Trailer Sway, Off-Tracking, and Rearward Amplification
This domain covers the three geometry problems the exam tests: where the trailer's rear actually goes in a turn, how far the last trailer swings in a quick maneuver, and how to answer a trailer that starts oscillating.
Off-tracking
Rear axles cut a shorter path than the front axle. In a 90-degree turn the trailer's rear tires can track several feet inside the tractor's path, and a triple off-tracks more than a 53-ft semitrailer. This is how rigs hit curbs, poles, and pedestrians. Turn wide enough to keep the trailer's rear clear, watch both mirrors through the turn, and remember that swinging too wide puts the tractor's front into oncoming traffic.
Rearward amplification
In a quick lane change or evasive steer, each trailer swings wider than the one ahead. For 28.5-ft triples at highway speed the last trailer's lateral acceleration can be roughly double the tractor's. Amplification grows with speed, with more trailers, with shorter trailers, and with light loads on the rear unit.
Trailer sway
Sway is a side-to-side oscillation, usually from speed, crosswind, abrupt steering, or a lightly loaded trailer. Correct response: ease off the accelerator smoothly, hold the wheel steady, and brake gently if you must slow. Slamming the brakes or counter-steering hard converts sway into a rollover or a jackknife.
Worked example
A driver running triples at 60 mph makes a quick lane change to avoid debris. The rear trailer swings about twice as far as the tractor did and crosses the lane line. The same maneuver at 50 mph produces roughly 30 percent less lateral acceleration (speed ratio squared), keeping the rear trailer inside its lane. Speed is the lever you control.
Traps
- Steering harder to "catch" a swaying trailer - it amplifies the oscillation.
- Cutting a corner because the cab cleared it.
- Assuming the mirrors show where the trailer's rear is tracking.
- Making a fast correction at highway speed in a multi-trailer set.