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Old 06-21-2009, 08:37 AM   #20 (permalink)
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scenic - '01 renault scenic
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megane - '97 renault megane classic
90 day: 72.53 mpg (US)

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Pale, I was wondering how to extend my uglyback down to the rear end of the notch while keeping it usable. My goal is to have a hatchback à la Citroen C4 coupé :

But while my notchback has a Cd of 0.31, I know that the 5 doors version of my megane has a Cd of 0.32 :

Certainly because the angle is too important, more than the one of my uglyback. I have to confess that I didn't calculated/measured the angle of my uglyback, but when I saw the 5 doors version side by side to my notchback I said me that I couldn't make worse and simply extending the roof should be OK.

From Hucho's book, the ways to improve the Cd of a notchback are to :
  • lower the angle of the rear window with the horizon
  • increase the length of the notch and
  • increase the height of the notch.
This book shows the notchbacks of the v.II vs the v.III of the audi 100 : respectively 57.5° vs 64.25° (Cd -0.01), +75mm (Cd -0.01) and 50mm (Cd -0.017).
I interpret this as : "the more the airflow reattach to the notchback, the more the Cd decreases".
It seams I lowered so much the angle of the rear window with my uglyback that the airflow doesn't touch the notch anymore. To make the airflow reattach, I would need something like this :

Which would also permit to open the hatch.
Now the questions are :
  • should I close it ?
  • do I close it at the front and/or at the rear side ?
  • if I don't close it then should I put 1 or more parallel spoilers inside it ?
I'm reaching the limits of my competence... didn't had a lot of opportunities to read Hucho's book and only consulted some chapters I needed.

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