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Engineering Design Handbook - Analysis and Design of Automotive Brake Systems

CHAPTER 1 
INTRODUCTION 
In this chapter some basic relationships are presented that show how stopping distance is 
dependent upon speed, deceleration, and time. The concept of tire-road friction utilization is 
introduced briefly. Significant problems of braking are im•roduced. Methods for improving 
braking perfomtance are reviewed briefly.

The vehicle is connected to the roadway by the 
traction forces produced by the tires. Consequently. 
only circumferential tire forces equal to or less than 
the product of normal force and tire-roadway friction 
coefficient can be transmitted by the wheels. Ex. 
ceptions are provided by special designs using aero. 
dynamic effects or rocket down thrusters resulting in 
greater normal forces On the tires than the vehicle 
weight. 
This fundamental consideration yields a possible 
all wheels locked minimum stopping distance Sg as 
given by the relationship

A significant problem of braking arises as a result 
of dynamic load transfer induced by vehicle de- 
ccleration. This is especially important in the design 
oi vehicles wherein a significant difference in center. 
ol'-gravity location exists between loaded and un- 
loaded cases, e.g., station wagons and trucks. For 
example, a typical 3/4.ton pickup truck will ex#r. 
ience a dynamic load transfer onto the front axle Of 
approximately 5(X) 1b for the empty case and 1b 
for the loaded case for a deceleration of 16 ft/sa, The 
static axle load distribution, the height of the center 
Of gravity above the road surface, the wheel base. as 
well as the level Of vehicle deceleration are factors in- 
fluencing dynamic load transfer. The relationships 
for determining the dynamic axle loads for a variety 
of vehicles are presented in detail in Chapters 8 and 9. 
For a typical two-axle tractor coupled to a single-axle 
trailer, commonly termed as a 2-SI combination, the 
dynamic axle loads as a function of vehicle de- 
celeration are illustrated in Fig. 1-3. These curves in. 
dicate that the rear axle load of the tractor is little af- 
fected by deceleration, whereas the front axle and the 
trailer axle show significant changes in their re- 
spective dynamic axle loads.