Page 33 - Driving Commercial Vehicles Manual

driving commercial vehicles

You might say the energy has gone full circle:

Heat Energy Energy of Motion Heat Energy
The basic principle behind
braking systems: friction (from the engine) (through the drivetrain) (through the brakes)
converts the energy of
motion to heat energy.

Drum brake Disc brake

Power to move and power to stop

A typical compact car weighs about 1,000 kg and has about 120 hp. It can
accelerate to 100 km/h within about 200 m and in less than 10 seconds.
A heavy tractor-trailer combination can weigh up to 63,500 kg when loaded.
Even though it may have a diesel engine producing over 400 hp, because of
the weight of the combination and its load, it might take over one kilometre
and over one minute to accelerate to 100 km/h.

Now think about stopping a tractor-trailer combination that's going 100 km/h.
How much energy is needed to stop it? You certainly want to stop it in a much
shorter distance than one kilometre and in much less than one minute. In an
emergency, the combination might have to stop in as little as seven seconds
— roughly / 9 of the time it took to reach 100 km/h.
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To stop the vehicle this quickly requires a stopping force of nine times the
acceleration force — the equivalent of approximately 4,000 hp.
With a tractor-trailer combination equipped with eight equally loaded axles
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fast fact and 16 brakes, each brake has to provide approximately / 16 of the braking
force. If one or two of these brakes isn’t correctly adjusted, the others have
to do more than their share of the braking, resulting in a longer stopping
Reaction time is the time it distance. If the axles are not equally loaded, wheels with lighter loads may
takes between recognizing a skid, also resulting in a longer stopping distance.
hazard ahead and applying
the brakes. The other brakes would also have to work harder and that might be more
An average driver’s reaction work than they’re manufactured to do. Excessive use of the brakes could
time is about / 4 of a second. result in a buildup of more heat than the brakes can absorb and dissipate. Too
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much heat can result in brake damage and possible failure.

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