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Old 08-28-2012, 12:58 AM   #33 (permalink)
christofoo
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00C - '00 Toyota Corolla
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Thanks for this tool, I love it.

I want to make a comment about using the template - hopefully some of the aero experts here will chime in.

While fitting my cars I kept wondering if I couldn't slide the template a little left of the roof's apex, to bias somewhat in favor of lower tail surface area. Essentially the question is, if my design is limited by length, is it better to err on the side of slightly greater turbulence (or should I say mildly detached flow) or slightly greater tail surface area?

(Asside from Cd, doing so would bias towards lift, but as long as the "fudging" is small then the lift may be small too. I doubt a Kammback would have more lift than a sedan. For a full boattail, they usually end up steeper than 10 deg on the bottom for clearance anyways, which should counteract lift.)

I thought to check this against some low Cd production cars - where I'd expect to see Cd-optimized length-limited designs.


The Prius is quite biased.

The 2011 Honda Insight is the closest to the template of these three. The fit is close enough it may be hard to see the deviations right away, but the template is a little left of the apex, and the tail still slopes just a little steeper than the template.

The Volt, even when I interpret it's little spoiler as the tail, is also biased in the same way, not as much as the Prius.

Cd's, if anything can be believed, might be 0.28 for the Volt, 0.32 for the Insight, and 0.30 for the Prius. (I get the impression from the pic this was test with mirrors on - Cd seems like an awfully fuzzy number anyway.) gm-volt.com/2009/12/04/chevy-volts-coefficient-of-drag-is-0-28-beats-prius-and-insight/

I'm tempted to conclude that if the full-time professionals are pinching the curves when they're short on length, then it's probably the best solution, even from a Cd point-of-view. By my fitting, they only pinch a little. But it doesn't take too much pinching on the curves to net several inches reduction in a Kammback.

To play devil's advocate, it could be that the big car-makers want to maximize rear windshield slope for other reasons, like visibility. It could be possible that they sacrificed more Cd in getting these curves than they gained by keeping the tail area down. (Earlier I suggested they may be making space for a trunk, but actually all 3 of these cars are hatchbacks.)

It just makes sense to me that the *optimal* solution should change a little if you add a constraint on a parameter.
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Last edited by christofoo; 08-28-2012 at 04:29 PM..
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