Semi-trailer truck

A semi-trailer is a road trailer designed to carry goods and supported partly by a tractor unit. Unlike a full trailer, it does not have its own front axle: its front section rests on the tractor through a coupling device called a fifth wheel. The semi-trailer itself has one or more axles toward the rear, while the tractor provides the engine, cab and most of the vehicle's driving systems.

When the two are connected, they form an articulated truck. Semi-trailers come in many forms, including enclosed box trailers, refrigerated units, tankers and flatbeds. Their construction and dimensions vary according to the type of goods they are designed to transport and the regulations of each country.

The coupling system allows the tractor to pull the semi-trailer while also articulating at the connection point, which helps the combination negotiate turns. During a turn, the trailer follows a different path from the tractor, so the driver must allow for its position and length. Braking systems are distributed across the combination, and modern vehicles may include electronic systems such as anti-lock braking and stability control.

The semi-trailer does not have its own engine, but it can have electrical and braking equipment supplied through connections with the tractor. This arrangement makes it possible to use different trailers with suitable tractor units, which is useful for long-distance freight transport. Semi-trailers are widely used to move food, manufactured goods, fuel, machinery and many other types of cargo.