Have you ever heard the term ‘fifth wheel’ being used in trucking? It can be a confusing term for many people, especially if you’re new to trucking.
We’re giving you the complete guide on the ‘fifth wheel’ below, from what it is to why it’s important and how you can adjust it.
Read on to find out more.
Here’s a Short Summary on How to Adjust a Fifth Wheel on a Semi Truck:
The fifth wheel on a semi-truck rarely needs to be adjusted. However, if there is too much play, you may need to tighten the jaws by loosening the adjustment bolt. The fifth wheel can be moved forward or backward. However, it should always be in front of the centerline of the two axles.
What is the Fifth Wheel on a Semi Truck?
The fifth wheel is the unit that couples the semi-truck tractor to the trailer. They consist of a kingpin and a horseshoe-shaped coupling device.
They are found on the back of the semi-truck tractor. The fifth wheel allows for the trailer to turn in a more stable manner with the tractor.
This is one of the reasons why a trailer hitch found other vehicles will not work on a semi-truck. The fifth wheel coupling is also much stronger and more durable, making it better for towing heavy loads.
You might be wondering why it’s called a fifth wheel. The interesting name has its origins traced back to horse-drawn carriages in the 1850s.
Back then, a similar set-up would be used to connect the carriages. However, in those days it would also have a wheel that would allow the carriage to pivot around it.
Carriages only had four wheels and adding this central pivot point was the addition’ fifth wheel’. Therefore, the name ‘fifth wheel’ was given to the system, which has stuck around ever since.
When is a Fifth Wheel on a Semi Used?
The fifth wheel on a semi-truck is used to tow heavy trailers. The fifth wheel is the best coupling device to haul heavy cargo around with a semi-truck since it is stronger and more durable.
The fifth wheel can also be used to tow other items that have the same coupling, for example, another semi-truck, heavy machinery, or RV campers.
Can You Adjust the Fifth Wheel on a Semi Truck?
The fifth wheel on a semi-truck can be adjusted. However, each manufacturer designs their fifth wheel differently, so the adjustments vary between different makes and models.
Some only require you to turn a bolt, whereas others require you to use a specialized tool to adjust the fifth wheel.
In the majority of cases, you can take it to a workshop to have it adjusted. However, the fifth wheel does not need to be adjusted often.
The fifth wheel is adjusted when there is a significantly heavier load that needs to be towed. Adjusting the fifth wheel will change the weight distribution.
Moving the fifth wheel forward will shift the weight onto the steer axle at the front of the semi-truck. Moving the fifth wheel backward will shift the weight onto the driven axles at the back of the semi-truck.
However, something that is very important to remember when it comes to the fifth wheel is to grease it regularly.
Since the fifth wheel consists of metal against metal, it can begin to grind instead of sliding smoothly if there is no grease regularly applied to the tops of the jaws.
What Tools Can You Use to Adjust the Fifth Wheel?
There are a few tools you can use to adjust the fifth wheel of your semi-truck. Each semi-truck manufacturer is unique, so not all tools work with each semi-truck.
It is best to check your manual to see which specific tools might be needed to adjust the fifth wheel.
What is a Fifth Wheel Adjustment Tool?
A fifth wheel adjustment tool is required to adjust the fifth wheel on some semi-trucks that have Holland fifth wheel.
The tool helps to lock the fifth wheel in place, and it also hooks in between the jaws, which allows you to adjust the fifth wheel.
The tool is usually stowed away underneath the fifth wheel. Placing it underneath the fifth wheel acts as a locking pin.
What is a Fifth Wheel Adjustment Bolt?
Some other semi-trucks, especially older models, have a bolt that can be used to adjust the fifth wheel’s jaws. This can be done simply by turning the bolt.
Turning the bolt looser will tighten the fifth wheel jaws, and the opposite is also true. The bolt can normally be found at the front of the fifth wheel, but sometimes on the sides depending on the model of your semi-truck.
You should only adjust it one complete turn and test it after the turn to see if there is still some slack on the trailer.
