A truck trailer is the combination of a tractor unit and one or more semi-trailers to carry freight. A truck trailer attaches to the tractor with a fifth wheel hitch, with much of its weight borne by the tractor. The result is that both the tractor and semi-trailer will have a distinctly different design than a rigid truck and trailer.

Types of trailers

There are many types of truck trailers in use, designed to haul a wide range of products.

Coupling and uncoupling

The cargo trailer is, by means of a king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel or a turntable hitch at the rear of the towing engine that allows easy hook up and release. The truck trailer cannot move by itself because it only has wheels at the rear end: it requires a forward axle, provided by the towing engine, to carry half the load weight. When braking hard at high speeds, the vehicle has a tendency to fold at the pivot point between the towing vehicle and the trailer. Such a truck accident is called a "trailer swing", although it is also commonly described as a "jackknife".Jackknifing is a condition where the tractive unit swings round against the trailer, and not vice versa.

Modern day truck trailers often operate as a part of a domestic or international transport infrastructure to support containerized cargo shipment.

Various types of rail flatbed train cars are modified to hold the cargo trailer or container with wheels or without. This is called Intermodal or piggyback. The system allows the cargo to switch from highway to railway or vice versa with relative ease by using gantry cranes.

The large trailers pulled by a tractor unit come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, side lifts and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins. The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws.