Page 360 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Braking systems C HAPTER 12.1

must be designed so as not to compromise the overall the secondary circuit. Conversely, if the secondary circuit
system quality that could result from design changes at develops a leak then the secondary piston moves forward
the component level. By way of example, a reduction in until it stops against the end of the master cylinder bore.
package space could lead to smaller diameter wheel This then allows trapped fluid between the two pistons to
brakes having to be fitted to the vehicle. This will change become pressurized and so the primary circuit remains
the brake force distribution unless checked, by say operative.
resizing the wheel cylinders, and in the worst case this
could lead to premature wheel lock and a violation of the
governing legislation. 12.1.1.3.4 Regulating valves
The dynamics of the braking process gives rise to need for
some means of reducing the magnitude of the brake force
12.1.1.3 Brake system components generated at the rear of a vehicle under the action of
and configurations increasing rates of deceleration. This need arises form the
load transfer that takes place from rear to front during
The principal components put together comprise a con- any braking event. This function is realized through the
ventional braking system that is outlined below together incorporation of some form of brake pressure regulating
with possible brake system layouts. The discussion of the valve into the rear brake circuit. The exact nature of the
components begins with the pedal assembly and moves valve depends upon the detail design but they fall into
through the brake system finishing with the foundation three generic types;
or wheel brakes.
Load sensitive pressure regulating valve: Valves of
this type are fitted to vehicles that experience large
12.1.1.3.1 Pedal assembly
in-service changes in axle load. The valve is anchored
A brake pedal consists of an arm, pad and pivot attach- to the vehicle body and is also connected to the rear
ments. The majority of passenger cars make use of hanging suspension through a mechanical linkage. This
pedals. A linkage is connected to the pedal and this trans- permits the valve to sense the relative displacement
mits both force and movement to the master cylinder. between the body and suspension and adjust the
valve performance to effect control over the rear line
12.1.1.3.2 Brake booster pressure and so enable the rear brakes to compensate
The brake booster serves to amplify the foot pressure for the change in axle load.
generated when the brake pedal is depressed. This has the Pressure-sensitive pressure regulating valve: Other-
effect of reducing the manual effort required for actua- wise known as a pressure limiter, this type of valve
tion. Boosters are invariably combined with the master isolates the rear brake circuit when the line pressure
cylinder assembly. A vacuum booster employs the nega- exceeds a predetermined value. They find applica-
tive pressure generated in the intake manifold of a spark tion on vehicles that are characterized by a low centre
ignition engine, whereas a hydraulic booster relies upon of gravity and a limited cargo volume.
the existence of a hydraulic energy source and typically Deceleration-sensitive pressure regulating valve: This
finds application in vehicles powered by diesel engines class of valve finds wide application. The actuation
that generate only a minimal amount of intake vacuum. point is determined by the rate of deceleration of the
vehicle and this is typically of the order of 0.3g.
12.1.1.3.3 Master cylinder A benefit of this type of valve is that it does provide for
a degree of load-sensitive operation as the overall
The master cylinder essentially initiates and controls the
decelerationofthevehicleisthefunctionofthevehicle
process of braking. The governing regulations demand that
weight and the line pressure. They are also sensitive to
passenger vehicles be equipped with two separate braking
braking ona slope.Mathematicalmodelsofthisclass of
circuits and this is satisfied by the so-called tandem master valve are developed later in the text and their influence
cylinder. A tandem master cylinder has two pistons housed on the performance of a brake system is demonstrated.
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 12.1.1.3.5 Foundation brakes
piston. Thus, if a leak develops within the primary circuit, Foundation, or wheel brakes, divide into two distinct
the primary piston moves forward until it bottoms against classes, namely disc (axial) and drum (radial) brakes.
the secondary piston. The push rod force is transmitted Modern vehicles are invariably fitted with disc units on
directly to the secondary piston through piston-to-piston the front axle and there is a growing tendency to fit
contact, thus allowing the secondary piston to pressurize similar units to the rear axle. If drum brakes are fitted to

363