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Old 05-29-2007, 04:00 AM   #1
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my airdam theory

there's a few other threads going on about airdams, but i'd like to start this one to get some feedback on my plan to build my own.

the stock airdam is to short by about 5-7cm imho
if i look underneath the car i see the following sticking out:

-suspention links sticking out
-underside of the engine block with exhoust "guts" in the airstream
-grearbox

my reasoning:

the air is going to hit something one way or another.

so i'd rather have it hit an aerodynamic shape that directs it where i want it to than a lot of rumble that causes turbulence.
or in other words, i can't reduce frontal area but i might be able to reduce the drag quoefficient of that area.
the fact the airdam is the way it is might be for a few reasons:
-the designers didn't want it to hit something
-they wanted a bit of a breeze underneeth the engine to cool the exhaust or more likely the breaks

i suppose there's different things that need to be done along the line of the airdam

a: the sides:
direct the air around the outer edges of the car, and maybe a little portion underneath it.
the extention will follow the curve of the bumper, and i figure it should either be straight or angeled slightly upward so as to motivate the airflow to go over the side rather than underneath. new cars (like some new mercedeses)seem to have a "buldge" there

b: the center:
since theres more pressure above the lower airdam than below the air will always want to go underneath the car. thus let us not interfear with it's direction, just it's flow (remember my aim was to reduce drag) so i think here i should use a plate that's angeled backwards.
or even curved so that it will release the air in the direction where i want it to go.

finally i'm wondering how the lower edges should look.... i suppose it's not a bad idea to have some sort of ( L ) shape rather than a flat end ( | or \ )
because the air might want to go back up after passing the obstruction.

anyway i'm no aerodynamic expert by any means but this is the most intelligent idea i could come up with.

any feedback is appreciated
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Old 05-29-2007, 05:47 AM   #2
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I know that on my car that the FE acutally decreased with a airdam. To much increase in frontal area and drag to overcome the aero benifits.The belly pan only on the other had improvements. When I was working with the airdam the break even point was to have the dam low on the out sides covering the wheels and then cut up to about 2 inches below the bumper across the middle. I think that if going fo FE improvements you won't see it with a airdam. With a belly pan the air dam is not necessary unless you are going for stability improvements. YMMV
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Old 05-29-2007, 07:05 AM   #3
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I agree with that notion in general, and here's some of the reasoning:

The lower drop on the sides is effective because the front wheels are relatively close to it and extend the furthest into the airpath, whereas what undercarriage extends down is typically further back and far less of an addition to frontal area.

At some point, any protrusion into the airflow creating a vortex or turbulence has an area behind it at which the air resumes less turbulent flow. For lack of formal education on the matter, I'll call it a turbulence envelope. Anything inside this turbulence envelope is unlikely to cause additional disruption or significant drag, and might even reduce it. Anything outside or beyond the tail of the turbulence envelope is effectively an additional surface to create turbulence and drag of it's own.

In a nutshell, the lower side dam is probably effective because the nearby tires are partially within the turbulence envelope, whereas things like suspension parts, oil pans, exhaust, etc. are too far back to remain within it.

I think that the flow pattern of the air with a lower dam on the sides is another issue to consider as well, as with a raised center and open area to the outside of the car, it creates an even larger area for the envelope to exist around the wheels. A flat across air dam by contrast likely propagates a rolling vortex underneath the undercarriage that does not provide the same drag reduction around the wheels.

This difference in behaviors would explain why fuel economy might improve with lower sides, and down force might improve with a lower center, as the lower sides do nothing to slow the airflow under the car, but the lower center does so by creating turbulence. In other words, it would seem that the two goals would be difficult to achieve together with an airdam alone. But that's where belly pans come in.

