CDL Doubles Triples Practice Test 1 — Questions and Answers
Question 1: What is rearward amplification?
- Weight transfer to the rear axles when going up a hill in a double/triple
- 'Crack-the-whip' effect when making quick lane changes (Correct answer)
- The increased weight on the rear axles of a double or triple trailer
- Sudden, dangerous weight shift from the front to the rear trailer
Correct answer: 'Crack-the-whip' effect when making quick lane changes
Rearward amplification, also known as the 'crack-the-whip' effect, describes how quick steering movements can be amplified down a set of multiple trailers. The last trailer in a double or triple combination will swing out much wider and with greater force than the tractor. This effect makes quick lane changes or sudden maneuvers extremely dangerous with multiple trailers.
Question 2: In a set of triples, where should the heaviest trailer be?
- Second trailer position
- Third trailer position
- Either second or third trailer position
- First trailer position (behind the tractor) (Correct answer)
Correct answer: First trailer position (behind the tractor)
In a set of triples, the heaviest trailer should always be placed in the first position, directly behind the tractor. This configuration provides the most stability and reduces the effects of rearward amplification. Placing lighter trailers further back helps maintain control and minimizes the 'crack-the-whip' effect.
Question 3: Why should you never disconnect the pintle hook with the dolly still under the rear trailer?
- The dolly tow bar may fly up, possibly causing injury (Correct answer)
- The kingpin may become damaged
- The pintle lock mechanism may become damaged
- The emergency brake will activate
Correct answer: The dolly tow bar may fly up, possibly causing injury
Never disconnect the pintle hook with the dolly still under the rear trailer because the dolly tow bar may fly up violently. The tow bar is under tension and can spring upwards with considerable force, potentially causing serious injury to anyone nearby. Always ensure the dolly is properly supported or disconnected from the trailer before uncoupling the pintle hook.
Question 4: When connecting the converter dolly to the front trailer, what do you do first?
- Secure converter gear support in the upright position
- Release dolly brakes
- Back first semitrailer in front of dolly tongue (Correct answer)
- Lock pintle hook
Correct answer: Back first semitrailer in front of dolly tongue
When connecting the converter dolly to the front trailer, the first step is to back the first semitrailer in front of the dolly tongue. This positions the trailer correctly for coupling. Once the trailer is aligned, you can then proceed with connecting the pintle hook and air lines.
Question 5: If a second trailer does not have spring brakes, how should you set the brakes when you are coupling?
- Connect the service air line to the trailer and use the Johnson bar to work the trailer brakes.
- Drive the tractor close to the trailer, connect the emergency line to charge the trailer air tank and disconnect the emergency line (Correct answer)
- Manually adjust the slack adjusters to move the brake shoes in contact with the trailer brake drums
- Connect the service air line to the trailer and press down on the brake pedal while you connect the trailer
Correct answer: Drive the tractor close to the trailer, connect the emergency line to charge the trailer air tank and disconnect the emergency line
If a second trailer does not have spring brakes, you must manually set its brakes for coupling. This is done by connecting the emergency line to charge the trailer's air tank, then disconnecting the emergency line. This will cause the trailer's service brakes to apply and hold, as they rely on air pressure to release, not apply.
Question 6: Pushing in the trailer air supply knob will do what?
- Supply air to the service air line
- Supply air to the emergency air line (Correct answer)
- Shut off air to the trailer air tank
- Put on the emergency brakes
Correct answer: Supply air to the emergency air line
Pushing in the trailer air supply knob (often red) supplies air to the emergency air line of the trailer. This action releases the trailer's emergency (spring) brakes, allowing the trailer to move. It's a critical step before driving a combination vehicle.
Question 7: How should you check that you have sufficient emergency airflow to all trailers in a set of doubles or triples?
- Use the trailer hand brake to supply air. Go to the rear of the last trailer, open the service line valve and listen for air escaping.
- Move the trailers and apply the emergency brakes.
- Push in the red trailer air supply knob, open the emergency line shut-off valve at the rear of the last trailer and listen for air escaping. (Correct answer)
- Fully charge the air system, activate the emergency brakes and look for a 15 psi air pressure reduction
Correct answer: Push in the red trailer air supply knob, open the emergency line shut-off valve at the rear of the last trailer and listen for air escaping.
To check for sufficient emergency airflow, you must activate the emergency air supply to the trailers. Pushing in the red trailer air supply knob sends air to the emergency lines. Opening the emergency line shut-off valve at the rear of the last trailer allows you to listen for escaping air, confirming that the emergency air system is fully charged and functional throughout the entire trailer combination.
Question 8: What is the main purpose of a trailer shut-off valve?
- It works the trailer brakes
- To be used only for emergency stopping
- A coupling device that connects the air lines between converter dollies and trailers
- To close the air lines of the last trailer to prevent air from leaking out (Correct answer)
Correct answer: To close the air lines of the last trailer to prevent air from leaking out
The main purpose of a trailer shut-off valve is to seal the air lines on the last trailer in a set of doubles or triples. By closing these valves, you prevent air from leaking out of the brake system. This ensures that adequate air pressure is maintained for the proper operation of the service and emergency brakes on all connected trailers.
Question 9: What is the correct position of the shut-off valves for the last trailer of a doubles or triples set?
- Fully closed (Correct answer)
- Fully open
- Partially open
- Emergency shut-off valve open; service shut-off valve closed
Correct answer: Fully closed
For the last trailer in a doubles or triples set, both the service and emergency shut-off valves must be fully closed. This action seals the air lines at the end of the trailer combination. Closing the valves prevents air from escaping the system, which is crucial for maintaining proper air pressure and ensuring the effective operation of the air brakes on all trailers.
Question 10: What is the correct position of the shut-off valves for the first trailer of a doubles or triples set?
- Fully closed
- Fully open (Correct answer)
- Partially closed
- Emergency shut-off valve closed; service shut-off valve open
Correct answer: Fully open
On the first trailer (and any intermediate trailers) in a doubles or triples set, the shut-off valves must be fully open. This allows air to flow freely through both the service and emergency air lines to the subsequent trailers and converter dollies. Proper airflow ensures that all trailers receive the necessary air pressure for both normal braking and emergency braking functions.
CDL Doubles Triples Practice Test 1
What is rearward amplification?