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```					                           Heavy Vehicle Braking
2

What You’ll Learn
After reading this chapter you will be able to:

explain why heavy vehicles take longer to stop than smaller
passenger vehicles

explain why air brakes are used on heavy vehicles

describe the basic scientiﬁc principles of braking
Deﬁnitions
identify the elements of stopping distance
Heat is a form of
energy. In an internal                 explain the effects of weight and speed on stopping distance
combustion engine, the
heat energy produced                   explain how to brake safely in different situations
by engine combustion is
converted to the energy
of Motion through the
moving parts of the
Starting and Stopping
engine and drive train,     To most people, stopping seems simple. When you drive a car and you
turning the wheels.         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
Even though the
energy of motion turns      gas pedal, and the car begins to move.
the vehicle’s wheels,       But what actually happens to cause the vehicle to stop? And what causes
the vehicle won’t go
a vehicle to move? To answer these questions, we need to know some
anywhere unless there
basic scientiﬁc principles.
is Traction between
the tires and the road      Your car’s engine, like the diesel engine of a truck, is a heat conversion
surface.                    machine, taking the energy of heat from the exploding mixture of fuel
In stopping a vehicle,      and air in the combustion chamber, and converting it to motion through
the energy of Motion is     the engine crankshaft and drivetrain to the wheels.
converted to the energy
of Heat through Friction    A brake — whether a brake for a car or a commercial vehicle — is also
between the lining/pad      a heat conversion machine, but works exactly opposite to an engine.
and drum/rotor surfaces     Brakes convert the energy of motion back into the energy of heat through
of the vehicle’s brakes.    the friction between the brake drums or rotors and the brake linings

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Chapter 2

Fast Fact                         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
The ﬁnal factor in                pavement to slow and stop the skater. If you were to feel the pad after
stopping is in the                the skater has stopped, it would be hot, as would the pavement. This
contact between the               is because the energy of motion has been converted to the energy of
vehicle and the road              heat through the friction between the pad and the pavement. Both the
through the tires.                pad and the pavement need to be able to absorb the heat created while
Bald or defective tires           stopping.
A car’s brakes work on these same principles, as do the brakes on a
performance.
truck or bus. Attached to each wheel is either a drum brake or a disc
brake. The drum or disc 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.

You might say the energy has gone full circle:

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

Drum brake                              Disc brake

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Heavy Vehicle Braking

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 the diagram above, the driver is pulling on an air brake slack adjuster
to measure if the brake is within adjustment tolerance.

lever. Leverage is a form of force multiplication.

Trucks and buses are much heavier than cars, so they need even more

What is 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.

The compressed air in the tire

Fast Fact

Air brakes were invented
by George Westinghouse
in 1869 to improve the
stopping power of trains.         The smaller the space into which air is squeezed, the greater the air’s
They were ﬁrst applied            resistance will be to being squeezed. This resistance creates pressure,
to trucks in the 1920s.           which can be converted into mechanical force to apply the brakes. Air
brakes generate more braking force than hydraulic brakes.

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Chapter 2

If a constant supply of compressed air were directed through a pipe that
is 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.

Fast Fact                        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
Air pressure is measured         would be 10 p.s.i. or 68.9 kPa.
in pounds per square
inch (p.s.i.). The metric        The more the air is compressed, (that is, the greater the air pressure), the
equivalent measurement           greater the force that would be exerted on the face of the plug.
is kilopascals (kPa),
named after Blaise               Leverage and air pressure
Pascal (1623-1662), a            Air chambers are very powerful. A typical type 30 chamber, if applied
French philosopher and           with air pressure at 100 p.s.i. (690 kPa), develops a pushrod force of
mathematician.                   3,000 pounds. This force is then applied to move the lever (the slack
10 p.s.i. = 68.9 kPa             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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Heavy Vehicle Braking

Power to Move and Power to Stop
A typical compact car weighs about 1,000 kilograms and has about 120
horsepower. It can accelerate to 100 km/h within about 200 metres and
in less than 10 seconds.

A heavy tractor-trailer combination
will take a lot longer to accelerate
than a car – and a lot longer to
stop, too.

A heavy tractor-trailer combination can weigh up to 63,500 kilograms
when loaded. Even though it may have a diesel engine producing over
400 horsepower, because of the weight of the combination and its load,
it might take over one kilometre to accelerate to 100 km/h and take over
one minute to do so.

Now think about stopping a tractor-trailer combination that is going 100
km/h. How much energy is needed to stop it? You certainly would 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 be
braked to a stop in as little as seven seconds — roughly 1/9 of the time it
took to reach 100 km/h.

