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Kinds of Semi-trailer

A semi-trailer is a trailer without a front axle. In the US, the term is also used to refer to the combination of a truck and a semitrailer, a tractor-trailer.

A large proportion of a semitrailer's weight is supported by a tractor unit, or a detachable front-axle assembly known as a dolly, or the tail of another trailer. A semitrailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled. Many semitrailers have wheels that are capable of being totally dismounted and are also relocatable to better distribute load to bearing wheel weight factors. Semitrailers are more popular for transport than full trailers, which have both front and rear axles. Ease of backing is cited as one of the semi's chief advantages. A road tractor coupled to a semi-trailer is often called a semitrailer truck or "semi" in North America & Australia, and an articulated lorry or "artic" in the UK.

Semi-trailers with two trailer units are called B-doubles (Australian English) or tandem tractor trailers, tandem rigs, or doubles (American English). Other terms used are "B-train" or (when there are three or more trailers) "road train". A B-double consists of a prime mover towing two semitrailers, where the first semitrailer is connected to the prime mover by a fifth wheel coupling (a.k.a. 'converter dolly') and the second semitrailer is connected to the first semitrailer by a fifth wheel coupling. In Australian English, the tractor unit is called a "prime-mover", and the combination of a prime-mover and trailer is known as a "semitrailer", "semi" or single.

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In road haulage, semi-trailers predominate over full trailers because of their flexibility. The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trunked between depots. If a power unit fails, another tractor can replace it without disturbing the cargo.

Compared with a full trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar attachment). Special tractors known as shunt trucks or shuttle trucks can easily maneuver semi-trailers at a depot or loading and unloading ferries. These tractors may lift the coupling so the trailer legs clear the ground.

A rigid truck and full trailer are articulated inside the cargo area length, so a semi-trailer can have a longer continuous cargo area. Because of this, a semi-trailer can haul longer objects (logs, pipe, beams, railway track). This depends on the legislation; in some European countries, a full trailer can be as long as a semi-trailer. However, since a rigid truck is longer than a semi-tractor, this increases the overall length of the combination, making it less maneuverable.


The two types of couplings are fifth wheel coupling and automatic. In some applications, no separable coupling is fitted, and the trailer is bolted to the tractor unit, using a bearing, and rocker feet as are used under a fifth wheel skid plate.

Fifth wheel coupling
The towing vehicle has a wide coupling plate known as a fifth wheel coupling bolted onto its chassis on which the semi-trailer rests and pivots. As the tractor reverses under the trailer, a kingpin under the front of the trailer slides into a slot in the skidplate, and the jaws of the fifth wheel close onto it. The driver has to raise the trailer legs manually, and couple the airbrake lines and electrical cables.

Automatic couplings
Many years ago, automatic couplings predominated,[citation needed] but are now quite rare. Automatic couplings were generally used for payloads of 12 short tons (11 t) or less, e.g. on the Scammell mechanical horse.

No coupling plate is used on the tractor; a turntable is permanently fixed to the underside of the trailer. This locks to the chassis of the tractor. When the tractor reverses under the trailer, its legs rise and the brake and electrical connections are made automatically. Almost the entire coupling and uncoupling procedure is operated by the driver from inside the cab, except that he or she has to descend to release (or apply) the trailer parking brake.
Side view and underside view of a conventional 18-wheeler semi-trailer truck with an enclosed cargo space.

The underside view shows the arrangement of the 18 tires (wheels). Shown in blue in the underside view are the axles, drive shaft, and differentials. The legend for labeled parts of the truck is as follows:
1. tractor unit
2. semi-trailer (detachable)
3. engine compartment
4. cabin
5. sleeper (not present in all trucks)
6. air dam
7. fuel tanks
8. fifth wheel coupling
9. enclosed cargo space
10. landing gear - legs for when semi-trailer is detached
11. tandem axles

Types of trailers

There are many types of semi-trailers in use, designed to haul a wide range of products.
 - Box, or dry van
 - Bus
 - Refrigerator, or "reefer"
 - Tanker
 - Dry bulk
 - Flatbed
 - Lowboy
 - Car hauler
 - Dump

Semi-truck manufacturers

 - Ford
 - Freightliner
 - Hino Motors (Canadian plant) 
 - Kenworth
 - Mack
 - Volvo
 - Caterpillar
 - Oshkosh
 - Crane Carrier Company