CDL - Combination Vehicle Practice Test Questions And Answers

CDL - Combination Vehicle Practice Test Questions And
Answers
 Course
 Cdl

Question 1:

What should you do to prevent a trailer from jackknifing when pulling a combination
vehicle?

 A) Increase speed while turning
 B) Apply trailer brakes sharply
 C) Apply gradual, steady pressure to the brakes
 D) Only use the truck’s engine brake

Answer: C) Apply gradual, steady pressure to the brakes.

Rationale: Jackknifing occurs when the trailer moves out of alignment with the tractor, often
due to sudden braking or sharp maneuvers. Applying gradual, steady pressure prevents the
trailer from swinging out.



Question 2:

Which type of vehicle is most likely to experience a “crack-the-whip” effect?

 A) Single-unit vehicles
 B) Double trailers
 C) Triple trailers
 D) Vehicles without air brakes

Answer: C) Triple trailers.

Rationale: The “crack-the-whip” effect is more common in double or triple trailers due to the
increased distance between the tractor and last trailer, making the last trailer prone to sudden,
swinging motion.



Question 3:

Why is it essential to ensure that the air lines between tractor and trailer are properly
connected?

 A) To keep the trailer tires inflated
 B) To ensure electrical connection to the trailer
 C) To maintain control over the trailer brakes

,  D) To provide additional traction to the tractor

Answer: C) To maintain control over the trailer brakes.

Rationale: Air lines supply air to the trailer’s braking system. Properly connected lines are
essential for synchronized braking and to avoid brake system failure.



Question 4:

What can result from having an overloaded or imbalanced load in a combination
vehicle?

 A) Improved fuel efficiency
 B) Difficulty controlling the vehicle
 C) Enhanced brake performance
 D) Increased tire life

Answer: B) Difficulty controlling the vehicle.

Rationale: Overloaded or imbalanced loads make steering and braking harder, increasing the
likelihood of rollovers and accidents, especially on curves and uneven terrain.



Question 5:

If you need to disconnect a trailer, what is the first step you should take?

 A) Lower the landing gear
 B) Lock the fifth-wheel latch
 C) Set the trailer brakes
 D) Disconnect the air lines

Answer: C) Set the trailer brakes.

Rationale: Setting the trailer brakes prevents the trailer from moving when disconnecting.
This is crucial to avoid injury or trailer damage.



Question 6:

What should you do to prevent the trailer from rolling over on a sharp curve?

 A) Accelerate through the curve
 B) Reduce speed and keep the load centered
 C) Steer aggressively through the curve
 D) Apply the brakes hard just before the curve

,Answer: B) Reduce speed and keep the load centered.

Rationale: Reducing speed lowers the vehicle’s center of gravity, and a centered load
reduces sway, both of which prevent rollovers in curves.



Question 7:

Which part of a combination vehicle should be checked for excessive wear to prevent
coupling issues?

 A) Landing gear
 B) Tires
 C) Fifth-wheel and kingpin
 D) Fuel tank

Answer: C) Fifth-wheel and kingpin.

Rationale: Excessive wear on the fifth-wheel and kingpin can lead to coupling failures,
increasing the risk of the trailer detaching from the tractor.



Question 8:

What should you check after coupling a trailer to make sure the trailer is securely
attached?

 A) That the landing gear is up
 B) The air lines are disconnected
 C) That the locking jaws are around the shank of the kingpin
 D) The fuel gauge

Answer: C) That the locking jaws are around the shank of the kingpin.

Rationale: This ensures that the trailer is securely attached to the tractor. The locking jaws
must fully enclose the kingpin to prevent separation.



Question 9:

What should you do if your trailer starts to skid while driving on a slippery road?

 A) Release the brakes and steer in the direction of the skid
 B) Increase speed to regain control
 C) Apply trailer brakes sharply
 D) Turn the steering wheel sharply in the opposite direction of the skid

, Answer: A) Release the brakes and steer in the direction of the skid.

Rationale: This helps regain traction and control by aligning the vehicle with the skid
direction. Overcorrecting or braking can worsen the skid.



Question 10:

What is the main purpose of the trailer hand valve?

 A) To disconnect the air brakes from the trailer
 B) To control the tractor’s brake pressure
 C) To control only the trailer brakes independently
 D) To speed up trailer unloading

Answer: C) To control only the trailer brakes independently.

Rationale: The trailer hand valve allows the driver to apply the trailer brakes without
engaging the tractor brakes. This is typically used for parking and certain maneuvers.

Question 11:

What is the purpose of the service line in a combination vehicle?

 A) It provides air to the emergency brakes
 B) It supplies air to inflate the tires
 C) It carries air to the trailer brakes during normal braking
 D) It supports trailer lights and signals

Answer: C) It carries air to the trailer brakes during normal braking.

Rationale: The service line, or “blue line,” controls the trailer brakes when the driver applies
the brakes in the cab, ensuring coordinated braking with the tractor.



Question 12:

How should you test the trailer emergency brakes?

 A) Press the brake pedal and listen for air leaks
 B) Engage the hand valve while moving slowly
 C) Pull the trailer air supply valve and gently tug on the trailer
 D) Disconnect the service line and listen for air pressure loss

Answer: C) Pull the trailer air supply valve and gently tug on the trailer.