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A vehicle frame, a.k.a. chassis, is the main structural element of a motor vehicle to which all other components are fastened. Another term for this design is body-on-frame construction.
In the 1920s, virtually every motor vehicle had a frame. Since then, nearly all cars have shifted to unit-body constructionThe last UK mass-produced car with a chassis was the Triumph Herald, which was discontinued in 1971. By contrast, today nearly all trucks and buses still use chassis frames.
The main functions of a frame in motor vehicles are:
- To support the vehicle's chassis 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 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.
There are three main designs for frame rails. Normally the material of construction for chassis and along with frame is carbon steel alloys or aluminum Alloys (Light Weight frames). 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.
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.
The rear rail is in the front.
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.
So named for its resemblance to a ladder, the ladder frame is the simplest and oldest of all designs. It consists merely of two symmetrical rails, or beams, and crossmembers connecting them. Originally seen on almost all vehicles, the ladder frame was gradually phased out on cars around the 1940s in favor of perimeter frames and is now seen mainly on trucks.
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 a unibody (also unit body, unitary construction, or unitized construction) design, the frame and body are constructed as a single unit. It first appeared in the 1930s on some Citroën automobiles, the Lincoln-Zephyr, and the 1941 Nash 600 but variations of the unibody (for rear wheel drive and front wheel drive) have been manufactured - including modern day sport utility vehicles (Jeep Grand Cherokee et.al.). The Nash 600's design was later used by Detroit's Big Three where its 1960 model year compacts (Ford Falcon, Chrysler's Valiant, and the Chevrolet Corvair, along with AMC's Rambler-badged automobiles) had variations of the unibody - this also included the early ponycars (Ford Mustang). Most of the American-manufactured unibody automobiles used torque boxes in their vehicle design to reduce vibrations and chassis flex with the exception of the Chevy II which had a bolt-on front apron (errorenously referred to as a subframe). 
The unibody is now the preferred construction for mass market automobiles and crossovers. This design provides both weight savings and ease of manufacture. Acceptance grew dramatically in the wake of the two energy crises of the 1970s and the mid-2000s oil price increases.
A backbone chassis is a type of automobile construction chassis that is similar to the body-on-frame design. Instead of a two-dimensional ladder type structure, it consists of a strong tubular backbone (usually rectangular in cross section) that connects the front and rear suspension attachment areas. A body is then placed on this structure.
This is the design used for the full-size American models of General Motors in the late 1950s and early 1960s in which the rails from alongside the engine seemed to cross in the passenger compartment, each continuing to the opposite end of the crossmember at the extreme rear of the vehicle. It was specifically chosen to decrease the overall height of the vehicles regardless of the increase in the size of the transmission and propeller shaft humps, since each row had to cover frame rails as well. Several models had the differential located not by the customary bar between axle and frame, but by a ball joint atop the differential connected to a socket in a wishbone hinged onto a crossmember of the frame.
The X-frame was claimed to improve on previous designs, but it lacked side rails and thus did not provide adequate side-impact and collision protection. This design was replaced by perimeter frames.
Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails just behind the rocker panels/sill panels. This was done to allow for a lower floor pan, and therefore lower overall vehicle in passenger cars. This was the prevalent design for cars in the United States, but not in the rest of the world, until the uni-body gained popularity and is still used on US Body-on-frame cars. It allowed for annual model changes introduced in the 1950s to increase sales, but without costly structural changes. As of 2014, there are no perimeter frame automobiles sold in the United States after the Ford Motor Company phased out the Panther platform in 2011, which ended the perimeter frame passenger car in the United States (the Chevrolet Corvette has used a variation of the perimeter frame since 1963 but its seventh generation variant has elements of the perimeter frame integrated with an internal endoskeleton which 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, hence the use of torque boxes, and soft suspension settings.
This is a modification of the perimeter frame in which the passenger compartment floor and often the luggage compartment floor were permanently attached to the frame, for extra strength. Neither floor pieces were sheet metal straight off the roll, but had been stamped with ridges and hollows for extra strength. This was used by the Germans on the Volkswagen Beetle (called body on pan construction) and the Mercedes-Benz "Ponton" cars of the 1950s and 1960s, where it was called in English-language advertisements as the "frame floor".
An Italian term (meaning "super-light") from Carrozzeria Touring for sports-car body construction using a three-dimensional frame that consists of a cage of narrow tubes that, besides being under the body, run up the fenders and over the radiator, cowl, and roof, and under the rear window; it resembles a geodesic structure. A skin is attached to the outside of the frame and is often made of aluminium. This construction is not stress-bearing.
A subframe is a frame component that attaches to a unibody. It is normally of box construction, but may be tubular. Subframes are often found at the front or rear end of cars, and are used to attach the suspension to the vehicle. A subframe may also contain the engine and transmission.
Examples of passenger car use include the 1967–1981 GM F platform, the numerous years and models built on the GM X platform (1962), and the unibody AMC Pacer that incorporated a front subframe to isolate the passenger compartment from engine, suspension, and steering loads.
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