Semi trucks

A semi-trailer truck (more commonly semi truck or simply "semi") is the combination of a tractor unit and one or more semi-trailers to carry freight. A semi-trailer attaches to the tractor with a fifth wheel hitch, with much of its weight borne by the tractor. The result is that both the tractor and semi-trailer will have a distinctly different design than a rigid truck and trailer.

It is variously known as a transport in Canada; semi or single in Australia and New Zealand; semi, tractor-trailer, big rig, or eighteen-wheeler in the United States; and articulated lorry, abbreviated artic, in the United Kingdom, Ireland and New Zealand.

In North America, the combination vehicles made up of a powered truck and one or more semitrailers are known as "semis", "semitrailers", "tractor-trailers", "big rigs", "semi trucks", "eighteen-wheelers", or "semi-tractor trailers".

The tractor unit typically has two or three axles; those built for hauling heavy-duty commercial-construction machinery may have as many as five, some often being lift axles.

The most common tractor-cab layout has a forward engine, one steering axle, and two drive axles. The fifth-wheel trailer coupling on most tractor trucks is movable fore and aft, to allow adjustment in the weight distribution over its rear axle(s).

Ubiquitous in Europe, but less common in North America since the 1990s, is the cabover engine configuration, where the driver sits next to, or over the engine. With changes in the US to the maximum length of the combined vehicle, the cabover was largely phased out of North American over-the-road (long-haul) service by 2007. Cabovers were difficult to service; for a long time the cab could not be lifted on its hinges to a full 90-degree forward tilt, severely limiting access to the front part of the engine.

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Wikipedia