What is a Converter Dolly?

October 26, 2021

Converter dolly

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Key Takeaways

A converter dolly enables a tractor unit to pull trailers. Learn about the different parts of a converter dolly and the different styles they come in.

What is a converter dolly?

A converter dolly is an unpowered commercial vehicle designed to connect with tractor units and enables them to pull trailers. It consists of a chassis equipped with one or more axles, a fifth wheel (coupling device), and a drawbar. Converter dollies support the front end of a semi-trailer when towing a second or third trailer. 

Trailer converter dollies are also known as road train dollies because they connect trailers like railroad cars. (The term “road trains” is more commonly used in Australia’s trucking industry.) When a tractor pulls two trailers, it’s known as a “doubles” combination; pulling three is a “triples” combination. 

Parts of a converter dolly

The main components of a converter dolly include:

  • Axles: Converter dollies have between one to three axles. Single axle dollies have one axle and one set of wheels. A tandem axle dolly has two sets of axles,  one right behind the other.

  • Fifth-wheel coupler: The sliding fifth wheel coupler provides the link between a semi-trailer kingpin and the dolly. 

  • Drawbar: Dollies may have a single- or double-tow drawbar for coupling to the towing trailer. (See types of dollies below.) Both kinds of towbars end in a towing eye.

  • Pintle hitch: The hitch itself consists of a hook that engages and supports the drawbar eye and a locking mechanism that ensures that the eye stays in place.  

  • Brakes: Converter dollies built on or after March 1, 1998, must have antilock brakes. These dollies will have a yellow lamp on the left side of the dolly.

  • Lighting: Dollies must have one stop lamp, one tail lamp, and two reflectors. Each converter dolly should have rear turn signals and hazard warning flasher lamps.

  • Suspension: Dollies will have either an air ride suspension or a mechanical spring suspension

Types of converter dollies

There are two types of converter dollies. Their drawbars differentiate them.


The A-dolly has a single drawbar with a single, centered hitching point. Single axle converter dollies are the most common type of converter dolly used in the U.S.


The C-dolly has two separate couplings, side-by-side. Originating in Canada, this dolly improves the stability of multiple trailer combinations.

Why use a converter dolly?

Converter dollies are useful for moving a large number of loads quickly. Although heavy-duty trucks can transport more loads, there are regulations involved in moving these larger amounts of freight.

In the U.S., trucks on public roads are limited to two trailers. While some states allow triples, they’re limited to less populated states. Occasionally, triples are used in less-than-truckload (LTL) freight hauling.

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