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, 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.
History 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 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), 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 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
= 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, 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 References
External links Ford assembly line featuring
body-on-frame car