Body-on-frame

Car frames.

2007 Toyota Tundra chassis.

Body-on-frame is an automobile construction technology. Mounting a separate body to a rigid frame which supports the drivetrain was the original method of building automobiles, and its use continues to this day. The original frames were made of wood (commonly ash), but steel ladder frames became common in the 1930s.

In the USA the frequent changes in automobile design made it necessary to use a ladder frame rather than monocoque 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 cost down and design time 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 monocoque 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 switch continued for several decades - even small SUVs typically use this construction method today. Body-on-frame remains the preferred construction method for heavy-duty commercial vehicles, especially those which are intended to carry and pull heavy loads, such as trucks.

A halfway house to full monocoque construction was the 'semi-monocoque' used by the Volkswagen Beetle and Citroen 2CV. 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 therefore easily lengthened for livery work.

Advantages and disadvantages compared to unibody

Advantages

• 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).
• More suited for heavy duty usage such as towing and off-roading; can be more durable.
• Easier to repair after accidents.
• In an environment where roads are salted, it will not rust through as quickly.
• Could allow a manufacturer to easily sub-contract portions of work, e.g. as when Austin subcontracted the aluminum body work of the Austin A40 Sports to Jensen Motors.

Austin A40 Sports, ca 1951. During production, A40 Sports alluminum bodies were built by Jensen (of West Bromwich) and transported to Austin's Longbridge plant for final assembly.

Disadvantages

• Heavier than unibody - lower performance and/or higher fuel consumption.
• Less resistant to torsional flexing (flexing of the whole car in corners) - compromising handling and road grip.
• No crumple zone - higher rate of death and serious injury.
• Higher production costs (Except for trucks and SUV's the only cars to continue using body on frame construction in the US is the Chevrolet Corvette.)
• Inferior packaging - comparatively smaller interior for total size

See also

• Frame (vehicle)
• Body Frame Integral
• Coachwork
• Monocoque
• Spaceframe
• Subframe
• Superleggera
• Chassis
• Backbone chassis

External links

• Article about body-on-frame in modern cars and trucks
• Body on frame and unibody in SUVs
• Ford assembly line featuring body-on-frame car (historic)