Obviously this is just my 'feeling' for the dynamics of it, but I think it's pretty close to the reality of what is going on.
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Old 05-29-2007, 07:12 AM   #4
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Quote:
Originally Posted by Snax View Post
I agree with that notion in general, and here's some of the reasoning:

The lower drop on the sides is effective because the front wheels are relatively close to it and extend the furthest into the airpath, whereas what undercarriage extends down is typically further back and far less of an addition to frontal area.

At some point, any protrusion into the airflow creating a vortex or turbulence has an area behind it at which the air resumes less turbulent flow. For lack of formal education on the matter, I'll call it a turbulence envelope. Anything inside this turbulence envelope is unlikely to cause additional disruption or significant drag, and might even reduce it. Anything outside or beyond the tail of the turbulence envelope is effectively an additional surface to create turbulence and drag of it's own.

In a nutshell, the lower side dam is probably effective because the nearby tires are partially within the turbulence envelope, whereas things like suspension parts, oil pans, exhaust, etc. are too far back to remain within it.

I think that the flow pattern of the air with a lower dam on the sides is another issue to consider as well, as with a raised center and open area to the outside of the car, it creates an even larger area for the envelope to exist around the wheels. A flat across air dam by contrast likely propagates a rolling vortex underneath the undercarriage that does not provide the same drag reduction around the wheels.

This difference in behaviors would explain why fuel economy might improve with lower sides, and down force might improve with a lower center, as the lower sides do nothing to slow the airflow under the car, but the lower center does so by creating turbulence. In other words, it would seem that the two goals would be difficult to achieve together with an airdam alone. But that's where belly pans come in.

Obviously this is just my 'feeling' for the dynamics of it, but I think it's pretty close to the reality of what is going on.
I think for optimum FE that the way to go is wheel deflectors and a belly pan if FE is the main objective. Looking at new cars the deflectors are pretty standard now and most have some kind of partial belly pan at least under the front of the car.
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Old 05-29-2007, 07:19 AM   #5
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Can you post some pics of how it is now?
Maybe a side view so we can see the front-to-rear dimensions of bumper + air dam + wheel cutout.
I'm trying to picture it in my mind's eye and could use some help.
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Old 05-29-2007, 08:15 AM   #6
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ok here's a few pics i already had on line. i have some showing the rumble underneath wich i could upload later





as you can see in the sideview the bumper is basically the only thing in front of the car... when removed you basically look straight onto the radiator,engine and into the front wheel wells!
the econd picture is deceptive... the airdam looks quite low untill you get down on all four's and look straight underneath... the lack of aerodynamics in this area is quite shocking giving the attention that was givven to it everywhere else.

my main reason for wanting to extend the airdam 2-3 inches
is this picture



this is the opel kadett GSI high power version on my car who's Cd was actually 0.01 to 0.02 lower than the standard kadett and this only by the addition of air extraction vents in teh hood, a rear spoiler and a different airdam.
right now it's around 0.31-0.32 wich is not terrible for something build in the 80's or ,sadly, even for a car build today but i think something can still be impoved .

while the airdam possibly contributes the least to the lower Cd it might count for something.
what i wanted to add is something similar so the lower strip on the red car's airdam along with the wheel spoilers yet with a center section that curves backward.

at best it would be a horisontal sheet that coveres the entire lower bumper and directs the air down and over the engine

i suppose i'd best keep the wheel spoilers and the center section sepparate so i could try out different configurations and don't have to overdo everything in case something works against me.
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Old 05-29-2007, 12:37 PM   #7
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this is more or less what the air sees that goes underneath the airdam. composite photo so the dimentions are not exactly right.

the airdam sits about 25cm - 10inch in front of this rumble. i can't help but think there has to be some way to direct the air underneath this in an orderly fashion.

what's the best thing to do? extentiong of the airdam in front of the weels? the original GSI had them so i guess that's a proven mod.

extend these fairings sideways so that they cover the suspention links...?
reading everything above i wonder if that will help?

oh yea i have a little oil leak around my ignition coil (or how's it called) othing serious but that's why the engines a little smugy... just need to find the right gasket to replace it
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