The fifth-wheel or circle of a carriage consists of two horizontal metal circles or sections of a circle 9or one of metal and one of wood), placed between the upper and lower portions of the front gearing, and generally connected with these parts by a king-bolt; these plates revolving one upon the other , allow the axle to turn laterally, and thus change the line of motion. Some fifth-wheels consist of two full circles, but there are alco “half-circle,” :three quarter,”, “D,” and “elliptical” fifth wheels.
- Appletons Clclopedia of Applied Mechanics: A Dictionary of Mechanical Engineering and the Mechanical Arts, New York, D. Appleton and Company, 1881