A dolly is a small trailer that can be coupled to a truck or trailer so as to support a semi-trailer. The dolly consists of a bogie equipped with a kingpin and a fifth wheel, to which the semi-trailer is coupled. This dolly needs its own rear lights and a registration plate. Some Americans call the whole dolly a bogie.
A tow dolly is little more than two wheels, an axle and a tow-hitch, used to tow a Front-wheel drive suspension car behind a recreational vehicle or other larger vehicle. It is designed to tow a vehicle with the front wheels on the tow dolly. Tow dollies are legal in all 50 US states and Canada. In the U.S. and Canada brakes are required on any loaded car tow dolly.
There are two basic types of dolly:
- Converter dolly, equipped with between one and three axles and designed to connect to a towbar on the rear of the truck or trailer in front. There are two variants of this:
- An A-dolly has a single drawbar with a centred coupling.
- A C-dolly has two separate couplings side-by-side.
- Low loader dolly, equipped with a gooseneck type drawbar that attaches to the fifth-wheel coupling on the rear of a prime mover to distribute the mass on the fifth wheel on the dolly between the prime mover and the wheels of the dolly. These are predominantly fitted with two axles.
The C-dolly design is not allowed in Australia, as it prevents articulation between the dolly wheels and the axles of the truck or trailer in front of the dolly. Australian rules require articulation between axle groups.
Low-loader dollies- which present a kingpin rather than a drawbar coupling- are used with many low loaders to allow heavy cargo to be carried without overloading the wheels of the prime mover or the low loader.