Tractor-trailer trucks, also called semis, haul much of the freight in the United States and a significant percentage of freight in the rest of the world. Trailers can deliver raw materials, refrigerated goods, and finished products to factories, warehouses, and markets. The tractor shown here includes a sleeping compartment so the driver can rest during long trips.
Heavy trucks have the largest frames and are usually powered by large diesel engines. They have multispeed transmissions, with as many as 18 gears, for pulling heavy loads. Most semis or tractor-trailer trucks have two rear drive axles, called dual tandems, each of which has dual tires on each side. Trucks with this arrangement are nicknamed 18-wheelers because of the total number of wheels under the truck and trailer.
When a heavy truck is assembled, the type of engine, transmission, brakes, axle arrangement, and other equipment used are usually specified by the individual or company that is ordering the truck. Almost all heavy trucks have power steering, cruise control, and air brakes. Air brakes use compressed air, rather than brake fluid, to activate and release the brakes. An air reservoir in the truck supplies air to the brakes through air hoses. The constant supply of air keeps the brakes released. Pressing on the brake pedal decreases the air pressure and applies the brake. If the brake system malfunctions or if the air hoses become punctured, the brakes apply themselves automatically as a safety precaution.
Most heavy trucks are designed for hauling goods over long distances, known as over-the-road hauling, although some are also used for local deliveries. The truck cab may be positioned behind the engine and hood or located directly over the engine. The latter is called a cab-over design, and it reduces the overall length of the truck-trailer combination. Cab-over trucks are hinged to tilt forward when engine access is necessary for maintenance or repairs. Most long-haul heavy trucks have a sleeper compartment, behind the cab, that contains a bed for the driver. The sleeper compartment is the truck driverís home away from home and may be equipped with a television, a microwave oven, and a refrigerator. Most heavy trucks are also equipped with citizens band radio for communication. Some truck fleets use satellite tracking equipment to monitor their trucks, so the fleet operator or dispatcher can know a truckís location at any time.