The fifth wheel is the heavy steel plate mounted on the back of a tractor frame. It connects the truck to the trailer by locking around the trailer's kingpin. The plate is greased heavily so the trailer can pivot smoothly during tight turns. A release handle on the side opens the locking jaws to unhook the trailer.
Keeping the fifth wheel lubricated is critical. If the grease wears off, the steel plates grind together, causing hard steering and dangerous binding on curves. Some trucks use plastic lube plates instead of grease to keep the connection slick. Drivers must verify that the locking jaws are closed completely around the kingpin shank before driving off to prevent the trailer from detaching on the road.