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Old 03-04-2008, 11:43 PM   #8 (permalink)
brucepick
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Outasight - '00 Honda Insight
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Quote:
Originally Posted by apgrok1 View Post
So a roof tail would be bad, VG's are good, and the longer the tail fairing, the better, right?
I've seen a lot of roof extensions that I think actually help, on squarish backed vehicles like hatchbacks, wagons, and SUV's. If the roof slopes gently downward towards the rear then an extension or roof spoiler can continue that line past the rear window. (A steep-sloped roof or hatch doesn't help; the air flow detaches rather than sliding along the surface.) The lengthened slope reduces the final cross sectional area at the rear, making for a smaller wake or turbulence zone. This is especially true if the sides are angled inwards a bit also.

Take a look at the writeup on Kammback in wikipedia:
http://en.wikipedia.org/wiki/Kammback

The roof extension type "spoilers" are really an attempt at a very short Kamm back. I'm sure one can argue a case that the overall benefit of this is very slight but that's the case for most of the features that contribute to better aerodynamics or fuel economy. And I don't think it's such a small gain. A roof that slopes down and sides that taper inwards towards the rear will have less drag than the same design with a roof and sides that continue level and straight towards a vertical back end - all other things being equal of course.
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