Combination Vehicles
Why combinations behave differently
A tractor-trailer is two or more units joined at a pivot point. That pivot changes how the rig turns, stops, and rolls over.
Concepts to master
- Off-tracking: The rear wheels follow a shorter path than the front wheels. In a right turn, the trailer cuts the corner, so swing wide left first.
- Swing-out: The rear of the tractor swings outward in a turn; watch mirrors and pedestrians.
- Brake connections: Connect the emergency (supply) line first, then the service line - the reverse of disconnecting.
- Trailer air supply: The tractor protection valve keeps air from escaping if the trailer breaks away.
- Rollover risk: High center of gravity plus speed in a turn equals rollover. Slow down before the curve, not in it.
- Rearward amplification: The last trailer swings widest in a turn, especially with doubles.
Traps
Common questions ask which line to connect first (supply/emergency), or what to do when the trailer begins to jackknife (release the brakes and steer). Another trap: the trailer hand valve should not be used for parking.
Worked example
Turning right from a two-lane road into a narrow street, you must swing left to keep the trailer's rear wheels from climbing the curb. But swinging too far left puts the tractor's front bumper into oncoming traffic - so check both mirrors and the front corner before committing.
Rule of thumb
Slow down before the turn, swing wide, and watch the trailer tires in your mirror.