Front air dam cross section - Fuelly Forums

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Old 05-28-2007, 09:32 AM   #1
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Front air dam cross section



OEM Factory air dam bottom edge: 11" from ground
Current air dam bottom edge: 7" from ground

Front face of lower dam is vertical.
I'm considering a lower lip at bottom, extending forward.
Also considering changing the angle of the added air dam to slope forward as it extends downward.

What do you think? I've been searching for air dam/spoiler design principles but I haven't found too much. My aim is mostly to keep air out from under the car. It does seem to be gliding better, having completed it as far as what you see here.

A friend of mine wants to mold what I have in fiberglass but we need to optimize the design first. Would a lower lip or a forward-sloping angle help?
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 05-28-2007, 10:29 AM   #2
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Like this sorta kinda ?



or like this ?



of the two the second seems to follow the Volvo lines better . The 70's Camaros used the cow catcher style - never was a big fan of that look .
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Old 05-28-2007, 11:03 AM   #3
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performance air dams often have a horozontal lip extending forward for a couple of inches. The purpose is to provide a surface for the high pressures area to act downward, improving the down force on the fornt of the car. This approach is probably only measurable at highers speeds, but in any event would not help mileage as it blocks more air and is designed to increase rolling resistance at high speeds.

One idea I've though about is to attempt to redirect more air upward over the better aerodynamic body shape, rather than under the car. A normal blunt nose would redirect air evenly. Perhaps a dam sloped up backward from the very bottom would redirect more air up...

But this is just a loose thought idea. I have not tested and usually things behave much differently than what we think, when it comes to aerodynamics... done properly you'd probably get some lift, which would be bad for traction, but maybe good for mileage (unless the lift had other negative altering effects like actually increasing air flow under the car... who knows.

The more I play with this stuff the more I see why the pros use wind tunnels and yarn tests. Even a small change can have unpredictable results (CFD programs just aint there yet for this kind of stuff, at least outside for us civilans
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Old 05-28-2007, 11:51 AM   #4
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MnFocus asked "like this?"

We're considering like this:



or maybe like this:


But based on Ica13's notes maybe we should stay with what it is now?
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 05-28-2007, 01:33 PM   #5
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My vote is for vertical as it is now . It works as OEM (just closer to the ground) and asthetically appears as OEM(meaning it won't look just tacked on there) .
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Old 05-28-2007, 06:36 PM   #6
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Quote:
Splitters supposedly work pretty well but you better make it either exceedingly stout
Yes,splitter, thats the word your looking for. They are supposed to force the air up over the front of the car instead of letting it slide underneath.
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Old 05-28-2007, 07:10 PM   #7
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I'd like to emphasize the original question as I clarified in the 2 small pics in my 2nd post in this thread.

Is there FE value to be gained by adding a protruding lower lip or by going to a forward-sloping front face - as in my two small pics? (Thanks, Ica13, for answering the question.)

Any other suggestions on optimizing the front face of this air dam?

Clencher, thanks for pointing out the durability thing. I've scraped the lower edge of the plywood vertical face two or three times and it's held up extremely well. Running a finger along the lower edge found no damage! Durability counts for a lot - after all it's a Volvo! Based on your notes I'll likely keep my wood creation as a backup if my friend makes it in fiberglass as he has suggested.

Re. a horizontal splitter:
I see that as definitely a good concept however I think it would need to be at least a foot forward of current air dam location to be able to scoot air up over the bumper and the front grill area. Then the air hits the windshield which is more vertical than on modern cars. So a splitter would be difficult and maybe not much gain due to a non-aero windshield. Better to push it out towards the sides is my guess.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 05-29-2007, 04:38 AM   #8
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sorry to run something of a parallel tread on the same subject but i didn't want to highjack this tread with my own project

anyway, here's some of my thoughts on airdams

http://www.gassavers.org/showthread.php?t=3570
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Old 05-29-2007, 07:54 AM   #9
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Paraphrasing very casually, in another parallel thread zpiloto suggested that an airdam will likely help FE less than a belly pan. Or possibly an airdam might actually be a negative for FE. Hmmm. I can see how that could be true.

One thing not shown in the pics above is the forward belly "tray" that I built. It has a curved front edge that follows the air dam's contour. In fact it determines the contour of the air dam's front (lower edge) and the side panels as all is attached to the tray's edge. The tray extends rearward as far as the air dam sides, and it has a center section that extends further rearward (between the tires) to meet the factory belly pan. So there's a fairly smooth path for air flow to approximately the front axles' line.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 05-29-2007, 05:39 PM   #10
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Since your air dam is downwind of the leading edge of your bumper, you are going to have a drag-inducing area of high pressure piling up in front of your air dam no matter how you shape it. IMO an air dam is a crutch, something to have on your car only if you don't have underbody panelling. The purpose of an air dam is to divert air away from the un-aerodynamic underside of your car, so if you have a smooth underside, you don't need an air dam. An air dam simply adds to the frontal area and can be damaged by collisions with road debris. I don't have an air dam on my car and would prefer to install underbody panelling over installing an air dam. Here's a shot of the front profile of my car.

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