The term big rig is a colloquialism for a semi-truck or a tractor-trailer, a two-part truck that consists of both a tractor, the truck portion with the engine, and a trailer. The driver sits in the tractor or cab and a trailer, usually filled with freight, is hooked to the tractor.
In the United States, tractor-trailers normally have three axles: the front or steer axle and two rear or drive axles. The steer axle usually has two wheels, while each drive axle has a pair of double wheels on each side of the axle. This translates into 10 wheels in total for the most common type, with eight wheels on the trailer. As the trailer only has wheels at its rear, it is not capable of moving on its own.
The cargo trailer of a big rig is hooked onto the tractor via a device called a fifth wheel, which is shaped like a horseshoe and is located at the rear of the tractor. It allows for convenient attachment and release of the trailer.
Due to the way the trailer’s wheels are positioned and the fact that it only carries half its weight, a semi-truck is subject to folding at the point of connection between the trailer and the tractor. Typically, such folding occurs when a driver brakes hard while driving at a high speed. This common accident is referred to as a jack-knife.
Big rigs have many other names, including 18-wheelers and semi-trailer trucks in the United States. In the United Kingdom, they are more likely to be called articulated lorries, artics, or truck and trailers. Australians and Canadians know them as semis. Exceptionally large articulated lorries are called juggernauts in the United Kingdom.
Some big rig tractors are designed with sleeping units behind the driver’s cab. They vary widely in design and can be as simple as a small bedding section or as luxurious as a tiny, but fully equipped, studio. Generally, such cabs are intended for use when long distance travel is required.
