The multi-material car

This method of construction accounts for 99.9 per cent of all the cars produced in the world. The remaining 0.1 per cent are mostly constructed with an aluminium BIW, while a very small number (less than 0.01 per cent) are constructed from carbon-fibre composite.

The material properties of steel (with its wide range of yield strength combined with high modulus) together with ease of manufacture and low cost, mean that steel intensive vehicles have by far the largest share of the market. The high cost of alternative materials such as aluminium or composites mean that steel’s position as the first-choice material is secure.

The BIW of a vehicle accounts for 20 per cent of the vehicle mass. The weight of the closures (doors, bonnet and boot/rear hatch), chassis (suspension parts) and driveline bring the total amount of steel and other ferrous metals to more than 60 per cent.

In recent years, the amount of ferrous metal has declined, mostly driven by manufacturers replacing iron with aluminium for engine castings. The percentage of sheet steel per car has also dropped, mainly due to:

• Higher levels of equipment, trim and soundproofing.
• More aluminium used in wheels and suspension parts.
• More moulded plastics, especially under the bonnet.

The environmental and economic requirements for reduced fuel consumption have also led to an increase in the use of lightweight materials for components that bolt on to a conventional steel vehicle, but at a cost.