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Old 04-10-2008, 09:04 AM   #31 (permalink)
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Originally Posted by SteveP View Post
OK, just to confuse things further, here's something I ran across late today. It's from an SAE paper, 951906, "Vehicle Design Strategies to Meet and Exceed PNGV Goals" by Timothy C. Moore and Amory B. Lovins, found here
Thanks for the link! I'm an avid areo paper reader

The following comes from page 11:

"Rather than smooth the underbody and attempt to tuck chassis
components up out of the flow, the industry strategy has
tended towards air dams below the front bumper to force much
of the flow around the vehicle rather than under.
Well, that is totally understandable! Lets suppose you have a given vehicule that looks like an aerodynamic wreck underside. What are you going to do? Redesign everything so you get a more or less streamlined underside? Or stick a plastic dam up in front of all that mess and acheive decent results? Accountants usually have the most convincing arguments when comes the time for management to make decisions.

This needlessly
increases frontal area and leads to the erroneous notion
that achieving very low aerodynamic drag requires extremely
low ground clearance[5]."
Since it actually reduced drag, I don't think you can say it needlessly increased frontal area. The guy seems to forget the answer depends on the vehicule you start with.

Footnote [5] says:

"If chassis components are streamlined or otherwise covered
by a smooth floorpan to prevent interference drag, there is
little reason, beyond the limited exposure of more of the tires’
frontal area, to prevent the airflow from passing under the car
(P. MacCready, AeroVironment, personal communication,
April 1995). Allowing the airflow to pass under the car can
actually aid in eliminating lift-induced drag."
I completely agree with that. However, I am inclined to beleive almost all the cars on the road today do not have the level of smoothness of the underside required not to benefit from reduced underside airflow.

It is clear in my mind that the average daily driver will see a drag reduction if lowered.

It is also clear in my mind that a streamlined concept vehicule will see a drag reduction if it is rised.

The only gray area that's left is for ground vehicules akin the lotus elise. Cause it's not just the smoothness that counts, the shape is at least as important.

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