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 that 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 intended to carry or 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).
• Quieter, because the stresses do not pass into the body, which is isolated from the frame with rubber pads around the attachment bolts. Less significant lately, but earlier bodies would squeak and rattle, ever more as they rusted, lubricants drained, and fasteners loosened. Isolated bodies had a lesser degree of these modes of aging.
• Smoother ride due to the same isolating pads.
• More suited for heavy duty usage such as towing and off-roading; can be more durable.
• Easier to repair after accidents. Grand-Am allows tubular spaceframe cars to replace their monocoque counterparts, as the cars can easily be repaired with new clips.
• 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 aluminum 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. Some cars have adopted a "front clip" and "rear clip" format similar to what is used in NASCAR race cars where the car is split into three sections, and the clips absorb the impact, allowing the "clip" to be replaced when repairing the car.

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)