Doubles and Triples
Doubles and Triples
This domain tests the extra skills for two or three trailing units. The key idea is rearward amplification: a small steering move at the tractor can become a large sideways move at the last trailer.
Stability and driving
Drive smoothly and below the speed you would use with a single trailer. Avoid sudden lane changes, hard braking, and rapid steering. Increase following distance, brake earlier, and slow down before curves. The last trailer swings widest, so allow extra room when turning or changing lanes.
Equipment checks
Check the converter dolly, its coupling devices, safety latches, air lines, electrical connections, and brakes. Make sure each trailer's brakes work and that shut-off valves are in the correct position. Inspect the load and the couplings after the first 50 miles and every 150 miles or 3 hours.
Worked example
At highway speed, a quick 2-foot steering input may move the last trailer several feet sideways. That is rearward amplification. The safe response is a gradual lane change with a signal, a mirror check, and enough gap so the last trailer does not strike the vehicle beside it.
Braking
Brake gently and in a straight line when possible. A hard stop can cause the trailers to push or jackknife. Do not use the trailer hand valve for parking.
Traps that cost marks
- Assuming the last trailer follows the tractor's exact path.
- Changing lanes as quickly as in a car.
- Skipping the converter-dolly inspection.
- Using the hand valve as a parking brake.