This is where rotor size and thermal capacity comes in.
if you plan on driving hard and doing a lot of hard braking, you need to make sure you have enough rotor mass to absorb all that heat because if you don't, you could exceed the thermal capacity of the rotors to act as heatsinks. When that happens, your rotors simply can't dissipate the heat as fast as you're putting them into the rotors and since any guven material of a set mass has a specific thermal capacity, once you reach this point, the rotor cannot efficiently absorb the heat anymore and the brake rotor and pad temperatures will rise very rapidly with each application of the brakes. The temps shoot up fast and high enough to easily exceed the MOT (maximum operating temp) of the brake pads and you get pad fade. You can also get things so hot that it boils the brake fluid in the calipers and this is also where it's possible to actually warp rotors. You can identify a rotor that has really been warped. It changes color. You'll see a bluish boundary somwhere between the rotor and hat as the heat at that point changes the molecular structure of the metal. The heat stress is visible.
Cross-drilling = less mass, = less capacity to absorb heat before this point is reached. when you're running close to the edge of the thermal capacity, the brake system will be consistently a little hotter because it's easier to push the system with less mass and thermal capacity over the edge.
There's also the problem that cross-drilling causes stress risers on the rotors. coupled with the thermal expansion that occurs during braking and repeated hard use, the holes make it much easier to develop deep stress cracks in the rotors. There's a reason NO race team that uses iron rotors has cross-drilled rotors. They use either plain or slotted rotors. The cross-drilling simply reduces the thermal capacity AND weakens the rotor courting the possibility of failure from stress cracks actually leading to a fragged rotor.