To stop the vehicle this quickly would require a stopping force of nine
times the acceleration force — the equivalent of approximately 4,000
horsepower.

With a tractor-trailer combination equipped with eight equally-loaded
axles and 16 brakes, each brake would have to provide approximately
1
/16 of the braking force. If one or two of these brakes were not correctly
adjusted, the others would have to do more than their share of the
braking, resulting in a longer stopping distance. If the axles were not
equally loaded, wheels with lighter loads may skid, also resulting in a
longer stopping distance.

As well, the other brakes would have to work harder and that might
be more work than they were manufactured to do. Excessive use of the
brakes would then result in a buildup of heat greater than the brakes
could absorb and dissipate. Too much heat would result in brake damage
and possible failure.

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Chapter 2

Fast Fact                   Stopping distance and stopping time
To stop a vehicle, you need to See-Think-Do. Total stopping distance is
Reaction time is the
the distance your vehicle will travel from the moment you:
time it takes between
recognizing a hazard            •   See — a hazard
brakes.                         •   Think — decide to stop
An average driver’s             •   Do — place your foot on the brake pedal until you stop
3
/4 of a second.            This distance can also be expressed as the time it takes to stop.

When you see a problem ahead while driving, it will take you about
3
/4 of a second of Perception Time (See-Think) and another 3/4 of a
second of Reaction Time (Do). Only then will your vehicle begin to slow.
Deﬁnition
Driver’s Perception Time    A car’s brakes begin to work almost instantaneously when you press
on the brake pedal. This is not the case with air brakes because there
+ Reaction Time         is a Brake Lag Time of approximately 4/ of a second between when
10
+ Brake Lag Time        you apply the brake and when the brakes begin to function. Add to this
+ Braking Time          the Braking Time, which is the time — or distance — the vehicle travels
before it stops.
= Total Stopping Time
This is why it is so important to allow enough distance when following
other vehicles

•   passenger cars and light truck drivers should use the two-
second rule. Watch the vehicle ahead pass some checkpoint on
the roadway, such as an overpass or sign post. Start to count:
Driving Tip
“One thousand and one, one thousand and two.” That’s two
Total Stopping Time                 seconds.
will be even longer if          •   drivers of buses, trucks and other large heavy vehicles should
brakes are incorrectly              never be less than ﬁve seconds behind the vehicle ahead at
adjusted or vehicle is on           highway speeds.
The most common                     on following distance.
air brake system
defect found during             •   stay even further back when conditions are less than ideal.
a commercial vehicle
inspection is brakes that

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Heavy Vehicle Braking

Fast Fact

Total stopping distance
or time depends on
the ability of the brake
produce friction, the
brake drums to dissipate
heat and the tires to

Total stopping distance includes
perception time, reaction time,
brake lag time and braking time.
Always allow enough following
distance or time and choose a
Speed and weight facts
speed that will allow you to stop   Speed and weight affect the stopping power required to stop any vehicle,
safely.                             and how far it will travel before it stops. You need more stopping power
whenever the speed you are travelling and/or the weight of your vehicle
increases:

Fast Fact                             •      2 times vehicle speed requires 4 times the stopping power
•      2 times vehicle weight requires 2 times the stopping power
Driving without due
care and attention and                •      2 times vehicle speed and 2 times vehicle weight requires 8
unsafe speed are the top                     times the stopping power
two factors assigned
to commercial drivers
involved in police-
Stopping Power Required to Stop a Vehicle
reported casualty                   Speed unchanged — weight doubled                           Weight unchanged — speed doubled
crashes.
Trafﬁc Collision
10,000kg 30 km/h                                  10,000kg 30 km/h
Statistics: British
Columbia (2002)

20,000kg 30 km/h                                  10,000kg 60 km/h

2 X vehicle weight requires 2 X the stopping power     2 X vehicle speed requires 4 X the stopping power

Both weight and speed doubled

20,000kg 60 km/h

2 X vehicle weight + 2 X vehicle speed requires 8 X the stopping power

A typical compact car weighs about 1,000 kilograms. The truck in the
illustration above may weigh 25,000 kilograms when fully loaded — over
25 times heavier than a car. And heavy tractor-trailer combinations can
weigh up to 63,500 kilograms.

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Chapter 2

There is only so much stopping power available, and the faster the
vehicle is travelling, the more power it needs to stop. This is why it is so
important to drive at a safe speed, at a safe following distance, and within
the vehicle’s stopping capability.

Compared to cars, trucks and buses have larger brake components,
more wheels and more brake linings/shoes or rotors/pads. Despite these
features, trucks and buses have longer stopping distances.

The truck is going to take much
longer to stop than the car — even
more so if going downhill.

Another factor involved in stopping distances is the slope or grade of the
road. A vehicle travelling down a hill will need a longer stopping distance
than a vehicle travelling at the same speed on a level surface because of
the effect of gravity. A vehicle travelling up a hill will stop in a shorter
distance than a vehicle travelling the same speed on a level surface, again
because of the effect of the grade.

How much heat can brakes handle?
Most brake components operate
best at around 120° C and should
not exceed 250° C.

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Heavy Vehicle Braking

Deﬁnition                      In normal stopping, the brake components heat up to about 120° C
(almost 250° F). The brake components are constructed to withstand
Brake fade occurs when         more heat, and can handle up to about 250° C under hard braking.
properly because they          If excessive hard braking is done, heat will continue to build up to a point
are overheated. Your           where it exceeds the capability of the brake components to absorb and
vehicle takes longer to        dissipate the heat. As brake drums heat up, they expand away from the
stop, or may not stop          brake linings. Too much heat can result in brake fade, brake damage
at all if you have not         and/or brake failure. Brake components or tires may actually catch ﬁre.
properly controlled the
speed.

Retarders
Retarders are designed to provide auxiliary slowing of the vehicle, such as
Deﬁnition                      for controlling the speed on long downgrades without the use of the main
braking system. Engine retarders (also known as engine brakes) help to
Retarders provide an
save the main braking system for emergency stopping requirements.
auxiliary slowing effect
and assist the main            There are four main types of retarders in use:
brake system to slow the
vehicle.                           •   Engine retarders
•   Exhaust retarders
•   Hydraulic driveline retarders
•   Electric driveline retarders

Many retarders can absorb as much or more horsepower as the engine
can develop.

Bauhinea
dcv_2-11

An engine brake control switch.

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Chapter 2

Using retarders
An engine retarder is more efﬁcient at a higher engine r.p.m. and a lower
Driving Tip                 vehicle speed, so it is important to select the right gear. Select your gear
before you start down a hill. You are more likely to miss a shift if you
Make sure you are           wait until you are on the downgrade.
familiar with the
manufacturer’s operating    Retarders/engine brakes should be used with caution on slippery road
procedures before           surfaces. Some can actually cause the drive wheels to lock up, stalling
operating any retarding     the engine, and in some cases result in loss of control or even cause a
device.                     tractor/trailer to jackknife.

All modern trucks are manufactured to meet safety
standards, including noise levels. A well-engineered
truck with an engine retarder and a properly
Deﬁnition
maintained mufﬂer system should not be noisy.
The term r.p.m. stands
for revolutions per        Many municipalities post signs restricting drivers
minute. This refers to     from using engine retarders/engine brakes. These
the number of complete     signs must be obeyed.
turns the crankshaft
makes in one minute.
Most trucks have a         Braking
tachometer in the          Bringing your vehicle to a stop on a level roadway usually means
instrument panel to        squeezing gradually and ﬁrmly on the brake pedal with your foot.
indicate the engine’s      However, as the speed or weight of your vehicle or the degree of
r.p.m.                     emergency increases, you may have to react more quickly.

Just before your vehicle comes to a full stop, reduce the pressure you
are placing on the brake pedal. This will prevent your vehicle from
Fast Fact                   jerking back. Practise stopping until you become familiar with how much
pressure you need to ease off the brake to smoothly stop where you want.
You may be ﬁned as
much as \$2,000 if you       Watch the driving technique of the driver ahead. If the driver in front
are operating a vehicle     of you is a tailgater, expect frequent panic stops. Professional drivers
with brakes that are not    maintain a safe following distance and plan for gradual stops. By making
adjusted correctly or not   your stops slowly you give the driver behind you plenty of notice that you

Heavy commercial vehicles have longer stopping times and distances
than most other vehicles. To stop safely, you must ensure that your brakes
condition. You should also be certain that your tires are properly inﬂated.
Tailgaters are drivers
who follow other            To ensure a smooth, controlled stop every time:
vehicles too closely.
These drivers are               •   travel at a speed that allows you to see a safe distance ahead
allowing the vehicle            •   maintain a safe following time or distance
ahead to be in their own
vehicle’s Danger Zone.          •   use good braking skills

