Semi-trailer

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a tractor unit, a detachable front axle assembly known as a dolly, or the tail of another trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.

Semi-trailers are more popular for transport than full trailers, which have both front and rear axles. Ease of backing is cited as one of the semi's chief advantages.

A road tractor coupled to a semi-trailer is often called a semi-trailer truck or "semi" in the US, and an articulated lorry or "artic" in the UK. The fifth wheel on a truck connects to a semi-trailer kingpin. Kingpins come in many forms, but the most common within the UK market is the 2.0-in EEC-approved type. This kingpin is fully interchangeable and, given a strict maintenance schedule, it should last the life of a trailer.

In Australian English, the tractor unit is usually referred to as a "prime-mover", and the combination of a prime-mover and trailer is known as a "semi-trailer" or "semi". Semi-trailers with two trailer units are called "B-doubles" (or in American English often just "doubles"), and in some cases (especially when there are three or more trailers), "road trains". A B-double consists of a prime mover towing two semi-trailers, where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling.