Combination Vehicles
Combination Vehicles tests the coupling, inspection, and handling differences that make a tractor-trailer unforgiving when something is missed.
Coupling and uncoupling
- Back under the trailer, listen for the click, then do two tug tests with the trailer spring brakes set. Pull gently - if the trailer moves with you, the jaws are not locked around the kingpin.
- Get out and look at the fifth wheel to confirm the jaws are closed. Connect the emergency (red) line and service (blue) line, then raise the landing gear fully.
- Before uncoupling, lower the landing gear, chock the wheels, and disconnect the lines only after the trailer brakes are set.
Handling
- Off-tracking: the rear wheels cut inside the curve, so swing wide without leaving your lane.
- Rearward amplification - the "crack-the-whip" effect - makes the last trailer swing most. Slow down before curves and lane changes.
- The trailer hand valve is a test tool only; never use it to park.
Worked example: You hear the fifth-wheel click and pull forward, but the trailer comes with you even though its spring brakes are set. The coupling has failed. Back under again, inspect the jaws, and repeat the tug test - do not hit the road until the trailer stays put.
Traps: trusting the click instead of the tug test; connecting the lines before checking the coupling; driving away with the landing gear down; using the trailer hand valve as a parking brake; forgetting a bobtail tractor handles and stops differently without trailer weight.