Although body-on-frame car construction once predominated, unit construction is now pretty well universal. It has made modern cars much safer with their strong metal passenger cells surrounded by deformable crumple zones designed to absorb crash energy.
In the early days, most automobiles evolved from horsedrawn buggies and carriages and the technology was carried over. The engine, running gear and suspension were attached to a channel-section steel frame, usually of the ladder type, which formed the car's foundation. On top of this was mounted the body, the majority of them open until the mid-1920s. They were pretty flimsy and flexible affairs.
A lot of wood was used in early bodies, but this gradually gave way to all-steel construction, principally through the sheetmetal-stamping research of Edward Budd's Budd Manufacturing Co. of Philadelphia in the early 20th century. The Dodge Brothers pioneered the Budd allsteel auto body in their first production car in 1914.
But the body and frame were still separate until Italy's Lancia started to show the way with the 1922 Lambda model that had its body/frame structure formed as one welded unit. Its substantial floorpan and two deep steel side members were held together at the front by strong cross members at the radiator and cowl, and at the rear by a box-shaped section.
In 1934, France's Citroën, inspired by Budd's work, applied the first true unit-construction body and frame to its front wheel-drive Traction Avant model. The design was so successful that Citroën built the Traction virtually unchanged for more than 20 years.
American manufacturers also became interested in unit construction in the 1930s. Chrysler Corp. consolidated its reputation for advanced engineering by exploring auto aerodynamics and taking the first step toward unit construction by attaching the body of its new Chrysler/DeSoto Airflow models to a light superstructure and bolting it to the main frame. This unit-type construction and an aerodynamic shape both appeared in the revolutionary 1934 Airflow.
The Airflow was very strong, and to quell competitors' rumours that its new body was unsafe, Chrysler engineers demonstrated its robustness by pushing one over an 11-storey cliff and then driving it away, bruised but still roadworthy.
Two years later, the Ford Motor Co. followed Chrysler's lead with its new, aerodynamic Lincoln Zephyr using a light body-frame. The Cord used similar construction.
Full unit construction came to the American industry in 1941 when the Nash Motor Co. boldly adopted a Budd-engineered onepiece body unit for the all-new Nash 600 model (600 miles on a tank of fuel). By eliminating the heavy frame on this popularly priced mass-produced car, Nash did more than any other company to legitimize the unibody in American cars. Nash standardized unit construction in all models, including its 1949 Airflyte series and compact 1950 Rambler.
When Nash and Hudson joined to form American Motors Corp. in 1954, Hudson had already been using unit construction since 1948 in its "Step Down" designs. Willys-Overland's Aero Willys compact, introduced in 1952, also had unit construction.
Monocoque is another body fabrication type that is often described as unit construction. Although it is a type of unit construction, there are differences. In monocoque, most of the stress is carried by the outer-body skin, like an egg.
In unit construction, the forces are borne predominantly by the floor, cowl and roof structures. Monocoque started in aircraft building and spread to buses and trains, including the famous Burlington Zephyr.
A disadvantage of monocoque construction for cars is that holes significantly weaken the structure, and cars need large door, window and trunk openings for access and visibility. Monocoque works well for aircraft, where only five or six per cent of the body area is holes. In cars it's closer to 50 per cent.
Monocoque construction has been used principally for competition cars. Colin Chapman of England was an innovative user of the technique in his lightweight Lotus cars. Jaguar used monocoque in its 1950s Jaguar D Type Le Mans racer, and the resulting 91-to 136-kilogram weight saving gave it a performance advantage.
As unit construction became more common, Ford proved it could be applied to large cars with its massive 1958 Lincoln Continental Mark III. In 1960, Chrysler adopted unit construction across its model line.
Unit construction is virtually universal today. Inherently lighter and stronger, it provides a bendand twist-resistant base for mounting suspension, steering, etc. It does, however, require much better corrosion protection than the old body-on-frame. Rust is the mortal enemy of the unibody approach, and it forced early adopters like American Motors to vastly improve corrosion resistance by paint dipping and galvanizing.
From Lancia to Citroën to Nash, with some help from Edward Budd, unit construction has a long and varied history. It contributes significantly to today's stronger, safer, better-handling cars.
