The underbody cavity is a plenum with one moving and one stationary wall, with all that that implies.
More clearance means less interaction between the road and vehicle, but allows rolling vortexes to move air in or out between the wheels. If one looks at the aeroCivic, it has dual side skirts between the front and rear wheels.
A central notch in the airdam can be beneficial, else wheel spats.
[IMG]
[/IMG]