Other Tools You Could Use to Adjust the Fifth Wheel
There are no other tools you can use to safely adjust the fifth wheel on a semi-truck. All semi-trucks will use either of the above mechanisms, or a slider to adjust the fifth wheel.
How do You Adjust a Fifth Wheel on a Semi Truck?
Adjusting the fifth wheel on a semi-truck is not always an easy task. It can sometimes be easier to send it in to a workshop and have them set it in the best possible position.
Where Should My Fifth Wheel Be Set?
Setting your fifth wheel in the correct position is crucial. Firstly, it helps to distribute the weight evenly across your semi-truck.
It will also help your semi-truck to steer properly. If the weight is shifted too far forward or too far back, you will have steering problems.
It should be the forward of the center between the axles. If your fifth wheel is behind the centreline of your two axles, it can become dangerous.
When the fifth wheel is too far back, it takes the weight off the steering axle, which takes away grip from the front wheels when you accelerate.
How Do You Adjust a Sliding 5th Wheel?
A sliding fifth wheel is a mechanism that puts the plate on rails. Pins are used to lock the plate in place, and these are controlled by a switch in the cabin.
To unlock the fifth wheel, you simply need to flick the switch. The plate can now slide forwards and backward as needed.
Always make sure that the pins have re-engaged properly as they can sometimes get caught in between the teeth and not slot incorrectly.
How to Adjust 5th Wheel Jaws?
Adjusting the jaws on your semi-trucks the fifth wheel depends on the type of semi-truck you have and the make and model of the semi-truck.
In most cases, there will be a bolt that you can turn to adjust the fifth wheel jaws. These work in the opposite way that you might think.
For example, if you loosen the bolt, the jaws will tighten. If you tighten the bolt, the jaws will become looser.
You can adjust the jaws to change the amount of slack on the trailer. Having the jaws tighter will give the kingpin less play, and therefore the trailer will have less slack.
How Do You Adjust a Fifth Wheel Pin Box?
The pin box will adjust the height of the fifth wheel on your semi-truck. If you want to lower the front of the fifth wheel, you’d need to raise the pin box and vice-versa.
You will want it to be set at the height that allows the trailer to be level and allows you at least 6 inches between the bottom of the trailer and the top of the fifth wheel plate.
When adjusting the height of a pin box, it is important to support the pin box when removing the bolts.
It’s recommended 2 or 3 other people give you a hand and make sure no one is beneath the pin box while raising or lowering it.
With the pin box supported, go ahead and unbolt it, raise or lower it as needed, and then put the bolts back in and tighten them down.
According to Lippert, manufacturers of pin boxes, you should use the following torque specs:
Torque 1/2″ bolts a minimum of 110 ft-lb.
Torque 5/8″ bolts a minimum of 160 ft-lb.
Torque 3/4″ bolts a minimum of 210 ft-lb.
How Much Play Can a Fifth Wheel Have?
There is normally a little play between the jaws and the kingpin. 3/8 inch to 1/2 inch is an average amount of play. This allows the trailer to turn smoothly while still keeping enough slack to effectively slow down and accelerate,
If there is any more play than this, you will notice it while driving as you will feel a disconnect between the tractor and the trailer.
In other words, If you being braking, the trailer will still have some momentum and ‘push’ the tractor forwards. You’ll feel this as a slight bump.
You may also notice it when you are accelerating. You might accelerate and feel the trailer being pulled along by the tractor.
When you have the correct amount of play on the fifth wheel, the tractor will be able to smoothly pull and stop the trailer without any kind of bumping or ‘chucking’.
How Do I Check My 5th Wheel Play?
The best way to test the play on your fifth wheel is to engage it by sliding a kingpin into the jaws and locking them in place.
Next, you can use a ZZ Class pin gauge to check the amount of play on the kingpin. You want to be looking at between .0400″ and .0800″.
This is the ideal amount of play that your kingpin should have when the fifth wheel jaws are locked in place.