Structural considerations

For aerodynamic components, weight and cost are easy parameters to quantify even before starting manufacture. We know how much material we are likely to use and approximately how much it weighs once combined with resin, and when mounting brackets and fasteners are attached. The only uncertainty we had was how much (if any) additional post-layup stiffening we would need. Our suspension and lap-time simulations will then give us an estimate of how this weight affects performance. Cost event points are awarded linearly, so we also can tell exactly how many points we will gain or lose compared to a baseline design without knowing the cost of our opponents' cars. 

Maximum downforce expected on each of the two separately-mounted floors was in the ballpark of 22kg, with drag at 10kg. The permanent chassis floor provided the rest of the downforce between the two side-mounted floors. Having been previously traumatised by dealing with EV18 and its undertray that took 2 people, 10 minutes, and a jacking bar to mount to the vehicle, I was determined to create a mounting system that required a single person, no jacks/stands, and could be done in a matter of seconds. This wasn't just for my sanity, but a couple of other reasons too:

• The Australasia competition has a tight schedule, so time between events has to be maximised. Access to the accumulator mounts is blocked when the floor is installed, so being able to remove it quickly directly impacts the available charging time between events, as well as general access to the accumulator area for maintenance.

• The car needs to be able to roll over our trailer ramp and over the curb outside our workshop with sufficient ground clearance, and the lowest point on the car is the floor. Having them be easily removable will encourage people to do so, limiting the chance they are left on and subsequently damaged. 

The solution I came up with was a forwards/backwards slide-mount design using pins and slots, with longitudinal motion secured with a single m4 bolt on each side. This design provides very little support for flex of the floor about the longitudinal/mount axis, so to support the outer extremity of the floor, 1mm steel cable with quick-disconnect hooks was used at points on the front and rear, connecting back to the upper side chassis tubes. Manufacture of the floors, their mounting, and the external suspension cover took two of us about three weeks full-time in October, although much of this was spent experimenting, researching, and fixing mistakes as we improved our understanding and knowledge of best practises and helpful tricks. There were also two full days of sanding the wooden moulds to smooth the steps between the layers, which is something we will improve in mould design for next time. 

The stiffness of the floor was designed around the strength and location of the supporting mount brackets and cables. While some flex is acceptable, too much could lead to excessive ground contact in high speed corners and increase the effective expansion of the diffuser to beyond what it was designed for, possibly inducing flow separation.

From the start, the plan was to use carbon fibre twill weave as the sole composite fabric. Carbon was preferred over glass due to its higher stiffness per weight, and its more favourable layup behaviour compared to the softer and easily deformable glass fibre twill or even standard weave. The competition points lost due to the higher cost of carbon were fewer than the points gained from the reduced weight. Twill was preferred over standard weave as the floor had complex curvatures and small local radii that a standard weave would struggle to adapt to. The reduced stiffness typical of the twill was minimised by laying each ply at an offset weave angle. 

The manufacture design utilised a "2D" flat sheet layup with no internal volume/mould or multi-surface construction. This was done to minimise weight and simplify manufacturing, given the team had little experience in making complex composite parts. This design required the use of a core material sandwiched in the layup to add thickness to support bending. There were two mainstream options for a core material: cork or foam. Foam had the benefits of being cheap, and easily conformable to more complex surface curvatures. The downside was it was more compressible, so didn't support bending as effectively. In the end I decided to use both materials. The foam would be used on the main inboard tunnel/diffuser section, and cork would be used on the flatter outboard skirt section. The inboard section had naturally greater stiffness due to its multi-axial curvature, but the flatter skirt needed the additional stiffness offered by the cork. 

Layup diagrams were produced to communicate the ply and core structure to anyone involved in the manufacture.

In general, 3 layers of fabric were used. One on the underside of the core (typically in tension), and two on top (for resistance to buckling). A strip of thin copper mesh was used surrounding the foam core within 100mm from the high voltage accumulator connections to meet grounding rules. 

