1950 Nash Ambassador. Click image to enlarge
Article and photo by Bill Vance
Although body-on-frame car construction once predominated, unit construction is now almost 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 latter 19th Century, as many automobiles evolved from horse-drawn buggies and carriages, 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. Those cars 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 metal stamping research of Edward Budd’s Budd Manufacturing Company of Philadelphia in the early twentieth century. The Dodge Brothers pioneered the Budd all-steel auto body in their first production car in 1914.
But body and frame were still separate until Italy’s Lancia company started showed the way in the 1922 Lambda model with 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 Citroen, 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 Citroen built the Traction virtually unchanged for more than 20 years.
American manufacturers also became interested in unit construction in the 1930s. Chrysler Corporation 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 strength by pushing one over an 11-storey cliff and then driving it away, bruised but still roadworthy.
Two years later the Ford Motor Company followed Chrysler’s lead with its new, aerodynamic Lincoln Zephyr using a light body-frame. The Cord used similar construction.