Engine, gearbox, suspension and brakes are the components that make a car able to move and to stop. All of them, however, need something to be anchored to, something that gathers up the forces they generate: the chassis. On modern production cars the separate chassis has all but disappeared, replaced by the unitary body, a sheet metal structure in which the bodywork and the load-bearing element are one and the same. Whatever construction method is chosen, the structure is always asked to do the same three things.
The first is stiffness, torsional stiffness in particular, that is the structure’s resistance to twisting around its own longitudinal axis when the wheels on the same axle meet different surfaces. If the body flexes, the suspension geometry changes in ways the engineer never intended and the car responds to the driver’s inputs imprecisely and with a delay. A stiff body, on the other hand, allows springs and dampers to be tuned without the structure distorting their work. The second task is managing impact energy: in a crash the front and rear ends have to deform in a controlled way, dissipating kinetic energy along what are known as crumple zones, while the passenger cell has to stay as intact as possible in order to preserve the survival space for the occupants. The third is weight: the body is one of the heaviest subassemblies in the vehicle and every kilogram saved means lower fuel consumption, shorter braking distances and greater agility.
These three objectives are partly in conflict with one another, and the way they are balanced depends to a large extent on the material used. There are essentially three families in use today:
- Steel
- Aluminium
- Carbon fibre
Steel
Steel is the material the vast majority of production cars are built from, and the reasons are above all industrial. It is cheap, it lends itself to pressing complex shapes at very high rates, it is welded with thoroughly proven technologies such as spot welding and it is easily recycled. Its limitation is density, close to 7.8 kg/dm³, which makes it the heaviest of the three materials considered here.
To keep weight down, bodyshells have for years been built not from a single type of steel but from a combination of panels with different characteristics. Alongside mild steels, easily drawn and used for the outer panels, high-strength steels are employed (grades such as HSLA, AHSS and UHSS), which make it possible to reduce panel thickness for the same structural performance. At the most critical points, such as the pillars, sills and cross members of the passenger cell, hot stamping is used: the sheet, often a boron alloy, is brought to a high temperature, formed and rapidly cooled in the die, producing components of very high strength. The result is a cell that is difficult to deform, flanked by side members in more ductile steels that absorb the impact by collapsing.
Steel also remains the easiest material to repair, because every body shop is equipped to work on it. It does have to be protected from corrosion, typically by galvanising, and hot-stamped components, because of their hardness, cannot be straightened and have to be replaced.
Aluminium
With a density of around 2.7 kg/dm³, aluminium weighs a little over a third of steel. The saving in practice, however, is never in the same proportion: its elastic modulus is also about a third of that of steel, so greater thicknesses and sections are needed for the same stiffness. The benefit is therefore obtained by redesigning the structure, not simply by substituting the material.
Aluminium is used in three forms: pressed sheet for the panels, extruded sections for the straight runs of the structure and castings for the nodes where several elements converge. This is the logic of the space frame, the lattice chassis first adopted on a large scale by Audi with the A8. On the generation presented in 2017 aluminium components account for around 58% of the bodyshell, which overall has gained torsional stiffness compared with its predecessor.
The drawbacks mainly concern manufacturing. Aluminium is harder to weld than steel, so the joints make extensive use of self-piercing riveting and structural bonding with epoxy adhesives. Repair work also calls for properly equipped workshops, with separate areas to avoid contamination between steel and aluminium dust, which would encourage corrosion. The cost of the material and of the processes is therefore higher, which is why aluminium is found mainly on premium and sports cars and, increasingly often, on electric models.
Carbon fibre
Carbon fibre properly means a composite material: carbon fibres, extremely strong in tension, embedded in an epoxy resin matrix that holds them in place and distributes the loads across them. Unlike metals, the composite is anisotropic: its properties depend on the orientation of the fibres, which are laid up following the directions of the expected loads. The result is the best strength-to-weight ratio available today.
The best known form is the one-piece monocoque, to which the engine, the suspension and the impact structures are anchored. The MonoCell introduced by McLaren on the MP4-12C weighs around 80 kg; on the volume production front, the composite passenger cell of the BMW i3, known as the Life Module, comes in at around 150 kg and was the first to be built on an industrial scale.
The limitations are cost and dealing with damage. Production requires long curing times and a significant amount of manual work, hard to reconcile with the volumes of a mass-produced car. What is more, the composite does not deform plastically the way a metal does: it absorbs energy by shattering in a controlled manner, a job entrusted to dedicated sacrificial elements. After an impact the damage may also be invisible from the outside, as in the case of delamination between the layers, and structural repair is often impractical: the whole element is replaced.
Multi-material construction
In reality modern cars do not belong to a single category. The design rule is to use the most suitable material in each area: hot-stamped steels where resistance to intrusion is needed, aluminium where weight counts, composite at the points where the gain justifies the expense. The very Audi A8 mentioned earlier combines aluminium, steel, magnesium and carbon-fibre-reinforced plastic.
The mixed approach does bring problems of its own, however: different materials expand by different amounts as temperature changes, and direct contact between them can trigger galvanic corrosion. Assembly therefore relies on combinations of bonding and mechanical joints rather than on welding alone, with a far greater number of production processes involved. There is no such thing, then, as a material that is best in absolute terms, only the compromise that best fits the performance required, the production volumes and the list price of the car.