On the Circuit des 24 Heures du Mans, perhaps more so than elsewhere, racing cars travel very, very fast. But to clock a quick lap time, drivers must decelerate before hitting the turns. Top performance is therefore as dependent on the brakes as it is on the engine.

Drumming up a winning appetite

Driveshaft brakes and cable brakes had come and gone. When the 24 Hours of Le Mans resumed in 1949, all the racers had switched to drum brakes, and almost all of them were hydraulically activated. The cars therefore had a large steel drum on each wheel, with aluminium subsequently replacing steel on some models. A drum brake comprises two steel “shoes”, lined with a friction material, that press against the circular inner surface of each drum. These shoes are articulated at one end. At the other end a hydraulic piston, pressurised by the brake pedal via the master cylinder, forces the shoes onto the drum. The system is a rather effective one as the surfaces in contact (drum/shoes) are quite large.

To improve the system, a second piston was soon introduced to ensure that each shoe retrieved all the pressure applied to the brake pedal so that the whole shoe was pressed against the drum. Springs are used to retract the shoes and release the drum when the brakes are not applied. Finally, the shoes’ position in relation to the drum has to be adjusted as accurately as possible. If they are too close, they will brake continuously. If they are too far away, the initial bite will suffer. Balancing the brakes on all four wheels could therefore pose some serious problems.

The principle of brakes is to convert kinetic energy (the car’s momentum) into heat by friction between the materials. In the case of drum brakes, however, heat is trapped by the bell-shaped drum. Cars would therefore frequently lose braking efficiency when the drum and/or the shoes simply got too hot. Many a driver has had a tremendous fright at Mulsanne or Arnage due to overheated brakes fading! 

Discs bring the crown to Britain in Coronation year

In the early fifties, Dunlop and Jaguar worked together to produce a new braking system for the 24 Hours of Le Mans based on innovations developed during the war for tanks and in aviation. The British marque had clinched victory in 1951 with an XK-120 C – soon to become known as the C-Type – which was fitted with traditional drum brakes.

The following year, however, the Mercedes 300 SL promised to put up a much stronger fight. To keep their German rival at bay, Jaguar worked on two new areas of development: aerodynamics and disc brakes. The new bodywork resulted in major overheating problems, leading to the Jaguars’ retirement and preventing the second major innovation from coming into its own.

So, how do these disc brakes work? Their principle is actually much simpler than drum brakes. A simple steel disc is gripped between two pads composed of friction materials over one-eighth of its surface. The remaining seven-eighths are exposed to the open air to allow the disc to cool efficiently. The pads are pressed onto the disc by means of a hydraulic device, known as a caliper, in which one or more pistons push the outer surface of said pads and increase the pressure on the discs. A further advantage is that there is no need for a complicated system to pull the pads away on releasing the brake pedal. The absence of pressure is enough to free the disc.

In 1953, the Jaguar C-Type was equipped with a well-developed, extremely effective system that proved decisive for the intense braking demands of Le Mans. Barely two weeks after Queen Elizabeth II’s Coronation, the British manufacturer continued the celebrations with a stunning one-two and succeeded in breaking the 4,000-kilometre barrier – the first time such a feat had been achieved in a 24-hour race. The distance covered (4,088 kilometres exactly) was 477 km further than the car’s older sister had managed in 1951! As the car’s engine output had only increased by around 20 horsepower over the two-year period, it is fair to say that the outstanding 1953 performance was largely down to disc brakes which had shown to be more powerful and more stable over time than drum brakes.

Gradually, disc brakes became the norm. Fins were later introduced between the two faces of “ventilated” discs. The size of the pads increased as did the number of caliper pistons (up to eight). Steel, on the other hand, continued to be used until carbon discs appeared – but that’s a story for another day in this Centenary series...

LE MANS (SARTHE, FRANCE), CIRCUIT DES 24 HEURES, 1950–1953 24 HOURS OF LE MANS – FROM TOP TO BOTTOM (© ACO ARCHIVES): In 1953, the Jaguar C-Type (#18) became the first car fitted with disc brakes to win the 24 Hours; the 1950 Jaguar XK-120 (#15), subsequently called the C-Type with a new streamlined body, still used drum brakes for its 1951 victory (#20); the 1953 winning line-up of Tony Rolt (at the wheel) and Duncan Hamilton (to his left).