Originally Posted by bellwilliam
1. if you have a flat bottom car, air going under the car is a good thing. speed up the flow, lower pressure, more downforce.
2. on a Miata, where it is an aero nightmare underneath. you wouldn't want air under the car.
also never look at F1 (or most spec race cars) for aero idea, they are most often regulated by rule.
this is essentially it. from what i've read about ride heights, ground clearances etc.
the idea is to keep air underneath low psi high velocity. if there is too much of an opening at the front, it will slow (bernoulli). slow air is closer to stagnant air. stagnant air is drag.
a production car with bad underbody aero can usually benefit from a higher ride height and may suffer from drag if it is lowered too much, whereas a car with flat underbody can benefit from being lower (again, pinch the air, low psi, high velocity, bernoulli).
the opel calibra has a Cd of 0.26 (today's prius's are 0.26, 0.25). opel made the middle section of their bumper higher than the outsides (similar to Racing Beat II bumper) to let more air under the car.. this was a big part of them reducing the Cd to 0.26. #foodforthought.
on my miata, with the drag / coast down tests i've done with it, my pinch height is 3.75/4.00 (95.25mm/101.6mm). front subframe is about 3.5" from ground. my tests show less drag with an airdam at 115mm vs my airdam at 65mm. the latter airdam also includes an undertray which reduces front lift.
my undertray is 2" higher than the lowest part of the car (approx 3.5" at front subframe) but my front airdam is 65mm (2.5") from the ground.