Belly pan ??? - Fuelly Forums

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Old 07-28-2006, 11:24 AM   #1
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Belly pan ???

I'm working on my belly pan this weekend. What is more effiecent. As the pan passes the rear suspension should it arc up to the rear bumper, Run the same parallel distance to the ground, or somewhere in the middle.

I am able to make side skirts to extend down to the pan behind the rear wheels if it needs to be level all the way. I hope to get the belly pan and grill block done this weekend. Also would there be any benefit to extending beyond the bumper.
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Old 07-28-2006, 11:42 AM   #2
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Quote:
I'm working on my belly pan this weekend. What is more effiecent. As the pan passes the rear suspension should it arc up to the rear bumper, Run the same parallel distance to the ground, or somewhere in the middle.
Depends on whether you consider lift or downforce to be important.
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Old 07-28-2006, 12:46 PM   #3
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Well I want the least amount of aero drag. I was thinking parallel to the ground but am unsure. The amount of lift or downforce generated is unimportant as long as it is not dangerous and adds FE. I realize this arm chair engineering so I'm looking for a best guess. Here is a pic of an SL

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Old 07-28-2006, 12:47 PM   #4
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I'll be deleting the front airdam and running parallel with the side rockers. I'm just not sure about the rear.
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Old 07-28-2006, 02:57 PM   #5
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The rear should be a smooth curve up the the bottom of the bumper, I'm sure mira or darin will give a better description than I have.
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Old 07-28-2006, 04:33 PM   #6
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I have been researching this. I'm worried that if I arc up to the bumper that it might cause the venturu effect (discussed here http://www.nas.nasa.gov/About/Educat...undeffect.html)

My reason for worry is the angle up will be sharp. How about a shallower arc with side skirts. This will allow me to cover the muffler, which is a large bump down.
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Old 07-28-2006, 04:35 PM   #7
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Indeed, from what I understand you do not want it to slope more than 11 degrees, I think it would be better to have it end lower than the bumper and not exceed 11 degrees than it would be to meet the bumper and be 12+ degrees of slope, but I'm no expert!
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Old 07-28-2006, 04:52 PM   #8
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I'm no expert either, but I think what Ben just said holds true. I've read that 10-15 degrees is the range of slopes where flow remains attached. Closer to 10 is "safer".

And like the lip of the trunk discussed elsewhere, you also want to terminate the tray abruptly - ie. avoid trailing features that end with radii to other planes. If the bottom of the bumper cover is curved (as most are), you might want to consider having the pan extend a little past the curve:

(bottom of bumper as viewed from the side)

*********
*********
********
*******
****
---------- (end of pan)

I think XFi used this approach on the rear tray of his car.
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Old 07-28-2006, 08:31 PM   #9
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Quote:
Originally Posted by MetroMPG
I'm no expert either, but I think what Ben just said holds true. I've read that 10-15 degrees is the range of slopes where flow remains attached. Closer to 10 is "safer".

And like the lip of the trunk discussed elsewhere, you also want to terminate the tray abruptly - ie. avoid trailing features that end with radii to other planes. If the bottom of the bumper cover is curved (as most are), you might want to consider having the pan extend a little past the curve:

(bottom of bumper as viewed from the side)

*********
*********
********
*******
****
---------- (end of pan)

I think XFi used this approach on the rear tray of his car.
This is what I had in mind. You guys have confirmed my theory.
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Old 07-29-2006, 04:50 AM   #10
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A slope up to the bumper should allow the underflow to help fill the low pressure area at the back?

Blending it in with the bumper should allow the flow to stay attached further into the low pressure area?

Adding a row of vgs right before the transition to the bumper might help it stay attached?
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