Erm..... For rear down force???
[The aerofoil on that makes the car more 'slippery' to quote JetBlick, he knows his stuff when it comes to aerodynamics.
I thought that with a front wheel drive car you need more front down force to give the car more grip - so you can get the power down as well as give better handly, and all that rear downforce does is effectivly slow the car by pushing the rear down, something like a front splitter would be better - saying this my knowledge of aerodynamics is quite minimal - feel free to tell me im wrong
