1935 Chevrolet Division General Motors Film about...
GM's YKAutomobile Frame
A Dramatization Of The Y-K Frame,
With Especial Attention to the Rigidity of the Frame & the Elimination of the Squeaks & Rattles.
First few minutes are a "haunted house" imagery, after that the explanation video begins...
Public domain film from the Prelinger Archives...
A frame is the main structure of the chassis of a motor vehicle. In 1920, other than a few cars based on motorcycles, every motor vehicle had a frame. Since then, nearly all cars have shifted to unit-body construction, while nearly all trucks and buses still use frames...
There are three main designs for frame rails. Their cross-sections include:
By far the most common, the C-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.
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. In the 1960s, the boxed frames of conventional American cars were spot-welded here and there down the seam; when turned into NASCAR "stock car" racers, the box was continuously welded from end to end for extra strength (as was that of the Land-Rover from its first series).
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. The first issue addressed is beam height, or the height of the vertical side of a frame. The taller the frame, the better it is 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 cross members.... Source: Jeff Quitney / Youtube