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Old 03-17-2008, 02:19 PM   #1 (permalink)
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Dealing with underside airflow & the front bumper (transition radius to undertray)

It seems to be warming up, and that means I can actually work outside more (this year's cabin fever has been horrible). So, I'm looking at a bellypan for the Paseo and have been thinking about how I want to make the transition between the front bumper and the pan. In the picture below you can see the car currently has a small air deflector on the bottom (all current stock equipment is gray). I'm thinking the best route to go with a bellypan would be to drop down to that level from the front of the bumper (see the red dots in my pic). Any ideas or comments?

Here are my less than great pictures:






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Old 03-18-2008, 10:17 AM   #2 (permalink)
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Sounds reasonable - you definitely want a radius between the bumper bottom and the pan, if possible. See the EV-1 for example:



There was a pic out there of Brigham Young U's EV1, shot from underneath the car on a lift that better shows the bumper/front pan transition. But I can't find it at the moment.

What does Hucho say on forward facing transition radii? 2 inches?
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Old 03-18-2008, 10:59 AM   #3 (permalink)
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Thanks for the pic. That is pretty darn close to what I was thinking of doing with my bumper actually. I was thinking of adding a vent for the radiator in the front radiused section of the underbody panel. In any case, I think I'll use the EV-1 as a template.

I'll have to look at Hucho's book. I haven't been reading it much lately and am only on chapter 4. But, I believe he said something like 2-4" when he was describing the VW bus. I remember the section, just not the exact number.
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Old 03-18-2008, 11:01 AM   #4 (permalink)
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Daox -

I like the shape you were thinking of. An alternative I have been thinking about is to find an old bumper of my car from a junk yard and "double-deck" it :



I think this would be harder for you than for me because it's easier for me to find a Saturn bumper.

I would call this the "Biplane Mod".

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Old 03-18-2008, 03:52 PM   #5 (permalink)
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What about doing something a bit more drastic? Reshaping the front a bit more like such? It would be much more of a bland look, but I would think a bit more aerodynamic.

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Old 03-18-2008, 03:57 PM   #6 (permalink)
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You'd be lowering the stagnation point, and sending more flow over the car than under (as well as increasing front downforce).

But will that reduce drag? Not sure. But I'd guess "yes", since you're diverting more air away from the dirty underside.
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Old 03-18-2008, 06:32 PM   #7 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
You'd be lowering the stagnation point, and sending more flow over the car than under (as well as increasing front downforce).

But will that reduce drag? Not sure. But I'd guess "yes", since you're diverting more air away from the dirty underside.
According to Hucho's book, the last proposed design would be better.
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Old 03-18-2008, 06:33 PM   #8 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Sounds reasonable - you definitely want a radius between the bumper bottom and the pan, if possible. See the EV-1 for example:
If I undertand you guy correctly, what you are saying is that the bottom edge of the bumper should have a rounded edge ( 2" ) instead of just a sharp cutoff ?
( When there is a bellypan installed . )

Why is this ? Does it add more / less velocity to the airflow travelling under the car ?
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Old 03-18-2008, 07:06 PM   #9 (permalink)
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It helps keep the flow attached. In the book it references A pillars more so than underbody airflow. However, the principle is still there.
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Old 03-18-2008, 07:16 PM   #10 (permalink)
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Raises an interesting question though: if there's no belly pan for the dam to transition to, is a lower radius strictly necessary?

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