Truck and Trailer Equipment – Attaching a Semi Trailer to a Road Tractor

A common question asked by people outside of the trucking industry is: “How is a road tractor connected to a semi-trailer?” The attachment between these two pieces of transportation equipment is made by two connecting devices. One is the “fifth wheel” that is mounted on the rear frame of the road tractor. The other is called the “kingpin”, which is located on the front end of the truck trailer. Here is a brief explanation.

What is a Fifth-Wheel?

Originally, a fifth wheel was a steering mechanism resembling a wheel that enabled the front axle of a horse-drawn wagon to rotate when making turns. Around 1910, John C. Endebrock invented a device for coupling trailers to motor vehicles. In its initial use, trailers were towed by Ford Model “T” passenger cars. This coupling scheme required three men to hook to and unhook the trailer from the automobile. In 1918, Endebrock designed a new coupling devise that allowed a single driver to easily connect the tractor and the trailer. The new fifth wheel design used a jaw with a spring locking device mounted on the fifth wheel plate that was attached to the tractor frame.

Today, the fifth wheel serves as a coupling device that attaches a road tractor to a semi-trailer. A fifth wheel is a heavy metal disc with a “V” shaped slot. It is attached horizontally to the chassis above the rear axles of the road tractor. The “V” slot is located at the rear and it contains a locking device. The fifth wheel mechanism used to connect tractors and trailers today is very similar to Endebrock’s design from the early twentieth century.

What is a Kingpin?

A kingpin is a heavy metal cylindrical pin that is located underneath the front end of the trailer. The kingpin is the mechanism on the trailer that locks it to a road tractor. It is typically positioned between 18″ to 48″ from the trailer nose. A greased metal grid plate surrounds the kingpin. This grid plate allows the fifth wheel to slide underneath the trailer. When the tractor is backed into the nose of a trailer, the fifth wheel pivots and slides underneath the trailer’s grid plate. The fifth wheel then locks onto the trailer’s king pin.

When coupled to the kingpin, the fifth wheel enables the semi trailer to rotate at the point where the fifth wheel and kingpin are joined. The rotating attachment allows the tractor-trailer combination to make turns and provides stability and maneuverability on the road. When the load is delivered, the driver disconnects the tractor from the trailer by rolling down the trailer’s dolly legs. He then pulls a lever to disengage the fifth wheel locking mechanism and drives the tractor away from the trailer.

For more information about semi-trailers, see American Trailer Exchange

Greg Pratt has a 32 year career in semi-trailer sales and leasing. He founded American Trailer Exchange in 1992. Greg is often consulted for equipment valuations and market trends. Greg has an M.B.A degree from Roosevelt University-Chicago, and a B.S. degree from Ball State University.

Greg Pratt
American Trailer Exchange, Inc.

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