What is a Big Rig?

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  • Written By: N. Madison
  • Edited By: O. Wallace
  • Last Modified Date: 17 October 2019
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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.


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Post 5

how many flat bed eighteen wheelers are on the road in the U.S.

Post 4

Trucks and trailers can last a very long time. The determining factor mechanically is primarily the engine, which can last well over a million miles; with regular maintenance and periodic overhauls, 2 or 3 million miles is not unheard of.

However, for insurance and political (emissions) reasons, most fleets run newer trucks. A typical guideline is to run the truck for 500,000 miles (the length of a typical drivetrain warranty on a new truck) and then trade it in.

That's not the end of the truck's life, however. It will typically be sold to a new owner to be used in regional or local transport, it may have its sleeper removed and be converted to a dump truck, or it may be exported for use in countries with more lax environmental and safety standards. For example, older US trucks are frequently found throughout the Caribbean and Latin America.

Post 3

how long does a tractor-trailer last in service?

Post 2

To answer the above question about dimension/weight regulations:

WIDTH: The Federal law mandates that every state allow a maximum of 102" in. If the state doesn't permit up to 102" in. wide vehicles to travel on all interstates, & some designated highways, it could lose Federal highway funds.

LENGTH: The Federal law (sec.411) mandates that every state allow a minimum of 48'ft for single trailers, 28'ft each for double trailers. A maximum of 53'ft (57'ft in some states) for sing trailers. This law overrides the state laws establishing limits less than 48'ft for interstates, & some designated highways. No state is allowed to set tractor length limits.

WEIGHT: The Federal law (sec.133) mandates that every state must allow a maximum gross weight of 80,000#lbs(40T) or lose Federal funds.

Additionally, each set of axles have a maximum weight limit. Steers: 12,000. Drives: 34,000. Trailer: 34,000 (adding up to the 80,000 lbs).

HEIGHT: 14 ft. (Most van trailers are 13'6")

Post 1

Can you provide me with the height, width, weight, and length of a cab over with box trailer attached.

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