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CDL · Coupling and Uncoupling

When coupling doubles, what is a converter dolly?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

A coupling device with a fifth wheel that connects the rear trailer to the front trailer — it has its own axle(s) and is used to convert a semi-trailer into a full trailer that can be pulled behind another trailer

  1. A A coupling device with a fifth wheel that connects the rear trailer to the front trailer — it has its own axle(s) and is used to convert a semi-trailer into a full trailer that can be pulled behind another trailer
  2. B A tool for changing tires
  3. C A type of engine
  4. D A trailer brake

Why this is the answer

Converter dolly: A coupling device used to connect a second (or third) trailer in a doubles or triples combination.

Components: A frame with one or more axles; a fifth wheel mounted on top (to receive the kingpin of the rear trailer); a tow bar/drawbar with an eye that connects to the pintle hook of the trailer ahead; its own brakes and lights.

Function: It 'converts' a semi-trailer (which needs a fifth wheel for support) into a full trailer that can be towed behind the first trailer; COUPLING WITH A DOLLY involves: connecting the dolly to the rear of the first trailer (pintle hook and safety chains), then backing the rear trailer onto the dolly's fifth wheel (like coupling a tractor to a trailer); the dolly creates an additional coupling point that must be inspected and secured carefully; DOUBLES/TRIPLES have multiple coupling points (tractor-to-trailer, dolly connections) — each must be properly secured; the converter dolly is central to doubles/triples operation and is a key inspection and coupling topic on the endorsement exam.

Source: FMCSA CDL Manual — Doubles/Triples, Converter Dolly

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About this topic: Coupling and Uncoupling

Coupling and uncoupling doubles and triples adds the converter dolly and the pintle hook to the usual fifth-wheel procedure, and the order matters — heaviest trailer first, lightest last. These questions walk through the steps and the checks that confirm a secure connection. This is one of 16 Doubles and Triples questions on coupling and uncoupling, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

Related questions

Coupling and Uncoupling

What is a 'converter dolly' in a doubles combination?

 Coupling and Uncoupling

Before coupling a converter dolly to the rear of the first trailer, you should:

 Coupling and Uncoupling

When uncoupling a converter dolly, what is the safe sequence?

 Coupling and Uncoupling

When inspecting the pintle hook on a converter dolly before coupling, what should you verify?

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