Ladder chassis

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.

Today some new vehicles still have a separate chassis, most commonly pickup trucks and their relatively low volume variants like crossover SUVs.

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.

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