Section I. PRINCIPLES OF BRAKING
moving vehicle, just as any other moving body, has what
34-1. Braking Action. Braking action is the use of a
is known as kinetic energy. Kinetic energy is the energy
controlled force to accomplish the three basic tasks of
an object possesses due to its relative motion and may
reducing speed, stopping, and holding an object in a
be expressed as (mass) x (velocity) . This kinetic
Braking action usually is
energy, which increases with the square of the speed,
accomplished by rubbing two surfaces together that
must be overcome by braking action. If the speed of a
cause friction and heat (fig. 34-1). Friction is the
vehicle is doubled, its kinetic energy is increased
resistance to relative motion between two surfaces in
fourfold; four times as much energy, therefore, must be
contact. The mechanical energy of reaction then is
overcome by the braking action. Brakes must not only
transformed into hear energy.
Heat energy is an
be capable of stopping a vehicle, but must stop it in as
unwanted product of friction and must be dissipated to
short a distance as possible. Because brakes are
the surrounding environment as efficiently as possible.
expected to decelerate a vehicle at a faster rate than the
Automotive vehicles use this rubbing action to develop
engine can accelerate it, they must be able to control a
the friction required for braking. Braking action also may
greater power than that developed by the engine. This is
be accomplished by establishing a rubbing contact with
the reason that well-designed, powerful brakes have to
the roadway, as is done by some trolleys, which apply a
be used to control the modern high-speed motor vehicle.
braking surface to the rails.
It is possible to accelerate an average passenger car
with an 80-hp engine from a standing start to 80 mph in
34.2. Braking Requirements.
It is known that to
about 36 seconds. By applying the full force of the
increase a vehicle's speed requires an increase in
brakes, such a vehicle can be decelerated from 80 mph
the power output of the engine. It also is true,
to a full stop in about 4.5 seconds. The time required to
although not so apparent, than an increase in speed
decelerate to a sop is one-eighth the time required to
requires an increase in the braking action required to
accelerate from a standing
bring a vehicle to a stop (fig. 34-2). A
Figure 34-1. Development of Friction and Heat.
Figure 34-2. Braking Requirements.