Body-on-frame

is an automobile construction method. Mounting a separate body to a rigid frame that supports the drivetrain was the original method of building automobiles, and continues to this day. Originally frames were made of wood (commonly ash), but steel ladder frames became common in the 1930s. It is technically not comparable to newer unibody designs, and almost no modern vehicle uses it (other than trucks).

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 in the 1960s, but the trend had started in the 1930s with cars like the Opel Olympia, and Citroen Traction Avant, leaving just trucks, some bus manufacturers, and large cars using conventional frames. The shift continues today: 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 increasing numbers of SUVs are also being converted to automobile-style unibody frames, and the market is also shifting from SUVs to crossover SUVs. Mass-market manufacturers Ford, General Motors and Chrysler are abandoning true body-on-frame SUVs, opting for cheaper-to-produce unibody construction.[2] Toyota currently manufactures the most body-on-frame SUVs with the 4Runner, FJ Cruiser, Land Cruiser and Sequoia followed by Nissan with the Armada, Patrol and Xterra. 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. Volkswagen made use of the bodyshell for structural strength as well as the chassis — hence 'semi-monocoque'.

The Lincoln Town Car dominates the American limousine market because it is the last American luxury car made with body-on-frame and is therefore easily lengthened for livery work. With the Town Car discontinued since 2011, the de facto replacement is the Lincoln Navigator.

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