What Is an 18 Wheel Tractor Trailer?

Have you ever seen or heard an 18-wheel tractor trailer? In fact, it is another name for semi-trailer. The 18 wheels include the number of tires for the tractor. On the semi-trailer, there are only 8 tires. This is the wheel arrangement on an 18-wheeler:


There are 10 tires on the tractor and 8 on the semi-trailer. Among them, 8 wheel hubs have two wheel rims, which is to better distribute the weight of the truck.

The meaning of Wheel

In fact, there has always been confusion about the definition of “wheel”, which has two different meanings. This has led to some people asking this question:

Why are semis called 18 wheelers when they have 10 wheels and 18 tires, and with more are only 10 tires?

1. Wheel Rim

This is the pneumatic tire connected to the cylindrical structure. There is only one tire on each wheel rim, no exceptions.


2. Wheel Hub

This is a structure in which the wheel rim or the wheel rim is attached with the stud. The wheel hub will rotate with the rim. Usually, the hub on the drive axle will rotate with the wheel rim; on a steering axle, it is the hub that steers which in turn steers the wheel rim. An axle must have a pair (that is, two) of wheel hubs, but the number of wheel rims on a wheel hub is not certain, it can be one or two.

Therefore, usually, the wheel in the 18 wheel tractor-trailer refers to the “wheel rim”, and the 10 wheels in the question refer to the “wheel hub”.


When does wheel refer to the wheel hub?

Usually when a driver introduces his driving configuration, the wheel refers to the wheel hub. For example, when you asked his truck’s driving configuration is, he would say: it is a “6×4″, or 4×2” or “8×6”, etc.

Take 6×4 as an example. Among them, 6 refers to 6 wheel hubs, and 4 refers to 4 wheel drive. So, the 6×4 is a three-axle truck with four-wheel drive and two-wheel steering.

But we can’t judge the number of rims based on this alone, because there may be one rim or two rims on its hub.

At present, this lexicon of the trucking industry has extended the 18 wheel tractor trailer to any big truck, whether it is has 18 wheel rims or not. Whether it is a heavy trailer has an extra axle to carry extra weight, or a double trailer or even triples. These trucks are still called 18 wheel tractor trailer by people outside the industry.

3 Axle semi trailer
Double Trailer
triples trailer

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