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When your speed has been reduced to
approximately five mph below your "safe" speed,
release the brakes. (This application should last for
about three seconds.)
When your speed has increased to your "safe"
speed, repeat steps 1 and 2.
For example, if your "safe" speed is 40 mph, you
would not apply the brakes until your speed reaches
40 mph. You now apply the brakes hard enough to
gradually reduce your speed to 35 mph and then
release the brakes. Repeat this as often as
necessary until you have reached the end of the
downgrade.
Figure 5.6
5.4.5 – Brake Fading or Failure
Brakes are designed so brake shoes or pads rub
against the brake drum or disks to slow the vehicle.
Braking creates heat, but brakes are designed to
take a lot of heat. However, brakes can fade or fail
from excessive heat caused by using them too
much and not relying on the engine braking effect.
Excessive use of the service brakes results in
overheating and leads to brake fade. Brake fade
results from excessive heat causing chemical
changes in the brake lining, which reduce friction,
and also causing expansion of the brake drums. As
the overheated drums expand, the brake shoes and
linings have to move farther to contact the drums,
and the force of this contact is reduced. Continued
overuse may increase brake fade until the vehicle
cannot be slowed down or stopped.
Brake fade is also affected by adjustment. To safely
control a vehicle, every brake must do its share of
the work. Brakes out of adjustment will stop doing
their share before those that are in adjustment. The
other brakes can then overheat and fade, and there
will not be enough braking available to control the
vehicle(s). Brakes can get out of adjustment quickly,
especially when they are hot. Therefore, check
brake adjustment often.
5.4.6 – Proper Braking Technique
Remember. The use of brakes on a long and/or
steep downgrade is only a supplement to the
braking effect of the engine. Once the vehicle is in
the proper low gear, the following is the proper
braking technique:
Apply the brakes just hard enough to feel a definite
slowdown.
5.4.7 – Low Air Pressure
If the low air pressure warning comes on, stop and
safely park your vehicle as soon as possible. There
might be an air leak in the system. Controlled
braking is possible only while enough air remains in
the air tanks. The spring brakes will come on when
the air pressure drops into the range of 20 to 45 psi.
A heavily loaded vehicle will take a long distance to
stop because the spring brakes do not work on all
axles. Lightly loaded vehicles or vehicles on slippery
roads may skid out of control when the spring
brakes come on. It is much safer to stop while there
is enough air in the tanks to use the foot brakes.
5.4.8 – Parking Brakes
Any time you park, use the parking brakes, except
as noted below. Pull the parking brake control knob
out to apply the parking brakes, push it in to release.
The control will be a yellow, diamond-shaped knob
labeled "parking brakes" on newer vehicles. On
older vehicles, it may be a round blue knob or some
other shape (including a lever that swings from side
to side or up and down).
Don't use the parking brakes if the brakes are very
hot (from just having come down a steep grade), or
if the brakes are very wet in freezing temperatures.
If they are used while they are very hot, they can be
damaged by the heat. If they are used in freezing
temperatures when the brakes are very wet, they
can freeze so the vehicle cannot move. Use wheel
chocks on a level surface to hold the vehicle. Let hot
brakes cool before using the parking brakes. If the
brakes are wet, use the brakes lightly while driving
in a low gear to heat and dry them.
If your vehicle does not have automatic air tank
drains, drain your air tanks at the end of each
working day to remove moisture and oil. Otherwise,
the brakes could fail.
Section 5 – Air Brakes Page 5-9
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Never leave your vehicle unattended without
applying the parking brakes or chocking the
wheels. Your vehicle might roll away and cause
injury and damage.
Subsection 5.4
Test Your Knowledge
1. Why should you be in the proper gear before
starting down a hill?
2. What factors can cause brakes to fade or fail?
3. The use of brakes on a long, steep downgrade
is only a supplement to the braking effect of
the engine. True or False?
4. If you are away from your vehicle only a short
time, you do not need to use the parking
brake. True or False?
5. How often should you drain air tanks?
6. How should you brake when you drive a
tractor-trailer combination with ABS?
7. You still have normal brake functions if your
ABS is not working. True or False?
