In the third step, information about connecting the chassis and upper body structure was added to the project: two pairs of mounts were placed, one at the longitudinal coordinate of the firewall, the second positioned just behind rear axle, in the vicinity of the rails. Hereby the firewall became more complex because, whilst it connected the front structure with the tunnel on one side, it also had to guarantee support for the front joints on the other. In this case, topographical optimisation results also helped in defining the shape of that part. 

In the fourth step, materials were associated to the parts of the chassis structure. In order to improve the material efficiency, it is important to delineate the prevalent loading condition for each component, with the purpose of obtaining a stress distribution as near as possible to the safety limit of the material.

Designing an automotive structure, it can be helpful first to individuate components which are mainly contributing to stiffness (e.g. pillars), from those requiring high strength (e.g. crash members). Also the definition of prevalent loading conditions and typical components can lead to a different choice for the most efficient material: that is, a hollow section mostly subjected to axial forces will probably require different material properties to a plate loaded by flexural torque. 

The Table 1 (see below) resumes different material performances in comparison to ultra high strength steels. In this project's case, the limited number of units per day produced for a niche vehicle permitted a degree of freedom in material choice. This is a typical field for Fibre Reinforced Plastic.

These basic considerations led to the following general statements: a) Carbon Fibre Reinforced Plastic (CFRP) allows the maximum lightness on flat panels under flexural loading conditions; b) CFRP is extremely efficient, too, for hollow sections under torsional loading and to reach a maximum strength target; c) High grade Titanium, despite its unaffordable cost, presents the highest specific energy absorption, that is, it represents the lightest choice for a predetermined crash member design; d) Mg, Al or Ti castings are efficient solutions whenever multi-functional integration leads to a complex design.

These considerations led inevitably to the conclusion that a multi-material approach was the only answer to ensure the highest structural efficiency and, consequently, the best lightweight construction. In the Sportiva Latina chassis, the above mentioned items were implemented as follows: a) CFRP composite was chosen to design the complex shape of the firewall and central floor, where the tunnel’s hollow section should withstand both torsional loading and strength requirements in crash conditions; b) Titanium was adopted for front and rear crash members and for the engine cradle, due to its structural performance and, last but not least, as an extremism for the multi-material concept; c) Al and Ti castings were designed for shock absorber towers and the body mounting brackets; d) Al was been chosen for front and rear cross members in order to reduce repair costs.

When designing the composite structure, shape and thickness optimisation analysis was adopted to define the structural needs.
 

CAE numerical optimisation led to find the best solution in terms of the number of layers and orientation for the composite firewall, as well as to determine the best thickness variation for the honeycomb reinforced panels (e.g. tunnel and floor walls). In order to assess the feasibility of the multi-material concept design and the actual level of performance achieved, the chassis design was then engineered.

In these terms, the most critical component was the large carbon fibre composite floor, featuring a complex, T-shaped, closed section tunnel with variable wall thickness along the axis to accommodate the gearbox, transmission shaft and rear differential. It was then coupled to a wide firewall, light as possible but stiff enough to manage crash forces from the main longitudinal members and to transfer these forces to the central structure.

In consideration of the different shapes and missions of the components it was decided to keep them divided and to differentiate the manufacturing technology. The main floor was fabricated from a hand-made lay-up deposition of carbon fibre layers and nomex honeycomb core, using a vacuum bag technique to obtain a hollow section for the tunnel; this technology is well established in Italy by leading and well known small and medium manufacturers. At the same time, the firewall was realised by Resin Transfer Moulding (RTM)): this technology was considered as quite promising due to low investment costs, relatively low production-cycle times and the possibility to partially automate the process in order to achieve a productive volume of 5-20 pieces per day.

Nevertheless, the application of RTM technology for high stressed components reinforced by long fibres of carbon has already been widely explored. In addition to that, the deeply drawn shape of the firewall design could have represented a serious obstacle to the uniform distribution of resin and reinforcement.