A Visual Look at Optimum Drag Coefficient (Cd) - Fuelly Forums

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Old 06-21-2006, 08:10 PM   #1
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A Visual Look at Optimum Drag Coefficient (Cd)

In the animal kingdom:

Swift: these can fly up to 106mph on the level.



Greyhound - dog bred for speed. Note legs very thin cross sectional area, stunted tail.



Dolphin - drag coefficient 0.0036 - do you see vortex generators anywhere here?


Eagle

Human Powered:

Varna Diablo II, Cd = 0.02.


Virtual Edge, Cd = 0.011.

The thing unifying all of these is that they are composed of streamlined bodies. Their leading edges all start out quite blunt, curve back smoothly, followed by a much more gentle slope at the rear. Even things that appear to "stick out" are actually just more streamlined bodies only on a smaller scale, such as the fins of the dolphin, the wheel fairings of the hpvs, the rear legs of the greyhound, and the wings of a bird.

We see no vortex generators on any of them, because there aren't any real areas of turbulence that a vortex generator could improve. And the shapes are very, very smooth.

To apply it to our cars, the first areas to obviously clean up are the underbodies of our cars, the wheel areas, and doing something about that rear turbulent area.
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Old 06-21-2006, 08:14 PM   #2
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A useful link comparing various drag coefficients:
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Old 06-21-2006, 09:06 PM   #3
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I'm going to find a picture for you mira...
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Old 06-21-2006, 09:10 PM   #4
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What do you think about that fugly crap for drag on my car?
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Old 06-21-2006, 10:05 PM   #5
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Quote:
Originally Posted by SVOboy
What do you think about that fugly crap for drag on my car?
Which?
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Old 06-21-2006, 10:06 PM   #6
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Haha, I meant that fugly spoiler, sorry!
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Old 06-21-2006, 10:44 PM   #7
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Quote:
Originally Posted by SVOboy
Haha, I meant that fugly spoiler, sorry!
Is that yours?

Man, that shape is a great base to work from. What year? Do you have the specs on it such as kerb weight, Cd (believe it to be 0.32), base mpg, etc?

How much cleaning up is there to do on the underbelly? I assume it would be fairly typical for such a car. The great thing about something like the CRX is that it's already pretty good for a car - small, and modest fastback design. So compared with something like mine which I imagine would get at least upwards of 0.35, probably a bit higher (it's shaped like a miniature minivan so should be similar), it gets most of its improved Cd from exterior modification - the basic shape.

Which is great, because in an attempt to get a low drag, they have done most of the work with the exterior and left all the easy bits for you - i.e. undertray, wheel skirts, wheel covers, deflectors, grille. Which means that there is more potential for drag reduction. You can get your car looking more like a streamlined body than my dustbuster - smooth a semi-streamlined body, it's always going to beat a smoothed dustbuster.

I'd hazard a guess that with a flat car design like yours, you'd get even BETTER results than mine for doing the undertray, simply because you have a wider undertray. And to compare the base CdA figures, mine probably has a Cd of 0.36. Using the gross approximation of the car having a rectangular cross section, my base time height is 3060 square inches. Yours is 3246. That means to compare the two, your CdA would be 6% better than mine, with more scope for improvement. I suppose it's not that much different, now that I think about it, but I think that your advantage would swell to something like 10-15% once we had both maxed out aero mods. (Actually now that I think about, your frontal area is better than it appears, because it tapers to the top. Which means that your CdA figure will beat mine by probably 12-17% once everything is said and done. Bugger.

I'm envious.

The only thing is, they've stymied an attempt to further reduce drag with a boattail by flaring the edges too early (but probably optimal for that length).

I wonder what sort of FE numbers you'd get if you dropped my 660cc EFI engine in there (daihatsu EF-EL)? Since the weight is comparable, you should get similar performance to my car.
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Old 06-21-2006, 10:47 PM   #8
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I suppose that my one saving grave is that I could remove both rearview mirrors, you only have one.
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Old 06-21-2006, 10:50 PM   #9
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It's not mine, that's just an example of the wing.

Mine is a 91, should be a little under 2k pounds, cd is .29 according to the sales brochure. Base mpg for the auto was 29/32 but since I have a random tranny who knows.

The underbelly has cleaning, cleaning it has to be done. I'm just mainly wondering if that little extension wing thing you see on the back does anything besides be ugly?
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Old 06-21-2006, 10:56 PM   #10
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Quote:
Originally Posted by SVOboy
It's not mine, that's just an example of the wing.

Mine is a 91, should be a little under 2k pounds, cd is .29 according to the sales brochure. Base mpg for the auto was 29/32 but since I have a random tranny who knows.

The underbelly has cleaning, cleaning it has to be done. I'm just mainly wondering if that little extension wing thing you see on the back does anything besides be ugly?
It may actually have some effect. Consider 2 vs 3, the hollowness behind 2 appears to lower the drag somewhat.

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