The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trucked between depots. In road haulage, semi-trailers predominate over full trailers because of their flexibility. With 1,170,117 million tonne-kilometers transported this way in the European Union, including the UK, road tractors and semi-trailers are 77.6% of the total tonne-kilometers transported in 2015, according to Eurostat. Road tractors and semi-trailers are responsible for carrying a large proportion of cargo. Semi-trailers were invented by August Fruehauf in 1914. In Australian English, the tractor unit is called a 'prime-mover', and the combination of a prime-mover and trailer is known as a 'semi-trailer', 'semi' or single. The first semi-trailer is connected to the power unit using the tractor's fifth wheel coupling while the converter dolly, already attached to the second semi-trailer, is connected to the first semi-trailer with a drawbar. A double-trailer combination is possible with the use of a dolly, or 'converter dolly' (Australian and American English), essentially one to three additional axles placed under the front of a second semi-trailer. Other terms used are ' B-train' or (when there are three or more trailers) ' road train'. Semi-trailers with two trailer units are called B-doubles (Australian English) or tandem tractor-trailers, tandem rigs, or doubles (American English). A road tractor coupled to a semi-trailer is often called a semi-trailer truck or 'semi' in North America & Australia, and an articulated lorry or 'artic' in the UK. Ease of backing is cited as one of the semi's chief advantages. Semi-trailers are more popular for transport than full trailers, which have both front and rear axles. Many semi-trailers have wheels that are capable of being totally dismounted and are also relocatable (repositionable) to better distribute load to bearing wheel weight factors. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled. The other portion of the semi-trailer's weight is semi-supported (half-supported) by its own wheels, which only support the rear of the semi-trailer.

Ī large proportion of a semi-trailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. In the United States, the term is also used to refer to the combination of a truck and a semi-trailer a tractor-trailer.

A truck pulling a semitrailer using a trailer dollyĪ semi-trailer is a trailer without a front axle.

Semi truck and trailers