Fifth Wheel

What is a Fifth Wheel

The fifth wheel refers to a horseshoe-shaped coupling device at the rear of the towing vehicle. It works in conjunction with the kingpin, a vertical steel pin on the semi-trailer, to connect the trailer with the tractor. As the truck turns, the semi-trailer’s surface rotates against the fixed fifth wheel, facilitating smooth movement​​.
Learn about Pintle Hitch here.
Learn more about Semi Trailers here.

Fixed or Sliding Fifth Wheel The Difference

Fixed Fifth Wheel

Definition
A fixed fifth wheel is mounted to the frame of the tractor and cannot move forward or backward along the frame rails​​.

Suitability
Ideal for hauling similar trailers with consistent loads. It’s beneficial when the weight distribution across the rig’s axles does not change significantly from load to load, as seen in bulk or tank carriers​​.

Stability
Fixed fifth wheels provide more stability as they offer a stationary connection between the tow vehicle and the trailer. They are preferable for higher speed towing or on uneven terrain​​.

Maintenance
Generally, they require less maintenance and potential wear due to fewer moving parts​​.

Sliding Fifth Wheel

Definition
A sliding fifth wheel is mounted to allow movement forward and backward along the frame rails​​.

Suitability
Suitable for hauling different trailers with varying weights and weight distributions. The sliding feature is useful for managing variable loads where the weight distribution across the rig’s axles changes significantly from load to load​​.

Maneuverability
They offer increased maneuverability, especially useful in tight spaces like parking lots or when making sharp turns. The ability to adjust the position of the fifth wheel helps in avoiding collisions between the cab and the trailer​​​​.

Weight Distribution
Sliding fifth wheels allow for adjusting the tongue weight on the tractor axles. This adjustment is crucial for maintaining the proper weight distribution within the axle weight limits and ensuring safe and legal transportation​​.

Factors in Choosing Between Fixed and Sliding Fifth Wheels

Towing Capacity and Compatibility
Consider the weight capacity of the hitch and ensure it is compatible with your vehicle​​.

Ease of Use
Sliding hitches often feature user-friendly mechanisms that make hitching and unhitching more efficient​​.

Cost and Installation
Assess the cost and installation requirements, including whether professional installation is needed​​.

Historical Development of the Fifth Wheel

The concept of the fifth wheel dates back to four-wheel horse-drawn carriages and wagons. The device enabled the front axle assembly to pivot for easier turning. The term ‘fifth wheel’ originated from an additional wheel placed on the rear frame of four-wheeled trucks, making it the ‘fifth wheel’ to facilitate turning. Notably, the invention of the fifth wheel for motorized trucks is credited to Charles H. Martin in 1915, revolutionizing the ease and safety of attaching large trailers to tractors​​.

The Importance of the Fifth Wheel in Modern Trucking

Today, the fifth wheel is integral to the trucking industry. It allows trailers to easily slide into place and lock securely, forming the only connection between the tractor and the trailer. This connection is crucial for the modern distribution system, enabling drivers to switch trailers efficiently and continue their journeys with minimal downtime​​.

Fifth Wheel Maintenance

Proper maintenance of the fifth wheel is vital for the safety and efficiency of trucking operations. It’s recommended to perform maintenance every three months or every 30,000 miles. This includes regular lubrication to reduce friction and thorough inspections for any signs of wear or damage. Particular attention should be paid before winter, as accumulated grease can pick up debris and freeze, potentially impairing the locking mechanism​​​​​​.

Maintenance Checklist

Cleaning and Degreasing
Regularly clean and degrease the fifth wheel to remove dirt, debris, and old grease, especially around the lock jaw, throat, and pivot points.

Inspection
Check for cracks, broken welds, and damaged or missing components. Ensure that the bracket liner thickness is adequate and replace if necessary.

Lubrication
Use 90-weight gear oil or lithium EP grease to ensure smooth operation. Pay special attention to lubricating the kingpin lock and all moving parts.

Seasonal Servicing
Service the fifth wheel in both winter and spring to address issues related to weather conditions and road treatments like deicing chemicals​​.

Learn more about Rigs and Trailers here.

External Links

Yanker Tool – Semi Truck 5th Wheel Fixed Vs Sliding – Differences And Applications – click here.