Engine cradle’s design
During the 2018 Shell Eco-Marathon, we found that the engine cradle was flawed. It was not stiff enough and the main materials were not strong enough to resist local compressive stresses made by the fastenings on the chassis. Thus, we thought of different options to resolve these issues.
The first one was molding the cradle instead of bending carbon plates and honeycomb, like we used to do. In the old one, the carbon fibers were not continuous where the edges were bent, yet having this continuity is essential to have a good force resistance which means a good rigidity.
Furthermore, we decided to increase the number of carbon layers to prevent as much as possible the distortions due to the various efforts. These additional layers will be placed according to the Solid Edge simulation’s results which will indicate us the efforts’ location and orientation.
Finally, we chose to keep a “sandwich structure”: carbon plate and honeycomb, for its bending strength. However, honeycomb is mainly made of air, meaning its strength when applied a local compression is very weak. Thus, we chose to drown it in epoxy resin (3,5GPa module, 100MPa compression resistance) to reinforce locally these areas without making the cradle too heavy.
Then, we have the preliminary phase of creation, preparation and realization. First, we had to test our solution with resin. To do that, we made a model of our piece to give the green light to our method. Then, the sizing phase: Solid Edge allowed us to pinpoint the efforts and their orientations. That’s how we could reinforce the fibers where it was needed. Once it was done, we cut the carbon to cover the wooden mold which had the old cradle’s size.
The implementation phase followed the process of presoaked composite material’s formatting: first carbon tapes laying, vacuum molding to press the fibers and remove any air bubbles, then, honeycomb laying without omitting to drown it locally, second carbon tapes laying and putting it under vacuum to cook for 16h at 80°C in a proofer.
During the implementation phase, we faced several issues. The resin has to be viscous and applied on a horizontal plane. The temperature gradient, in the proofer must be controlled. The engine cradle is big compared to the proofer so we have to make sure that the air can circulate around it to avoid a temperature gradient and an inconsistent heating.
The engine cradle assured us to have a rigid rear suspension. Also, less risks to do damage, now that is it well sized! By the way, we went to the 2019 Shell Eco-Marathon with this cradle.