I believe it's 0.34 with top up... at least that's what i read

 

its high b/c of the windshield and the rear window regardless of hardtop or soft top...rear window is to steep causing flow separation aft of the roof...its the reason why the mooncraft hardtop is designed the way it is to keep the flow along the top reducing cd.

 

most roadster have a relatively high Cd....the MB slk's are pretty high too IIRC, somewhere around .38

is it to make the relatively small car more stable at higher speeds? not sure...

 

in this case you are associating drag with downforce and they do not go hand in hand...near 100mph the s2k has a lifting force near 100lbs on the rear...i would hardly call that stable...

 

ah thanks for the clarification

 

it's not a bad thing consider there's regulation for new roadster to have it support it's weight upside down without crashing down on the human thing inside

 

The meaning of a drag coefficient of .38 is that the effective frontal area of the vehicle (in terms of actual drag force generated by air resistance at a given speed) is 38% of the physical frontal area. A modern sedan may be more in the .26 - .30 range. Of course, the effective frontal area of the much more low-slung S2000 is likely to be less than the effective frontal area of that sedan, and less drag force will be generated at the same highway speed. If you look at the S2000 in profile, its shape is a bit awkward compared to the sedan, and the air just does not stream around it smoothly.

 

Drag = 1/2 * rho * V^2 * Area * Cd

(rho is the air density and V^2 is the speed squared)

1/2 * rho * V^2 = "dynamic pressure" (air pressure due to speed)

So you could interpret the Cd * Area as an "effective area", but that would be a subtle misinterpretation. Really any area could be used for the calculation, as long as the Cd was adjusted accordingly. The point is that for a fixed 3D geometric shape, the drag scales with length^2 (ie. "area"), air density, and the velocity^2. The Cd is just a non-dimensional coefficient. In fact, the 1/2 could be included in the Cd if people chose to do it that way. The only reason they keep it out separate is because the dynamic pressure is a recognized term that is also used in many other calculations.

 

^oh boy..