Brakes Part 2: Brake Discs

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.

INCREASING DISC DIAMETER

The problem with increasing the effective radius of the discs is that, since the designers used the largest rotor that would fit inside the wheel. Typically, increasing the rotor diameter means increasing the wheel size. The expense involved is only one issue, a major issue is the impact on race suspension geometry.

The camber curves and roll resistance characteristics of any proper suspension system are designed for tyres with a specific sidewall height and stiffness. Increasing the wheel diameter means decreasing the sidewall height and the compliance of the tyre. Carried to an extreme, this will hurt cornering capability and might actually result in a loss of braking traction due to “edging” the front tyres under heavy braking.

FLOATING DISCS

All metals “grow” when heated. The diameter of cast iron brake discs can increase as much as 2mm 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 aluminium. 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 aluminium billets that are pre-stressed and relieved, not from 6061 or from plate stock.