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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 listed below.

7.2.1 – 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.

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

A converter gear on a 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.

Position Converter Dolly in Front of Second (Rear) Trailer

Release dolly brakes by opening the air tank petcock (if the dolly has spring brakes, use the dolly parking brake control). If the distance is not too great, wheel the 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 semitrailer 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 semitrailer 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 the second semitrailer.
  • 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 trailer brakes (push “air supply” knob in) and check for air at rear of second trailer by opening the emergency line shut-off. If air pressure isn't there, something is wrong and the brakes will not work.

Multiple-Choice Questions:

Question #349 (1 of 1)

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

  • The weight of the individual trailers will have no noticeable effect on the handling of the vehicle
  • 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.
  • The weight of all trailers should be nearly the same, within 2% of each other
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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