Pulling Double/Triple Trailers: Converter Dollies, Couplers, and Air Supply
The pulling hardware
A double or triple carries its extra trailers on converter dollies. A converter dolly is a small frame with a fifth wheel, one or two axles, and a drawbar ending in a pintle eye (lunette) that drops over the pintle hook on the rear of the trailer ahead. The dolly also has air lines and an electrical cord that plug into the rear of the trailer ahead, and its own rear couplers - two glad hands and a receptacle - plus shut-off valves that feed the trailer behind it.
Rules a question will test
- A trailer is never towed off another trailer's pintle hook without a dolly carrying its own fifth wheel.
- Shut-off valves open toward a trailer, closed toward nothing. If the valve feeding a trailer is shut, that trailer gets no air: its spring brakes will not release, or it has no service brakes at all.
- The emergency (blue) line must be connected and supplied, or the trailer's spring brakes stay applied and the trailer drags.
- Safety chains must be crossed under the drawbar (left chain to the right side), and the pintle hook latch must be closed with its safety pin in place.
- Every glad hand needs a good gasket, and lines must be supported so they do not drag or chafe.
Traps
Questions describe a symptom instead of a rule. "The second trailer's brakes drag and smoke" points to a closed shut-off valve or an unconnected emergency line; "a trailer separated on a bump" points to an unlatched pintle hook or missing chains. Another favorite asks which trailer's brakes the trailer hand valve controls.
Rule of thumb
One valve per coupler: open toward a trailer, closed toward empty air.