semitruck

A semitruck, also called a tractor-trailer or tractor-trailer rig in some regions, is a combination vehicle used for freight transport. It consists of two main parts: a tractor unit (the cab that contains the engine and driver controls) and a semi-trailer (the cargo-carrying trailer). The two are connected by a fifth-wheel coupling on the tractor and a kingpin on the trailer. The trailer has no front axle and is supported by the tractor when connected and by landing gear when detached.

The tractor provides propulsion, steering, and braking control, while the trailer serves as the load platform. The configuration enables a single tractor to pull multiple trailers in some regions, though in many countries a tractor is paired with one trailer for standard operations. The term “18-wheeler” is often used in North America to refer to a typical tractor-trailer combination because the overall vehicle commonly has 18 wheels.

Uses and regulations vary by country but semitrucks are central to long-haul and regional freight networks, moving a wide range of goods between factories, distribution centers, and ports. Common safety and performance concerns include braking efficiency, tire wear, visibility, and the risk of rollover given high payloads and long stopping distances. Vehicles employ air braking systems, lighting, and other safety features, and are subject to regular inspections and operating rules.

History and impact: the concept emerged in the early 20th century with evolving couplings and trailer designs, becoming widespread in the mid-20th century. Today semitrucks are a dominant mode of road freight, reflecting the logistics needs of modern economies.

See also: tractor unit, semi-trailer, trucking industry, intermodal freight.