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MetroMPG,
Here's an "authoritative" reference (Road Vehicle Aerodynamics, mentioned in my Intro), page 51:
"The effects of ground clearance would be different for very rough or smooth undersides. Some researchers suggest that a very rough underside and a large clearance, as in trucks and lorries, actually causes an increase of drag by creating conditiions of greater freedom for the formation of eddies in the underside flow[34]. On the other hand, a general increase in ground clearance leads to a more unobstructed airflow which by bleeding air from the other flow regions produces, in effect, a decrease in the aerodynamic force."
He then shows a diagram that indicates that for a rough underbelly, the Cd increases with increasing ground clearance, while for a smooth underbelly, Cd decreases with increasing ground clearance.
He then goes on to say "These diagrams were produced for cars with an average underside roughness and show that vehicles with bad aerodynamic styling, characterised by large total drag coefficients, display a slight increase in drag while those of good aerodynamic shape display a rapid decrease in drag with increasing ground clearance."
I would also note that both the Insight and the EV1 do not have particularly low ground clearances (5" or thereabouts) and still manage to achieve Cds of .19 and .25 respectively.
One thing I'm picking up from my reading is that it is difficult to generalize about this stuff--it is all very interrelated and what you do in one area greatly influences what happens in other areas (e.g., underbody vs. front air dam vs. air around the tires) and that what works for one car may well be disaster for another.
--Steve
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