Towing Multiple Trailers: Tracking, Off-Tracking, and Handling
Domain 3 tests the geometry of a long combination: where the rear trailer actually goes when you steer, and how much of the road it needs.
Off-tracking
Every trailer's rear wheels track inside the front wheels in a turn. The more trailers in the set, the greater the off-tracking at the last trailer, so triples cut a corner noticeably more than a single semi. In a right turn the last trailer's right wheels may ride the curb even though the tractor cleared it. In a left turn the same geometry pulls the last trailer toward the inside line while its body swings toward the outside as it pivots.
Managing a turn
- Approach from a position that leaves room on the inside. In some cases you may legally use part of an adjacent lane, but only if it is clear.
- Start the turn later than you would in a car, and watch the last trailer in the mirror as you complete it.
- Avoid stopping mid-turn: a stalled set of triples blocks everything and is hard to restart without swinging.
- Mirrors must be set to show the last trailer's flank. A mirror set only for the tractor hides the problem.
Rearward amplification
In a quick lane change or sudden steer, the last trailer swings sideways much farther than the tractor did, several times the lateral movement at the front. That swing can clip a car, take out a sign, or roll the last trailer. The defence is anticipation: look far ahead, plan lane changes early, and make one smooth small input instead of a late correction.
Backing
Doubles and triples can be backed only in a straight line for short distances, and never without a helper watching the rear trailer. Backing around a corner with two or three trailers is effectively impossible.
Worked example
A driver turning right from a two-lane road into a narrow street swings into the left lane and starts the turn early. The tractor clears the curb, but the last trailer's right wheels strike it and the trailer leans. Two faults: turning too early and too sharply, and never watching the last trailer in the mirror. Correct approach: keep as far left in the lane as is legal to open the turn, begin the turn when the tractor's rear axle reaches the corner, and complete it slowly while the mirror shows the last trailer clearing the curb.
Traps that cost marks
- Assuming the rear trailer follows the tractor's path.
- Returning to the right lane before the last trailer has cleared.
- Turning wide without re-checking the left mirror for a vehicle that moved up.
- Backing without a spotter to save time.