Semi trucks, also known as 18-wheelers, use tires to support the weight of the rig and its cargo, distributing the load evenly promoting stability in case of tire failure. Typically, the configuration has 10 tires on the tractor section and eight on the trailer. Another 18-wheeler combination allowed is a tractor pulling two trailers, each with a single axle with four tires on each trailer. A ring-hitch coupling joins the first and second trailers.
The tractor of a semi truck combination has a front steering axle with two tires, while the rear axles have a set of tandem wheels with four tires on each. The tires sit directly beneath the fifth wheel coupling that connects to the trailer. It must support the tongue weight of the trailer, which includes part of the cargo.
The tires on the trailer are on the rear. Most rigs allow the rear axles to move forward and backward to help with the turning radius and weight distribution. In the United States, the steering axle on tractor may legally support 12,000 pounds, the drive axles on the tractor 34,000 pounds and the dual trailer axles 34,000 pounds. When a truck goes through the highway scales, the weight of the entire load is taken as well as the individual axle loads. Having dual tires on each side of an axle makes a semi truck safer to drive. If one tire blows, there is still one tire left to support the load.