Page 359 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 2. 1 Braking systems

When simply stated, as above, the importance of the role (2) Modulation system
played by the brakes/braking system in controlling the This embraces those elements of the brake system
vehicle motion is grossly understated. Consideration of which are used to control the level of braking effort
the diverse conditions under which the brakes must applied to each brake. Included in this system are
operate leads to a better appreciation of their role. These the driver, pressure limiting/modulating values and,
include, but are not limited to, the following: if fitted, anti-lock braking systems (ABSs).
(3) Transmission system
slippery wet and dry roads.
The components through which energy travels to
rough or smooth road; the wheel brakes comprise the transmission
split friction surfaces; system. Brake lines (rigid tubes) and brake hoses
straight line braking or when braking on a curve; (flexible tubes) are used in hydraulic and air brake
wet or dry brakes; systems. Mechanical brakes make use of rods,
new or worn linings; levers, cams and cables to transmit energy. The
laden or unladen vehicle; parking brake of a car quite often makes use of
vehicle pulling a trailer or caravan; a mechanical transmission system.
frequent or infrequent applications of short or (4) Foundation brakes
lengthy duration; These assemblies generate the forces that oppose
high or low rates of deceleration; the motion of the vehicle and in doing so convert
skilled or unskilled drivers. the kinetic energy associated with the longitudinal
motion of the vehicle into heat.
Clearly the brakes, together with the steering compo-
nents and tyres, represent the most important accident There are four main stages involved in the design of
avoidance systems present on a motor vehicle which a brake system. The first, and perhaps most fundamental
must reliably operate under various conditions. The stage, is the choice of brake force distribution between
effectiveness of any braking system is, however, limited the axles of the vehicle. This is primarily a function of the
by the amount of traction available at the tyre–road vehicle dimensions and its weight distribution. Next is
interface. the design of the transmission system and this activity
embraces the sizing of the master cylinder together with
12.1.1.2 System design methodology the front and rear wheel cylinders. Additional compo-
nents, such as special valves that modulate the hydraulic
The primary functions of a brake system, listed above, pressure applied to each wheel are physically accounted
must be fulfilled at all times. In the event of a system for at this stage. The foundation brakes form the focus of
failure, the same functions must also be performed albeit the third stage of the process. As well as being able to
with a reduced efficiency. Consequently the braking react the applied loads and torques, the foundation brakes
system of a typical passenger car comprises a service must be endowed with adequate thermal performance,
brake for normal braking, a secondary/emergency brake wear and noise characteristics. The last phase in the
used in the event of a service brake failure and a parking process results in the incorporation of the pedal assembly
brake. Current practice permits service brake compo- and vacuum boost system into the brake system. To
nents to be used in the secondary/parking brake systems. accomplish this design task, the engineer requires access
Irrespective of the detail design considerations all to several fundamental vehicle parameters. These
brake systems divide into the following subsystems: include:
laden and unladen vehicle mass;
(1) Energy source static weight distribution when laden and unladen;
This includes all those components which generate, wheelbase;
store or release energy required by the braking height of centre of gravity when laden and unladen;
system. In standard passenger cars muscular pedal maximum vehicle speed;
effort, applied by the driver, in combination with tyre and rim size;
a vacuum boost system comprise the energy source.
In the event of a boost failure, the driver can still vehicle function;
apply the brakes by muscular effort alone. Alter- braking standards.
native sources of energy include power braking It is essential to recognize that each of the preceding
systems, surge brakes, drop weight brakes, electric stages are closely linked and that the final design will take
and spring brakes. many iterations to realize. Thus any formal methodology

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