CDL - Commercial Driver's License Combination Vehicles Practice Test 1 — Questions and Answers
Question 1: Which type of jackknife is considered the most dangerous because it is the most difficult to correct?
- A trailer jackknife caused by the trailer wheels locking up
- A tractor jackknife caused by the drive wheels locking up (Correct answer)
- A front-axle jackknife caused by the steering wheels locking up
- A converter dolly jackknife caused by the dolly wheels skidding
Correct answer: A tractor jackknife caused by the drive wheels locking up
When the drive wheels lock up, the tractor can spin around the trailer — this tractor jackknife is the most dangerous because once the angle becomes severe, there is almost no way to recover. Trailer jackknifes, while serious, can sometimes be corrected by releasing the brakes.
Question 2: On a combination vehicle, what color is the emergency (supply) air line glad hand?
- Blue
- Green
- Yellow
- Red (Correct answer)
Correct answer: Red
Federal regulations require the emergency (supply) air line glad hand to be red and the service (control) air line to be blue. This color coding prevents connecting the lines to the wrong couplers, which would leave the trailer without proper brake control.
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?
- Shift into reverse and push back against the trailer until resistance is felt
- With the trailer brakes released, pull forward slowly to verify the kingpin slides free
- With the trailer brakes applied, gently pull forward with the tractor and check that the trailer does not separate (Correct answer)
- Drive forward 10 feet, then apply full brakes to stress-test the coupling
Correct answer: With the trailer brakes applied, gently pull forward with the tractor and check that the trailer does not separate
The tug test is performed with the trailer brakes set — you then gently pull forward with the tractor. If the fifth wheel is properly locked around the kingpin, the trailer will not move. If it separates, the fifth wheel was not latched correctly.
Question 4: When uncoupling a semi-trailer, you should lower the landing gear BEFORE disconnecting the air lines. Why?
- The air pressure is needed to power the electric–over–air landing gear motor
- So the emergency air line stays pressurized to keep the trailer brakes released while you crank
- To support the trailer's front end and prevent it from dropping and damaging the air lines and electrical cord (Correct answer)
- Because the service line must stay connected until the landing gear locks into position
Correct answer: To support the trailer's front end and prevent it from dropping and damaging the air lines and electrical cord
If you disconnect the air and electrical lines before the landing gear is down and bearing weight, the front of the trailer can drop suddenly, tearing apart the lines and potentially injuring anyone nearby or damaging equipment.
Question 5: What is 'rearward amplification' in a combination vehicle?
- The tendency of the trailer to track wider than the tractor on highway curves
- The magnification of sway toward the rear of a combination, so the last trailer swings more violently than the tractor (Correct answer)
- The increase in stopping distance caused by the rear trailer brakes applying last
- The buildup of air pressure delay from the tractor to the rearmost trailer axle
Correct answer: The magnification of sway toward the rear of a combination, so the last trailer swings more violently than the tractor
Rearward amplification means that a lane-change or swerving motion grows progressively larger toward the rear of the vehicle. A triple-trailer combination can see the last trailer sway several times more than the tractor, making rollovers or loss of control far more likely at highway speeds.
Question 6: What is the correct use of the trailer hand valve (also called the trolley valve) in a combination vehicle?
- Use it to apply the trailer brakes smoothly during all normal stops to reduce wear on tractor brakes
- Use it only to test that the trailer brakes are working before driving, not as a brake while in motion (Correct answer)
- Use it to release trailer brakes when the tractor brake system fails
- Use it to hold the trailer stationary when parking on grades steeper than 7 percent
Correct answer: Use it only to test that the trailer brakes are working before driving, not as a brake while in motion
The trailer hand valve applies only the trailer brakes, with no front-axle braking to stabilize the rig. Using it while driving can lock up the trailer wheels and cause a trailer jackknife. Its correct use is a quick pre-trip check to confirm the trailer brakes respond.
CDL - Commercial Driver's License Combination Vehicles Practice Test 1
Which type of jackknife is considered the most dangerous because it is the most difficult to correct?