can you elaborate on why NOT drilled......
Drilled rotors are VERY prone to cracking. The edges of the holes create significant stress concentrations during the thermal expansion of the rotor under heavy braking.
The whole purpose of drilled/slotted/dimpled rotors is to allow the gasses caused by heavy braking to escape. When you really mash on the brakes, the pads are converting energy. Kinetic energy (rotational and translational) is converted into thermal energy. That thermal energy or heat, is then dissipated to the air around it. Create enough heat quick enough (heavy or prolonged braking), and the pads literally expel some gas as the material wears down. This off gassing can create what is perceived as brake fade. By allowing these gasses out via holes/dimples/slots, you prevent that separation of pad and rotor and maintain a more consistent feel during heavy braking. A secondary effect of the holes/dimples/slots is increased pedal feel. The amount of leading edge is directly related to how much initial "bite" you have between the pad and rotor. The more leading edge the more easily you can modulate that brake. This is why "wave" rotors are very common on bicycles and motor cycles. A great increase in lever feel that adds to control.
Dimples and slots also create stress concentration areas, however since they don't fully penetrate the base geometry they likelyhood of developing cracks is significantly less.
Another note. "warping" of rotors isn't actually a change in physical geometry of the rotor itself. The needed stresses and temperatures required to alter the structure of that cast iron are simply not possible under even the most extreme braking situations. What most often happens (outside of physical damage to a rotor) is pad material unevenly builds up on the rotor face, and that is perceived as a pulsating action in the brake pedal. On the average late model braking system, the human body can perceive as little as 0.0005 difference in pad/rotor interface surface variation. This uneven deposit of pad material comes from getting them very hot, and then stopping and letting the pad sit in one position. Towing can create this situation in a hurry, think of coming down a long hill, then exiting the expressway. You've heated the brakes coming down the hill, then have to stop the whole rig at the end of the ramp, and then you sit still for a few moments waiting to turn. You've just laid down a tiny deposit of pad material onto the rotor, and moving forward it will continue to "grow" that deposit until it's large enough to feel through the pedal, and affect braking performance. Often turning the rotor will help remove this feeling, however it will often return do to the reduced thermal mass available to dissipate the heat.
Cryo treated rotors will help with this due to the harder outer surface from the heat treatment process. The best defense against warpage is multi-pronged. The easiest is to bed the brakes properly and deposit a thin layer of brake material all around the rotor face upon initial installation, then driving with the "don't heat them and then stop" mentality. The second is easiest is to get higher duty pads. These are less likely to deposit material to the rotor face under extreme heat, and then won't show the symptoms of "warpage". The third and most costly is to increase rotor/pad size. This simply keeps the heat form being built in the first place, and the benefits that come from a less stressed system.
I sat through a seminar by Brembo for aftermarket shops doing suspension/braking upgrades at SEMA in 2003. A lot of this was news to me at the time, and I find those old theories are still around in a lot of places.
