Troubleshooting guide

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Now let’s reintroduce weight and speed into the

comparison. If the same stopping power is used, a 5,000

pound vehicle needing only 30 feet to stop from 20 miles

per hour will require 18 times the stopping distance – or

540 feet – when loaded to 10,000 pounds and traveling

at 60 miles per hour. Note: Many other factors, including

road surface, brake friction material, and tire condition

also affect stopping distance.

Leverage

Now that we’ve reviewed the forces involved in braking

a vehicle, let’s consider how these forces are developed

and directed to do the braking work. Almost all braking

systems make use of the lever, one of the oldest

mechanical devices governing the transmission and

modiﬁ cation of force and motion.

A lever is an inﬂ exible rod or beam capable of motion

about a ﬁ xed point called a fulcrum, and it is used to

transmit and modify force and motion.

Figure 5 illustrates three simple types of levers. The only

difference among them is the location of the fulcrum in

relation to the applied force and the delivered force. All

shapes and sizes of levers used in a typical brake system

are one of these three types.

A simple law governs levers: The applied force multiplied

by the perpendicular distance between the line of force

and the fulcrum always equals the delivered force

multiplied by the perpendicular distance between the

fulcrum and the line of force.

That means, with a leverage arrangement as shown in

view 5(a), an applied force of 100 pounds two feet from

the fulcrum will give a delivered force of 200 pounds

at a point one foot from the fulcrum. With a leverage

arrangement as shown in Figure 5(b), an applied force

of 100 pounds three feet from the fulcrum will lift 300

pounds at a point one foot from the fulcrum.

In both cases, note that the delivered force exceeds the

applied force because the applied force is farther from

the fulcrum than the delivered force. With a leverage

arrangement as shown in Figure 5(c), the delivered force

is the farthest from the fulcrum; therefore, it is less than

the applied force. If the applied force in this case is 300

pounds at a point two feet from the fulcrum, the delivered

force at a point three feet from the fulcrum will be 200

pounds.

To calculate the delivered force of any lever, ﬁ rst multiply

the applied force by its distance from the fulcrum. Then

divide this answer by the distance between the delivered

force and the fulcrum.

In determining the distance at which any force is acting on

a lever, the true length of the lever arm is the perpendicular

distance from the force to the fulcrum, regardless of the

shape of the lever. The lever arm is always measured at

right angles to the direction of the force.

The product of the force acting on a lever, multiplied by the

distance between the force and the fulcrum, is called the

turning moment. When the turning moment relates to a

shaft, it is called torque. The turning moment – or torque

– is usually expressed in inch-pounds, foot-pounds, foot-

tons, etc. The designation depends on whether the force

is measured in pounds or tons, and whether the distance

is measured in inches or feet. For example, a force of

100 pounds acting on a lever arm ﬁ ve inches long would

result in a turning moment or torque of 500 inch pounds.

FIGURE 5 - LEVERAGE

5(a)

5(b)

5(c)

Leverage