What is "semi-trailer"

also semitrailer, 1910 in reference to motor vehicles (late 19c. in botany), from semi- + trailer.Short form semi is attested from 1942.

n. 1 A trailer without a front axle and with wheels only at the trailing end, designed to be pulled via a pivoting arrangement which also partially supports its weight. 2 A tractor-trailer or big rig: a semi-trailer plus the truck or tractor pulling it.

A semi-trailer is a trailer without a front axle. A large proportion of a semitrailer's 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. 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. 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 North America & Australia, and an articulated lorry or "artic" in the UK, New Zealand & Australia. Semi-trailers with two trailer units are called "B-doubles" (or in American English just "doubles"), and are often referred to as , " B-Trains", or (when there are three or more trailers), " road trains". A B-double or 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 (aka 'converter dolly') and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. 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".