These questions may be on your test. If you can’t
answer them all, re-read subsection 5.4.
Section 5 – Air Brakes Page 5-10
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 6
COMBINATION VEHICLES
This Section Covers
• Driving Combinations
• Combination Vehicle Air Brakes
• Antilock Brake Systems
• Coupling and Uncoupling
• Inspecting Combinations
This section provides information needed to pass
the tests for combination vehicles (tractor-trailer,
doubles, triples, straight truck with trailer). The
information is only to give you the minimum
knowledge needed for driving common combination
vehicles. You should also study Section 7 if you
need to pass the test for doubles and triples.
6.1 – Driving Combination Vehicles
Safely
Combination vehicles are usually heavier, longer,
and require more driving skill than single
commercial vehicles. This means that drivers of
combination vehicles need more knowledge and
skill than drivers of single vehicles. In this section,
we talk about some important safety factors that
apply specifically to combination vehicles.
6.1.1 – Rollover Risks
More than half of truck driver deaths in crashes are
the result of truck rollovers. When more cargo is
piled up in a truck, the "center of gravity" moves
higher up from the road. The truck becomes easier
to turn over. Fully loaded rigs are ten times more
likely to roll over in a crash than empty rigs.
The following two things will help you prevent
rollover--keep the cargo as close to the ground as
possible, and drive slowly around turns. Keeping
cargo low is even more important in combination
vehicles than in straight trucks. Also, keep the load
centered on your rig. If the load is to one side so it
makes a trailer lean, a rollover is more likely. Make
sure your cargo is centered and spread out as much
as possible. (Cargo distribution is covered in
Section 3 of this manual.)
Rollovers happen when you turn too fast. Drive
slowly around corners, on ramps, and off ramps.
Avoid quick lane changes, especially when fully
loaded.
6.1.2 – Steer Gently
Trucks with trailers have a dangerous "crack-the-
whip" effect. When you make a quick lane change,
the crack-the-whip effect can turn the trailer over.
There are many accidents where only the trailer has
overturned.
"Rearward amplification" causes the crack-the-whip
effect. Figure 6.1 shows eight types of combination
vehicles and the rearward amplification each has in
a quick lane change. Rigs with the least crack-the-
whip effect are shown at the top and those with the
most, at the bottom. Rearward amplification of 2.0
in the chart means that the rear trailer is twice as
likely to turn over as the tractor. You can see that
triples have a rearward amplification of 3.5. This
means you can roll the last trailer of triples 3.5 times
as easily as a five-axle tractor.
Steer gently and smoothly when you are pulling
trailers. If you make a sudden movement with your
steering wheel, your trailer could tip over. Follow far
enough behind other vehicles (at least 1 second for
each 10 feet of your vehicle length, plus another
second if going over 40 mph). Look far enough
down the road to avoid being surprised and having
to make a sudden lane change. At night, drive
slowly enough to see obstacles with your headlights
before it is too late to change lanes or stop gently.
Slow down to a safe speed before going into a turn.
6.1.3 – Brake Early
Control your speed whether fully loaded or empty.
Large combination vehicles take longer to stop
when they are empty than when they are fully
loaded. When lightly loaded, the very stiff
suspension springs and strong brakes give poor
traction and make it very easy to lock up the wheels.
Your trailer can swing out and strike other vehicles.
Your tractor can jackknife very quickly. You also
must be very careful about driving "bobtail" tractors
(tractors without semitrailers). Tests have shown
that bobtails can be very hard to stop smoothly. It
takes them longer to stop than a tractor-semitrailer
loaded to maximum gross weight.
In any combination rig, allow lots of following
distance and look far ahead, so you can brake early.
Don't be caught by surprise and have to make a
"panic" stop.
Section 6 - Combination Vehicles Page 6-1
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Figure 6.1
6.1.4 – Railroad-highway Crossings
Railroad-highway crossings can also cause
problems, particularly when pulling trailers with low
underneath clearance.
These trailers can get stuck on raised crossings:
Low slung units (lowboy, car carrier, moving van,
possum-belly livestock trailer).
Single-axle tractor pulling a long trailer with its
landing gear set to accommodate a tandem-axle
tractor.
