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Old 08-22-2012, 10:03 AM   #10 (permalink)
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Originally Posted by drmiller100 View Post

if you have squared sides, and things are not perfect, you will induce vortices which will "fix" the imperfections.

build the thing.

keep side and top tapers to 20 degrees or less, and bottom taper to 10 degrees or less.

i would not worry about the corners.
Mr Miller,

I think you need to reconsider this. Creating a vortex off a c-pillar is not what you want. You may be thinking cute little burbles of air that help the air "Re-attach". This would be an almost impossible thing to engineer with a team of scientists, a wind tunnel, mission statement, and $2 million NASA grant. So I doubt a guy in his driveway with duct tape and coroplast will nail it.

The Vortex creation that needs to be avoided are the long trailing tornados like you see off the wing tips of airplanes. These are very common. The energy required to create and maintain these is tremendous and it can only come from one place, the cars engine. We all know what that means.

The C-pillar is the most likely spot for these to form and they're caused when large pressure differences are within close proximity to each other, the air tries to move sideways from high to low pressure and in doing so, sets itself spinning.

So, in rough numbers terms, if Neon were to "Just Build it" yeah, anything is likely better than nothing, and he may realize 30% of the potential gain out of building a kamm extension just due to the fact that he's "Stuffing the Wake". But if he builds it properly, and radiuses the edges, he'll have a more optimized device that will see 70% of the potential gain. So 25MPG base, if he stuffs the wake, 26.5MPG, builds it better, 28MPG. Would he be happy with 26.5? Probably, but why build something half-baked when if you take the time to do it right, the result is so much better?

Also the 20 thing is not a very good rule of thumb. One has to consider what you're starting out with. If the slope at the top of rear glass is already at 10 or so, then yeah, 20 works, but if it's flat, 20 would be too drastic a change and would not be optimal. I really don't think you want more than a 10 change at any one point. This is why a blended curve works so well when adhereing to the template, it allows the air to transition as undisturbed as possible optimizing the result.
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