The role of aluminium in suspension systems (Chapter 8)

All vehicles require a suspension system in order to provide comfort for their passengers and make the car stable and controllable in all its attitudes with respect to the road surface. The evolution of technology has led many car manufacturing companies to develop their suspension systems in innovative materials, one of the most exploited of which is aluminium. Over the years, its structural strength and workability, combined with its rather competitive cost and above all limited weight, have reached such a level that it has become one of the most intensely used materials. In fact, it is well known that weight affects the fuel consumption of cars; the lower the weight, the better the consumption and therefore the lower the pollutant emissions, today a priority element because of increasingly stricter norms on this subject. This reason further encouraged the choice of aluminium in the entire complex of the elements between the wheels and the body to which suspension systems belong. By way of an example, the new model of the Audi A4 uses a front suspension system with independent aluminium arms which makes it possible to reduce the value of the car’s non-suspended weight by 8.5 kg. Similarly, vehicles like the S class Mercedes-Benz, the BMW X7, the Volkswagen Phaeton, the Ferrari 360, the Audi A6/A8, and the Maserati GT use aluminium in many of their components, especially in the suspension systems. The current C class Mercedes-Benz uses single aluminium arms in the structure of the MacPherson suspension system. In the case of suspension systems, as well as helping to reduce the overall weight of the vehicle, the use of light alloys improves its handling and dynamic behaviour. Today this is a highly significant aspect since, in the design phase of a motor vehicle, the priority is not so much to produce a car that allows the driver to “go fast” as to achieve a remarkably easy drive (for example, drivers highly appreciate the fact that they have to effect a minimum number of corrections when they drive through a bend). Reducing the weight of these elements implies a reduction of non-suspended weights, in other words of the parts in direct contact with the ground (wheels, tyres, brakes and components of the suspension system); as we know, these are the ones which have the greatest effect on the vehicle’s road holding. The development of suspension systems in aluminium also helps to limit the vehicle’s moment of inertia with respect to the vertical axis, the one which has the greatest effect on the vehicle’s dynamic behaviour when it changes its trajectory. Suspension systems are ‘peripheral’ masses, so even a slight change to them makes a great contribution towards limiting this parameter. A highly evident example of this is to be found in Formula 1 cars, whose suspension systems are extremely small-sized. In mass-produced vehicles, such extreme configurations are never achieved because during the sizing process extremely restrictive safety criteria have to be respected for all the components of the front axle, the steering system and the rear axle, which ensure that no fractures ever occur for the entire lifecycle of the vehicle, even in extreme or overloaded conditions. Another advantage of using aluminium in suspension systems derives from the material’s high level of ductility which makes it possible to achieve all sorts of shapes: this is an extremely important aspect if the kinematic and dynamic characteristics of the suspension system call for the presence of leverages with complex geometries which would be difficult to achieve using machining operations on steel or cast irons. Special emphasis is placed on aluminium’s better crash properties compared with steel (to the advantage of passenger safety) and its excellent behaviour towards corrosion (so that pieces no longer need to be surface treated).