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Heavy Vehicle Braking

To stop on icy roads, use extremely light pressure on the brake to control
Driving Tip
your vehicle’s speed. This will help prevent the wheels from locking up.
Don’t wait until you spin         A slowly revolving wheel on an icy surface will be more effective than
out on a hill to put on           a locked wheel skidding on an icy surface. ABS equipped vehicles will
the tire chains.                  require a different technique for braking on an icy road surface, as shown
later in this chapter.
It is extremely difﬁcult
to mount chains                   Using snow tires and/or chains on icy roads is
when you are on a
a good idea. Many highways have designated             CHAIN UP
chain-up areas to allow you to mount chains
designated chain-up               before getting onto steep hills. Make sure you           AREA
area.
are familiar with how to mount chains on tires
Use your “chain sense”                                                                         E X IT
— practise mounting them before you ﬁnd
to stay out of danger in          yourself in conditions where you need to put

Make certain tire chains are properly sized for your tires and tightly
mounted (do not deﬂate tires to install tire chains). You should stop and
Fast Fact                         check tire chain tightness at the ﬁrst safe opportunity. Reduce speed
when driving with tire chains.
Frequent use of the
service brakes will cause         In extremely bad conditions, it may be safer to park your vehicle than
the service brakes to             to continue driving. Check weather and road conditions before you
heat up, reducing the             encounter icy roads.
stopping ability of the
The result can be a               There is a practical limit to the amount of heat that brakes can absorb
dangerous condition               and dissipate. The highest brake temperatures occur when braking from
called brake fade.                highway speeds while descending long downgrades, or from repeated use
of the brakes without enough time between applications for the brakes to
cool. Almost all brake failures and downhill runaway crashes are caused
by overdriving the ability of the brakes to deal with heat. In other words
– poor speed control.

Whether in town or on a highway, in most cases,
Some steep hill warning signs
you will need to descend a hill at a speed below the
show the steepness of the grade.                          speed of other trafﬁc in order to avoid overdriving
The higher the percentage, the                            your brakes. With today’s diesel engines, you
steeper the hill. This hill has an                        should be in a lower gear to go down the hill than
18 per cent gradient, which is                            used to climb it. Never shift to a higher gear on
be controlled with a retarding device or engine
compression.

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Chapter 2

Fast Fact                           On highways
The braking effect from             where posted. Use your four-way ﬂashers to alert other drivers that you
engine compression is               are driving slowly down the hill. Stay in the far right lane where possible.
greatest near governed              You must control your speed all the way down every hill so you can
maximum engine r.p.m.               respond to any emergency.
and with the transmission
in a low gear. Save your                •   select a safe speed that is not too fast for the weight of your vehicle,
brakes so you will be                       length, and steepness of the grade, weather, and road conditions
able to stop if road and
trafﬁc conditions require               •   select an appropriate low gear to hold that speed
their use.                              •   use the vehicle’s retarding device

If you have selected an appropriate safe speed, are in the right low
Fast Fact                           gear and are using your vehicle’s retarding device, you should be able
to go down the hill without using the service brakes.
Always obey the
regulatory stop and                 If this does not control your speed, and speed is increasing above your
check brakes signs.

These road signs may be placed
before a downhill grade. The one
in the top left corner shows the
speed suggested for descending the
grade. The other signs direct you to

chosen speed:
•   apply the brakes hard enough to reduce speed by 10 – 15 km/h
— the brakes are cold at this point
•   downshift to a lower main gear (do not use the splitter valve for
shifting on a downhill)
Continue down the grade, using engine compression, transmission
gearing and your vehicle’s retarding device to control your speed. If the
speed again increases to above your chosen speed, repeat this process. Be
careful using this procedure on icy roads.

Keep your vehicle in gear all the way down the hill.

In town
•   select a safe speed that is not too fast for the weight of your vehicle,
•   select an appropriate low gear to hold that speed
•   use the vehicle’s retarding device unless signs prohibit their use

You must control the speed of your vehicle so it can be stopped at any
time for any emergency while descending a hill.

30
Heavy Vehicle Braking

Water entering the brake drums will reduce their braking efﬁciency.
Avoid driving through large amounts of water whenever possible. If it is
necessary to drive through water on the roadway, cover the brake pedal
with your foot as you approach the water. Place a slight drag (that is,
apply a slight constant pressure) on the brakes while you drive through
the water. The slight drag placed on the brakes will reduce the amount of
water entering the brake drums and shoes. Always reduce your vehicle’s
speed before driving through large pools of water on the roadway.

During extremely wet conditions, or after driving through water, test
your brakes for safe operation. Do this by applying a slight pressure with
your foot on the brake pedal. Keep this pressure on for a short distance
to dry out the brakes.

