What is the fifth wheel on a Semi-trailer?

1. It is a popular expression meaning the steering wheel.
2. A fixture, shaped like a wheel, designed to lift the driver’s compartment during engine treatment.
3. Correct answer: A fixture (plate) that is designed to connect between the cab and the trailer.
4. It is a spare wheel to replace flat (punctured) tires.

WhyThe fifth wheel is the coupling plate on the tractor unit that the semi-trailer sits on. The steering wheel and spare tire have nothing to do with it.