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Originally Posted by aki
The purpose of cross-drilled rotors is not for air to blow through them, it's to serve as a heat sink with the increased exposed surface area.
Also, there's is some debate as to the durability of cross-drilled rotors; some say rotors cracking in them are a result of poor manufacturing. Whether that's true or not I'm not sure, but I think slotted rotors are safer in general.
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in contrary to what you are saying cross dilling was intended for "outgassing" its a term used in racing that gave brake gases a place to go while giving the driver a btter brake pedal feel. this process occurs at racing temps, but now these days many pads because of their composition will exihibit these characteristics as well. although cross drilling provides a welcome path for brake gases for when and if they do develop, cross drilling is primarily a visual effect for the open wheel design. now cracking due to stress is an issue, but if you go with a high quality brake company they do take this into account whne the manufacture their rotors. venting the disc is primarily the job of the vanes that are put into the rotor...depending on the angle is to how much air will flow through them....
i'll go ahead and do my best at explaining the purpose of slotting as well...slotting also hepls in the outgassing effect as well. mainly slots in the rotors in a sense "micro-shave" the pad to help de-glaze or prevent glazing from building up. when you have glazed rotors we all know that braking performance is decreased tremendously....so most manufacturers do not chamfer or radius the edge of the slot for this reason....
the combination of both cross drilled/slotted rotors from a high quality brake manufacturer can help decrease braking distance by providing decreased brake fade along w/ better pedal firm to an extent....