CDL - Commercial Driver's License Combination Vehicles Practice Test 1

Question 1

Which type of jackknife is considered the most dangerous because it is the most difficult to correct?

Accepted Answer

A tractor jackknife caused by the drive wheels locking up

Answer

A trailer jackknife caused by the trailer wheels locking up

Answer

A front-axle jackknife caused by the steering wheels locking up

Answer

A converter dolly jackknife caused by the dolly wheels skidding

Question 2

On a combination vehicle, what color is the emergency (supply) air line glad hand?

Accepted Answer

Red

Answer

Blue

Answer

Green

Answer

Yellow

Question 3

After coupling a tractor to a semi-trailer, what is the correct way to perform the 'tug test' to confirm the fifth wheel is locked?

Accepted Answer

With the trailer brakes applied, gently pull forward with the tractor and check that the trailer does not separate

Answer

Shift into reverse and push back against the trailer until resistance is felt

Answer

With the trailer brakes released, pull forward slowly to verify the kingpin slides free

Answer

Drive forward 10 feet, then apply full brakes to stress-test the coupling

Question 4

When uncoupling a semi-trailer, you should lower the landing gear BEFORE disconnecting the air lines. Why?

Accepted Answer

To support the trailer's front end and prevent it from dropping and damaging the air lines and electrical cord

Answer

The air pressure is needed to power the electric–over–air landing gear motor

Answer

So the emergency air line stays pressurized to keep the trailer brakes released while you crank

Answer

Because the service line must stay connected until the landing gear locks into position

Question 5

What is 'rearward amplification' in a combination vehicle?

Accepted Answer

The magnification of sway toward the rear of a combination, so the last trailer swings more violently than the tractor

Answer

The tendency of the trailer to track wider than the tractor on highway curves

Answer

The increase in stopping distance caused by the rear trailer brakes applying last

Answer

The buildup of air pressure delay from the tractor to the rearmost trailer axle

Question 6

What is the correct use of the trailer hand valve (also called the trolley valve) in a combination vehicle?

Accepted Answer

Use it only to test that the trailer brakes are working before driving, not as a brake while in motion

Answer

Use it to apply the trailer brakes smoothly during all normal stops to reduce wear on tractor brakes

Answer

Use it to release trailer brakes when the tractor brake system fails

Answer

Use it to hold the trailer stationary when parking on grades steeper than 7 percent