When cars were first introduced they used the conventional system of frame with a body fitted on top. This style of construction enabled many manufacturers to develop chassis and mechanical combinations that were given to coachbuilders who then fitted a luxury body on top.
Rolls Royce, Bentley and many others gave their cars to coachbuilders like Thrupp & Maberley or Mulliner Park Ward who would add on bodies that used certain styling concepts and radiator grilles that tied them back to the main car manufacturer.
As mass production took hold, the car industry started to share technologies with the aviation industry. Engines were shared, production processes were adopted by car manufacturers and the concept of a “shell” for a body migrated from aircraft to cars. Aircraft designers had found that they could build lighter and stronger fuselages using aluminium that was getting cheaper to buy.
A car with a body on a separate chassis uses the chassis as a stress and load bearer so it needs to be strong, often heavy (until the space frame was designed) and it needed to be rigid. Cars then, by default would be heavy with poor handling, however on the plus side, they were easy to repair as panels could be removed and replaced without having to do too much work.
Then in the mid 1920s, Lancia in Italy, took the aviation idea of creating a body that was a load bearer known as a “unibody” or Unitary Construction. The most common description is monocoque a mix of Greek – “mono” meaning single, and the French word “coque”, meaning shell. Lancia released the Lambda with the body acting as a shared load bearer for the engine, suspension and other mechanical components. Soon after, Citroen, Chrysler and Lincoln brought out models using the same design concepts.
The first company to put monocoques on the production line was Nash in the US with their 600 model in 1941. They used an all steel shell with the chassis welded in. They were quoted as achieving a huge weight loss that helped the fuel economy of the car. Today most mass produced cars use an evolution of this type of shell.
Chevrolet took the concept further in the 1960s with the Corvair by styling strength into the panels so that the main shell could be lighter still. Over the years the chassis frames got smaller and many cars used a monocoque with a sub frame that the engine and other components were mounted on. Engineers found that because the subframe was bolted to the unibody, the car suffered from vibrations caused by all the stress being absorbed in one place.
With advances in design and production facilities – especially the use of spot welding robots, the unibody design started to spread the loads over more of the body with more panels and stressed components being stamped in one go and then welded together so that the whole body absorbed the stress of the weight and loads.
Monocoques are used extensively in single seater racing. Colin Chapman started the trend by using a space frame with the engine as a stressed member. This allowed a small monocoque around the driver that everything was bolted to. Today with the use of carbon fibres, Kevlar and other composite materials these monocoques can add to the aerodynamic efficiency of the car by being designed with sculpted sides and diffusers.
Citroen introduced unitary monoque construction on their production .line for their F.W.D. Traction saloon cars in 1934 seven years before Nash and continues on to this day.: