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Originally Posted by brucepick
When I originally suggested this thread (which was split off from another) my intent was to see more information on diesel_john's car. However this thread got titled with my own car's description.
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If diesel_john replies to your request to write more about his aero mods, I'll make it into a new thread as well. The more the merrier!
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Neat drawings. Don't forget: the initial angle may be in the range of 12-15 degrees, but if you use a gentle ellipse rather than a straight line, you can continue to curve towards that theoretical point behind the car where all planes meet, and exceed the initial transition angle where you depart the roof line.
Also: don't think so much in terms of adding a pyramid shape to the back of the car. The "hard" corners between the planes (e.g. sides & top) promote vortex formation; You want to round those transitions generously if possible.
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I don't know if I dare build this thing.
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You don't have to go to extreme lengths. Any boat tailing will help some. My 2 foot long cardboard kammback (which was mostly a roof extension, and had poorly shaped top-to-side transitions) made a measurable (if small) difference @ 55 mph:
http://ecomodder.com/forum/showthread.php?t=94