semitrailer

A semi-trailer is a trailer designed to be attached to a tractor unit through a fifth-wheel coupling. When connected, the front portion of the trailer rests on the tractor and only part of the load is borne by the trailer itself. The combination is commonly called a tractor-semi-trailer or articulated lorry.

Design and function: The front end carries a kingpin that engages the fifth wheel. The trailer has landing gear for independent support when uncoupled and usually one or more rear axles. The articulation allows turning with a long payload, while the tractor provides propulsion and some braking.

Sizes and uses: Semi-trailers come in many body styles, including dry vans, refrigerated trailers, flatbeds, curtain-siders, and tank trailers. They are used for long-haul road transport and can operate in intermodal networks, where trailers or containers are exchanged between road and rail or ship.

Operation and regulation: Semi-trailers function as a two-unit combination. Vehicle weight and dimensions are subject to road rules, and load securing, axle configurations, and braking systems are regulated. Modern trailers may feature air-ride suspensions, multiple axles, and advanced electronic braking for safety.

History: The semi-trailer concept emerged in the early 20th century with the development of fifth-wheel couplings, enabling detachable trailers. It enabled higher payloads and easier maintenance, and became standard for road freight in many regions.