frame rail

A vehicle frame, also historically known as its chassis, is the main supporting structure of a motor vehicle to which all other components are attached, comparable to the skeleton of an organism.
Until the 1930s, virtually every car had a structural frame separate from its body, known as body-on-frame construction. Both mass production of completed vehicles by a manufacturer using this method, epitomized by the Ford Model T, and supply of rolling chassis to coachbuilders for both mass production (as by Fisher Body in the United States) and to smaller firms (such as Hooper) for bespoke bodies and interiors was practiced.
By the 1960s, unibody construction in passenger cars had become common, and the trend towards building unibody passenger cars continued over the ensuing decades.
Nearly all trucks, buses, and most pickups continue to use a separate frame as their chassis.

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