"Warning - disc brakes" used to be a familiar sign on the rear of cars in the 60s, when the newly arrived braking system meant that cars fitted with it stopped much more quickly than those without.
Disc brakes were also used by Jaguar at Le Mans in the 1950s to help its racers outbrake other cars into corners to gain a speed advantage. At first, other racing teams were mystified by the Jaguars' ability.
Disc brakes are a simple, but very effective, idea. The disc is fixed to the same shaft as the wheel so that it spins at the same speed. The disc passes through at least one calliper, sometimes two, and these house the brake pads.
When pressure is applied to the brake pedal, hydraulic fluid is forced through the system and moves pistons in the brake callipers, which in turn press the brake pads against the discs.
The pads are covered with a friction material which rubs against the discs to slow them, at the same time slowing the road wheels by the same amount.
Ventilated discs on high performance cars have air passages through them to provide extra cooling, as hot brakes are far less efficient. But for many cars, non-ventilated discs are good enough.
In theory, the old-fashioned drum brakes should be more efficient at stopping cars because they have a larger surface area for the friction material. However, the efficiency is greatly reduced because drum brakes are more difficult to keep cool.
Some manufacturers have audible warnings built into the disc pads, simply no more than metal particles which grate against the disc to let you know when the pads are wearing down to their safety limit. Other cars have an electrical circuit connected to the discs which is completed when the pads wear down to expose wires they contain. This then lights a warning lamp in the fascia to inform you that it's time to get the brakes attended to.
Maurice Hardy
