CDL Combination Vehicles practice test

Required for a Class A CDL (tractor-trailers). The real test has 20 questions in most states.

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Study all 30 questions with answers

What is the 'crack-the-whip' effect?
Rearward amplification, where a quick steering move can tip over the trailer
Sudden steering is amplified toward the back of a combination. The rear trailer can tip over even if the tractor stays upright.
What color is the emergency (supply) air line glad hand?
Red
The emergency or supply line is red. The service line is blue.
What color is the service air line glad hand?
Blue
The blue service line carries air controlled by the brake pedal or trailer hand brake to apply the trailer brakes.
What does the trailer air supply control look like?
A red, eight-sided knob
Push the red eight-sided knob in to supply the trailer with air. Pull it out to shut off air and apply the trailer emergency brakes.
What happens if the emergency line breaks or comes apart while driving?
The trailer emergency brakes come on
Loss of pressure in the emergency line causes the trailer's emergency (spring) brakes to apply.
The tractor protection valve closes automatically when air pressure drops to:
Between 20 and 45 psi
The valve keeps air in the tractor if the trailer lines break or leak badly. It closes in the 20-45 psi range.
Should you use the trailer hand valve (trolley valve) to park?
No, use the parking brakes
The hand valve uses service air, which can leak off and let the trailer roll. Always park with the spring parking brakes.
When coupling, how high should the trailer be compared to the fifth wheel?
Just low enough that the trailer is raised slightly when the tractor backs under it
If the trailer is too high it may not couple right. Set it so the fifth wheel lifts it slightly as you back under.
After coupling, what should you see when you look at the fifth wheel?
No space between the upper and lower fifth wheel, and the jaws locked around the shank of the kingpin
A gap means the trailer is riding on the jaws or is high-coupled. The jaws must close around the kingpin shank, not the head.
How do you test that the coupling is secure?
With the trailer brakes locked, pull forward gently in low gear (a tug test)
A gentle tug against the locked trailer brakes confirms the kingpin is locked in the fifth wheel.
Before driving away after coupling, the landing gear should be:
Fully raised, with the crank handle secured
Landing gear left partly down can catch on railroad tracks or other objects.
During a turn, the trailer wheels follow a path that is:
Shorter than the tractor wheels' path (off-tracking)
Off-tracking means the rear wheels cut inside the path of the front wheels, which is why you need to turn wide.
Your trailer starts to swing out in a jackknife skid. What should you do?
Release the brakes to let the wheels grip again
Trailer jackknifes usually come from locked trailer wheels. Releasing the brakes lets the tires regain traction.
Compared to an empty rig, a fully loaded rig is about how much more likely to roll over in a crash?
About 10 times more likely
The high center of gravity of a loaded trailer makes rollovers far more likely. Steer gently and take curves slowly.
Which statement about combination vehicles and rollovers is true?
Keeping cargo low and steering gently reduces rollover risk
Keep the load as close to the ground as possible and avoid quick lane changes or tight turns at speed.
When backing a trailer, which way do you turn the steering wheel?
The opposite direction from where you want the trailer to go
When backing with a trailer, turn the steering wheel opposite to the way you want the trailer to move.
Once the trailer starts turning while backing, you should:
Turn the wheel the other way to follow the trailer
Once the trailer begins to turn, follow it by turning the wheel toward the bend.
Before backing under a trailer, the fifth wheel should be:
Tilted down toward the back of the tractor with the jaws open
The fifth wheel plate should be greased, tilted down toward the rear and the jaws open and ready.
Before you back under a trailer, what should you do with the trailer?
Make sure the trailer brakes are locked or the wheels are chocked
The trailer must not roll as you back under it. Use its spring brakes or chocks.
After backing until the fifth wheel just touches the trailer, what's next?
Set the parking brake, connect the air lines and charge the trailer's air
Connect the air lines and supply air to the trailer before backing under, so you can test the trailer brakes.
When uncoupling, why should you ease pressure on the fifth wheel locking jaws?
So the release handle is easier to pull
Backing up gently and setting the brakes takes pressure off the jaws so the locking lever releases easily.
If the air lines are crossed on an older trailer without spring brakes, what can happen?
You could drive away with no trailer brakes
Crossed lines send supply air to the wrong place. On older trailers you might not notice until you need the brakes.
What do the rubber seals in the glad hands do?
Prevent air leaks at the connection
The seals keep air from leaking where the glad hands connect. Check they're clean and not cracked.
A trailer skid usually happens when:
The trailer wheels lock up
Locked trailer wheels lose traction and the trailer swings out.
To help prevent a jackknife, you should:
Avoid braking hard in a curve or on slippery roads
Brake gently and before curves. Sudden braking, especially in a turn, can cause a jackknife.
Longer combination vehicles off-track:
More than shorter ones
The longer the vehicle, the more the rear wheels cut inside in a turn.
What should you check after coupling, at the trailer's front?
That the electrical cord and air lines aren't tangled and won't be pinched
Make sure lines and the cord are secured so they won't snag or be crushed when you turn.
When should you raise the landing gear after coupling?
After you confirm the coupling is secure and the lines are connected
Raise the landing gear fully once coupling is confirmed. Never drive with it partly raised.
What is the safety catch on the fifth wheel locking lever for?
To keep the lever locked so the trailer can't come loose
Make sure the safety catch is in place over the locking lever after coupling.
Should you use the trailer hand valve to test the trailer brakes?
Yes, it's used to test trailer brakes, not for parking
The hand valve is for testing trailer brakes. Don't use it while driving because of the skid danger, and never for parking.

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