Converter Dollies: Function and Configuration Types
A converter dolly is a short chassis assembly with one or two axles and a fifth wheel on top, used to convert a semi-trailer (which has a kingpin at the front) into the equivalent of a pull trailer. The front of the second trailer's kingpin connects to the dolly's fifth wheel; the dolly's coupler connects to a drawbar that hooks to the rear of the lead trailer. This creates a 'doubles' combination: a tractor pulling a lead trailer, connected via the converter dolly to a second (pup) trailer. A-dolly configuration: the single drawbar connects to the rear of the lead trailer at a single coupling point. This allows the dolly and second trailer to pivot freely side to side — the second trailer can potentially swing wide (crack-the-whip effect) on curves or emergency maneuvers. The A-train name refers to the 'A' shaped dolly drawbar configuration (two arms converging to a single coupler). B-train configuration: instead of a dolly, a B-train uses a second fifth wheel mounted on the rear of the lead trailer itself. The second trailer's kingpin connects directly to this rear fifth wheel — there is no separate dolly chassis. Advantages of B-train: more stable than A-dolly (no converter dolly, tighter geometric coupling), better handling characteristics in high-speed maneuvers, and fewer connection points to fail. B-trains are common in Canada and less common in the U.S.