As well as providing the location for the suspension system and being sufficiently durable, a car body needs to protect the bodies of the occupants. And to look alright.
If we compare the smooth bodies of contemporary vehicles with early attempts at safety engineering you notice how safety was first ‘added on’ by means of obviously larger bumpers and also by the use of safety padding inside the car. Volvo took this approach as did the GM ESV (1972) and Fiat with the ESV (1973). GM did also provide for passive safety by removing the A-pillars and fitting airbags.
These days the approach is to use fully integrated crash structures, passive safety devices and clever combinations of materials.
One way to achieve safety is to consider the construction overall. Early research ameliorated existing structures. The most usual material for a car body was and is steel. In recent decades, engineers have redesigned every element of the body and tried variation in material choice. Aluminium has been used, notably in Honda’s NSX (1991), Audi’s A8 (1994) and A2 (1999) and the Jaguar XJ (2003). Carbon fibre is still too costly. More recently, hybrid bodies of steel and aluminium have been used, allowing a better fit of material to the task required, balancing safety and functionality. The thickness of the steel and aluminium is varied depending on requirements too.
The body of the 2011 Audi A6 used 20% aluminium which allowed for a weight reduction of 15% over an all-steel body while also being less costly than all-aluminium body. That produced a reduction of 30 kg compared to the outgoing car along with achieving excellent safety ratings. The surface panels used aluminium sheet. Aluminium castings served for the front suspension towers. Audi deployed hot formed steel for the central structures of the b-pillars, a-pillars and parts of the floor. Extrusions of aluminium were used for the front crash structures while cold-formed steel was used for the rest. What this amounts to is a transitional body-type between the monococque and the aluminium space frame concept used on the A8.
This mix of materials, particularly at the front, allows for the engineers to design for a variety of crash scenarios as well as meeting the needs of production and normal performance. The extrusions in particular are particularly amenable to controlled collapse, deforming in a progressive manner to allow for the absorption of the impact energy.
GM’s lamented EV-1 also used aluminium for the car’s understructure but with plastic panels for the skin. This combination called for adhesive bonding, using methods more normally found in aerospace, an automotive industry first. For this car, the aluminium body did all the energy dissipation work while the exterior panels had little or no part to play in crash performance.
The safety trend in car design parallels the aero trend in that at first both advances were signalled with overt differences to the cars’ physical appearance. By the time the necessary research had been done, the advances had been skillfully blended into acceptable-looking forms. You wouldn’t suspect from modern cars’ appearances that they were so safe or had such low cDs.