![]() A wheel would be placed on the rear frame section of the truck, which at the time had only four wheels, making the additional wheel the "fifth wheel". The device allowed the front axle assembly to pivot in the horizontal plane, to facilitate turning. The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. Some camper trailers also use a fifth-wheel configuration, with the coupling installed in the bed of a pickup truck as a towing vehicle "fifth wheel" is therefore sometimes used as a synonym for such campers. The advantage of this type of coupling is towing stability. The configuration is sometimes called a turn-table in Australia and New Zealand, especially if it is a rotating ball-race-bearing type. To reduce friction, grease is applied to the surface of the fifth wheel. As the connected truck turns, the downward-facing surface of the semi-trailer (with the kingpin at the center) rotates against the upward-facing surface of the fixed fifth wheel, which does not rotate. The coupling consists of a kingpin, a 2-or- 3 + 1⁄ 2-inch-diameter (50.8 or 88.9 mm) vertical steel pin protruding from the bottom of the front of the semi-trailer, and a horseshoe-shaped coupling device called a fifth wheel on the rear of the towing vehicle. The fifth-wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. ![]() A fifth wheel, already coupled, side view ![]()
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