Troubleshooting guide

55
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Deceleration
In discussing brakes, the term deceleration is often
used. This term expresses the rate at which vehicle
speed is reduced. Deceleration usually denotes the
speed being reduced each second, in miles per hour or
feet per second.
An example is shown in Figure 6. If a vehicle is moving
at the rate of 20 miles per hour, and one second later
its speed is 18 miles per hour, the vehicle has reduced
its speed by two miles per hour during one second. Its
deceleration rate is two miles per hour per second.
In the same way, consider a vehicle moving at a rate of
30 feet per second. If, one second later, its speed is 20
feet per second, then the vehicle is decelerating at the
rate of 10 feet per second per second.
Therefore, the change in a vehicle’s speed during a
slowdown or stop is expressed by fi rst stating the
rate of speed being lost – such as miles per hour
or feet per second – and then by stating the time
required for this rate of speed to be lost.
Let’s examine a deceleration rate of 10 feet per second
per second. The fi rst part – “10 feet per second” – is the
rate of speed being lost. The second part – “per second”
– is the time in which the loss of ten feet per second takes
place.
If a vehicle is moving at a known rate and decelerating at
a known rate, the stopping time will be the initial speed
divided by the deceleration rate, provided both the rate
of speed and the deceleration rate are expressed on the
same basis.
This may be more easily understood as follows: A vehicle
is moving at the rate of 30 feet per second and begins to
decelerate at the rate of 10 feet per second per second.
At the end of the fi rst second, it will be traveling 20 feet
per second. At the end of the second second, it will
be traveling 10 feet per second. And, at the end of the
third second, it will be stopped. Thus, by losing speed
at the rate of 10 feet per second per second, the vehicle
would lose its initial speed of 30 feet per second in three
seconds.
Similarly, if the initial speed is 20 miles per hour and the
deceleration rate is two miles per hour per second, the
stopping time will be 10 seconds.
Key Takeaway:
To understand the concept of vehicles being brought
to a stop, remember this important point: While the
deceleration rate may be constant for each second
during the stop, the distance the vehicle travels each
second during the stop varies greatly as the speed
decreases.
The point is illustrated in Figure 7. A vehicle is decelerat-
ing at the rate of 10 feet per second per second from an
initial speed of 30 feet per second. The changing position
of the vehicle is shown in relation to the distance traveled
each second during the stop. Though the rate of decel-
eration remains constant throughout the stop, the vehicle
travels 25 feet during the fi rst second after the brakes
were applied, 15 feet during the second, and only fi ve
feet during the third second.
Stopping Distance
The distance traveled each second during the stop is
always greater at the beginning of the stop. To keep
stopping distance as short as possible, the brakes must
become fully effective when the driver depresses the
pedal. Any lag affects stopping distance.
A driver is mainly interested in the time and the distance
required to safely stop the vehicle under emergency
conditions, as measured from the instant he or she
depresses the brake pedal. Time lost between that
instant and the instant deceleration begins is critical.
During this time, the vehicle continues to travel at close
to its initial speed.
In the case of the vehicle in Figure 7, a lag time of only
one second between the depressing of the brake pedal
and the application of the brake system will increase the
stopping distance by 30 feet. Adding that extra second
– changing the stopping time from three seconds to four
seconds – lengthens the stopping distance from 45 feet
to 75 feet. In reducing the stopping time under these
conditions by only one second (or 25%), the stopping
distance is reduced by 30 feet (or 40%).
FIGURE 7 - DECELERATION AT 10
FEET PER SECOND PER SECOND
Deceleration