SD CDL - South Dakota Commercial Drivers License Combination Vehicle Operations Questions and Answers — Questions and Answers
Question 1: You are driving a combination vehicle and make a sudden lane change. What is the 'crack-the-whip' effect?
- The front of the tractor swinging wide in a turn.
- A delayed braking response from the rearmost trailer.
- The tendency of the rear trailer to swing out with greater force than the tractor. (Correct answer)
- A loud noise made by the air brakes when applied suddenly.
Correct answer: The tendency of the rear trailer to swing out with greater force than the tractor.
The 'crack-the-whip' effect, also known as rearward amplification, occurs when a driver makes a quick steering maneuver. The force is magnified down the length of the combination vehicle, causing the rearmost trailer to swing out more dramatically than the tractor, which can lead to a rollover of the trailer.
Question 2: When coupling a tractor to a semitrailer, what is the correct height for the trailer?
- The trailer should be about one foot lower than the fifth wheel.
- The trailer should be high enough that the tractor has to be lowered to get under it.
- The trailer will be raised slightly when the tractor is backed under it. (Correct answer)
- The fifth wheel should make contact with the kingpin before it touches the trailer apron.
Correct answer: The trailer will be raised slightly when the tractor is backed under it.
The trailer should be set at a height where backing the tractor under it causes the trailer to be lifted slightly. This ensures a secure connection with the fifth wheel and prevents damage to the landing gear or the tractor.
Question 3: Which of the following is a primary factor that contributes to the rollover risk of combination vehicles?
- Low tire pressure.
- A high center of gravity. (Correct answer)
- An empty trailer.
- Newly paved road surfaces.
Correct answer: A high center of gravity.
A high center of gravity is a major reason why combination vehicles are prone to rollovers. The higher the load is stacked, the higher the center of gravity, making the vehicle less stable, especially in turns or during sudden maneuvers.
Question 4: A driver is operating a combination vehicle with a dual air brake system. With the engine off and the brakes released, what is the maximum allowable air pressure leakage rate per minute?
- 1 PSI
- 4 PSI
- 2 PSI
- 3 PSI (Correct answer)
Correct answer: 3 PSI
For a combination vehicle, the Federal Motor Carrier Safety Administration (FMCSA) regulations state that the air pressure drop should not exceed 3 PSI in one minute when the engine is off and the service brakes are released.
Question 5: While uncoupling a trailer, after you have lowered the landing gear and disconnected the air and electrical lines, what is the next step?
- Drive the tractor completely out from under the trailer.
- Unlock the fifth wheel by pulling the release handle. (Correct answer)
- Chock the trailer wheels.
- Set the trailer brakes.
Correct answer: Unlock the fifth wheel by pulling the release handle.
After lowering the landing gear and disconnecting the lines, the next step is to unlock the fifth wheel. This is done by pulling the release handle, which disengages the locking jaws from the trailer's kingpin, preparing the tractor to be pulled away.
Question 6: Under what condition does a combination vehicle, particularly a tractor-trailer, take longer to stop?
- When it is fully loaded.
- When it is empty or lightly loaded. (Correct answer)
- When driving on a downhill grade.
- When the road is wet.
Correct answer: When it is empty or lightly loaded.
Counterintuitively, large combination vehicles often take longer to stop when empty or lightly loaded. This is because the stiff suspension and powerful brakes have poor traction on the light vehicle, making it very easy for the wheels to lock up and skid.
SD CDL - South Dakota Commercial Drivers License Combination Vehicle Operations Questions and Answers
You are driving a combination vehicle and make a sudden lane change.
What is the 'crack-the-whip' effect?