Body-on-frame is an automobile construction method by which a separate body is mounted on a relatively rigid frame or chassis that carries the engine and drivetrain. It was the original method of building automobiles and continues to this day. In the late 19th century frames might be made of wood reinforced by steel flitch plates but in the early 20th century steel ladder frames or chassis rapidly became standard.
Mass production of all-metal bodies began with Edward G Budd and the Dodge Brothers but prior to that bodies, like those of the carriages they replaced, were made of metal panels fastened to wooden (commonly ash) (body)-frames.
Mass production of all-metal bodies became general in the 1920s but Europe, with exceptions, followed almost a decade later. Europe's custom-made or "coachbuilt" cars usually contained some wood framing or used aluminium alloy castings.
Unibody designs where panels within the body supported the car on its suspension were developed by European manufacturers in the late 1920s with Budd USA (which had a number of large factories in Europe) and its technical knowhow but not in USA until the 1950s and generally later. Because of the high cost of designing and developing these structures and the high cost of specialised machinery to make the large pressings required by this style of construction it is not used by low-volume manufacturers who might construct an equivalent by welding steel tube to form a suitable space frame.
The Ford Model T carried the tradition of body-on-frame over from horse-drawn buggies, helping to facilitate high volume manufacturing on a moving assembly line. In the USA the frequent changes in automotive design made it necessary to use a ladder frame rather than unibody construction to make it possible to change the design without having to change the chassis, allowing frequent changes and improvements to the car's bodywork and interior (where they were most noticeable to customers) while leaving the chassis and driveline unchanged, and thus keeping costs down and design times short. It was also easy to use the same chassis and driveline for several very different cars. Especially in the days before computer-aided design, this was a big advantage.
Most small passenger vehicles switched to unibody construction by the end of the 1930s. The trend had started with cars like the Citroen Traction Avant (1934) and Opel Olympia (a General Motors design) introduced in 1935. Trucks, bus manufacturers and large low volume cars or those made in USA continued to use separate bodies on "conventional" frames. Body-on-frame remains the preferred construction method for heavy-duty commercial vehicles (especially those intended to carry or pull heavy loads, such as trucks and some sport utility vehicles (SUVs)) but as production volumes rise increasing numbers of SUVs and crossover SUVs are switching to unibody frames. Mass-market manufacturers Ford, General Motors and Chrysler are abandoning true body-on-frame SUVs, opting, when sales volume permits, for more efficient unibody construction. Toyota currently manufactures the most body-on-frame SUVs with the 4Runner, FJ Cruiser, Land Cruiser, Sequoia, Lexus GX and LX followed by Nissan with the Armada, Patrol, Xterra and Infiniti QX56/80. The Ford Panther platform, which was discontinued in 2011, was the last series of traditional passenger cars to be built in this manner.
One variant used by Chevrolet for its Corvette incorporates the inner skeleton to the frame.
An intermediate to full monocoque construction was the 'semi-monocoque' used by the Volkswagen Beetle and Renault 4. These used a lightweight separate chassis made from pressed sheet steel panels forming a 'platform chassis', to give the benefits of a traditional chassis, but with lower weight and greater stiffness. Both of these chassis were used for several different models. The mid-1930s designed Volkswagen made use of the bodyshell for structural strength as well as the chassis — hence 'semi-monocoque'. Traditionally chassis had "compliance", they were designed to allow some twisting. As suspensions improved they could not perform correctly unless supporting a rigid structure like that intended to be provided by unibody or monocoque construction.
The Lincoln Town Car once dominated the American limousine market because it was the last American luxury car made on the body-on-frame system and it was easily lengthened for livery work. With the Town Car discontinued since 2011, the de facto replacement is the Lincoln Navigator SUV.
Advantages and disadvantages
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- Easier to design, build and modify (less of an issue now that computer-assisted design (CAD) is commonplace, but still an advantage for coach-built vehicles).
- Quieter, because the stresses do not pass into the body, which is isolated from the frame with rubber pads around the attachment bolts. This is less significant now, but historically bodies would squeak and rattle more as they rusted, lubricants drained, and fasteners loosened. Isolated bodies were affected to a lesser degree by these modes of aging.
- Easier to repair after accidents. This is crucial for first responders (police, fire, EMS) and taxicabs, because damaged bolt-on fenders can be replaced in the firm's own garage for petty cash, with the cab returned to earning status immediately, whereas a unibody body would require straightening by paid specialists on a machine expensive to rent — with the cab laid up for repair longer. Grand-Am allows tubular space frame cars to replace their unibody counterparts, as the cars can easily be repaired with new clips.
- Can allow a manufacturer to easily subcontract portions of work, e.g. as when Austin subcontracted the aluminium body work of the Austin A40 Sports to Jensen Motors.
- Can allow more torsional flexing before yielding (trucks, truck-base SUVs, off roading) 
- Heavier than unibody, causing lower performance and/or higher fuel consumption, although reconstruction of a chassis in a metal like titanium can ensure a more lightweight material.
- Far less resistant to torsional flexing (flexing of the whole car in corners), compromising handling and road grip.
- Lack of a crumple zone causes higher rate of death and serious injury. Some cars have adopted a "front clip" and "rear clip" format similar to what is used in NASCAR where the car is split into three sections, and the clips absorb the impact, allowing the "clip" to be replaced when repairing the car.
The following is a list of production cars, SUVs, and light-duty pickup trucks available in the United States that currently have a body-on-frame construction.
- BMW i3
- BMW i8
- Cadillac Escalade (and ESV)
- Chevrolet Colorado
- Chevrolet Silverado
- Chevrolet Suburban
- Chevrolet Tahoe
- Ford Econoline (1977-2014)
- Ford Expedition (and EL)
- Ford F-150
- GMC Canyon
- GMC Sierra
- GMC Yukon (and XL)
- Infiniti QX80
- Jeep Wrangler (and Unlimited)
- Lexus GX
- Lexus LX
- Lincoln Navigator (and L)
- Mercedes-Benz G-Class
- Nissan Armada
- Nissan Frontier
- Nissan Titan
- Ram 1500
- Toyota 4Runner
- Toyota Land Cruiser
- Toyota Sequoia
- Toyota Tacoma
- Toyota Tundra
- page 106, George A Oliver, A History of Coachbuilding, Cassell, London, 1962
- Great Moments in Body-on-Frame Car Construction WSJ, May 2, 2014
- Sawyer, Christopher A. (2009-06-02). "Framing the question | Automotive Design & Production | Find Articles at BNET". Findarticles.com. Retrieved 2009-12-08.
- "Austin A40 Sports". Austin Memories.
- "Archived copy". Archived from the original on 2008-12-02. Retrieved 2009-06-28.