If for any reason you get stuck on the tracks, get out
of the vehicle and away from the tracks. Check
signposts or signal housing at the crossing for
emergency notification information. Call 911 or
other emergency number. Give the location of the
crossing using all identifiable landmarks, especially
the DOT number, if posted.
6.1.5 – Prevent Trailer Skids
When the wheels of a trailer lock up, the trailer will
tend to swing around. This is more likely to happen
when the trailer is empty or lightly loaded. This type
of jackknife is often called a "trailer jackknife." See
Figure 6.2.
The procedure for stopping a trailer skid is:
Recognize the Skid. The earliest and best way to
recognize that the trailer has started to skid is by
seeing it in your mirrors. Any time you apply the
brakes hard, check the mirrors to make sure the
trailer is staying where it should be. Once the trailer
swings out of your lane, it's very difficult to prevent
a jackknife.
* (From R.D. Ervin, R.L. Nisconger, C.C. MacAdam,
and P.S. Fancher, “Influence of size and weight
variables on the stability and control properties of
heavy trucks, “University of Michigan Transportation
Research Institute, 1983).
Section 6 - Combination Vehicles Page 6-2
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Commercial Driver’s License Manual – 2005 CDL Testing System
without entering another traffic lane, turn wide as
you complete the turn. This is better than swinging
wide to the left before starting the turn because it
will keep other drivers from passing you on the right.
See Figure 6.4.
Figure 6.3
Figure 6.2
Stop Using the Brake. Release the brakes to get
traction back. Do not use the trailer hand brake (if
you have one) to "straighten out the rig." This is the
wrong thing to do since the brakes on the trailer
wheels caused the skid in the first place. Once the
trailer wheels grip the road again, the trailer will start
to follow the tractor and straighten out.
6.1.6 – Turn Wide
When a vehicle goes around a corner, the rear
wheels follow a different path than the front wheels.
This is called off-tracking or "cheating." Figure 6.3
shows how off-tracking causes the path followed by
a tractor to be wider than the rig itself. Longer
vehicles will off-track more. The rear wheels of the
powered unit (truck or tractor) will off-track some,
and the rear wheels of the trailer will off-track even
more. If there is more than one trailer, the rear
wheels of the last trailer will off-track the most. Steer
the front end wide enough around a corner so the
rear end does not run over the curb, pedestrians,
etc. However, keep the rear of your vehicle close to
the curb. This will stop other drivers from passing
you on the right. If you cannot complete your turn
Figure 6.4
6.1.7 – Backing with a Trailer.
Backing with a Trailer. When backing a car,
straight truck, or bus, you turn the top of the steering
wheel in the direction you want to go. When backing
a trailer, you turn the steering wheel in the opposite
direction. Once the trailer starts to turn, you must
turn the wheel the other way to follow the trailer.
Whenever you back up with a trailer, try to position
your vehicle so you can back in a straight line. If you
must back on a curved path, back to the driver's side
so you can see. See Figure 6.5.
Section 6 - Combination Vehicles Page 6-3
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Subsection 6.1
Test Your Knowledge
1. What two things are important to prevent
rollover?
2. When you turn suddenly while pulling
doubles, which trailer is most likely to turn
over?
3. Why should you not use the trailer hand
brake to straighten out a jackknifing trailer?
4. What is off-tracking?
5. When you back a trailer, you should
position your vehicle so you can back in a
curved path to the driver’s side. True or
False?
6. What type of trailers can get stuck on
railroad-highway crossings?
These questions may be on your test. If you can’t
answer them all, re-read subsection 6.1.
6.2 – Combination Vehicle Air Brakes
Figure 6.5
Look at Your Path. Look at your line of travel
before you begin. Get out and walk around the
vehicle. Check your clearance to the sides and
overhead, in and near the path your vehicle.
Use Mirrors on Both Sides. Check the outside
mirrors on both sides frequently. Get out of the
vehicle and re-inspect your path if you are unsure.
Back Slowly. This will let you make corrections
before you get too far off course.
Correct Drift Immediately. As soon as you see the
trailer getting off the proper path, correct it by turning
the top of the steering wheel in the direction of the
drift.
