DRILLED VS SLOTTED ROTORS
For many years, most racing rotors were drilled. There were two reasons-the holes gave the "fireband" boundary layer of gasses and particulate matter someplace to go and the edges of the holes gave the pad a better "bite."
Unfortunately, the drilled holes also reduced the thermal capacity of the discs and served as very effective "stress raisers," significantly decreasing disc life. Improvements in friction materials have pretty much made the drilled rotor a thing of the past in racing. Most racing rotors currently feature a series of tangential slots or channels that serve the same purpose without the attendant disadvantages.
PAD AREA
We have seen that brake torque is directly proportional to Piston Area, System Pressure, Friction Coefficient and Effective Radii, and is not affected by pad area. Pad area and geometry are however important for several reasons:
1) Pad service life. Since pad material is consumed, an increase in pad area results in an increase in the time interval between pad replacements. OE designs often make slight sacrifices in pad life by including tapered ends for reduction of noise, vibration and pad taper. In some OE designs, the pads on the two sides of the caliper are even shaped differently, with the inside pad being shorter in arc-length in the direction of rotation and wider radially than the outside pad for system design and integration reasons.
2) Heat dispersion and dissipation over a larger surface area and greater mass. Although in the case of a larger pad, the pad masks a larger portion of the rotor face, absorbing more radiant energy and shielding the area from cooling that may cancel any actual benefit.
3) Geometry: Since rubbing speed between the disc and the pad is greater at the periphery of the disc, the pad geometry will sometimes be designed to reduce the area toward the center of the disc. This is done in an effort to produce even temperature and pressure distribution across the face of the pad.
FLOATING DISCS
All metals "grow" when heated. The diameter of cast iron brake discs can increase as much as 2mm (0.080 inch) at elevated braking temperatures. When the disc is radially restrained from growing (as in all one-piece discs) the friction plates are forced into a cone shape as temperature increases, adversely effecting both temperature and pressure distribution within the pads and the feel of the pedal.
Racing and high-performance street discs are mounted on separate hats or bells, usually of aluminum. The fastening system is designed to allow radial growth and minimal axial float resulting in a mechanically stable system. Hats or bells should be made from 7075 or 2024 heat-treated aluminum billets that are pre-stressed and relieved, not from 6061 or from plate stock.