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depends on the proper location of the cam in respect

to the location of the brake shoes. The transmission of

the proper force is partially determined by the effective

lever length of the cam. If the effective lever length of

the cam is too long or too short, the brake shoe force

will be correspondingly too little or too much.

It is also important that the effective lever length of

the cam remains constant as the lining wears and the

shoes have to be spread further; otherwise, the brake

performance would vary as the lining wears.

Another form of lever found in drum-brake forms of

braking systems is the brake shoe. This is one of the

simpler forms because it is easily recognized as a

beam, fulcrumed at one end on the hinge pin, which

forces the brake lining against the drum when the

brake cam force is applied to the other end.

Perhaps the least easily recognized lever in a drum

brake system is the relation of the brake drum diameter

to the tire diameter. In order to understand this fully

it must be remembered that although the brakes stop

the brake drums and wheels, it is always the tires

and road surface that stop the vehicle. This is clearly

demonstrated when quick stops are attempted on

wet or icy roads. Under these conditions the brake

equipment may still be as efﬁ cient as ever in stopping

the wheels, but its ability to stop the vehicle quickly

diminishes because there is not sufﬁ cient friction

between the tire and road to develop the necessary

retarding force.

Returning to the principles of leverage involved in the

relation of the tire and brake drum size, the retarding

force developed by the brake shoes acting against

the drum is working on an effective lever length of the

brake drum radius. Counteracting this is the retarding

force developed between the tire and the road, working

on an effective lever length of the rolling radius of the

tire. Since it is not practical to have brake drums as

large as the tires, the principles of leverage require

development of a greater retarding force between the

brake shoes and the drums than between the tire and

the road. Also, since a rubber tire on a smooth, dry

road surface has a higher coefﬁ cient of friction than

brake lining against a brake drum, it is necessary to

develop additional retarding force between the brake

shoes and brake drum in order to overcome the

difference in friction.

Deceleration

In discussing brakes, the term deceleration is often

used. This term expresses the actual rate at which

vehicle speed is reduced and usually denotes the

speed being reduced each second, in terms of miles

per hour or feet per second.

As an example as shown in Figure 6 - if a vehicle

is moving at the rate of 20 miles per hour, and one

second later its speed is only 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, if a vehicle is moving at a rate of 30

feet per second, and one second later its speed is only

20 feet per second, then it is decelerating at the rate

of ten feet per second per second.

Therefore, the change in the rate of speed of a vehicle

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.

Thus, in examining the expression covering a

deceleration rate of say, “ten feet per second per

second,” the ﬁ rst part – “ten feet per second” – is the

rate of speed being lost, and 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 is

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. As an example

– if a vehicle is moving at the rate of 30 feet per

FIGURE 6 - DECELERATION

Leverage (continued), Deceleration

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 69 70

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BENDIX MC-12 MODULATOR CONTROLLER ASSY Guida alla Risoluzione dei Problemi Pagina 53

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