Body-on-frame

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).

The BMW i3 electric car is one of the rare modern cars with a separate body and frame design (2013).

History

2007 Toyota Tundra chassis holding the vehicle's engine, drivetrain, suspension and wheels.

The Ford Model T carried the tradition of body-on-frame over from horse-drawn buggies. 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. 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.

Advantages and disadvantages

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. 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 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 spaceframe 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)

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

Disadvantages

• Heavier than unibody, causing lower performance and/or higher fuel consumption. Though 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.

See also

• Vehicle frame
• Coachwork
• Spaceframe
• Subframe
• Superleggera
• Backbone chassis

External links

• Ford assembly line featuring body-on-frame car (historic)