fifth wheel
A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.
fifth wheel: the pivot pin that turns truck-trailer combinations
A fifth wheel is a coupling device mounted on the frame of a truck or tractor unit that locks onto the kingpin of a semi-trailer. The mechanism consists of a horseshoe-shaped plate that rotates on a ball or tapered bearing, allowing the trailer to articulate vertically and horizontally while remaining firmly connected. When a trailer is backed underneath, the kingpin (a heavy steel pin protruding from the trailer's upper coupler plate) slides into the horseshoe and is secured by a spring-loaded locking jaw and safety pin.
Fifth wheels are classified mainly by capacity and mounting style. Standard fifth wheels handle trailers up to 30,000 pounds; heavy-duty units rated for 45,000 pounds or more are common in long-haul trucking. Gooseneck fifth wheels sit lower on the truck bed for stability with certain cargo configurations, while conventional units mount higher on the tractor frame. Some models incorporate a sliding mechanism that allows fore-and-aft adjustment of the coupling point to distribute load weight between the truck's axles.
Operation and wear patterns
The pivot point experiences significant stress during cornering, braking, and loading transitions. Lubricating grease in the bearing cavity reduces friction, but uneven wear occurs if the kingpin is misaligned or the locking jaw is damaged. A worn fifth wheel produces excessive play in the coupling, leading to trailer sway, premature tire wear, and potential loss of control. A missing or bent safety pin creates a critical hazard, as the locking jaw alone cannot reliably hold the kingpin under severe jolting.
The term derives from the fifth wheel of a carriage, a non-load-bearing wheel used on horse-drawn wagons to assist turning. The modern trucking device performs an analogous function: enabling articulation without being part of the primary load path. Inspection and maintenance of the fifth wheel are federally mandated; regulations require checking for secure locking, proper lubrication, and absence of cracks in the plate or mounting brackets before each journey.