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Old 10-29-2021, 07:16 PM   #635 (permalink)
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BicycleBoB : The fins were a quick trial to reduce airflow wrapping around the box corner (towards the tail light "cavity / recess") based on tuft tests I did. I haven't found a good article what would give guidance what aspects of fins / end plates need to be factored in (other than the vertical angle) so I gave it a shot and made them to that size and shape... Fortunately, it gave me fairly significant improvement on gas mileage but before saying that for certainty, I need to run more tests... but looks promising... As part of that, I realized the fins / end plates can be further out to "scoop" more air from the blended side and top surface between them so that's next what I do. At the same time moving the fins / end plates more "out" (towards the side wall of the bed), it also reduces the size of the cavity / recess shape around the tail lights... which I assume just helps...

Aerohead: totally agree with you about the side winds and yaw -moment. Though, putting it in perspective by looking size of overall side surface of the truck, those fins are fairly small... but they are there, true. Plan was to try to find more info about what factors of fins / end plates are important but haven't found good articles yet. And I don't, back to tuft testing I go....

Looking side surface difference between the cap + the tailbox vs. with the cap + the tailbox + fins - yes, fins/end plates do increase the surface area but doesn't look that big increase. And like said, those were initial prototypes just to see impact and now those can be optimized more as they seems to help... In overall, that whole tail light corner area needs some thinking how to factor all aspects in (thanks for ideas, suggestions, comments, and feedback) and see how to incorporate them into design ....
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