Runaway lanes
runaway lanes. The sign on the left
is posted in advance of a runaway
lane; the one on the right is found
at the entrance to a runaway lane.

These lanes are there to help slow and stop vehicles if their brakes fail
as they are going downhill. Use a runaway lane only if you are unable to

These lanes are there for safety. Do not use them for any other purpose.

Combination unit braking
When you step on the brake pedal of an air brake equipped vehicle, all of
the brakes are applied at the same time and at the same pressure.

Trailers equipped with electric or vacuum brakes work in a similar way.
Applying brake pressure too forcefully may cause the wheels to lock up
which can cause a vehicle to jackknife or skid. Use extra caution when
applying brakes on or before a curve, on wet or icy surfaces or when

Remember that the brakes, tires and suspension of a combination vehicle
work best when the vehicle is legally loaded and the load is properly
distributed among axles. When the cargo area is empty and there is
no ABS, the vehicle’s wheels may bounce and lock up. This can make
braking more difﬁcult and may mean you will need a longer distance to
stop, especially when going down hills.

You need to take extra care to safely tow two trailers. It can be difﬁcult
in an emergency to stop a multiple trailer combination in a straight line.
In bad weather, slippery conditions, and mountain driving, the risk of
skidding and loss of traction increases. Look far ahead, leave plenty
of following distance, and slow down to help avoid having to make
emergency stops.

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Chapter 2

Anti-lock braking systems
Anti-lock braking systems (ABS) are mandatory in Canada on all trucks
and buses over 4,560 kilograms manufactured since April 1, 2000 and on
all commercial trailers equipped with air brakes.

ABS is an addition to a normal air brake system. The purpose of ABS is
to prevent wheel lockup or skids caused by over-braking on slippery road
surfaces.

Contrary to what many people believe, anti-lock braking systems do not
allow you to drive faster and they don’t always allow you to stop sooner.
In fact, on some surfaces such as gravel, the braking distance needed with
ABS may be longer.

Anti-lock brakes can help prevent wheel lock-up on surfaces where
conventional brakes usually lock up including surfaces that may be
slippery because of water, ice, wet leaves, grease or spilled ﬂuids. This
means you can brake hard without skidding and losing steering control.

Anti-lock braking systems are only as good as the driver. Learn the
correct technique and practise it so that you are ready in an emergency.
Read the manual from your vehicle manufacturer to learn how to use

Deﬁnition

A full jackknife occurs
when a tractor and the
trailer form an angle
of 90 degrees or less,
relative to one another.
Warning: this can
happen even at very
low speeds. Jackkniﬁng
is often caused by poor
braking techniques,
including improper use
of retarders.

A fully jackknifed tractor/trailer
combination.

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Heavy Vehicle Braking

A few tips for emergency stopping with ABS:

•   Apply ﬁrm, hard continuous pressure to the brake pedal until
Driving Tip                     the vehicle stops.
It’s a good idea to         •   Don’t pump the brakes as this turns the system on and off.
practice using ABS so       •   Don’t be alarmed by brake noise, pedal movement or shudder.
that you are familiar           This is normal. Keep applying ﬁrm pressure.
with how it works and
feels.                      •   Be extremely cautious when attempting to steer around
obstacles. Remember that steering and handling characteristics
will be affected by the size and load of the vehicle you are
driving, and by the trailer you may be towing.

•   When operating combination vehicles, make sure you know
which of the units have ABS. If the tractor and trailers do
not all have ABS, apply the brakes as if you were operating a
combination without ABS.

Automatic traction control
Automatic Traction Control (ATC) is an optional addition to an ABS
system. It uses the pulsating signal from the speed sensors to detect loss
of drive wheel traction on slippery road surfaces.

Using the same wheel sensors used by ABS, ATC can sense if a wheel
has lost traction and is spinning instead of helping to accelerate the
vehicle. If a drive wheel begins to spin, the traction control system will
apply that brake, transferring power to the opposite side. In extremely
slippery conditions, the traction control system will reduce engine power,
allowing the wheels to regain traction.

Most vehicles equipped with ATC will have an information decal on the
dashboard and a dash-mounted lamp that will light if wheel spin occurs.

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Chapter 2

Review Questions
1. What is the ﬁnal factor that will determine if the vehicle will
move?

2. How is brake heat dissipated?

3. If the weight of the vehicle is doubled, how many times must
the stopping power be increased?

5. What can cause jackkniﬁng?

6. What is the difference between ABS and ATC?

7.   What is one of the hazards of stopping a tractor/trailer
combination?

8. When should a retarding device be used?

9. What is brake lag time?

34

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