Vehicle frame

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 and buses, and most pickups continue to use a separate frame as their chassis.

Functions

The main functions of a frame in a motor vehicle are:

To support the vehicle's mechanical components and body

To deal with static and dynamic loads without undue deflection or distortion

These include:

Weight of the body, passengers, and cargo loads.

Vertical and torsional twisting transmitted by going over uneven surfaces

Transverse lateral forces caused by road conditions, side wind, and steering of the vehicle

Torque from the engine and transmission

Longitudinal tensile forces from starting and acceleration, as well as compression from braking

Sudden impacts from collisions

Frame rails

Typically, the material used to construct vehicle chassis and frames include carbon steel for strength or aluminum alloys to achieve a more lightweight construction. In the case of a separate chassis, the frame is made up of structural elements called the rails or beams. These are ordinarily made of steel channel sections by folding, rolling, or pressing steel plate.

There are three main designs for these.