Combination Vehicles
The Core Idea
Combination vehicle questions focus on coupling, uncoupling, and the dynamics of a tractor-trailer - especially the danger of off-tracking and rollover.
Concepts You Must Know
- Fifth wheel and kingpin: the locking jaws must close around the kingpin. Always tug test after coupling by pulling gently with the trailer brakes set to confirm the coupling holds.
- Air lines: connect the emergency (supply) line first (usually blue) and the service line (red) last; disconnect in reverse order. Check that trailer air pressure builds and that the tractor protection valve works.
- Landing gear: raise fully before driving; lower and crank down when uncoupling.
- Off-tracking: the rear wheels of a long trailer cut a shorter path than the front wheels, so you must swing wider on turns.
- Rollover: high center of gravity plus speed in a turn equals rollover. Slow down before curves.
- Brake checks: with a combination, test that the trailer brakes apply and release, and that the tractor protection valve pops out if trailer air is lost.
Traps to Watch
Questions often ask the order of connecting air lines or what happens if you forget the tug test. Another trap is assuming the trailer follows exactly in the tractor's path - it does not.
Rule of Thumb
"Blue first, red last" when connecting; reverse when disconnecting. And always tug test - if the trailer moves against set brakes, the coupling is wrong.