ntsqd
Heretic Car Camper
The reason that holes promote cracking is that the rotor expands when heated. That's obvious. What isn't so obvious is that the growth isn't even. Heat a disc of sheet metal and watch how it distorts from the differential growth.
That difference in expansion results in internal stresses in the rotor. Holes anywhere are evil, avoid them at all costs. Anywhere there is a discontunity in the metal or a change in cross section will cause an increase in stress.
Factor in that most rotors are made from cast iron. In comparison to other ferrous metals the group of metals known as cast iron are fairly brittle. So now there is non-uniform stress, and stress risers in a brittle material. That your A4 rotors haven't cracked is the exception rather than the rule. My guess is that if you had them Magnafluxed that there would be cracks, they're just not yet visible at ambient temperature.
A very odd and esoteric example of drilled rotors being desirable:
It was done for a very specific reason, that being to intentionally increase the temperature rise of the rotors by decreasing their mass. The nature of the course combined with the available pad compound for the application resulted in a pad that was slow to "come on" (meaning that it took too long to warm up to operating temp) and that there were long periods between brake uses. The pads & rotors had cooled considerably by the next use of the brakes.
By drilling the rotor they were able to increase the rate that the rotor and pads came up to temperature by enough to stop the driver from complaining about the lack of brakes. This is very much an exceptional use of drilled rotors.
That difference in expansion results in internal stresses in the rotor. Holes anywhere are evil, avoid them at all costs. Anywhere there is a discontunity in the metal or a change in cross section will cause an increase in stress.
Factor in that most rotors are made from cast iron. In comparison to other ferrous metals the group of metals known as cast iron are fairly brittle. So now there is non-uniform stress, and stress risers in a brittle material. That your A4 rotors haven't cracked is the exception rather than the rule. My guess is that if you had them Magnafluxed that there would be cracks, they're just not yet visible at ambient temperature.
A very odd and esoteric example of drilled rotors being desirable:
It was done for a very specific reason, that being to intentionally increase the temperature rise of the rotors by decreasing their mass. The nature of the course combined with the available pad compound for the application resulted in a pad that was slow to "come on" (meaning that it took too long to warm up to operating temp) and that there were long periods between brake uses. The pads & rotors had cooled considerably by the next use of the brakes.
By drilling the rotor they were able to increase the rate that the rotor and pads came up to temperature by enough to stop the driver from complaining about the lack of brakes. This is very much an exceptional use of drilled rotors.