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Coupling and uncoupling doubles and triples with a converter dolly

Doubles and Triples (T)5 minManual section 7.2

A second or third trailer is attached with a converter dolly. The order of the steps protects you from a rolling trailer, a flying tow bar and a trailer with no brakes. Learn the order and the reason for each step.

What a converter dolly is, and trailer order

A converter dolly (also called converter gear) is a coupling device with one or two axles and a fifth wheel. It lets you couple a semitrailer to the rear of a tractor-trailer, making a double bottom rig. The dolly's tongue has a pintle eye that hooks into the pintle hook at the back of the trailer in front.

Trailer order: for the safest handling, put the more heavily loaded semitrailer first, right behind the tractor. The lighter trailer goes in the rear.

Coupling twin trailers (7.2.1)

1. Secure the second (rear) trailer. If it has no spring brakes, drive the tractor close, connect the emergency line, charge the trailer air tank, then disconnect the emergency line. This sets the trailer emergency brakes (if the slack adjusters are adjusted right). Chock the wheels if you have any doubt about the brakes.

2. Position the dolly in front of the second trailer. Free the dolly brakes by opening the air tank petcock (if the dolly has spring brakes, use its parking brake control instead). If it is not far, roll the dolly into place by hand, lined up with the kingpin. Otherwise, use the tractor and first trailer to fetch it: get as close as you can, bring the dolly to the back of the first trailer and hook it on, lock the pintle hook, lock the dolly support in the up position, tow the dolly as near as you can to the nose of the second trailer, lower the dolly support, unhook it from the first trailer, and roll it into line with the kingpin.

3. Connect the dolly to the front trailer. Back the first semitrailer into position in front of the dolly tongue, hook the dolly to the front trailer, lock the pintle hook, and secure the converter gear support in the raised position.

4. Connect the dolly to the rear trailer. Be sure the trailer brakes are locked and/or the wheels are chocked. Check trailer height: the trailer should sit a bit below the center of the dolly's fifth wheel, so it gets lifted slightly as the dolly goes under. Back the dolly under the rear trailer. Lift the landing gear just off the ground (so nothing is damaged if the trailer moves). Tug against the second trailer's kingpin to test the coupling. Look at the coupling: no gap between upper and lower fifth wheel, and the jaws closed on the kingpin. Hook up the safety chains, air hoses and light cords. Close the dolly air tank petcock and the service and emergency shut-off valves at the back of the second trailer. Open the shut-off valves at the back of the first trailer (and on the dolly, if it has them). Crank the landing gear all the way up. Charge the trailer brakes (push in the air supply knob), then open the emergency line shut-off at the back of the second trailer to confirm air is there. No air means a problem — the brakes will not work.

Uncoupling twin trailers (7.2.2)

Uncouple the rear trailer: park in a straight line on firm, level ground. Apply the parking brakes. Chock the wheels of the second trailer if it has no spring brakes. Lower the landing gear of the second trailer enough to take some weight off the dolly. Close the air shut-offs at the rear of the first trailer (and on the dolly, if equipped). Disconnect all dolly air and electric lines and secure them. Release the dolly brakes. Release the converter dolly fifth wheel latch. Slowly pull the tractor, first trailer and dolly forward to pull the dolly out from under the rear trailer.

Uncouple the converter dolly: lower the dolly landing gear. Disconnect the safety chains. Apply the converter gear spring brakes or chock the wheels. Release the pintle hook on the first trailer. Slowly pull clear of the dolly.

Do not unlock the pintle hook while the dolly is still under the rear trailer. The dolly tow bar can spring up, which may hurt someone and makes re-coupling very hard. So pull the dolly out first, then unhook it.

Triples and other combinations (7.2.3–7.2.4)

Coupling triples: couple the tractor to the first trailer the normal tractor-semitrailer way. Then move a converter dolly into position and couple the first trailer to the second using the doubles method. (The second and third trailers are joined the same way.) The triples rig is then complete.

Uncoupling triples: uncouple the third trailer first by pulling the dolly out, then unhitching the dolly, using the doubles method. Then uncouple the rest like any double-bottom rig.

Other combinations: there are many other truck-trailer and tractor-trailer combinations, too many to list. Learn the correct way to couple and uncouple your vehicle from the manufacturer and/or owner specifications.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 7.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. What is a converter dolly?
    1. A special trailer for hauling cars
    2. A hand truck for loading cargo
    3. A coupling device with one or two axles and a fifth wheel
    4. A device to check air pressure
    Show answer
    A coupling device with one or two axles and a fifth wheel

    A converter dolly has one or two axles and a fifth wheel, used to couple a semitrailer to the rear of a tractor-trailer.

  2. For the safest handling, where should the more heavily loaded trailer be?
    1. In first position, right behind the tractor
    2. In the rear position
    3. In the middle of a triple only
    4. It does not matter
    Show answer
    In first position, right behind the tractor

    Put the heavier trailer first behind the tractor and the lighter one in the rear.

  3. The second trailer has no spring brakes. How can you set its emergency brakes before coupling?
    1. Pull the trailer hand valve
    2. Open the shut-off valves
    3. Chain it to the dolly
    4. Connect the emergency line, charge the trailer tank, then disconnect the emergency line
    Show answer
    Connect the emergency line, charge the trailer tank, then disconnect the emergency line

    Charging the tank and then disconnecting the emergency line sets the emergency brakes (if slack adjusters are right).

