Browse over 9,000 car reviews

Sorry, there are no cars that match your search

25 February 2019

Coachbuilding: The advent of mass production

By Vivek ShahVivek Shah
This Speedster body would sit on top of the frame. (image credit: Silodrome)

This series takes a journey through the historic practice of coachbuilding and how it has evolved to remain relevant to the present day.

Following the introduction of the automobile in the early 20th century, manufacturers quickly realised the potential of a car lay not as a luxury for the wealthy, but as a form of mass-produced transport. In order to make this possible, manufacturers made fundamental changes to the engineering of the car, with the introduction of a unibody design, and with how cars were produced and sold. In turn, this had a substantial impact on the traditional practice of outsourcing a rolling chassis to specialist builders to create unique bodywork. 

The switch from body-on-frame to unibody vehicle construction was perhaps the most significant innovation to affect coachbuilding in the 20th century. As per its namesake, traditional body-on-frame manufacturing meant that the vehicle body constituted a separate part that sat on top of a steel or wood frame chassis underneath. Therefore, with this type of construction the chassis handled vehicle stresses whilst the body largely ‘sat pretty’, so to speak. 

  • The frame of the 1921 Duesenberg Model A. (image credit: Heacock Classic) The frame of the 1921 Duesenberg Model A. (image credit: Heacock Classic)
  • This Speedster body would sit on top of the frame. (image credit: Silodrome) This Speedster body would sit on top of the frame. (image credit: Silodrome)

In contrast, the unibody (also known as the unitised body) combined the body and frame as one piece that could take the entire structural load applied to the vehicle. Of course, it was difficult to make an entire car out of a single unibody, so manufacturers typically created separate unibodies for different sections of the vehicle (e.g. discrete unibodies for the engine compartment and passenger cell) that were then welded together.

  • The unibody for the passenger cell of the 1934 Citroen Traction Avant. The Traction Avant (which translated literally, means ‘forward traction’) was the first production unibody vehicle and notably also the first to feature front-wheel-drive. (image credit: Carbody Design) The unibody for the passenger cell of the 1934 Citroen Traction Avant. The Traction Avant (which translated literally, means ‘forward traction’) was the first production unibody vehicle and notably also the first to feature front-wheel-drive. (image credit: Carbody Design)
  • The 1934 Citroen Traction Avant. (image credit: Wiki Commons) The 1934 Citroen Traction Avant. (image credit: Wiki Commons)

The unibody method of construction enabled manufacturers to deliver complete vehicles to their customers more cost efficiently, as it required less material than building a separate frame and chassis. This also allowed for lighter vehicles to be developed, which consequently delivered benefits in fuel economy, handling and ride comfort. 

Most significantly, with parts such as the roof and side pillars now being key to the structural integrity of the car, there was much less scope for creativity with the vehicle body. Cars couldn’t simply be sent to a specialist coachbuilder for a new or altered body; any changes would require substantial modification to the underlying engineering of the vehicle. As a result, manufacturers in the mid-20th century brought design and styling capabilities in-house. For the specialist coachbuilder, this meant being acquired (e.g. Ghia by Ford) or transforming their business into design and general manufacturing houses (such as Pininfarina), which would have work subcontracted to them.     

Previously a specialist coachbuilder, Ford subsequently used ‘Ghia’ to typically denote the most luxurious trim level of its cars, such as the 1990 Ford Fairmont Ghia. (image credit: Wiki Commons) Previously a specialist coachbuilder, Ford subsequently used ‘Ghia’ to typically denote the most luxurious trim level of its cars, such as the 1990 Ford Fairmont Ghia. (image credit: Wiki Commons)

Check out our other coachbuilding stories:

NEXT: Modern coachbuilding revived

Is the unibody design superior to the body-on-frame layout? Let us know in the comments.