The fifth wheel coupling provides the link between a semi-trailer

Semi-trailer

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer...

 and the towing truck

Truck

A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

, tractor unit

Tractor unit

A tractor unit, prime mover , road tractor, or traction unit is a heavy-duty commercial vehicle within the large goods vehicle category, usually with a large displacement diesel engine, and several axles. The tractor unit serves as a method of moving trailers...

, leading trailer or dolly

Dolly (trailer)

A dolly is a small trailer that can be coupled to a truck or trailer so as to support a semi-trailer. The dolly is equipped with a fifth wheel to which the semi-trailer is coupled...

. Some recreational vehicle

Recreational vehicle

Recreational vehicle or RV is, in North America, the usual term for a Motor vehicle or trailer equipped with living space and amenities found in a home.-Features:...

s use a fifth wheel configuration, requiring the coupling

Coupling

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...

 to be installed in the bed of a pickup truck

Pickup truck

A pickup truck is a light motor vehicle with an open-top rear cargo area .-Definition:...

 as a towing vehicle. The coupling consists of a coupling pin (or kingpin) on the front of the semi-trailer, and a horseshoe

Horseshoe

A horseshoe, is a fabricated product, normally made of metal, although sometimes made partially or wholly of modern synthetic materials, designed to protect a horse's hoof from wear and tear. Shoes are attached on the palmar surface of the hooves, usually nailed through the insensitive hoof wall...

-shaped coupling device called a fifth wheel on the rear of the towing vehicle. The surface of the semi-trailer 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

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, especially if it's a rotating ball-race-bearing type.

Origin

The term 'fifth wheel' comes from a similar coupling used on four-wheel horse-drawn carriages and wagon

Wagon

A wagon is a heavy four-wheeled vehicle pulled by draught animals; it was formerly often called a wain, and if low and sideless may be called a dray, trolley or float....

s. 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. Today's fifth wheels allow the trailers to slide into the fifth wheel and lock into it, and are a very reliable unit when maintained and serviced properly. The engagement of the king pin into the fifth wheel locking mechanism is the only means of connection between tractor and trailer, no other device or safety mechanism is used. Couplers and pintle hooks will use safety chains in the event of a trailer separation while going down the road. Trailer to trailer connection can also be made by using fifth wheels.

The invention of the fifth wheel is often credited to U.S. inventor Charles H. Martin of the Martin Rocking Fifth Wheel Co. who invented the device in 1911. The earliest documented patent of a fifth wheel is in patent number 2,053,812 issued to Charles E. Bradshaw of Wellville, VA filed March 18, 1936 and granted September 8, 1936. One third of the patent was assigned to Charles Martin, also of Wellville, VA. With that being said, Alaskan Dick Rubbins' maintains he invented the device. Engaged in a long time feud with Martin, Rubbins also claims the Martin not only stole his invention, but his wife. Always eccentric, Rubbins passed in 1996 and requested a controversial epitaph in his last will and testament. His tombstone reads: "Here lies Dick Rubbins, Inventor of the fifth wheel and holder of 17 other patents. I requested to buried face down, so that Charles Martin can kiss my royal Irish ass."