Based on the mobility of their supports, fifth wheels are classified into fixed, sliding, and lifting types. In a fixed fifth wheel, the left and right supports are bolted to the base plate, which is in turn secured to the tractor unit's chassis frame. The fixed type is the most widely used. Sliding fifth wheels come in various styles but share a similar structure to the fixed type; the key difference lies in the connection between the base plate and the chassis frame, which utilizes a multi-hole or toothed design. This allows the base plate to be repositioned longitudinally to accommodate different trailers for smooth coupling. Some sliding models feature an automatic pneumatic control system that uses air pressure to slide the unit back and forth along a track. Lifting fifth wheels are primarily used on tractors at port terminals; they can move vertically within a specific range to accommodate semi-trailers of varying heights.
They are also categorized by their degrees of freedom into single-degree-of-freedom and dual-degree-of-freedom types. A single-degree-of-freedom fifth wheel-also known as a single-axis type-allows only for longitudinal tilting (pitching) of approximately 8 degrees. This type offers high driving stability and is suitable for high-speed, lightly loaded semi-trailers with high centers of gravity operating on good road surfaces. Its disadvantage is that the inability to oscillate laterally subjects the vehicle frame to significant torsional stress. A dual-degree-of-freedom fifth wheel-or dual-axis type-allows for both lateral and longitudinal oscillation, helping to protect the frames of heavy-duty transport tractors operating across diverse road conditions. However, because dual-axis fifth wheels sit significantly higher than single-axis models, they offer poorer lateral stability during high-speed cornering; consequently, they are rarely used in standard highway transport.
