Driving Combination Vehicles Safely
The physics you must respect
A combination vehicle is two or more units connected by a hitch, and its handling differs from a single truck in three ways: it is heavier, it is longer, and it bends at the coupling point. The most tested concept is off-tracking - the rear wheels of the trailer follow a shorter path than the front wheels of the tractor, so the trailer cuts the corner. The longer the wheelbase and the sharper the turn, the more the trailer cuts inside. A related idea is swing-out: the rear of the trailer swings wide in the opposite direction when you turn, which endangers curbs, poles, and pedestrians.
Stopping and following distance
Because of weight and the air-brake lag (see the brake section), a loaded combination needs far more stopping distance than a bobtail or a single-unit truck. Keep one second of following distance for each 10 feet of your vehicle length at speeds under 40 mph, and add one extra second at higher speeds. That means a 60-foot rig needs at least six seconds below 40 mph and seven seconds above it.
Traps in questions
- Questions often describe a right turn and ask where the trailer will go. Answer: the trailer cuts the corner, so swing wide to the left before turning.
- "Swing-out" questions reverse the direction: the trailer's rear swings left when you steer right.
- A common trick is to ask whether a light load stops faster. It does, but not as much as candidates assume; the air-brake lag still applies.
Rule of thumb
When turning, go wide, go slow, and watch the trailer's rear wheels in the mirror - if you cannot see them, you are not ready to turn.