Here's an easy way to put it, surprised i didn't think of it before:

If cars naturally produce downforce as they speed up (as some of you are suggesting), then why are spoilers and airdams needed?

http://wings.avkids.com/Curriculums/Sports/racing_summary.html
Look under "Teacher Text:", second paragraph.
http://www.heronsgate.milton-keynes.sch.uk/TFR2/aerodynamics/page1.html
check that out

Cars naturally become unstable at speed due to lift. The lift decreases the effective weight of the vehicle on the tires and in turn decreases the contact patch (I feel like i've said this before).

All i'm presenting are elementary concepts. If you have a solid concept all the math in the world can't undo it.

Justin