<P> The fifth - wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly . Some camper trailers use a fifth - wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle, and "fifth wheel" is therefore sometimes used as a synonym for such campers in North America . The coupling consists of a kingpin, a 2 - or - 3 ⁄ - inch - diameter (50.8 or 88.9 mm) steel pin on the front of the semi-trailer, and a horseshoe - shaped coupling device called a fifth wheel on the rear of the towing vehicle . The surface of the semi-trailer (with the kingpin at the center) rotates against the surface of the fixed fifth wheel, which does not rotate . To reduce friction, grease is applied to the surface of the fifth wheel . The configuration is sometimes called a turn - table in Australia and New Zealand, especially if it is a rotating ball - race - bearing type . The advantage of this coupling is improved towing stability . </P> <P> The term fifth wheel comes from a similar coupling used on four - wheel horse - drawn carriages and wagons . The device allowed the front axle assembly to pivot in the horizontal plane, to facilitate turning . Basically a wheel was placed on the rear frame section of the truck, which back then only had four wheels; this wheel that was placed on the frame was the "fifth wheel", hence the name . The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the fifth wheel . </P> <P> Fifth wheels were originally not a complete circle and were hand forged . When mass production of buggy parts began in the early 19th century, fifth wheels were among the first products to be made . There were a number of patents awarded for fifth - wheel design . Edward and Charles Everett, Quincy, Illinois patented a type of fifth wheel in 1850, followed by Gutches' metallic head block and fifth wheel in 1870 and Wilcox fifth wheel in 1905 . </P>

Where did the term 5th wheel come from
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