A Thought Experiment on Aero Mods - Fuelly Forums

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Old 06-18-2007, 09:28 PM   #1
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A Thought Experiment on Aero Mods

I am starting to think that there might be a simple practical rule when playing around with aero mods.... practically: if it adds cross sectional area, it probably hurts.

On paper, however, one could alter a shape and increase its cross sectional area and still reduce aerodynamic drag.... the shape simply must exhibit a low enough Cd to offset the increase in area.

But in practice.... we are not replacing our entire blunt front ends with parabolic shapes and generally are not adding full tear drop boat tails (OK, Basjoos being the exception). Instead we are doing less major tweaks here and there and thus are having smaller affects on the total aero characteristics. And thus there is a limit to the overall Cd effects that can be achieved by our smaller Cd improvement tweaks.

Let's try an example: A deflector in front of the blunt surface of a tire will help redirect air around the tire.... good. But if the deflector extends outward any at all, it adds area, and since no matter how good the deflector is, its Cd is not zero, thus drag is added in addition to that avoided by the deflector addition in the first place.

Now to be rigorous one could show that there is a continuous transistion from "no area add, better shape" to "some area add, but better shape" to "a lot of area add, doesn't matter how good the shape is" scenarios. But again, I am thinking more of a defacto rule or let's say a guideline to follow, which could be:

If it adds area it probably hurts overall..... or, if it does add area, it better be a small amount and it better provide a much better Cd for the area the mod is being considered for.

Any thoughts on this? Some of this applies to the ideas I have seen where one tries to improve a bad aerodynamic shape be altering the airflow out and away from the it. But I can't figure out how to do that without adding area, thus back to my supposition.
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Old 06-19-2007, 06:11 AM   #2
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Quote:
Originally Posted by lca13 View Post
And thus there is a limit to the overall Cd effects that can be achieved by our smaller Cd improvement tweaks.
In the neverending quest to save pennys, I suppose the most appropriate answer is : 'Every little bit helps'.

or

'You can put a dress on a pig and its still a pig....but at least it'll look better when you take it to the prom.'
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Old 06-19-2007, 06:21 AM   #3
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Once I get the belly pan on the toaster I need to do some ABA coast down tests on the air dam. I think that maybe the frontal area it adds is not worth any improvement that there may be from diverting air away from the underside. It sure does drive better with the airdam, though...
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Old 06-19-2007, 09:13 AM   #4
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I'm getting better FE with my airdam extension + forward belly pan. It seems to be gliding better. However I also upped the tire pressure and refinded my P&G technique - so I may never know if the airdam + FBP are actually helping.

If my friend comes through and makes a fiberglass copy of my construction as promised, I'll be using the OEM setup for a while so I'll be able to compare.
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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.

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Old 06-19-2007, 09:25 AM   #5
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I am completely convinced that enhancing Aerodynamics can have a significant effect on improving the mileage on a vehicle. I am entirely unconvinced that their is any degree of totally correct understanding of the whole process, when it comes to existing vehicles, what works, what does not, why it works, why it doesn't work, why it works on some vehicles, but doesn't seem to work on other vehicles and so forth.

My conclusion has been that if you really want to know if something in aerodynamics will work, on your car, you have to try it out, on your car. If you have some other gizmo, dudad, widget which is unique to your car, then their is a high liklihood that will skew either for or against, any modifications you might make, or try.

One of the great things about this site is that people are willing to share what seems to work, or not, for them, on their vehicle, in their experimenting. Most of the time people are willing to accept that just because one person got some sort of results, it doesn't necessarily make it the be all, end all solution for everybody, because everybody doesn't drive the same identical vehicle, in the same setting.

I have had some experiences with roof top carriers, where I had problems which I figured I was just stuck with, but I accidentally found a solution which made a unbelievable improvement in aerodynamics, but I wouldn't have ever considered making a change in the direction I stumbled upon.

I guess all I'm trying to suggest is to be careful to not unduly block possibilities because their might be possiblities you or I don't really understand, comprehend, or even grasp.
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Old 06-19-2007, 10:24 AM   #6
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[quote=Gary Palmer;59296]I am completely convinced that enhancing Aerodynamics can have a significant effect on improving the mileage on a vehicle. I am entirely unconvinced that their is any degree of totally correct understanding of the whole process, when it comes to existing vehicles, what works, what does not, why it works, why it doesn't work, why it works on some vehicles, but doesn't seem to work on other vehicles and so forth.

CO ZX2 writes:
Amen, Gary. I would say that approximately 50% of 'Old Reliable's' FE gains stem from aero mods. I pay little attention to what experts have to say in regard to what will and what won't work. Some of the mods on my car fly in the face of all analysis, opinions, statements and recommendations.

I'm wondering if the experts that write the books everyone references shouldn't be looking a lot closer at what's going on in the real world. ala GasSavers.org.

Thanx to all the guys here that keep testing any and everything they even suspect will produce results. Anyone that says they've tried everything and nothing works just needs to go sit down and watch the other guys do it. Yes??
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