Why Semi-Trucks Have 18 Wheels: The Design Behind the Extra Wheels

When people picture a semi-truck, they often wonder about those “all those extra wheels.” The number of wheels is not the main concern; rather, it is the number of axles and how the load is distributed. This article will explore why semi-trucks typically have 18 wheels and the advantages of this design choice in the context of road and bridge loading standards.

Number of Axles: The Real Key

The design of a truck is more about the number of axles, not the number of wheels. The most common design for a semi-truck has four axles, with dual wheels on each end and a steer axle with a single wheel on each end. This combination, known as an 18-wheeler, is not strictly tied to the number of wheels but rather the number of axles. In fact, newer designs have started to replace dual wheels with a single larger wheel, offering different benefits.

Load Distribution: The Critical Factor

Loading for roads and bridges is based on the number of axles and the spacing between them. For instance, most axles are spaced at 60 inches. In the common 18-wheeler design:

There are 4 axles with dual wheels on each end. The steer axle has a single wheel on each end. The loading of the trailer is supported by 8 wheels. The tractor pulling the trailer has 2 steering wheels and 6 wheels that bear the cargo load.

Assuming a truck carrying 16,000 pounds of cargo plus trailer weight, each wheel would bear 1,000 pounds, and each axle would bear 4,000 pounds. This is in contrast to a scenario where the entire load is placed on just 4 wheels, with each wheel bearing 4,000 pounds, and each axle bearing 8,000 pounds. The latter scenario would require much stronger components and place significantly greater pressure on the pavement.

Advantages of 18-Wheeler Design

Reduced Loads on Each Wheel: By distributing the load over more wheels, the stress on each wheel is significantly reduced. This results in better tire durability, lower tire pressure needs, and stronger wheel and axle components. Lower Weight Distribution: The reduction in pressure on each wheel also means less stress on the roadbed and structures like bridges. This makes it easier and less expensive to build and maintain roads and bridges, even with the same total load. Redundancy in Case of Failure: Having 4 wheels at each of the four load-bearing corners provides redundancy. In the event of a single tire failure, the truck can continue to operate and seek repair rather than stopping immediately.

Conclusion

The design of semi-trucks with 18 wheels is a result of engineering principles that prioritize load distribution and strength. While the number of wheels is a significant visual factor, the real design focus is on the number of axles and how the load is distributed to optimize performance and safety. This design plays a crucial role in ensuring that our roads and bridges can handle the heavy loads of modern transportation, and it’s why they are called 18-wheelers.