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Old 03-13-2008, 07:53 PM   #1 (permalink)
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Square shapes for aerodynamics ?

How is it that cars such as the Opel g90 ( point 22 .Cd ) the Ford Prodigy, and others such as Cadillacs can have a front end that literally has a completely flat front end with no curvature , and huge gapping grills as well, yet superb drag coefficients ?
Also, there are no wheel covers on their wheels ( which are also open spoked versus smooth. )

I don't understand.

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Old 03-13-2008, 09:07 PM   #2 (permalink)
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The simple answer would be it's not how much you disturb the flow up front, it's how smooth you leave it behind.

The complete answer can be found by reading the aero forum and the references you'll find in it.
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Old 03-13-2008, 11:21 PM   #3 (permalink)
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I don't think the Opel is as bad as it looks, at first glance. There's quite a bit of roundness to the front in plan view (to my eye), and the front/side transition doesn't look horrible.

On top of that, the designers undoubtedly sweated the "detail optimization" portion of the design to compensate for some styling compromises. EG - partial rear skirts, wheels fill the arches, the wheel covers are fairly smooth, low ride height... there doesn't even appear to be the usual rain channels on the leading edge of the A pillars.
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Old 03-14-2008, 03:16 AM   #4 (permalink)
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The low cD is probably due to windtunnel data. Surprisingly minor changes to a non-aerodynamic shape can create significant differences in drag. These minor changes are pretty much impossible to find outside of the wind tunnel.

Food for thought: the lowest cD of a wheeled vehicle in ground effect - ~0.15

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Old 03-14-2008, 07:43 AM   #5 (permalink)
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Quote:
Originally Posted by LostCause View Post
The low cD is probably due to windtunnel data. Surprisingly minor changes to a non-aerodynamic shape can create significant differences in drag. These minor changes are pretty much impossible to find outside of the wind tunnel.

Food for thought: the lowest cD of a wheeled vehicle in ground effect - ~0.15

- LostCause
I'd like to add that .15 is assuming a completely smooth body.
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Old 03-14-2008, 09:14 AM   #6 (permalink)
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Quote:
Originally Posted by LostCause View Post

Food for thought: the lowest cD of a wheeled vehicle in ground effect - ~0.15

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I thought the lowest Cd for a ground vehicle was 0.11, which is what they claim is the Cd for the Aptera.
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Old 03-14-2008, 09:20 AM   #7 (permalink)
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Quote:
Originally Posted by MetroMPG View Post


I don't think the Opel is as bad as it looks, at first glance. There's quite a bit of roundness to the front in plan view (to my eye), and the front/side transition doesn't look horrible.

On top of that, the designers undoubtedly sweated the "detail optimization" portion of the design to compensate for some styling compromises. EG - partial rear skirts, wheels fill the arches, the wheel covers are fairly smooth, low ride height... there doesn't even appear to be the usual rain channels on the leading edge of the A pillars.
Rain channels are one of the biggest culprits to poor aerodynamics, especially the way they did them in the 60's.
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Old 03-14-2008, 11:00 AM   #8 (permalink)
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The back end of a vehicle is what needs to be relatively smooth for good aero. Pressure drag is a killer...
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Old 03-14-2008, 01:02 PM   #9 (permalink)
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Quote:
Originally Posted by basjoos View Post
I thought the lowest Cd for a ground vehicle was 0.11, which is what they claim is the Cd for the Aptera.
Ground effect is the key part. The Aptera isn't exactly your average ground vehicule. The 0.15 reference is for a basic bluff body with wheels at regular car height. The higher from the ground you go, the lower the Cd will be for a given shape.
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Last edited by tasdrouille; 03-14-2008 at 01:23 PM..
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Old 03-14-2008, 02:58 PM   #10 (permalink)
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Interesting to note how the mirrors are not at the base of the A pillars.

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