Semitrailers

Semitrailers are trailers designed to be pulled by a truck tractor. The front of the trailer connects to the tractor via a kingpin and a fifth-wheel coupling, so the tractor bears the front-end weight in motion. Unlike a full trailer, a semitrailer does not have a front axle of its own; when detached, the trailer rests on its landing gear, and the tractor provides the front support when coupled.

Construction and features vary, but most semitrailers have two or more rear axles on a frame or chassis, a front kingpin, nose-mounted landing gear, and a suspension system that may be air-ride or leaf-spring. Some trailers include a steerable front axle to improve handling. They are commonly built from steel or aluminum and configured for different cargo types.

Types and uses cover a range of configurations. The most common are dry van box trailers, refrigerated (reefer) trailers, flatbeds, and curtain-side trailers. Other variants include tank trailers for liquids, lowboys or step-decks for heavy equipment, and specialty trailers for bulk or hazardous materials. Semitrailers are widely used in long-haul freight, regional distribution, and intermodal transport, often moving between road and rail systems.

Regulation and safety considerations vary by region. Typical highway semitrailers in many areas reach about 53 feet in length and operate within an upper gross vehicle weight limit (for example, roughly 80,000 pounds in the United States). Braking is usually by air brakes, with safety features such as anti-lock braking systems and electronic stability control common on modern trailers.