A frame is the main structure of an automobile chassis. All other components fasten to it.
There are three main designs for frame rails. These are:
By far the most common, the C-rail has been used on nearly every type of vehicle at one time or another. It's 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.
Originally, boxed frames were made by welding two matching c-rails together to form a rectangular tube. Modern techniques, however, use a process similar to making c-rails in that a piece of steel is bent into four sides and then welded where both ends meet.
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.
Abandoned for a while, the hat frame gained popularity again when companies started welding it to the bottom of unibody cars, in effect creating a boxed frame.
While appearing at first glance as a simple hunk of metal, frames encounter great amounts of stress and are built accordingly. They first issue addressed is beam height, or the height of the vertical side of a frame. The taller the frame, the better it's able to resist vertical flex when force is applied to the top of the frame. This is the reason semi-trucks have taller frame rails than other vehicles instead of just being thicker.
Another factor considered when engineering a frame is torsional resistance, or the ability to resist twisting. This, and diamonding (one rail moving backwards or forwards in relation to the other rail), are countered by crossmembers. While hat-shaped crossmembers are the norm, these forces are best countered with "K" or "X"-shaped crossmembers.
As looks, ride quality, and handling became more of an issue with consumers, new shapes were incorporated into frames. The most obvious of these are arches and kick-ups. Instead of running straight over both axles, arched frames sit roughly level with their axles and curve up over the axles and then back down on the other side for bumper placement. Kick-ups do the same thing, but don't curve down on the other side, and are more common on front ends.
On perimeter frames, the areas where the rails connect from front to center and center to rear are weak compared to regular frames, so that section is boxed in, creating what's known as torque boxes.
Another feature seen are tapered rails that narrow vertically and/or horizontally in front of a vehicle's cabin. This is done mainly on trucks to save weight and slightly increase room for the engine since the front of the vehicle doesn't bear as much of a load as the back.
The latest design element is frames that use more than one shape in the same frame rail. For example, the new Toyota Tundra uses a boxed frame in front of the cab, shorter, narrower rails underneath the cab for ride quality, and regular c-rails under the bed.
This design offers good beam resistance because of its continuous rails from front to rear, but poor resistance to torsion or warping if simple, perpendicular crossmembers are used. Also, the vehicle's overall height will be higher due to the floor pan sitting above the frame instead of inside it.
In addition to a lowered roof, the perimeter frame allows for more comfortable seating positions and offers better safety in the event of a side impact. However, the reason this design isn't used on all vehicles is the transition areas from front to center and center to rear reduce beam and torsional resistance, hence the use of torque boxes.
By far the most common type of frame in use today.
The sub frame, or stub frame, is a boxed frame section that attaches to a unibody. Seen primarily on the front end of cars, it's also sometimes used in the rear. Both the front and rear are used to attach the suspension to the vehicle and either may contain the engine and transmission.
The most prolific example is the 1967-1981 Chevrolet Camaro.