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Originally Posted by EcoVan
Is this the rule of thumb simply " at most covering the low hanging component of the vehicle" or some other mathematically derived number? I could certainly see in my mind's eye that a shorter air dam would shield the low hanging components from creating drag if the air stream is sculpted downward by the shape of the air dam or valance. This would allow greater ground clearance but still maintain the gains possible. The cross sectional view of the airdam/valance suggests it is creating downward velocity with the sloped section and then the smaller flat air dam portion is at the bottom.....Maybe the blast fence as described previously.
Unfortunately, its going to be a few weeks before I can test this installation, as the van needs a new sensor and some front end work. However, I have nearly 80,000 miles of experience already with it, so a base is already established. Also, with my experience with my 93 escort, the air dam by itself did'nt seem to be particularly beneficial and neither did smoothing out the front underside. However, when the belly pan was attached to the bottom of the air dam, that was when the magic happened. I suspect that this configuration allowed the cooling air being pushed/pulled into the engine compartment filling the vacuum behind the air dam, greatly reducing it drag.
I already started mocking up a belly pan that will attach to the bottom of the van's air dam and close up the front of the front wheel wells.
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1) In science, we're cautioned not to use them.
2) Over 45-years though, one observes certain design behaviors.
3) All my sources for airdams reveal inflexion points for drag, as a function of 'face height', whereas, at some 'height' drag begins to increase again, as also seen in SAE Paper 2018-04-03, if you'll look beyond the superficial presentation as presented. Context. Conditions. Caveats.( world records live between the lines ).
4) Over the decades, you'd be hard-pressed to find a 'street car' in which the airdam extended below the 'belly'.
5) If you acted upon Larsen & Woodiga's apparent conclusion, you'd be shooting yourself in the foot, dragwise.
5) Wolf Heinrich Hucho, in his 2nd-Edition, cautioned us about doing anything which would aggravate frontal area. I happen to believe that it's a good recommendation, even for the billionaires.
6) The F-150's minimum drag airdam does not occur where SAE 2018-04-03 implies.
7) And it just happened, that the 'rule-of-thumb' solution ended up within a fraction of a percent of the optimum, without spending one second on testing. Just sayin'.
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By the way, I brought a level and tape measure with me this morning and measured curbs and parking space, pre-cast concrete minicurbs. If the F-150 'settled' as much as 1 millimeter in ground clearance when boarded, then, there's not a place you could park in Denton, Texas where Larsen & Woodiga's airdam wouldn't scrub, be damaged, or knocked off.