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7.2 Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to the safe operation of doubles and triples. Wrong coupling and uncoupling can be very dangerous. Coupling and uncoupling steps for doubles and triples are outlined below.

Coupling Twin Trailers

Secure second (rear) trailer:

If the second trailer does not have spring brakes, drive the tractor close to the trailer, connect the emergency line, charge the trailer air tank, and disconnect the emergency line. This will set the trailer emergency brakes (if the slack adjusters are correctly adjusted). Chock the wheels if you have any doubt about the brakes.

Couple tractor and first semitrailer as described earlier:

A converter gear or dolly is a coupling device of one or two axles and a fifth wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig.

Caution: For the safest handling on the road, the more heavily loaded semi-trailer should be in first position behind the tractor. The lighter trailer should be in the rear.

Position converter dolly in front of second (rear) trailer:
  • Release dolly brakes by opening the air tank petcock. (Or, if the dolly has spring brakes, use the dolly parking brake control.)
  • If the distance is not too great, wheel dolly into position by hand so it is in line with the kingpin.
  • Or, use the tractor and first semitrailer to pick up the converter dolly:
    • Position combination as close as possible to converter dolly.
    • Move dolly to rear of first semi-trailer and couple it to the trailer.
    • Lock pintle hook.
    • Secure dolly support in raised position.
    • Pull dolly into position as close as possible to nose of the second semitrailer.
    • Lower dolly support.
    • Unhook dolly from first trailer.
    • Wheel dolly into position in front of second trailer in line with the kingpin.
Connect converter dolly to front trailer:
  • Back first semi-trailer into position in front of dolly tongue.
  • Hook dolly to front trailer.
    • Lock pintle hook.
    • Secure converter gear support in raised position.
Connect converter dolly to rear trailer:
  • Make sure trailer brakes are locked and/or wheels chocked.
  • Make sure trailer height is correct (it must be slightly lower than the center of the fifth wheel, so trailer is raised slightly when dolly is pushed under).
  • Back converter dolly under rear trailer.
  • Raise landing gear slightly off ground to prevent damage if trailer moves.
  • Test coupling by pulling against pin of number two semi-trailer.
  • Make visual check of coupling no space between upper and lower fifth wheel. Locking jaws closed on kingpin).
  • Connect safety chains, air hoses and light cords.
  • Close converter dolly air tank petcock and shut-off valves at rear of second trailer (service and emergency shut-offs).
  • Open shut-off valves at rear of first trailer (and on dolly if so equipped).
  • Raise landing gear completely.
  • Charge trailers (push “air supply” knob in) and check for air at rear of second trailer by opening the emergency line shut-off. If air pressure is not there, something is wrong and the brakes will not work.
It isn't very likely you will connect to a trailer without spring brakes, but a question about this still comes up from time to time on the written exam.
While you don't necessarily need to memorize the definition of a converter gear or dolly, you should have a solid understanding of what its purpose is.
This is very important to remember! Always have the heaviest trailer closest to the tractor. There is a very good chance this will show up on the written exam.
Written exam questions commonly refer to trailer height positioning. The trailer should raise slightly when the dolly is pushed under it.
You should always check the emergency line shut off valve instead of the service line shut off valve. The service line shut off valve will not have any air escaping it unless somebody is depressing the brake pedal or the trailer hand valve is engaged. It's better practice to check the emergency line.

Doubles:

Refers to pulling two trailers at the same time, otherwise known as "pups" or "pup trailers" because they're only about 28 feet long. However there are some states that allow doubles that are each 48 feet in length.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

The following are proper steps to connecting the converter dolly to the front trailer, except:
  • Back first semi-trailer into position in front of dolly tongue
  • Secure converter gear support in the lowered position
  • Lock pintle hook
  • Hook dolly to front trailer

Quote From The CDL Manual:

Connect converter dolly to front trailer:

  • Back first semi-trailer into position in front of dolly tongue.
  • Hook dolly to front trailer.
  • Lock pintle hook.
  • Secure converter gear support in raised position
Next
When pulling more than one trailer:
  • If there are three trailers, the heaviest should be in the middle
  • The position of the heaviest trailer is not important as long as the brakes are working properly
  • The heaviest trailer should be the very last trailer
  • The heaviest trailer should be in first position behind the tractor

Quote From The CDL Manual:

For the safest handling on the road, the more heavily loaded semi-trailer should be in first position behind the tractor. The lighter trailer should be in the rear.

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All of these methods can be used to secure a second trailer before coupling, except:
  • Back another trailer to the rear of the trailer you want to hook to which will act as a brace and keep the trailer from rolling
  • If the second trailer does not have spring brakes, drive the tractor close to the trailer, connect the emergency line, charge the trailer air tank, and disconnect the emergency line
  • Chock the wheels
  • Make sure the trailer spring brakes are locked

Quote From The CDL Manual:

  • Release dolly brakes by opening the air tank petcock. (Or, if the dolly has spring brakes, use the dolly parking brake control.)
  • Make sure trailer brakes are locked and/or wheels chocked.
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When pulling 3 trailers, the lightest trailer should:
  • Be the first trailer
  • Be the middle trailer
  • Be the last trailer
  • The placement of the lightest trailer doesn't matter

Quote From The CDL Manual:

For the safest handling on the road, the more heavily loaded semi-trailer should be in first position behind the tractor. The lighter trailer should be in the rear.

