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Old 07-03-2008, 04:48 PM   #5 (permalink)
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Darin,This is a little scattered so forgive please.I thought I had a photo of an ideal teardrop shape over at maxmpg.I tried to go over and check and my brain froze and couldn't get in.Forgot my password.I'm in town and copy center will be closed tomorrow and for long weekend and I can't get to the house and back to get my password.

Here's where I'm going...

That 2.5:1 teardrop can serve you as a template for a proper roofline profile.This is a little weird but try and visualise this.The teardrop,if it's width (at it's widest point) is twice the height of your car (any car), when a line is drawn through the center of it lengthwise,defines the ideal roofline template form, along with it's mirror image projected under the ground.

If you were to park a scaled photo of your car under the form of the teardrop,with the bottom of the wheels on the line bisecting the teardrop( the ground plane),such that the point where your roofline just begins to descend, is exactly at the point of max cross-section of the teardrop,the upper arc described by the teardrop rotating towards the rear,describes the perfect roof curvature for any car of any height as long as the teardrop's width,is twice the height of the car .

I've been working on this for weeks now.Kamm's car fits the profile.Jaray's car fits.Lay's car fits.The Gentoo Penguin fits. Mair's boat-tail fits under perfectly.If you go "outside the box" you aggravate skin friction.If you go "inside the box",the boundary layer separates and your screwed.As long as you stay on the curve you're streamlined.

If you extend it all the way,it intersects the ground at a point which is 1.78 times the width of the teardrop,as measured from where the roofline just begins it's downward curve. Chop it off anywhere you like,and you're "clean" up to that point!

I was going to wait to say anything about this until I could post it with graphics, then I just now saw your post.I think this is like a universal aerodynamic constant.If the flow is good up to the vehicles point of maximum cross-section,if you follow the outline of the teardrop,you'll have attached flow the whole way.

By the way,all the talk about "phantom" boat-tails behind cropped aft-bodies does not appear to hold up under scrutiny.I'll be in touch soon.Happy fabricating!

P.S.,with this relationship,you can reliably predict the Cd of any vehicle who's roofline stucture basically mirrors that of the body.
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