[MetroMPG]

Originally Posted by SteveP

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.

Hucho's book says something similar:

p. 219: The effect of ground clearance (e) on lift and drag is less clear-cut... In "normal" vehicles, ie., vehicles with structural roughness on the underbody, the drag decreases as a car is set closer to the ground.

However, vehicles with a smooth underbody, in this case the Citroen DS 19, have the opposite tendency. For these vehicles the drag increases with reduced ground clearance, in the same way as for a streamlined body [example given is the Schlor car - on right, in image below - which has an entirely smooth underbelly with cutouts for the wheels.]



This rise in drag can be traced to the increasing effective thickness of the body with reduced ground clearance... This thickness effect is more than offset on vehicles with rough underbody, as the (high) underbody drag decreases when the flow between vehicle and road is impeded.

There's a corresponding chart which shows drag for several vehicles relative to ground clearance:

Citroen DS 19, Cd @ clearance: .405 @ 90 mm; .379 @ 160 mm; .385 @ 280 mm

Note the plot isn't linear for the Citroen. Drag increases as clearance changes in either direction from around 160 mm.

Porsche 914, Cd @ clearance: .34 @ 140 mm; .357 @ 165 mm; .367 @ 180 mm

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.

But the Impact/EV1 which was used to set the electric drive speed record apparently had a reduced ride height from the production version. (Based only on images I've seen - I don't have figures). Doesn't prove anything related to Cd, but I'd bet it was done to reduce it.