Not all discs are created equal...
The market price of brake discs is being driven down, and in order to produce cheaper discs, some manufacturers reduce the weight/mass and quality of the material they use in the manufacturing of the disc. They do this by reducing the thickness of the friction plates (thus increasing the air gap Fig 1.), vane size and configuration, heat dam length, and carbon content.
However, despite reducing the cost of the disc and creating the perception that so called ‘lightweight’ discs are what you need to reduce the weight of your vehicle in order to increase fuel economy, it also affects the ability of the disc to dissipate heat, which therefore has a detrimental effect on the on the efficiency of the brakes.
When brakes are applied energy is transferred from the moving disc to the static brake pad in the form of friction, which in turn creates heat. The amount of heat generated depends on the speed and weight of the vehicle, and how hard the brakes are being applied. Under normal operating conditions the temperature of the brakes will be between 150-250 degrees Celsius – however, prolonged or heavy braking applications in quick succession can see the temperature of the brakes exceed 700 degrees.
Prolonged, heavy or aggressive braking can see the temperature of the brakes rise so high that they start to become less effective and fade. When this happens, the driver has to press harder on the brake pedal to slow the vehicle down. In extreme cases, there may be a point where the brakes get so hot that they cannot generate enough friction, no matter how hard the brake pedal is pressed. This leads to a downward spiral of braking efficiency with potentially catastrophic results.
Overheated discs suffer with excessive wear, cracking, Disc Thickness Variation (DTV), and potential break-up. If not given the chance to cool down, cast iron discs will transform into a super hard material called ‘Cementite’ if subjected to prolonged temperatures of between 650-700 degrees Celsius. This typically happens around the areas of the disc that fall between the cooling vanes, leading to uneven hardness and consequentially uneven wear; DTV.
DTV is commonly misdiagnosed as ‘warped’ discs, as the frequency of the brake pad moving over the uneven surface is resonated through the hydraulics to the brake pedal and through the steering column to the steering wheel.
Some distributors and technicians are unknowingly supplying and fitting lightweight discs and depending on driving style, vehicle application or both, this can lead to increased warranty claims and potentially total failure of the disc.
So the most important thing to consider here is the brake disc’s ability to dissipate heat quickly and effectively in the right areas. In order to do this the disc must be of the right weight/mass/thickness; be of the necessary material/carbon composition; have the specific vane size and configuration, and have the correctly specified heat dam. This prevents heat radiating to the ‘top hat’, which in turn prevents the hub bearings from overheating (Fig 2.) – In short, they must exactly match the OE specification.