Commercial Driving Combination Vehicles 2 — Questions and Answers
Question 1: At what angle should you position the tractor when backing a semi-trailer into a tight dock?
- Straight in at 180 degrees to the dock
- At a 45-degree angle to begin the back (Correct answer)
- Fully perpendicular to the trailer
- At a 90-degree offset to the building
Correct answer: At a 45-degree angle to begin the back
Starting the backing maneuver at approximately a 45-degree angle to the dock allows the driver to steer the trailer into the bay with controlled corrections.
Question 2: What is the converter dolly used for in a doubles or triples combination?
- It connects the tractor fifth wheel to the trailer
- It converts a semi-trailer into a full trailer by providing a set of axles and a fifth wheel (Correct answer)
- It provides an extra fuel tank between trailers
- It houses the air brake relay valve for the rear trailer
Correct answer: It converts a semi-trailer into a full trailer by providing a set of axles and a fifth wheel
A converter dolly attaches to the rear of a lead trailer and provides a fifth wheel for coupling the next trailer, effectively converting a semi-trailer into a full trailer.
Question 3: When driving a combination vehicle, how does the off-tracking of a long trailer affect turning?
- The rear wheels track wider than the front wheels
- The rear wheels cut inside the path of the front wheels, requiring a wider turn (Correct answer)
- The trailer pushes the tractor outward on turns
- Off-tracking only occurs in reverse
Correct answer: The rear wheels cut inside the path of the front wheels, requiring a wider turn
Off-tracking causes the rear trailer wheels to follow a shorter, inside arc compared to the front tractor wheels, requiring drivers to swing wider to keep all wheels in the lane.
Question 4: What is the maximum number of trailers that can be towed in a triples combination under federal regulations?
- 2 trailers
- 3 trailers (Correct answer)
- 4 trailers
- 5 trailers
Correct answer: 3 trailers
Federal regulations permit a maximum of three trailers (triples) in a combination vehicle, and even this is restricted to certain states.
Question 5: When should the tractor protection valve close automatically?
- When trailer air pressure rises above 120 psi
- When air pressure in the tractor drops to approximately 20–45 psi (Correct answer)
- When the engine is shut off
- When the service brakes are applied for more than 10 seconds
Correct answer: When air pressure in the tractor drops to approximately 20–45 psi
The tractor protection valve closes automatically when tractor air pressure drops to roughly 20–45 psi, sealing off the trailer to prevent the tractor's air supply from being fully depleted.
Question 6: How should the heavier trailer be positioned when pulling doubles?
- The heavier trailer should always be at the rear
- The heavier trailer should be in the middle if a tripler
- The heavier trailer should be closest to the tractor (Correct answer)
- Weight distribution does not matter in doubles
Correct answer: The heavier trailer should be closest to the tractor
The heavier trailer must be placed directly behind the tractor because a heavier rear trailer increases the risk of rearward amplification and rollover.
Commercial Driving Combination Vehicles 2
At what angle should you position the tractor when backing a semi-trailer into a tight dock?