Page 32 - Driving Commercial Vehicles Manual

2 heavy vehicle braking

What you’ll learn

After reading this chapter you’ll be able to:
❏ explain why heavy vehicles take longer to stop than smaller
passenger vehicles

❏ explain why air brakes are used on heavy vehicles
definition ❏ describe the basic scientific principles of braking

Heat is a form of energy. ❏ identify the elements of stopping distance
In an internal combustion ❏ explain the effects of weight and speed on stopping distance
engine, the heat energy
produced by engine ❏ explain how to brake safely in different situations.
combustion is converted
to the energy of motion
through the moving parts of
the engine and drive train,
turning the wheels. Starting and stopping
Even though the energy of
motion turns the vehicle’s Stopping seems simple. When you drive a car and you want to stop, you
wheels, the vehicle won’t press on the brake pedal and the car comes to a stop. And when you want
go anywhere unless there’s to go somewhere in a car, you start it, press on the accelerator, and the car
traction between the tires begins to move.
and the road surface.
But what actually happens to cause the vehicle to stop? And what causes a
In stopping a vehicle, vehicle to move? To answer these questions, we need to know some basic
the energy of motion is scientific principles.
converted to the energy
of heat through friction Your car’s engine, like the diesel engine of a truck, is a heat conversion
between the lining/pad and machine, taking the energy of heat from the exploding mixture of fuel and air
drum/rotor surfaces of the in the combustion chamber, and converting it to motion through the engine
vehicle’s brakes. crankshaft and drivetrain to the wheels.
A brake — whether a brake for a car or a commercial vehicle — is also
a heat conversion machine, but works exactly opposite to an engine. Brakes
convert the energy of motion back into the energy of heat through the friction
between the brake drums or rotors and the brake linings or pads.
A simple kind of brake is that used by a skater on rollerblades — the skater
fast fact tips the rollerblade to the rear, and a pad rubs against the pavement to slow
and stop the skater. The pad gets hot, and so does the pavement. This is
The final factor in stopping because the energy of motion has been converted to the energy of heat
is in the contact between through the friction between the pad and the pavement. Both the pad and
the vehicle and the road the pavement need to be able to absorb the heat created while stopping.
through the tires.
A vehicle’s brakes work on these same principles. Attached to each wheel is
Bald or defective tires may either a drum brake or a disc brake that rotates with the wheel. To stop the
degrade braking performance.
vehicle, brake linings rub against the brake drum, or in the case of disc brakes,
brake pads rub against the brake rotor. This creates friction, converting the
vehicle’s energy of motion into the energy of heat, which stops the vehicle.
The heat is absorbed and dissipated into the atmosphere, primarily through
the brake drums/rotors.

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