Doubles and Triples
Doubles and Triples tests coupling and controlling multiple trailers. The extra articulation points increase off-tracking, rearward amplification, and the chance of a rollover or jackknife.
Core rules
- Inspect every coupling: tractor to first trailer, converter dolly, and second trailer. Confirm kingpins, fifth wheels, safety latches, air lines, and electrical connections.
- Check that shutoff valves are in the correct position and that all trailer brakes work. Test each coupling with a tug test.
- Drive smoothly. Hard braking or sudden steering can make the rear trailer swing out or amplify movement.
- Allow more room for turns and lane changes. The rear trailer cuts a tighter path than the tractor.
- Watch for surge, especially when starting, stopping, or changing speed with partly loaded trailers.
Worked example
A driver turns a set of doubles at a city intersection. The tractor clears the curb, but the second trailer's rear wheels track inside the turn. The driver swings wider before the turn and checks both mirrors through the turn. Reasoning: each articulation point adds off-tracking, so the last trailer needs more clearance than a single trailer.
Traps
Assuming a coupling that looks closed is locked; forgetting the converter dolly's air lines; and braking hard in a curve, which can fold the combination.