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Next
Which of these steps to connecting the converter dolly to the rear trailer is incorrect?
  • Back converter dolly under rear trailer
  • Close shut-off valves at rear of first trailer (and on dolly if so equipped)
  • Make sure trailer brakes are locked and/or wheels chocked
  • Connect safety chains, air hoses and light cords

Quote From The CDL Manual:

Connect converter dolly to rear trailer:

  • Make sure trailer brakes are locked and/or wheels chocked.
  • Make sure trailer height is correct. (It must be slightly lower than the center of the fifth wheel, so trailer is raised slightly when dolly is pushed under.)
  • Back converter dolly under rear trailer.
  • Raise landing gear slightly off ground to prevent damage if trailer moves.
  • Test coupling by pulling against pin of number two semi-trailer.
  • Make visual check of coupling. (No space between upper and lower fifth wheel. Locking jaws closed on kingpin.)
  • Connect safety chains, air hoses and light cords.
  • Close converter dolly air tank petcock and shut-off valves at rear of second trailer (service and emergency shut-offs).
  • Open shut-off valves at rear of first trailer (and on dolly if so equipped).
  • Raise landing gear completely.
  • Charge trailers (push "air supply" knob in) and check for air at rear of second trailer by opening the emergency line shut-off. If air pressure is not there, something is wrong and the brakes will not work.
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Next
How do you check to make sure the trailer height is correct before coupling?
  • The trailer landing gear should be lowered completely so that you can push the dolly under the trailer and be ready to drive once attached
  • The trailer must be slightly lower than the center of the fifth wheel, so the trailer is raised slightly when dolly is pushed under.
  • The trailer should be slightly higher than the center of the fifth wheel, so the trailer doesn't have to raise up when the dolly is pushed under
  • The trailer should be a minimum of 1ft higher than the fifth wheel, so the trailer king pin will not strike the fifth wheel plate and cause damage

Quote From The CDL Manual:

Connect converter dolly to rear trailer:

  • Make sure trailer brakes are locked and/or wheels chocked.
  • Make sure trailer height is correct. (It must be slightly lower than the center of the fifth wheel, so trailer is raised slightly when dolly is pushed under.)
  • Back converter dolly under rear trailer.
  • Raise landing gear slightly off ground to prevent damage if trailer moves.
  • Test coupling by pulling against pin of number two semi-trailer.
  • Make visual check of coupling. (No space between upper and lower fifth wheel. Locking jaws closed on kingpin.)
  • Connect safety chains, air hoses and light cords.
  • Close converter dolly air tank petcock and shut-off valves at rear of second trailer (service and emergency shut-offs).
  • Open shut-off valves at rear of first trailer (and on dolly if so equipped).
  • Raise landing gear completely.
  • Charge trailers (push "air supply" knob in) and check for air at rear of second trailer by opening the emergency line shut-off. If air pressure is not there, something is wrong and the brakes will not work.
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Next
Do converter dollies have spring brakes?
  • Some converter dollies have spring brakes, but dollies are not required to have them
  • While some states don't require dollies to have spring brakes, it is required for dollies to have spring brakes on the interstate system
  • Converter dollies don't ever have spring brakes
  • All converter dollies are required to have spring brakes if built after 1975

Quote From The CDL Manual:

A converter gear or dolly is a coupling device of one or two axles and a fifth wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig.

  • Release dolly brakes by opening the air tank petcock. (Or, if the dolly has spring brakes, use the dolly parking brake control.
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How are dolly brakes released when not equipped with spring brakes?
  • Brakes will be released when the electrical cable is attached
  • If a dolly doesn't have spring brakes, it doesn't have brakes at all
  • Hooking airlines to the dolly will automatically release the brakes
  • Opening the air tank petcock

Quote From The CDL Manual:

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

Prev
Next
All of the following are correct steps to positioning the converter dolly in front of the second (rear) trailer except:
  • Be sure the pintle hook is unlocked
  • Wheel dolly into position in front of second trailer in line with the kingpin
  • Move the dolly to rear of first trailer and couple it to the trailer
  • Pull dolly into position as close as possible to nose of the second semitrailer

Quote From The CDL Manual:

Position converter dolly in front of second (rear) trailer:

  • Release dolly brakes by opening the air tank petcock. (Or, if the dolly has spring brakes, use the dolly parking brake control.)
  • If the distance is not too great, wheel dolly into position by hand so it is in line with the kingpin.
  • Or, use the tractor and first semitrailer to pick up the converter dolly:
  • Position combination as close as possible to converter dolly.
  • Move dolly to rear of first semi-trailer and couple it to the trailer.
  • Lock pintle hook.
  • Secure dolly support in raised position.
  • Pull dolly into position as close as possible to nose of the second semitrailer.
  • Lower dolly support.
  • Unhook dolly from first trailer.
  • Wheel dolly into position in front of second trailer in line with the kingpin
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What is a converter gear or dolly?
  • A special accessory used to supply hydraulic pressure to the rear trailer brakes
  • A special device to indicate any airline failures or leaks in the rear most trailer
  • An anti-jackknife braking system similar to ABS used in the second and/or third trailer
  • A coupling device of one or two axles and a fifth wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig.

Quote From The CDL Manual:

A converter gear or dolly is a coupling device of one or two axles and a fifth wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig.

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