Choose The Best Freightliner Semi Truck Bumpers

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A semi truck, also known as a semi, tractor-trailer, or articulated truck or articulated lorry, is an articulated vehicle consisting of a towing engine and a semi-trailer that carries the freight. A semi-trailer does not trail completely behind the towing vehicle, but is attached at a point that is forward of the rear-most axle of the towing unit, so that some fraction of the weight of the trailer is carried by the rear axle of the prime mover. This arrangement requires both tractor and semi-trailer to be distinct in design from a rigid truck and trailer. Colloquial terms for semi-trailer truck include truck and trailer, transfer truck, 18 wheeler, semi, Mack truck, big rig, transport, lorry.

The cargo trailer is, by means of a nipple called the king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel or a turntable hitch at the rear of the towing engine that allows easy hook up and release. Semi trucks use air pressure, rather than hydraulic fluid, to actuate the brakes mainly due to the much larger braking forces required. This also allows for ease of coupling and uncoupling of trailers ...
... from the tractor unit, as well as reducing the potential for problems common to hydraulic systems, such as leakage or brake failure caused when overheated brake fluid vaporizes in the hydraulic lines. 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. However, all truck manufacturers now offer semi-automatic transmissions (manual gearboxes with automated gear change), as well as automatic transmissions. Older trucks often had twin gearshifts, usually with 4 or 5 ratios on main shift and 3 or 4 on the auxiliary shift. This example has 5 main ratios and 3 auxiliaries, thus a "5x3."

"Semi" truck transmissions usually provide at least nine or ten gear ratios, but possibly as many as eighteen. A large number of transmission ratios means the internal combustion engine itself can operate within a narrow range of speeds.

An electrical connection is made between the tractor and the trailer through a cable often referred to as a "pigtail." This cable is a bundle of wires in a single casing. Each wire controls one of the electrical circuits on the trailer, such as running lights, brake lights, turn signals, etc. A standard cable would break when the rig went around corners so it is coiled and retains these coils when not under tension. It is these coils that cause the cable to look like a pigtail.

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