On a gen 3 car, the amount of unsupported material is not that large. with the front splitter cables coming from the core support down to the front of the splitter and the mounting pattern on the chassis itself flexion is pretty nil. The tensile strength of the product is immaterial as you are not pulling on it as you would a rope to try and break it.
I'm not here to debate, just some friendly conversation. The splitter cables don't see a significant load because the carbon fiber splitter is flat and rigid, it's in a horizontal neutral zone, limited surface wind force upward or downward and no airfoil effects. The function of the cables / links is to make sure the splitter doesn't change angle which would be significant forces at high speed affecting the vehicle dynamics, unlike support for a front / rear wing providing downforce.
If the splitter shape distorts along any part of the surface (due to bumps at high speed, sag if the starboard material was too thin, thermal material property change sagging the ploymer, etc..), resulting with downward wind force. Typically there are only 2 cables on a carbon fiber splitter, because the carbon fiber splitter is very rigid. On something weaker like thin aluminum sheet, it may require multiple cables or gussets / ribs to maintain a flat surface, due to material deflection. This is the only reason of emphasis for rigid material. As long as the starboard material is thick enough and supported, shouldn't be a problem as a cost effective solution to withstand bumping. I would just buy a 4x8 sheet of aluminum, make a template, fab up some riveted angle section supports on the underside. If it'***** beyond repair, just cut a new splitter from the template and replace the rivets, etc..
Have fun.. sorry for the long replies in your post Gavin.
Surely Chuck's suggestion of fibreglass would work too.. dave6666 - do you have a link to the manufacturer of this unobtanium?
