Why Are Semi-Trucks Called 18-Wheelers? (And How Many Wheels They Really Have)

A semi-truck is a crucial part of supply chains across the world because of its ability to move goods over long distances. Designed to carry large, heavy, or large quantities of cargo, semi-trucks are huge machines. Their size is essential to transport loads inexpensively and efficiently, and require powerful tractor trucks to pull the trailers that contain the cargo. Semi-trucks are sometimes called other names, including big rig, semis, or 18-wheelers, and for the last moniker, this is because of the combined number of wheels of both the tractor and the trailer. 

These two parts of a semi-truck each have their own set of wheels. The tractor, or power unit for the whole rig, has ten wheels, which are divided between the front steering axle and the pair of rear drive axles, which have two and four wheels respectively. Trailers can have between two and three axles. The most common type is the double axle, which has four wheels attached to each, for a total of eight wheels for the trailer. 

Semi-trucks have many wheels for several reasons, with weight distribution as the foremost. On semi-trucks, the weight is split between the three axles. This weight distribution is further emphasized for the trailer when carrying loads, with an industry guideline of 60% of the weight in front of the trailer axle and 40% behind it. In essence, proper weight distribution is essential for on-road stability, which impacts the performance of semi-truck operations. Additionally, there are federal regulations that govern weight limits and the load distances per axle that semi-trucks must comply with to avoid hefty fines and penalties.

There are other important reasons why a semi-truck needs several wheels. One big reason is to avoid damaging road surfaces, as the weight of the truck is spread across all wheels instead of just a single point. Also, having more wheels and axles allows the semi-truck to be more firmly anchored to the road, which gives the driver better control, a critical factor for a vehicle of its size and weight. Finally, each tire on the semi-truck is federally mandated to have independent brakes, which means more tires mean more stopping power for the truck.

Of course, the number of wheels on a semi-truck also has its downsides, including increased costs from repairs and replacement and higher fuel consumption. Today, the average mpg of a semi-truck is around 7.1 mpg, and this is substantially impacted by its total weight, including the wheels. For a semi-truck, its tires alone weigh 110 pounds each, while the average steel wheel is 78 pounds, totaling 738 pounds for just the eight wheels, which is heavier than a Harley Davidson Dyna. In addition, the greater number of tires causes greater rolling resistance, which also negatively impacts fuel consumption.

While they may all look the same, it is important to note that a semi-truck has three different tire types. The first is the steering tire, which is used in the front of the tractor for steering and handling. The drive tires give traction and power the tractor, and the trailer tires are specifically designed for load handling.