CDL Practice Tests: Doubles And Triples

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Question #348 (1 of 6)

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Doubles and triples are more likely to turn over than other combination vehicles because of what effect?

  • Crack-the-whip effect
  • S-curve effect
  • Compression effect
  • Cylindrical effect
Doubles and triples are more likely to turn over than other combination vehicles because of the "crack- the-whip" effect. You must steer gently when pulling trailers. The last trailer in a combination is most likely to turn over.
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Question #350 (2 of 6)

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When uncoupling doubles or triples, what is important to know about unlocking the pintle hook?

  • Always unlock the pintle hook with the dolly still under the rear trailer. Otherwise, the dolly tow bar may fly up, possibly causing injury and making it very difficult to re-couple.
  • Unlock the pintle hook only if the trailer it is under is loaded, otherwise the dolly tow bar may fly up, possibly causing injury and making it very difficult to re-couple.
  • Never unlock the pintle hook with the dolly still under the rear trailer. The dolly tow bar may fly up, possibly causing injury and making it very difficult to re-couple.
  • None of these are correct
Never unlock the pintle hook with the dolly still under the rear trailer. The dolly tow bar may fly up, possibly causing injury and making it very difficult to re-couple.
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Question #352 (3 of 6)

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When inspecting the air brakes on doubles and triples, you wait for air pressure to reach normal, then push in the red “trailer air supply” knob. When you go to the rear of the last trailer, what should you do to complete this check?

  • Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged.
  • Close the emergency line shut-off valve at the rear of the trailer. Make sure you can hear air escaping with the valve closed
  • Open the emergency line shut-off valve at the rear of the last trailer. You should not hear air escaping. If you do, there is a problem.
  • Leave the emergency line shut-off valve closed and open the service line valve. With the red knob pushed in on the tractor dash, the service line should have air in it. If not, there is a problem.
Wait for air pressure to reach normal, then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged.
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Question #349 (4 of 6)

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When pulling doubles or triples, what is important to know about the order of the trailers?

  • The weight of all trailers should be nearly the same, within 2% of each other
  • The weight of the individual trailers will have no noticeable effect on the handling of the vehicle
  • The more heavily loaded semitrailer should be in first position behind the tractor. The lighter trailer should be in the rear.
  • The lighter semitrailer should be in first position behind the tractor. The heavier trailer should be in the rear.
The more heavily loaded semitrailer should be in first position behind the tractor. The lighter trailer should be in the rear.
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Question #353 (5 of 6)

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When inspecting doubles and triples you must make sure there is air all the way to the back of the last trailer, otherwise what may not work?

  • The converter dolly
  • The brakes
  • The lights
  • The pintle hook
You MUST have air all the way to the back for all the brakes to work.
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Question #351 (6 of 6)

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When inspecting doubles or triples, in what position should the air shut-off valves be in?

  • Air valves should be open on all trailers
  • Rear of front trailers: open, rear of last trailer: closed
  • Rear of front trailers: closed, rear of last trailer: open
  • Air valves should be closed on all trailers

Shut-off valves (at rear of trailers, in service and emergency lines):

  • Rear of front trailers: OPEN.
  • Rear of last trailer: CLOSED.
  • Converter dolly air tank drain valve: CLOSED.
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About The Doubles and Triples CDL Exam

The Doubles and Triples written CDL Exam is required for your doubles and triples endorsement. This section explains how important it is to be careful when driving with more than one trailer, how to couple and uncouple correctly, and about inspecting doubles and triples carefully.

This section covers:

  • Pulling Double/Triple Trailers
  • Coupling and Uncoupling
  • Inspecting Doubles and Triples
  • Checking Air Brakes

Advice For Safe Driving With Doubles And Triples

Pulling Double/Triple Trailers

Take special care when pulling two and three trailers. There are more things that can go wrong, and doubles/triples are less stable than other commercial vehicles. Some areas of concern are discussed below.

Prevent Trailer from Rolling Over

To prevent trailers from rolling over, you must steer gently and go slowly around corners, on ramps, off ramps and curves. A safe speed on a curve for a straight truck or a single trailer combination vehicle may be too fast for a set of doubles or triples.

Beware of the Crack-the-Whip Effect

Doubles and triples are more likely to turn over than other combination vehicles because of the "crack-the-whip" effect. You must steer gently when pulling trailers. The last trailer in a combination is most likely to turn over.

Inspect Completely

There are more critical parts to check when you have two or three trailers. Check them all. Follow the procedures described later in this section.

More Safe Driving Advice For Doubles And Triples

Look Far Ahead

Doubles and triples must be driven very smoothly to avoid rollover or jackknife. Therefore, look far ahead so you can slow down or change lanes gradually when necessary.

Manage Space

Doubles and triples take up more space than other commercial vehicles. They are not only longer, but also need more space because they cannot be turned or stopped suddenly. Allow more following distance. Make sure you have large enough gaps before entering or crossing traffic. Be certain you are clear at the sides before changing lanes.

Adverse Conditions

Be more careful in adverse conditions. In bad weather, slippery conditions and mountain driving, you must be especially careful if you drive double and triple bottoms. You will have greater length and more dead axles to pull with your drive axles than other drivers. There is more chance for skids and loss of traction.

Parking the Vehicle

Make sure you do not get in a spot you cannot pull straight through. You need to be aware of how parking lots are arranged in order to avoid a long and difficult escape.

Antilock Braking Systems on Converter Dollies

Converter dollies built on or after March 1, 1998, are required to have antilock brakes. These dollies will have a yellow lamp on the left side of the dolly

Questions You Should Know For Doubles And Triples

  • 1. What is a converter dolly?
  • 2. Do converter dollies have spring brakes?
  • 3. What three methods can you use to secure a second trailer before coupling?
  • 4. How do you check to make sure trailer height is correct before coupling?
  • 5. What do you check when making a visual check of coupling?

More Questions You Should Know For Doubles And Triples

  • 6. Why should you pull a dolly out from under a trailer before you disconnect it from the trailer in front?
  • 7. What should you check for when inspecting the converter dolly? The pintle hook?
  • 8. Should the shut-off valves on the rear of the last trailer be open or closed? On the first trailer in a set of doubles? On the middle trailer of a set of triples?
  • 9. How can you test that air flows to all trailers?
  • 10. How do you know if your converter dolly is equipped with antilock brakes?

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