Unitary construction

A vehicle frame 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. This construction design is known as body-on-frame. Over time, nearly all passenger cars have migrated to unibody construction, meaning their chassis and bodywork have been integrated into one another.

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

The main functions of a frame in motor vehicles are:

Types of frame according to the construction:

Typically the material used to construct vehicle chassis and frames is carbon steel; or aluminum alloys to achieve a more light-weight 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, made by folding, rolling or pressing steel plate.

There are three main designs for these. If the material is folded twice, an open-ended cross-section, either C-shaped or hat-shaped (U-shaped) results. "Boxed" frames contain chassis rails that are closed, either by somehow welding them up, or by using premanufactured metal tubing

C-shape
By far the most common, the C-channel rail has been used on nearly every type of vehicle at one time or another. It is made by taking a flat piece of steel (usually ranging in thickness from 1/8" to 3/16") and rolling both sides over to form a c-shaped beam running the length of the vehicle.

Hat
Hat frames resemble a "U" and may be either right-side-up or inverted with the open area facing down. Not commonly used due to weakness and a propensity to rust, however they can be found on 1936–1954 Chevrolet cars and some Studebakers.

...
Wikipedia