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.

Jost Fifth Wheel 2

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.