Dolly Classification by Axle Configuration
There are two types of dolly bogie:
Full trailer - 2 axle (4 wheels), with a draw bar which also controls the trailer's front axle steering. The draw bar does not take load of the full trailer. Heavy full trailer needs to have its own brakes remotely controlled by the prime mover vehicle.
Semi-trailer - 1 axle (2 wheels), without the front axle but have a landing gear. Large semi-trailer of truck size is designed for connection via the fifth wheel on the tractor unit or the semi-trailer truck. Small semi-trailer such as travel trailer and boat trailer is design for connection via a tow hitch of a passenger vehicle. Either the fifth wheel or the tow hitch takes up to half to load of the semi-trailer.
Special large dolly bogie is equipped with a fifth wheel for further connection by the gooseneck type drawbar of another similar dolly and form a road train. The last dolly in a road train needs its own rear lights, brakes remotely controlled by the prime mover vehicle, and registration plate.
A semi-trailer designed as automobile rescue equipment. It is designed to couple to the concerned automobile's powered wheel, i.e. the front wheel of a Front-wheel drive automobile, or the rear wheel of a rear-wheel drive automobile, by locking the powered wheels onto the tow dolly's tray. The tow dolly is tow hitch connected to a tractor or truck. 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.
Dolly Classification by Coupling Configuration
There are two basic types:
- 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:
- A 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.