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Old 05-30-2008, 07:19 PM   #2 (permalink)
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Vibe Kamm back / boat-tail

trikkonceptz, good question.And your in luck,as you already have the Kamm-back.GM and Toyota have done a really good job on the Matrix/Vibe aero and you should have clean,fully attached flow,up to where your Vibe ends.This was the whole point behind Kamm's work.And with the wagon-back body style you get to steer around C-pillar vortices which I must be careful about with my fastback.And even better for you,is that the body of the Vibe/Matrix begins plan-taper contraction quite a bit forward along it's length,adding pressure along the sides and also top,as the top is also carefully sloping downward as well. All of this means that you can have more aggressive sloping on the tail.So for you,the decision is,how long do you want to make it? If you merely follow the natural progression of the Vibes contours,you'll get a beautiful converging boat-tail and you can chop it off wherever you like ( truncating ).The further back you go,the steeper the slope can be.I posted a thread about the Gentoo Penguin with Cd 0.07 at GOOGLE Images,page 3,showing a single penguin swimming underwater.That's your boat-tail.And a parting thought,as this place is going to close down at 5:30. In the picture of the Mercedes-Benz C-111,they used 28-inches of boat-tail,and it dropped the Cd from 0.2378,to 0.195.Had Mercedes gone ahead and used the full length ( 60-inches ) the Cd would have dropped to 0.178 ( another 25 % drop! ) and the car would have beat it's own record.Consider some concession on length,as you've got to be able to park that thing,and you don't want to knock pedestrians over when you round a corner.If you want to go really crazy like me,consider a very light streamliner trailer for extended highway travel,that would plug into a receiver hitch.It could cut drag to bare minimum on the open road,then you could yank it for daily commuting.Compromise sucks but somehow reality has intruded into my imaginary drag-free world,and even I have had to make some compromises to survive in traffic.Keep us posted,good luck,Phil
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