The principal components put together comprise a conventional braking system that is outlined below together with possible brake system layouts. The discussion of the components begins with the pedal assembly and moves through the brake system finishing with the foundation or wheel brakes.
A brake pedal consists of an arm, pad and pivot attachments. The majority of passenger cars make use of hanging pedals. A linkage is connected to the pedal and this transmits both force and movement to the master cylinder.
The brake booster serves to amplify the foot pressure generatedwhen the brake pedal is depressed. This has the effect of reducing the manual effort required for actuation. Boosters are invariably combined with the master cylinder assembly. A vacuum booster employs the negative pressure generated in the intake manifold of a spark ignition engine, whereas a hydraulic booster relies upon the existence of a hydraulic energy source and typically finds application in vehicles powered by diesel engines that generate only a minimal amount of intake vacuum.
The master cylinder essentially initiates and controls the process of braking. The governing regulations demand that passenger vehicles be equipped with two separate braking circuits and this is satisfied by the so-called tandemmaster cylinder.Atandemmaster cylinder has two pistons housed within a single bore. Each section of the unit acts as a single cylinder and the piston closest to the brake pedal is called the primary piston whilst the other is called the secondary piston. Thus, if a leak develops within the primary circuit, the primary piston moves forward until it bottoms against the secondary piston. The push rod force is transmitted directly to the secondary piston through piston-to-piston contact, thus allowing the secondary piston to pressurize the secondary circuit. Conversely, if the secondary circuit develops a leak then the secondary piston moves forward until it stops against the end of the master cylinder bore. This then allows trapped fluid between the two pistons to become pressurized and so the primary circuit remains operative.
The dynamics of the braking process gives rise to need for some means of reducing the magnitude of the brake force generated at the rear of a vehicle under the action of increasing rates of deceleration. This need arises form the load transfer that takes place from rear to front during any braking event. This function is realized through the incorporation of some form of brake pressure regulating valve into the rear brake circuit.
Foundation, or wheel brakes, divide into two distinct classes, namely disc (axial) and drum (radial) brakes. Modern vehicles are invariably fitted with disc units on the front axle and there is a growing tendency to fit similar units to the rear axle. If drum brakes are fitted to the rear axle then these are typically of the Simplex type which employs a leading and trailing shoe configuration to generate the required brake torque. The torque output of this type of drum brake is not sensitive to change in vehicle direction. On vehicles fitted entirely with disc brakes, then a small drum unit is often employed to act as a parking brake on the rear axle of the vehicle. Issues surrounding the selection of the materials used to manufacture both discs and drums together with their friction material partners are discussed in more detail later in the text.