Truck, motor vehicle designed primarily for hauling cargo or for special work or service purposes. Trucks are usually larger and heavier than automobiles and differ in basic construction. Most modern automobiles have a unibody construction, in which the body itself provides structural support for the vehicle. Trucks, by comparison, are built around a strong metal frame, called a chassis, that supports the rest of the truck. Trucks usually have larger, more powerful engines and stronger suspensions than automobiles have. Large trucks have additional axles and wheels for carrying heavy loads.
Trucks come in many different varieties and are classified by weight, type, and the job they perform. Light trucks have a vehicle weight of up to 6,300 kg (14,000 lb) and are used for light hauling or towing, as well as for everyday transportation. Medium trucks have a weight of up to 15,000 kg (33,000 lb) and are typically used as dump trucks, garbage trucks, local freight-delivery trucks, and utility vehicles. Light and medium trucks are usually powered by gasoline internal-combustion engines, although some may be powered by diesel engines. Heavy trucks are over 15,000 kg (33,000 lb) in weight and are used primarily to pull trailers. Such trucks are often called semis (short for semitrailer), or tractor-trailer trucks, and use large diesel engines for power. Heavy trucks also include large vehicles designed for off-road work, such as large construction and mining trucks.
Trucks are also classified by their construction. Straight trucks have the body, or cab, and cargo compartment mounted on the same frame. All light trucks and most medium trucks are straight trucks. In a tractor-trailer truck, the engine and cab are part of the tractor unit. The trailer is a detachable unit, is separate from the tractor, and has its own suspension and wheels. It is joined to the tractor by a hinged platelike mounting device called a fifth wheel.