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second and is decelerating at the rate of ten feet per

second, the stopping time will be the initial speed of

30 feet per second divided by the deceleration rate of

ten feet per second per second, or a stopping time of

three seconds.

This perhaps can be more easily understood if

explained in the following manner; if a vehicle is

moving at the rate of 30 feet per second and begins

to decelerate at the rate of ten 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 ten feet per second, and at the end

of the third second, it will be stopped. Thus, by losing

speed at the rate of ten feet per second per second,

it 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 ten seconds.

One important thing to remember in respect to

stopping vehicles is the fact that 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.

This is illustrated in Figure 7 which also shows a

vehicle decelerating at the rate of ten feet per second

per second from an initial speed of 30 feet per second,

but the positions of the vehicles are shown in relation

to the distance traveled each second during the stop.

This shows that although the rate of deceleration

remains constant throughout the stop, the vehicle

actually travels 25 feet during the ﬁ rst second after

the brakes were applied, 15 feet during the second

second, and only ﬁ ve feet during the third second.

The distance being traveled each second during the

stop is always greater at the beginning of the stop.

To keep stopping distance as short as possible, it is

important that the brakes become fully effective when

the pedal is depressed by the driver.

Any time lost between the instant the brake pedal is

depressed and the instant actual deceleration begins

is important because the vehicle continues to travel

at close to its initial speed. In this case, the loss

of only one second between the instant the driver

depresses the brake pedal and the point where the

brakes are really applied will result in lengthening

the actual stopping distance by 30 feet. Thus, if four

seconds instead of three elapse between the instant

the driver depresses the brake pedal and the instant

the vehicle stops, the actual stopping distance will be

increased from 45 feet to 75 feet. In other words, by

reducing the stopping time under these conditions by

only one second or 25%, the actual stopping distance

is reduced by 30 feet or 40%.

It is this part of brake fundamentals which is not

often considered in evaluating brake performance,

particularly when different forms of brakes are

involved. A common method of testing brakes is by the

use of a decelerometer–a device that determines the

maximum rate of deceleration developed during a stop

and which shows a calculated stopping distance from

a speed of 20 miles per hour based on the maximum

rate of deceleration developed during a stop. Such

instruments do not, however, make allowances for lost

time before the braking system develops full power

and therefore are not suitable for analyzing time lag

factors in brake performance.

The true performance of any type of brake system

in terms of stopping time or stopping distance can

only be determined by actually measuring the time

and distance the vehicle travels from the instant the

driver depresses the brake pedal to the point where

the vehicle actually stops. Such tests can, of course,

be made comparative only by using instruments to

determine accurately the speed of the vehicle at the

instant the brake pedal is depressed.

In so far as brakes are concerned, a driver is mainly

interested in the amount of time and the distance

required to bring the vehicle safely to a stop under

emergency conditions as measured from the instant he

or she depresses the brake pedal. Any lag in the time

between the instant the brake pedal is depressed, and

the instant the brakes apply, affects stopping distance.

FIGURE 7 - DECELERATION AT 10 FEET PER SECOND PER

SECOND

Deceleration (continued)

1 2 ... 49 50 51 52 53 54 55 56 57 58 59 ... 69 70

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