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 and to smaller firms 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
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. If the material is folded twice, an open-ended cross-section, either C-shaped or hat-shaped, results.
"Boxed" frames contain closed chassis rails, either by welding them up or by using premanufactured metal tubing.

C-Shaped

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 and rolling both sides over to form a C-shaped beam running the length of the vehicle. C-channel is typically more flexible than boxed of the same gauge.

Hat

Hat frames resemble a "U" and may be either right-side-up or inverted, with the open area facing down. They are 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 regained popularity when companies started welding it to the bottom of unibody cars, effectively creating a boxed frame.

Boxed

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 in multiple places down the seam; when turned into NASCAR "stock car" racers, the box was continuously welded from end to end for extra strength.

Design features

While appearing at first glance as a simple form made of metal, frames encounter significant 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 can 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.
As looks, ride quality, and handling became more important to consumers, new shapes were incorporated into frames. The most visible of these are arches and kick-ups. Instead of running straight over both axles, arched frames sit lower—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 without curving down on the other side and are more common on the front ends.
Another feature are the tapered rails that narrow vertically 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 does not bear as much load as the back. Design developments include frames that use multiple shapes in the same frame rail. For example, some pickup trucks have a boxed frame in front of the cab, shorter, narrower rails underneath the cab, and regular C-rails under the bed.
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 are called "torque boxes".

Types

Full under-body frames

Ladder frame

Named for its resemblance to a ladder, the ladder frame is one of the oldest, simplest, and most frequently used under-body, separate chassis/frame designs. It consists of two symmetrical beams, rails, or channels, running the length of the vehicle, connected by several transverse cross-members. Initially seen on almost all vehicles, the ladder frame was gradually phased out on cars in favor of perimeter frames and unitized body construction. It is now seen mainly on large trucks. This design offers good side impact resistance because of its continuous rails from front to rear, but poor resistance to torsion or warping if simple, perpendicular cross-members are used. The vehicle's overall height will be greater due to the floor pan sitting above the frame instead of inside it.

Backbone tube

A backbone chassis is a type of automotive construction with chassis that is similar to the body-on-frame design. Instead of a relatively flat, ladder-like structure with two longitudinal, parallel frame rails, it consists of a central, strong tubular backbone that carries the power-train and connects the front and rear suspension attachment structures. Although the backbone is frequently drawn upward into, and mostly above the floor of the vehicle, the body is still placed on or over this structure from above.

X-frame

An X-frame is built generally in the shape of the letter X, beginning in its simplest form with two frame rails parallel to one another in the engine compartment, crossing in the middle, then returning to parallel at or after the rear axle. The purpose of the design was to allow the floor pan to be placed lower than had been possible sitting atop a full ladder frame. Centerline humps, however, provided for the power train and central crossmember, intruded into the cabin space.
The X-frame varied in stiffness depending on the gauge and proportion of its cross-section, but could be rigid when heavy enough.
It was widely used, as in the exclusive Mercedes-Benz 300 "Adenaeur" limousines, and for some full-sized GM cars of the late 1950s and early 1960s.
A shortcoming was weakness to side-impact, resulting in the addition of side rails, spurring development of the perimeter frame.

Perimeter frame

Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails, routed around the passenger footwells, inside the rocker and sill panels. This allowed the floor pan to be lowered, especially the passenger footwells, lowering the passengers' seating height and thereby reducing both the roof-line and overall vehicle height, as well as the center of gravity, thus improving handling and road-holding in passenger cars.
This became the prevalent design for body-on-frame cars in the United States, but not in the rest of the world, until the unibody gained popularity. For example, Hudson introduced this construction on their third generation Commodore models in 1948. This frame type allowed for annual model changes, and lower cars, introduced in the 1950s to increase sales – without costly structural changes.
The Ford Panther platform, discontinued in 2011, was one of the last perimeter frame passenger car platforms in the United States. The fourth to seventh generation Chevrolet Corvette used a perimeter frame integrated with an internal skeleton that serves as a clamshell.
In addition to a lowered roof, the perimeter frame allows lower seating positions when that is desirable, and offers better safety in the event of a side impact. However, the design lacks stiffness because the transition areas from front to center and center to rear reduce beam and torsional resistance and is used in combination with torque boxes and soft suspension settings.

Platform frame

This is a modification of the perimeter frame, or of the backbone frame, in which the passenger compartment floor, and sometimes the luggage compartment floor, have been integrated into the frame as loadbearing parts for strength and rigidity. The sheet metal used to assemble the components needs to be stamped with ridges and hollows to give it strength.
Platform chassis were used on several successful European cars, most notably the Volkswagen Beetle, where it was called "body-on-pan" construction. Another German example are the Mercedes-Benz "Ponton" cars of the 1950s and 1960s, where it was called a "frame floor" in English-language advertisements.
The French Renault 4, of which over eight million were made, also used a platform frame. The frame of the Citroën 2CV used a minimal interpretation of a platform chassis under its body.

Space frame

Originally known as a "tubular frame", the space frame utilizes tubular steel, alloy, or carbon fibre to create a load-bearing three-dimensional skeleton, to which the suspension, engine, and body panels are attached. As the body panels have limited or no structural function, geometry is used to maximize rigidity and minimize weight, frequently employing triangles where all the forces in each strut are either tensile or compressive. The lack of bending forces allows members to be kept to a minimum weight and cross-section.
The first true spaceframe chassis were designed and produced in the 1930s by Buckminster Fuller and William Bushnell Stout, who understood the theory supporting them from either architecture or aircraft design, resulting in the bus-like Dymaxion and Stout Scarab. Maximizing space while minimizing weight were the goals.
With its high strength-to-weight ratio, the space frame was adapted to automobile racing following World War II. The 1951 Jaguar C-Type racing sports car utilized a lightweight, multi-tubular, triangulated frame over which an aerodynamic aluminum body was crafted. The form saw mass production with the 1954 introduction of the Mercedes-Benz 300 SL "Gullwing" sports car, the fastest road-going automobile of its day. The car's exceptionally high sills made conventional doors impractical, spawning the model's iconic gullwing doors.
In 1994, the Audi A8 was the first mass-market car with an aluminium chassis, made feasible by integrating an aluminium space-frame into the bodywork. Audi A8 models have since used this construction method co-developed with Alcoa, and marketed as the Audi Space Frame.
A tubular frame that is not load-bearing is not a true space frame. The Italian term Superleggera was trademarked for lightweight sports-car body construction that still requires its own chassis, and thus only resembles a space-frame chassis in general look and construction method. It utilizes a geodesic-like network of narrow tubes running under the body, up the fenders and over the radiator, cowl, and roof, and under the rear window, to provide form and attachment points for a sheetmetal skin, typically aluminum for weight-savings, as rigidity is not a consideration.