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Old 06-12-2007, 04:57 AM   #1
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straight wheel and engine deflector theory

i have a few questions/theorys about (underbody) deflectors and fairings i'd like to bounce around... perhaps i'm drawing wrong conclusions but i'm trying to make sense of things i see on production cars to use this info to make my own mods.

i've strated thinking why car makers would sometimes use flat vertical tabs ahead of the wheels and sometimes engines, and what the difference would be with smooth fairings (wich are sometimes seen in place of or in combination with the flat plates).

intuitively smooth fairing seem better but in a lot of cases where it's impractical to fully enclose a drag causeing element, smooth fairings might not be the best option.

the reason why i think this, is that while flat vertical obstructuons to the airflow have a much greater drag coefficient they also have a larger wake.
(as a result to get the same wake one would need a bigger smooth fairing)
and since drag is the sum of the drag coefficient and frontal area, in some cases this may work in the advantage of a small flat deflector.)

usually a big turbulent wake is not what you want but if you want the air to go over lets say an exposed engine underside, or around an area of the wheel where it would otherwise cause greater disturbance in the airflow, a small strip in front of it might create a big turbulent bubble that causes the airflow to pass over the engine and reattache to the hopefully flat undersurface behind the car. otherwise the air would hit the engine and the entire engine would cause a turbulent wake that would extend far beyond the engine. does this make sense or am i drawing wrong conclusions?

the engine's just an example but i've seen certain cars that seem to have a lip ahead of the engine.

the reason why i'm thinking about this is i'm redesigning front wheel fairings of my car. i currently have a smooth fairing in front of the wheels wich is ideal, only i can't make than big enough to cover the suspention links as well so i was wondering if a small vertical ridge in front of the suspention links could work, as this might spill a wake big enough to carry over the links.

i'll try to analise both configurations but i suppose gains will be small so it'll be difficult to tell wich is best.


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Old 06-12-2007, 05:32 AM   #2
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Drawings and/or pictures might help me visualize them.

Reality is merely an illusion, albeit a very persistent one. - Albert Einstein
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Old 06-12-2007, 07:22 AM   #3
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I thought it was because flat tabs are cheaper to make...
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Old 06-12-2007, 09:15 AM   #4
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So you are saying that the purpose of say a front airdam could be to simply force the air flow around the engine bay area rather than under it?
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Old 06-12-2007, 10:33 AM   #5
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Originally Posted by Red View Post
So you are saying that the purpose of say a front airdam could be to simply force the air flow around the engine bay area rather than under it?
That's what I read somewhere before I started my airdam project.

basjoos pointed out that you get more benefit from having a long(er) sloping nose like his which ends up flowng the air over the hood + windshield. I'm sure he's right as his mpg is phenomenal. However my project - with the belly pan reaching from the air dam's lower edge to approx. the front axle line - did have good results.
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 06-12-2007, 01:55 PM   #6
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sorry if my explanation is a little confusing.... my aerodynamic knowledge is just scraped together from the web... and therefore might not be in touch with reality at all....

here's a little drawing that perhaps explains more clear what i think is going on:

it's a car driving from the right to the left.

i'm talking about the underside of the car because that's where there's more room for improvement. bit this could be used anywere where appropriate...

first picture is the way things are stock. you've got something verry irregular sticking out of the bottom of the car... like the bottom of the engine or a suspention link.

now i got most of my inspiration for this theory from this picture

now perhaps this picture isn't representative for car aerodynamics, but when you compare the first two drawings you see that with the same frontal area the flat piece has a far bigger wake than the round shape (smooth fairing)

wich is exactly what you want if a big wake would be what you need to "hide" something in.

does that make sence? of course everything will need to be proportionate because one could easily create more drag than one solves.

the reason why i'm interested in all this is this:

this looks bad doesn't it?

this makes if very hard to attach anything that would cure it.

it's not impossible i'm sure bit more that i can produce for now so something else will have to do.
so finally these where my options:

the third desing is currently installed (i'll post some pictures later). it's a tradeof between closing as much as a could and still enable the fairing to support itself without the need of elaborate metal frameworks.

but i'm considdering makeing a straight downwardextention at the trailing edge of the fairing. some sort of hibrid between a smooth fairing and a flat deflector...

because the smooth deflector ends quite a distance before the suspention link.. especially since i'm worried the airflow will go up again, and hit the link anyway.

so far i have the "impression" there's less roadnoise, so i take that as a good sign. anyway i hope i'm still makeing sence here

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Old 06-12-2007, 02:49 PM   #7
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no matter what shape... gaps can mess you up. Note the grey car pictures, right to left mention above with the triangular versus straight down fairing.... in either case what is miising from the diagram is the turbulence in the gaps.

the middle (triangle) case and the bottom (straight) are probably equivalent from an "after the fairing" perspective. The middle would have less pressure in front though, so of the three I would take the middle.

Best however is to completely enclose the bottom protrusion with a fully streamlined shape.... ie enclose the bottom exacly like in the third picture.

Check out some pictures of airplane landing gear fairings for the best of shapes.

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