DESIGN ANALYSIS
of AC's post-war 2 Litre SaloonPage 5
The type of brakes employed on the 2 Litre are of interest. The first system used was Girling's hydro-mechanical brakes, that is, hydraulic at the front and mechanical at the rear. The front brakes were the Girling "hydrastatic" system. The linings were in continuous contact with the drums to reduce pedal travel, and also make them self-adjusting. There were no return springs on the shoes. A balance spring (or "bias spring") was fitted across the shoes near the pivot, to take up the weight of the shoes and to balance the spring that is inside the cylinder.
The rear brakes featured a compensating unit mounted on the rear axle to keep the two rear brakes in balance. This was made by AC rather than Girling. Note that the mechanical expander is mounted on the backplate in slotted holes. Its fixing nuts have double-coil spring washers under them, and are not fully tightened. This allows the expander housing to float slightly. The reason for this feature tends to be incorrectly explained in articles. The real reason is to equalise the forces applied to each shoe. On a fixed expander, the force applied to the leading shoe is reduced by the self-energising effect of the shoe. A floating expander thus increases the force on the leading shoe (in the same way that a hydraulic cylinder equalises the loads).
The pedal operates via a balance linkage to distribute the force between the mechanical brake-rod and the master-cylinder. The handbrake operates through the same brake-rod. It is a push-pull type handbrake control which achieves the required leverage via a cable to the front of the car, and a large lever visible in the engine compartment. The first time I saw it tested on a rolling road (for its MoT test), the car almost lept out of the garage!
Very large 12 inch (305mm) drums are fitted front and rear, and the deep wells of the wheels keep the drums exposed to the air-stream. Slow speed braking is very effective even by modern standards, with a stopping distance of about 31 feet (9.5m) from 30mph (48km/h). This could be attributed to the long wheelbase and low gravity centre, reducing weight transfer under braking. Also, the high pedal force required (up to 160lbs/700N) allows careful control on the limit of adhesion with plenty of 'feel'.
