Publish Time: 2025-07-09 Origin: Site
The fifth wheel is the coupling device that connects the tractor to the trailer. It consists of a large plate with a locking mechanism that secures the trailer's kingpin. This connection allows the trailer to pivot smoothly while distributing the freight's weight between the tractor and trailer axles.
Over time or due to different loads, the fifth wheel may develop slack or improper height alignment. This can cause trailer sway, uneven tire wear, or excessive stress on the truck's drivetrain. Adjusting a fifth wheel ensures a tight, secure connection and proper weight distribution, which is essential for safety and compliance with road regulations.
You should consider fifth wheel adjustment if you notice:
Excessive play or looseness between the tractor and trailer when starting or stopping
Trailer fishtailing or instability during turns
Uneven or excessive wear on tires or axles
Difficulty in coupling or uncoupling the trailer
After changing trailers with different kingpin heights or configurations
The first step when adjusting the fifth wheel is to remove the weight from the fifth wheel connection. This is usually done by lowering the trailer's landing gear to support the trailer's weight. Doing this prevents the fifth wheel from binding or jamming during adjustment and protects the mechanism from damage.
Additionally, you should:
Set the trailer brakes and chock the wheels for safety
Slightly deflate the tractor's air suspension to ease the sliding or movement of the fifth wheel if it is a sliding model
Ensure the ground is level and firm before starting any adjustment
Many modern semi trucks have a sliding fifth wheel designed to move forward or backward to adjust the weight distribution between the tractor's steer and drive axles for safe and efficient towing. Here's how to perform a 5th wheel adjustment on a sliding model:
Lower the trailer landing gear to take the weight off the fifth wheel.
Release the fifth wheel slider lock from inside the cab, usually by flipping a switch or pulling a lever.
Verify the slider pins are fully disengaged by exiting the cab and visually inspecting the locking mechanism.
Mark the current position of the fifth wheel and the target position on the frame or ground for reference.
Gently move the tractor forward or backward to slide the fifth wheel to the desired position. Move slowly to avoid damage.
Re-engage the slider locks once the fifth wheel is in position. Rock the truck slightly to ensure the locks are fully seated.
Check the position and lock security before driving.
This adjustment can shift weight from the drive axles to the steer axle or vice versa, helping you comply with axle weight limits and improve handling. For example, moving the fifth wheel forward by 8 inches can transfer about 800 pounds from the drives to the steers.
If your trailer connection feels loose or you notice play between the king pin and the fifth wheel, you may need to tighten the locking jaws. The jaws grip the king pin and keep the trailer securely attached. Here’s a general guide on how to adjust fifth wheel jaws:
Locate the adjustment bolt or screw on the side of the fifth wheel. For JOST 5th wheel adjustment, this is typically a lock bar adjustment screw.
Loosen the jam nut that secures the adjustment bolt.
Turn the adjustment bolt counterclockwise to tighten the jaws and reduce play. Turn slowly, about a quarter to half turn at a time.
After adjustment, tighten the jam nut to lock the setting.
Test the locking mechanism by coupling and uncoupling the trailer to ensure it holds securely without excessive tightness.
Note that different manufacturers may have specific procedures, so consult your fifth wheel’s manual or manufacturer instructions. The JOST fifth wheel adjustment tutorial videos provide detailed guidance for their models.
Always disconnect the trailer or unload weight from the fifth wheel before making height or jaw adjustments.
Use wheel chocks and set the trailer brakes to prevent accidental movement.
Lubricate the fifth wheel plate and locking components regularly to avoid sticking or wear.
Avoid coupling or uncoupling at an angle to prevent damage to the kingpin or fifth wheel.
Take a trailer coupling course or training to learn proper adjustment and coupling techniques.
Regularly inspect the fifth wheel for cracks, excessive wear, or damaged parts. Replace worn components promptly.
The height of the fifth wheel relative to the trailer’s skid plate is important. If the fifth wheel is too high or too low, it can cause the trailer deck to slope, leading to:
Increased risk of the kingpin jumping the fifth wheel locking mechanism
Uneven weight distribution, causing axle overload or instability
Damage to the tractor or trailer components
Most standard fifth wheels are set around 1250mm from the ground, but this can vary depending on the trailer and load. Adjusting the height usually involves lowering or raising the trailer landing gear and sometimes using an adjustable fifth wheel or slider mechanism.
Be aware of maximum legal height limits (often around 4.3 meters) to avoid permits or fines.
Knowing how to adjust a fifth wheel on your semi truck is essential for safe, efficient, and legal tractor-trailer operation. The process involves:
Removing weight from the fifth wheel by lowering the trailer landing gear
Sliding the fifth wheel forward or backward to adjust weight distribution
Tightening the locking jaws using the adjustment bolt, especially for brands like JOST
Performing safety checks and lubrication regularly
By following these steps, you can reduce trailer sway, improve load handling, and extend the life of your equipment. For specific models, always consult your manufacturer’s instructions or seek professional guidance.