Peel ply was used on the top surface of the floor to provide a uniform finish. Perforated plastic film was later used for the same purpose on the strakes (which was not vacuum sealed for practical reasons), for which a smoother finish was desired due to the difficulty of conducting post-work on the thin part.

Additional separately-attached parts consisted of the squirt suppression wishbone element, the brow wing, the diffuser flap, and the large diffuser strake. 

The diffuser strake was simple as it was intentionally designed to be perfectly flat. This meant it could be pre-cured in a flat compression mould giving a perfect finish. The cured part was then trimmed to shape using a printed template and mounted to the diffuser roof using a single strip of carbon tape on each side. Resistance to bending at the interface with the floor came naturally from the diffuser's curvature. CFD suggested a maximum of 0.25 kg of lateral force on the strake, which it held fine when tested. 

The brow wing and flap shared the same manufacture method: a 2D polystyrene foam core cut with a hot wire and templates/guides, sealed with spray adhesive to prevent excessive resin absorption, then covered with a single ply of fibreglass (to avoid the electrical grounding rules issues that come with using carbon). We decided not to use vacuum bags but in hindsight we should have put in the effort as we weren't able to get a good surface finish. In the end a sheet of adhesive vinyl was applied to these parts to give them a smoother surface. 

The squirt-suppression element was purely solely of foam to save time given it didn't need the strength. This also led to a predictably poor surface finish, but it wasn't expected to influence its function.

The diffuser flap was bolted securely to the floor on both sides, providing a critical contribution to the overall stiffness of the floor. The angle was adjustable for the acceleration event. 

The brow wing and squirt suppression element were both at risk of being damaged if the car hits a cone. The latter is so simple to make that it doesn't matter; we already made half a dozen spared. For the brow wing though, I decided to make the mounting for these parts as weak as I could get away with. While the brow wing itself is reasonably simple to make, it ran the risk of damaging the floor if the mounting was too strong. Instead, the brow wing and mount should separate in a cone impact after absorbing some of the energy. 

Finally, the chassis mounts were aligned in-situ on the car. They attached to the top side of the floor, but a better option would have been to remove some core material and insert the mounts between the plies of the floor, concealing them from airflow. 

Overall, the manufacturing and structural aspects of the floor were pretty successful given this was our first attempt at trying something of this scale and complexity. The mass was particularly impressive, with each floor weighing just 1.1 kg. The total weight of 2.2 kg was 0.3 kg lighter than our pre-manufacture target, and 2.8 kg below a worst-case estimate from the start of the year. Plugging the final CFD estimates, weight, and standardised cost into our lap simulator suggests the floors provide a 14-point benefit throughout the competition. This is pretty similar to our original estimate of 11 points, which was based on 12% higher downforce but double the mass and 40% higher cost.

As with all things though, we identified a few areas for future improvement.  

The cable mounting locations were not ideal as they didn’t really consider the effects of deflection in the floor. The deflection let the floor to move in ways not opposed by the cable tension, allowing the centre section to sag more than intended. Some more careful consideration of the pickup points and cable angles will be needed next time.  

The chassis mount brackets and studs worked well in concept for their ease of use but the aluminium brackets had some deflection under load due to their required mounting angle not being perfectly vertical/perpendicular. This translated to deflection in the floor and some difficulty when attempting to slide the floor into or out of its position. This will be solved for next year by simply increasing the tab thickness. 

The diffuser flap mount will also be in the scope for improvement next year, with the ideal implementation being to move the mounting interface to the side of the floor rather than the top. This would sandwich it between the floor and chassis, minimising losses sent through the slot gap.

Lastly, quality of the clear coat on the top of the floor was sub-par from a combination of limited experience, not using the right resin, and running out of time to build up the required thickness. 

Despite all this, less went wrong that I was expecting, and I think a seriously good looking and strong design can be created for the next iteration.