Page 363 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 2. 1 Braking systems

the vehicle is limited so that the braking system does not distance travelled by the vehicle moving with constant
become too inefficient. A detailed interpretation of this velocity U is:
requirement is outlined later in the text.
S 1 ¼ Ut 1 (12.1.1)
12.1.3 The fundamentals of braking In part 2 the vehicle is decelerated at a constant rate
until such time as the vehicle comes to rest. The distance
12.1.3.1 Kinematics of a braking vehicle travelled is:

12.1.3.1.1 Kinematic analysis of a braking Ut 2 U 2
vehicle S 2 ¼ 2 ¼ 2a (12.1.2)
The distance travelled by a vehicle when braking, either
during a stop or snub (when the final velocity is non-zero) Thus, the total stopping distance is simply
is a basic measure of the effectiveness of a brake system.
Before addressing issues coupled to the forces which act U 2
on a vehicle during a braking manoeuvre it is worthwhile S i ¼ S 1 þ S 2 ¼ Ut 1 þ 2a (12.1.3)
to first consider the kinematic behaviour of the vehicle. A
straightforward kinematic analysis assuming straight line The preceding analysis assumes the vehicle de-
(one-dimensional) motion and constant deceleration celeration is achieved instantaneously and is sustained
provides a ready indication of stopping distance. Pre- for the duration of the stop. No account is taken of
dictions of stopping distance made by this analysis find driver reaction time, initial system response time, de-
use in accident reconstruction. celeration rise time, change in deceleration during the
With reference to Figure 12.1-3, the total distance period of actual braking and, if applicable, release time.
travelled is made up of two parts. In part 1, the total These factors are now defined with reference to

Deceleration time history
2.5
Deceleration (m s –2 ) 1.5
2.0
1.0
0.5
0
0 5 10 15 20
Time (s)

Velocity time history
35
30
Velocity (m s –1 ) 20
25
15
10

0 5
0 5 10 15 20
Time (s)

Displacement time history
400
Displacement (m) 200
300

100
0
0 5 10 15 20
Time (s)
Figure 12.1-3 Kinematics of a simple stop.

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