  4. You have doubts about whether the second trailer's emergency brakes will hold. What should you do?
    1. Couple quickly before it rolls
    2. Chock the wheels
    3. Open the petcock
    4. Raise the landing gear
    Show answer
    Chock the wheels

    Chock the wheels if you have any doubt about the brakes.

  5. How do you release the brakes on a converter dolly without spring brakes?
    1. Push in the air supply knob
    2. Pull the trolley valve
    3. Open the air tank petcock
    4. Close the shut-off valves
    Show answer
    Open the air tank petcock

    Release dolly brakes by opening the air tank petcock (or use the dolly parking brake control if it has spring brakes).

  6. How should the trailer height compare with the dolly fifth wheel before you push the dolly under?
    1. Slightly lower than the center of the fifth wheel
    2. Slightly higher than the fifth wheel
    3. Exactly level with the top of the jaws
    4. As high as possible
    Show answer
    Slightly lower than the center of the fifth wheel

    The trailer should sit a bit below the center of the dolly's fifth wheel, so the dolly lifts it slightly as it goes under.

  7. After backing the dolly under the rear trailer, why raise the landing gear slightly off the ground before the tug test?
    1. To check the ABS
    2. To let air into the tanks
    3. To drain moisture
    4. To prevent damage if the trailer moves
    Show answer
    To prevent damage if the trailer moves

    Raising the landing gear slightly prevents damage if the trailer moves during the test.

  8. What do you check when making a visual check of the dolly coupling?
    1. That the ABS lamp is lit
    2. No space between upper and lower fifth wheel and the locking jaws closed on the kingpin
    3. That the landing gear is on the ground
    4. That the glad hands are crossed
    Show answer
    No space between upper and lower fifth wheel and the locking jaws closed on the kingpin

    Look for no gap between the upper and lower fifth wheel, and for jaws closed on the kingpin.

  9. After coupling the second trailer, which valves should you close?
    1. The shut-off valves at the rear of the first trailer
    2. All shut-off valves
    3. The dolly air tank petcock and the shut-off valves at the rear of the second trailer
    4. None
    Show answer
    The dolly air tank petcock and the shut-off valves at the rear of the second trailer

    Close the dolly petcock and the rear shut-offs of the second (last) trailer; open those on the first trailer.

  10. After charging the trailer brakes, how do you check that air reaches the second trailer?
    1. Open the emergency line shut-off at the rear of the second trailer
    2. Look at the dash gauge only
    3. Pull the dolly petcock
    4. Tug against the kingpin
    Show answer
    Open the emergency line shut-off at the rear of the second trailer

    Open the emergency line shut-off at the rear of the second trailer; no air means the brakes won't work.

  11. What three things connect the dolly and trailers besides the coupling itself?
    1. Landing gear, crank and chocks
    2. ABS lamp, ECU and sensors
    3. Mud flaps, reflectors and tires
    4. Safety chains, air hoses and light cords
    Show answer
    Safety chains, air hoses and light cords

    Connect safety chains, air hoses and light cords.

  12. When uncoupling the rear trailer, where should you park the rig?
    1. On a slight hill
    2. In a straight line on firm, level ground
    3. On soft ground to cushion the landing gear
    4. In a curve so you can see the trailer
    Show answer
    In a straight line on firm, level ground

    Park in a straight line on firm, level ground.

  13. When uncoupling doubles, why lower the second trailer's landing gear before pulling the dolly out?
    1. To drain the tanks
    2. To lock the pintle hook
    3. To remove some weight from the dolly
    4. To turn on the ABS
    Show answer
    To remove some weight from the dolly

    Lower the landing gear enough to take some weight off the dolly.

  14. Why should you pull the dolly out from under the rear trailer before disconnecting it from the trailer in front?
    1. If you unlock the pintle hook with the dolly under the trailer, the tow bar may fly up
    2. The dolly brakes will lock
    3. The ABS will turn off
    4. The rear trailer will roll away
    Show answer
    If you unlock the pintle hook with the dolly under the trailer, the tow bar may fly up

    Unlocking the pintle hook while the dolly is still under the rear trailer can make the tow bar spring up and hurt you.

  15. Before releasing the pintle hook to uncouple the dolly, what should you do?
    1. Open the shut-off valves
    2. Raise the dolly landing gear
    3. Connect the safety chains
    4. Apply the converter gear spring brakes or chock the wheels
    Show answer
    Apply the converter gear spring brakes or chock the wheels

    Lower the dolly landing gear, disconnect the chains, set the dolly spring brakes or chock, then release the pintle hook.

  16. When uncoupling the rear trailer, what do you do with the air shut-offs at the rear of the first trailer?
    1. Open them
    2. Close them
    3. Remove them
    4. Leave them as they are
    Show answer
    Close them

    Close the air shut-offs at the rear of the first trailer (and on the dolly, if equipped).

  17. When uncoupling a triple-trailer rig, which trailer do you uncouple first?
    1. The first trailer
    2. The second trailer
    3. The third trailer
    4. Any order is fine
    Show answer
    The third trailer

    Uncouple the third trailer first by pulling the dolly out, then unhitching the dolly.

  18. For truck-trailer combinations not covered in the manual, how should you learn to couple and uncouple?
    1. From the manufacturer and/or owner specifications
    2. By trial and error
    3. From other drivers only
    4. Use the tractor-semitrailer method for all
    Show answer
    From the manufacturer and/or owner specifications

    Learn the correct method for your vehicle from the manufacturer and/or owner specifications.