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A semi-trailer truck or tractor-trailer (colloquially known as an 18-wheeler, semi, or big-rig in the US; as a semi in Australia, and Canada; and as an articulated lorry, artic, or truck and trailer in the UK, Ireland, and New Zealand) is an articulated truck or lorry consisting of a towing engine (tractor in the US, prime mover in Australia, and truck in the UK and New Zealand), and a trailer that carries the freight. In the UK, the term juggernaut is sometimes used for especially large articulated lorries, whilst the term 'semi-trailer' is almost unknown.
In North America, semi tractors usually have 3 axles, the front, or "steer" axle having two wheels, and each of the two rear "drive" axles having a pair of "dualies" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. The cargo trailer usually has two "tandem" axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. The maximum weight for a tractor-trailer in this configuration is at least 80,000 pounds (36.2 t), although some states allow up to 99,000 pounds (44.9 t).
Although dual wheels are most common, use of a single, wider tire (known as "super singles") on each axle is becoming popular, particularly among bulk cargo carriers and other weight-sensitive operators. The advantages of this configuration are two: the lighter weight allows a truck to be loaded with more weight, and the single wheel covers less of the brake unit, which allows faster cooling. The biggest disadvantage is that when a tire becomes deflated or destroyed, it is not possible to drive the vehicle to a service location without risking damage to the rim, as it is with dual wheels... more.
