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
definition
Heat is a form of energy.
In an internal combustion
engine, the heat energy
produced by engine
combustion is converted
to the energy of motion
through the moving parts of
the engine and drive train,
turning the wheels.
Even though the energy of
motion turns the vehicle’s
wheels, the vehicle won’t
go anywhere unless there's
traction between the tires
and the road surface.
In stopping a vehicle,
the energy of motion is
converted to the energy
of heat through friction
between the lining/pad and
drum/rotor surfaces of the
vehicle’s brakes.
-- explain why air brakes are used on heavy vehicles
-- describe the basic scientific principles of braking
-- identify the elements of stopping distance
-- explain the effects of weight and speed on stopping distance
-- explain how to brake safely in different situations.
Starting and stopping
Stopping seems simple. When you drive a car and you want to stop, you
press on the brake pedal and the car comes to a stop. And when you want
to go somewhere in a car, you start it, press on the accelerator, and the car
begins to move.
But what actually happens to cause the vehicle to stop? And what causes a
vehicle to move? To answer these questions, we need to know some basic
scientific principles.
Your car’s engine, like the diesel engine of a truck, is a heat conversion
machine, taking the energy of heat from the exploding mixture of fuel and air
in the combustion chamber, and converting it to motion through the engine
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.
fast fact
The final factor in stopping is
in the contact between the
vehicle and the road through
the tires.
Bald or defective tires may
degrade braking performance.
A simple kind of brake is that used by a skater on rollerblades — the skater
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
because the energy of motion has been converted to the energy of heat
through the friction between the pad and the pavement. Both the pad and
the pavement need to be able to absorb the heat created while stopping.
A vehicle’s brakes work on these same principles. Attached to each wheel is
either a drum brake or a disc brake that rotates with the wheel. To stop the
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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driving commercial vehicles
You might say the energy has gone full circle:
The basic principle behind
braking systems: friction
converts the energy of
motion to heat energy.
Heat Energy
Energy of Motion
Heat Energy
(from the engine)
(through the drivetrain)
(through the brakes)
Drum brake
Disc brake
Force multipliers
The force generated at the wheel to stop is a lot more than the force you
apply when pushing down on the brake pedal.
The driver is using a tool to
gain leverage to measure
the brake chamber pushrod
travel.
In this diagram, the driver is pulling on an air brake slack adjuster to measure
if the brake is within adjustment tolerance.
The slack adjuster, besides adjusting for brake wear, acts as a lever. Leverage
is a form of force multiplication.
Trucks and buses are much heavier than cars, so they need even more
mechanical advantage.
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chapter 2 — heavy vehicle braking
What's compressed air?
Air can be compressed (squeezed) into a much smaller space than it would
normally occupy. The tires on a car are filled with compressed air to support
the weight of a vehicle.
Squeezing air into a smaller space increases the air’s resistance. This resistance
creates pressure, which can be converted into mechanical force to apply the
brakes. Air brakes generate more braking force than hydraulic brakes.
If a constant supply of compressed air were directed through a pipe that's
one-inch square (see diagram below), and if a one-inch square plug were
placed in the pipe, the compressed air would push against the plug. Holding
a scale against the plug would register how many pounds of force the air was
exerting against the plug.
If the scale registered 10 pounds, for example, then it could be said that the
force was 10 pounds on the one square inch surface of the plug. This would
be 10 p.s.i. or 68.9 kPa.
The more the air's compressed (that is, the greater the air pressure), the
greater the force that would be exerted on the face of the plug.
Leverage and air pressure
Air chambers are very powerful. A typical type 30 chamber, if applied with air
pressure at 100 p.s.i. (690 kPa), develops a pushrod force of 3,000 pounds.
This force is then applied to move the lever (the slack adjuster) to apply the
brakes.
Through force multiplication,
100 p.s.i. (690 kPa) of air
pressure produces a pushrod
force of 3,000 pounds.
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