For the driving arcade game, see 18-Wheeler: American Pro Trucker.
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semi-trailer truck with 'sleeper' behind the cab.

A semi-trailer truck (colloquially known as an 18-wheeler, semi, tractor trailer or big-rig in the US, as a semi in Australia, US, and Canada, and as an articulated lorry or artic in the UK and Ireland) is an articulated truck consisting of a towing engine (tractor in the US, prime mover in Australia), and a trailer that carries the freight. (See below for the etymology of the name "semi-trailer").

In United States, semi tractors usually have 3 axles, the front, or "steer" axle having two wheels, and each of the two rear "drive" axles having a pair of "dual" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. The cargo trailer usually has two axles at the rear, each of which has dual wheels, or 8 wheels on the trailer.

However, the United States also allows 2-axle tractors to tow two 1-axle 28-foot (8.5 m) semi-trailers known colloquially as doubles, a set, or a set of joints. Some states also allow towing up to three 28-foot trailers known coloquially as triples. A 2-axle full-sized semi-trailer pulling a 28-foot "pup" trailer known as a Rocky Mountain Double is also permitted in some regions. A very few states allow two full-sized semi trailers similar to the Australian road trains.

In Europe, most semi tractors have 2 axles, again with the front, steer, having two wheels, and rear, drive, having a pair of double wheels on each side. Thus, the most common configuration has 6 wheels. Conversely, the cargo trailer usually has three axles at the rear, each with dual wheels, or 12 wheels in total. One way or the other, the entire vehicle thus usually has 5 axles and 18 wheels in total, although the trailers can vary in number of wheels. Overall lengths often range from 50 to 70 ft (15 to 25 m) in the US, and most US states limit the overall weight to 80,000 lb (36 tonne). The long-haul towing engines used in interstate travel are often equipped with a "sleeper" behind the driver's cab, which can be anything from a small bunk to a rather elaborate miniature apartment.

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An articulated lorry in London, England.

The noticeable difference between trucks in the US and lorries in Europe is the lack of a nose on European models. While some US trucks are built without a nose, they are not as common. In European design, the driver's cab is positioned above the engine. For repairs, the entire cab hinges forward to allow maintenance access. European lorries, whether small or fully articulated, have a sheer face on the front. This allows greater manoeuvrability when steering, as the driver need only gauge distances behind his seating point, and this allows for shorter trucks with longer trailers (with larger freight capacity) within the legal maximum total length.

Most semis use air rather than hydraulic fluid to actuate the brakes. If the air lines fail and the pressure drops during driving, the trailer brakes will actuate and stay applied, causing the wheels of the trailer to lock and skid on the road. Because of the wide variety of loads the semi may carry, they usually have a manual transmission to allow the driver to have as much control as possible. A special driver's license is required to operate a semi-trailer in most countries.

The cargo trailer is hooked to a horseshoe-shaped coupling device called a "fifth wheel" at the rear of the towing engine that allows easy hook up and release. The trailer cannot move by itself because it only has wheels at the rear end, hence the name semi-trailer: it only carries half its own weight. The vehicle has a tendency to fold at the pivot point between the semi and the trailer when braking hard at high speeds. Such a truck accident is appropriately called a jack-knife, or jack-knifing.

Modern day semi-trailer trucks often operate as a part of an international transport infrastructure to support containerized cargo shipment. Some flat bed train cars are modified to hold the cargo trailer with wheels and all. This is called "piggy-back" in North America. The system allows the cargo to switch from the highway to railway or vice versa with ease.

The large trailers pulled by a semi come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, containerlifts and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins (thick unthreaded bolts). The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws.

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Road train, Australia. This one travels from the port city of Darwin to the tourist centre of Alice Springs.

On some interstate highways in the US, long-haul semi-trailer trucks can tow another full trailer at the end, which makes the vehicle look like a two-car small train. Some of the second cars are full trailers with wheels on both ends, while others are just regular semi-trailer cars hooked to the standard coupling device on another set of wheels in tow (sometimes referred to as a "dolly"). Some states further allow a third trailer to be added to the vehicle, against the objections of some car drivers who must share the highways with these longer trucks.

In Australia, semi-trailers with more than one trailer are known as road trains. In certain areas "B-doubles" are permitted. These include a modified trailer with a turntable at the rear to allow a second trailer to be tightly coupled to the rig without the extra cost and handling problems of a dolly.

See also

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