A brake booster having a fluid actuated piston in the form of a diaphragm (5) to which the supply of fluid is controlled by a valve mechanism (8A, 14A, 17), actuated by a driver-operated force input member (16) which acts via a reaction element (19) on an output member (10) providing power assisted input to a master cylinder. The booster includes a force transmission assembly (14,8) providing surfaces through which force is transmitted between the input and output members (16,10). A spring (24) is arranged to yield under a predetermined force to permit relative movement between parts (14) and (8) such as to change the ratio of the operative surface areas and thereby increase the boost ratio.
BACKGROUND OF THE INVENTION
This invention relates to a brake booster, primarily for use in a vehicle hydraulic braking system for the purpose of providing power assistance in the actuation of a brake master cylinder, and of the general kind in which the supply of working fluid to a fluid-actuated servo piston is controlled by a valve mechanism actuated under the influence of a driver-operated force input member which acts via a reaction element on an output member arranged to provide power assisted input to the master cylinder.
A conventional brake booster of the aforesaid general kind provides a constant boost ratio throughout its operating range so that the boost increases progressively and rapidly with increased driver input, typically to a brake foot pedal. Whilst this provides satisfactory results in general use, it has become recognised that some drivers fail to exert enough force on the brake pedal during emergency braking, with the result that the available maximum deceleration of which a vehicle is capable without skidding is not always achieved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a brake booster of the aforesaid general kind which provides improved braking performance when relatively high levels of vehicle deceleration are required.
According to the invention, a brake booster of the aforesaid general kind includes a force transmission assembly providing surfaces through which force is transmitted between the input and output members, and yieldable means arranged to yield under a predetermined force to permit relative movement between parts of the force transmission assembly such as to change the ratio of the operative surface areas and thereby increase the boost ratio.