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Old 03-25-2010, 02:39 PM   #1 (permalink)
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New bug, .38 CD why?

Title pretty much says it all,

For being round and what looks fairly aero, it isn't. Could it be the back wake creating most of the drag?



That doesn't seem like it would have much drag, does it?

cant post images... need 2 more posts

The link states that the rear bumper is where the flow is disrupted, and that causes the lift and severe drag as well.


Last edited by Daox; 03-25-2010 at 03:53 PM.. Reason: add image code
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Old 03-25-2010, 02:56 PM   #2 (permalink)
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Think it is the rounded rear part that is detrimental to it's cd.
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Old 03-25-2010, 03:29 PM   #3 (permalink)
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Why? The Bug is a fashion statement, not an aero one. That pretty much says it all.
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Old 03-25-2010, 03:32 PM   #4 (permalink)
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I thought that the new Bug had a Cd of 0.45? The curve in the rear is way too steep. I doubt it has attached flow as far down the back as the picture shows...
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Old 03-25-2010, 03:35 PM   #5 (permalink)
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I love drawings with arrows that purport to show airflow. :heart:
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Old 03-25-2010, 03:50 PM   #6 (permalink)
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.38

The New Beetle is short.It has a very low fineness ratio.While it has good penetration,once it gets to the frontal area cross-section,the curves rapidly approach those that cannot support attached flow,creating a significantly large wake,'poison of mpg'.
For performance,VW/Porsche/Audi squashes this shape down to create the Audi TT.
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Old 03-25-2010, 04:21 PM   #7 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I thought that the new Bug had a Cd of 0.45? The curve in the rear is way too steep. I doubt it has attached flow as far down the back as the picture shows...
In this case, the sooner the airflow detaches the better.

EDIT: The following in red isn't exactly true, see post #10 for details.

I saw a chart somewhere (can't find it right now, grrrr). Pretty much it showed that as the rear angle gets steeper (starting from 0°=flat) lift decreases until 12°-14°, then it increases again until 26°-28° when airflow detaches and lift goes down again.

So, if you can find the spot where the tangent is 12° and make a small "step", then the airflow will detach there, reducing lift at the best point. Many new cars already have this small "step" around the top of the rear window.

Quote:
Originally Posted by aerohead View Post
While it has good penetration,once it gets to the frontal area cross-section,the curves rapidly approach those that cannot support attached flow,creating a significantly large wake,'poison of mpg'.
If you extend this "step" into a Kammback, you keep the angle right while reducing wake area, and keeping Aerohead happy at the same time
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Old 03-25-2010, 04:23 PM   #8 (permalink)
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for good aero you basically want a teardrop shape with a long tail. less ideal but still good you could have a shorter shape with sharp rear edges that allow for a clean sepparation of the airflow.

the beetle has none of this, the air separate very early and will become turbulent,
also the wheel wells will cause the air to sepparate and not reattach to the body.

i'd assume the air starts to sepparate much sooner, i think at the top of the rear window, and also the wheel fairings will greate much more turbulence than the image shows
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Old 03-25-2010, 06:44 PM   #9 (permalink)
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Well the link shows video of the tuft testing, and it does look connected up to the end of the trunk.
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Old 03-26-2010, 10:19 AM   #10 (permalink)
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OK, I looked for the chart and found it. I was a little off, I got angles right, but mixed up lift (Cz) and drag (Cx). At my age, it's good I remember anything
Here are two charts, they are from Piechna's book. The data is compiled by the author from a few sources.



The first chart shows how drag first decreases, then increases, and finally airflow detaches, with the rear angle getting steeper.



The second chart is lift. When the roofline of the vehicle is flat lift is actually negative (=downforce). It grows linearly as the rear angle steepens, zeroing around 6°-7°, then abruptly falls to almost zero after 30°.

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e·co·mod·ding: the art of turning vehicles into what they should be

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