Combination vehicles and coupling
Combinations are harder to drive than single vehicles. Quick steering moves get amplified toward the back of the rig, the crack-the-whip effect, and can tip the trailer over even when the tractor stays upright. A loaded rig is far more likely to roll over than an empty one, so steer gently and slow down before curves.
In turns, trailer wheels follow a shorter path than the tractor's (off-tracking), so plan wide turns. If the trailer wheels lock and the trailer swings out, release the brakes to let them grip again.
Two air lines connect tractor and trailer. The red emergency (supply) line keeps the trailer tanks filled and controls its emergency brakes; if it breaks, the trailer brakes come on. The blue service line carries the air that applies the trailer brakes when you press the pedal. The tractor protection valve keeps air in the tractor if the trailer breaks away. The red eight-sided knob supplies air to the trailer; the yellow diamond knob is the parking brake.
To couple, chock the trailer or set its brakes, and set the trailer just low enough that it lifts slightly when you back under. Back until the fifth wheel touches, connect the air lines, then back under slowly until it locks. Check that there's no gap between the upper and lower fifth wheel and that the jaws are locked around the kingpin's shank. Do a tug test in low gear against the trailer brakes, then raise the landing gear fully.
