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Old 03-17-2008, 03:14 AM   #1 (permalink)
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Strange OEM Aerodynamic Shape (longitudinal roof indents)

While reading the square shapes for aerodynamics thread, I noticed the Opel G90 (cD 0.22) had a weird indent on the top of its roof. At first I thought nothing of it, as it could easily just be a stylistic feature...

G90 Roof Channel


However, while looking around the internet, I came across the Daihatsu UFE (cD 0.26). I noticed it also has the same hatchback shape as the G90 and uses a similar roof channel...

UFE Roof Channel


I can only imagine the feature for three reasons: Style, Stiffness, Aerodynamics. If the first two are discounted, then I imagine they help aerodynamically by shedding vortices just as a NACA duct does.

If you excuse the poor PAINT graphic, you'll see why NACA ducts operate so efficiently. Air sheds over the sharp sides into the trough by forming vortices. I wonder if the OEM roof channels are acting as mild vortex generators?

Vortex Shedding


Maybe I'm just looking a little too deep for something that's not there. It might be something hatchback owners should look into, though. Thoughts?

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Old 03-17-2008, 06:36 AM   #2 (permalink)
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The ( last generation ) Mazda RX-7 and Dodge Viper GTS ( as well as some other sports cars have this feature as well.
I remember reading the RX-7 brochure and the roof dent was mentioned as an aerodynamic aid.
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Old 03-17-2008, 11:51 AM   #3 (permalink)
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Re: strange shape

I agree on your three points.A number of cars have used the abbreviated roofline over the decades.The "indent" shaves a little frontal area off 4-seaters,where there will be no need of headroom in the center.The rolled edge offers stiffness to a thinner (lighter!) sheetmetal panel,and the whole package provides an evocative "look".I think its intelligent design and one of the small details stylists can capitalize on.
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Old 03-17-2008, 12:14 PM   #4 (permalink)
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Interesting observation, lostcause.

Re: sports cars - often they have raised roofs over the seats to accomodate helmet headroom for competition.

The Prius also has the longitudinal lines in its roof.


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Old 03-17-2008, 01:23 PM   #5 (permalink)
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I've been observing these types of line, for a while. Their are a number of vehicles which also use a number of parallel shapes, which remind me a lot of a piece of wood molding, mounted along the top of the car, down it's length. These are typically minivans, vans and vehicles of that sort. I have started to wonder if those don't have some aero benefits, as well as thin roof strengtheners. If anyone knows for sure, I would be interested.

I have a 89 Honda wagon, which does not have these stiffeners and I do have some need of something to keep it from popping, anyway.
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Old 03-17-2008, 06:17 PM   #6 (permalink)
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longitudinal roof "dents"

These are indeed aero aids - it's a small way to lower the roofline in that particular area and thus lessen the frontal area of the car. It was highlighted in an article in Corvette Quarterly sometime ago.
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Old 03-17-2008, 06:19 PM   #7 (permalink)
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Quote:
Originally Posted by technoshark View Post
These are indeed aero aids - it's a small way to lower the roofline in that particular area and thus lessen the frontal area of the car. It was highlighted in an article in Corvette Quarterly sometime ago.
Welcome to the site! Do you perhaps have a link to this article? It would be interesting reading.
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Old 03-17-2008, 06:55 PM   #8 (permalink)
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Old 03-17-2008, 07:10 PM   #9 (permalink)
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Unless it has a greater effect than the few inches of frontal area that it saves, you would think that a mirror delete would be more beneficial.
Does it create a low pressure area that helps the air stick to the car better ?
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Old 03-17-2008, 08:15 PM   #10 (permalink)
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Cool Corvette

Sorry SVO, I got the print version (and I have no idea how I got on the mailing list, by the way) and it is not in the archives on the website.

The article was an examination on how Corvette owners (the current model) were reporting great hwy gas mileage. .Cd, yes the frontal area saved is indeed small - like many aero tweaks, its in the small things adding up together. The other item they specifically mentioned spending design time on, aero-wise, was the shape and placement of the exterior mirrors.

You can't argue with the results - the story reported (anecdotally) that 'vette owners were reporting an average of 34 hwy mpg - from a sports car with a 400+hp V8. Shows you what is possible when you truly sweat the TINIEST details - aero, engine efficiency, and weight. Shoot, my little Focus wagon 4-banger doesn't get hwy mileage that good - and I'm spotting the Corvette oh, ~270 horses.

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