The large trailers pulled by semi tractors have their own rear suspension and wheels, with the front of the trailer being supported by the fifth wheel on the tractor. Semi trailers also have folding supports under the front that are lowered when the trailer is detached from the tractor and parked. The brakes on the trailer’s axles have air hoses that attach to the tractor’s brake system, so the tractor and trailer brakes work together. Trailers have their own signal, tail, and brake lights, all of which are powered by the tractor’s electrical system.
Trailers come in many different designs, depending on the intended cargo. Enclosed, or standard box-type trailers are used to haul a wide variety of goods and merchandise. Double trailers are often used on roads that have sharp turns. Double trailers resemble two smaller trailers linked together and can maneuver through tight turns more easily than standard trailers can. Size and weight restrictions apply and vary from state to state. In the United States, tractor and single trailer combinations generally must be less than 16 m (53 ft) in length and are limited to a maximum weight of 36,000 kg (80,000 lb). Separate weight limits apply to trailers with single or tandem axles and to double trailers. Maximum trailer height and width are dictated by state law and vary from state to state. For most states, the maximum height is 4.11 m (13.5 ft), and the maximum width is 2.6 m (8.5 ft). In some states, trailers may be equipped with additional wheels and axles to carry heavier loads.
A special type of enclosed trailer is an insulated and refrigerated “refer” unit, used for transporting perishable food items. Refrigerated trailers have a small engine mounted on the trailer for powering the refrigeration system. This allows the refrigeration unit to run continuously, even when the trailer is parked or disconnected from the tractor.
Piggyback trailers are enclosed trailers designed to be mounted on railroad flatcars for cross-country transport. Some have their own wheels and suspension, while others are sealed containers that are lifted off and placed on a trailer chassis. Sealed containers are also used on special ships, called container ships, to transport goods overseas.
Flatbed trailers are used to transport large objects such as construction equipment, industrial machinery, and oversized objects. Such trucks may be equipped with an Oversize Load warning sign and flashing lights, and may be accompanied by an escort vehicle to warn other motorists.
Platform trailers are essentially large containers with open tops for transporting produce and grain. Special trailers are also designed for hauling livestock, automobiles, and beverages.
Tank trailers, known as tankers, are used to haul chemicals, milk, gasoline, and other liquids. Tankers, as well as other trucks that carry flammable or toxic products, must display special warning emblems to warn police and firefighters in case of an accident.