A fifth wheel is a part that almost anyone in industrial trucking is familiar with. We know it works to connect the semi truck tractor and trailer, and if you’ve been in the business for a while, you might even be familiar with a few of the key features that are nice to have in top-of-the-line models.

But when there are so many options to choose from on the market today, it can be helpful to know what you truly need to have and what options are add-ons. Since all fifth wheels are manufactured a little differently, we’re going to dive into the 5 essential parts of a fifth wheel. This should give you the information you need to find the best fifth wheel for your unique application.

01. Top Plate

The top plate is the cast part of the fifth wheel. It usually looks like a large horseshoe shape. The top plate secures coupling components and vertical trailer or cargo load. It’s the largest component of a fifth wheel.

02. Release Handle

Not all fifth wheels have this type of handle — the release handle (shown blue in the image above) is a unique part specific to JOST fifth wheels. In a JOST fifth wheel, the release handle makes it easy for operators to couple and uncouple a fifth wheel while ensuring ultimate safety.

The release handle also provides operators with secondary locks and visually indicates that the coupling is secure. While not all fifth wheels feature a release handle, JOST has prioritized this part for the safety and security of all our customers. When operators can visually confirm that tractor-trailer coupling is successful, there’s no guesswork, and you know you’re hauling safely.

03. Lever

The lever is a relatively small component of a fifth wheel, but that does not make it unessential. The lever links the release handle to the locking bar. Together, all three components work to slide the locking bar across the opening of the cushion ring, supporting and securing the lock jaw and ensuring the tractor and trailer are appropriately coupled for transit.

04. Lock Jaw

The lock jaw is the coupling mechanism that locks around the kingpin to hold it in place. In the above image, you can identify the lock jaw by it’s “C” shape. This shape allows the jaw to fit snugly around the kingpin. It is connected to the lever and release handle, and is the primary securing mechanism holding the tractor and trailer together.

05. Lock Bar

A fifth wheel’s lock bar secures the lock jaw. When engaged, the lock bar slides across the opening of the cushion ring, under the lock jaw, and ensures the trailer kingpin is held firmly in the fifth wheel and is secure for transit. The lock bar provides superior structure to support trailer drawbar load.

Are these the only components of a fifth wheel?

While these five components are essential to the safe functionality of any fifth wheel, they are not the only components. For example, JOST fifth wheels also offer a range of features to ease operation and extend product life cycle, including:

• Adjusting Screw. This allows the locking bar to maintain a snug coupling even as the lock jaw and cushion ring wear.
• Grease Fitting. This delivers grease directly to the kingpin and lock jaw contact point to ensure that all parts are properly lubricated to extend the life of both products.
• Cushion Ring Insert. This works to protect the kingpin and top plate casting from wear and damage.

There are a number of additional components featured on JOST fifth wheels, but the five mentioned above are essential to the functionality of every fifth wheel. At its core, a fifth wheel must have:

• A top plate for the trailer to rest on
• A securing mechanism to hold and secure the kingpin
• An operational mechanism that allows the driver to couple and uncouple tractor and trailer

With a better understanding of each of these components and their importance to the safety of your daily operations, you’ll be able to select the ideal fifth wheel product for your specific application’s requirements.

If safety and durability are priorities for your fleet, JOST fifth wheel products deliver. Check out our parts online, or get in touch with our team to find a dealer near you.

JOST International has been manufacturing fifth wheel coupling devices since 1956. In fact, we submitted the patent for the first-ever steel cast fifth wheel coupling. As a leading manufacturer of fifth wheels, we want to make sure that anyone in the industry, new or experienced, has access to helpful, informative content about all of our components. With that goal in mind, we’re tackling a popular question from those new to the semi-truck and industrial transportation industry: “how does a fifth wheel work on a semi?”.

We’ll cover everything you need to know, from what a fifth wheel is, to how it works and what kind of fifth wheel might be best for your application. Let’s start with the basics.

What is a Fifth Wheel?

Not to be confused with an RV trailer, a fifth wheel is an important component of a semi truck’s tractor. A fifth wheel is the plate above the tractor’s back wheels where a semi trailer’s kingpin hooks to the tractor. The fifth wheel is what allows the tractor to turn with the trailer still attached. A fifth wheel is also designed to spread the trailer’s freight load evenly across the tractor and trailer so that the operator can safely haul, no matter where they’re headed.

How Does a Fifth Wheel Work on a Semi?

As mentioned, a fifth wheel is a key connection point between a semi-truck tractor and trailer.

To understand how this works, it’s important to understand the key components.

First, you have a kingpin, which is a metal component that protrudes from the trailer.

Then, you have a fifth wheel, where the kingpin engages. A fifth wheel is a flat, non-rotating horseshoe-shaped coupling device that is installed on the semi-truck tractor.

The kingpin on the trailer installs into the fifth wheel on the tractor. The semi trailer’s surface, with the kingpin at the center, rotates against the upward-facing fifth wheel. Grease is applied to reduce the friction of these two components rubbing together and works to extend the life of both the kingpin and the fifth wheel.

What is the Purpose of a Fifth Wheel?

A fifth wheel has two key purposes:

1. A fifth wheel connects the semi-truck tractor and trailer, while allowing both to articulate, move around, and steer comfortably. Without a fifth wheel, it would be almost impossible for a semi-truck to make a turn.
2. A fifth wheel evenly distributes weight between a semi-tractor and trailer.

While the first purpose is fairly self-explanatory, the second can be a bit more complicated. Since every load is a little different, fifth wheels must accommodate to ensure that semi-truck weight is balanced appropriately. To do so, there are two key types of fifth wheels, fixed and sliding.

Fixed and Sliding Fifth Wheels: What’s the Difference?

Fixed and sliding fifth wheels perform the same function, but they do it a little differently.

A fixed fifth wheel stays in exactly the same place on a semi-truck tractor. It is “fixed” on the vehicle and is typically used for rigs that are hauling about the same size load, no matter what. Because the load is the same for every trip, there’s no need to adjust where the fifth wheel sits to accommodate different types of loads.

A sliding fifth wheel is designed to support a variety of load weights. They can slide backward and forward to facilitate safer hauling. As the operator slides the fifth wheel forward or backward, they can better distribute the weight of lighter or heavier loads.

In general, if you or your operators are hauling about the same size load for every run, a fixed fifth wheel is sufficient. But, if you’re an owner/operator, or if your team often deals with varied load types, then a sliding fifth wheel is likely the better option.

Looking for a New Fifth Wheel or a Fifth Wheel Replacement?

JOST International has you covered. As an industry leader in fifth-wheel innovation, we’re proud to supply the safest fifth wheels on the market, all of which feature our original locking mechanism for safe, simple use. From lightweight options to reinforced fifth wheel solutions built to stand the tests of time, get in touch with our team to find a dealer near you.