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Old 05-08-2010, 03:11 AM   #11 (permalink)
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I was thinking more of a complete kammback with a hingd mechanism to keep the trunk functioning. He never did post the results of the "uggyback", though. I'd also want to get some clear material so I can still see out the back window.
Even a short Kammy is better than none, as long as you get the angles right. In fact, the first foot will help more than the second. Diminishing returns.

Maybe make a permanent shorty for short trips, and a removable extension for long, high speed trips? The short Kamm need not be transparent - in my case the only thing it covers are the headlights of trunks behind me at a traffic light, other than that everything's OK. In fact, it's better, since it keeps the sun off the rear window (doesn't "light up" the dust and keeps the temperature down). The extension could be clear, and since it would hook up to the existing Kammy, it would be shorter, lighter and easier to work with.

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Old 05-08-2010, 10:04 AM   #12 (permalink)
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Filled up the gas tank, and the average is 26.3 MPG. It seems to be staying pretty consistent now, which is nice. I'm going on a 330 mile round-trip drive today, so that'll use up a tank.

I also thought that if the kamm was too short (such as simply detaching airflow off of the roofline), then it would just make it seem like the rear end was larger and produce a larger wake, since the airflow does reattach to the trunk right now. I guess I wasn't thinking about the flow around and above the vehicle as well.
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Old 05-08-2010, 11:35 AM   #13 (permalink)
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I also thought that if the kamm was too short (such as simply detaching airflow off of the roofline), then it would just make it seem like the rear end was larger and produce a larger wake, since the airflow does reattach to the trunk right now. I guess I wasn't thinking about the flow around and above the vehicle as well.
A short roof extension wouldn't reduce your wake as much as reduce Cd (Cx) and lift (Cz). As ShadeTreeMech mentioned, your rear window is steeper than it should be. As the rear angle becomes steeper (starting from 0°), Cd gets smaller, with a minimum between 12° and 14°, and then grows until about 28°-30°, when airflow detaches. Lift increases with steepness, also until about 30°. For nice charts, see this post.

So what you want to start with is a short roof extension that will direct the airflow in the right direction, instead of allowing it "stick" to the rear window, increasing drag and lift. With a longer Kammback you start to get the added benefit of wake area reduction.

A second option is to raise and extend your trunk line. I believe that Frank Lee was interested in doing this with his Tempo, and Moonmonkey played with it on his Echo. Here are two threads on the idea:
Reattaching wake and drag penalty
Decklid mock-up photos
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Old 05-08-2010, 12:11 PM   #14 (permalink)
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And that's perfect, because I'm looking at reducing both drag and lift.

Come to think of it, I noticed a small spoiler on the roof of a BMW once.



Also, would it be a good idea to straighten out the sides as well? The C-pillars curve around to the rear glass, and I'm thinking a clean, sharp edge to detach flow would be better.

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