The term big rig is a colloquialism for a semi-truck or a tractor-trailer. A big rig is a two-part truck that consists of both a tractor and a trailer. The driver sits in the tractor or cab and a trailer is hooked to the tractor. Freight is transported in the trailer portion.

In the United States, big rig tractors normally have three axles: the front or steer axle and two rear or drive axles. The steer axle of a big rig 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 for the most common type of big rig tractor.

Commonly, a big rig trailer will have two tandem axles that are located at the rear portion of the trailer. Each axle has double wheels on both sides. Thus, it is typical for big rig trailers to have eight wheels in all. As a 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. The fifth wheel 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, the big rig 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 big rig accident is referred to as a jack-knife.

Big rigs have many other names. They are called 18-wheelers, semis, semi-trailer trucks, and tractor-trailers 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 the big rig as a semi. 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. These sleeper units 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 interstate travel is required.