Pull Forward. When backing a trailer, make pull-
ups to re-position your vehicle as needed.
You should study Section 5: Air Brakes before
reading this. In combination vehicles the braking
system has parts to control the trailer brakes, in
addition to the parts described in Section 5. These
parts are described below.
6.2.1 – Trailer Hand Valve
The trailer hand valve (also called the trolley valve
or Johnson bar) works the trailer brakes. The trailer
hand valve should be used only to test the trailer
brakes. Do not use it in driving because of the
danger of making the trailer skid. The foot brake
sends air to all of the brakes on the vehicle
(including the trailer(s)). There is much less danger
of causing a skid or jackknife when using just the
foot brake.
Never use the hand valve for parking because all
the air might leak out unlocking the brakes (in
trailers that don't have spring brakes). Always use
the parking brakes when parking. If the trailer does
not have spring brakes, use wheel chocks to keep
the trailer from moving.
Section 6 - Combination Vehicles Page 6-4
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
6.2.2 – Tractor Protection Valve
The tractor protection valve keeps air in the tractor
or truck brake system should the trailer break away
or develop a bad leak. The tractor protection valve
is controlled by the "trailer air supply" control valve
in the cab. The control valve allows you to open and
shut the tractor protection valve. The tractor
protection valve will close automatically if air
pressure is low (in the range of 20 to 45 psi). When
the tractor protection valve closes, it stops any air
from going out of the tractor. It also lets the air out
of the trailer emergency line. This causes the trailer
emergency brakes to come on, with possible loss of
control. (Emergency brakes are covered later.)
6.2.3 – Trailer Air Supply Control
The trailer air supply control on newer vehicles is a
red eight-sided knob, which you use to control the
tractor protection valve. You push it in to supply the
trailer with air, and pull it out to shut the air off and
put on the trailer emergency brakes. The valve will
pop out (thus closing the tractor protection valve)
when the air pressure drops into the range of 20 to
45 psi. Tractor protection valve controls or
"emergency" valves on older vehicles may not
operate automatically. There may be a lever rather
than a knob. The "normal" position is used for
pulling a trailer. The "emergency" position is used to
shut the air off and put on the trailer emergency
brakes.
6.2.4 – Trailer Air-lines
Every combination vehicle has two air-lines, the
service line and the emergency line. They run
between each vehicle (tractor to trailer, trailer to
dolly, dolly to second trailer, etc.)
Service Air-line. The service line (also called the
control line or signal line) carries air, which is
controlled by the foot brake or the trailer hand brake.
Depending on how hard you press the foot brake or
hand valve, the pressure in the service line will
similarly change. The service line is connected to
relay valves. These valves allow the trailer brakes
to be applied more quickly than would otherwise be
possible.
Emergency Air-line. The emergency line (also
called the supply line) has two purposes. First, it
supplies air to the trailer air tanks. Second, the
emergency line controls the emergency brakes on
combination vehicles. Loss of air pressure in the
emergency line causes the trailer emergency
brakes to come on. The pressure loss could be
caused by a trailer breaking loose, thus tearing
apart the emergency air hose. Or it could be caused
by a hose, metal tubing, or other part breaking,
letting the air out. When the emergency line loses
pressure, it also causes the tractor protection valve
to close (the air supply knob will pop out).
Emergency lines are often coded with the color red
(red hose, red couplers, or other parts) to keep from
getting them mixed up with the blue service line.
6.2.5 – Hose Couplers (Glad Hands)
Glad hands are coupling devices used to connect
the service and emergency air-lines from the truck
or tractor to the trailer. The couplers have a rubber
seal, which prevents air from escaping. Clean the
couplers and rubber seals before a connection is
made. When connecting the glad hands, press the
two seals together with the couplers at a 90 degree
angle to each other. A turn of the glad hand attached
to the hose will join and lock the couplers.
When coupling, make sure to couple the proper glad
hands together. To help avoid mistakes, colors are
sometimes used. Blue is used for the service lines
and red for the emergency (supply) lines.
Sometimes, metal tags are attached to the lines with
the words "service" and "emergency" stamped on
them. See Figure 6.6
Figure 6.6
Section 6 - Combination Vehicles Page 6-5
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
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