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Old 10-14-2011, 03:52 PM   #1 (permalink)
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van luggage rack/shuttle van aerodynamics


After "rediscovering" this site by following links from an otherpower.com discussion - I found aerohead's summary of aerodynamics quite useful and informative. I'm hoping someone can shed some light on a question regarding aerodynamics of a "luggage rack" atop a (high profile) van. The vehicle is not decided yet, but might be a regular van, minivan (sienna), or sprinter van. 99% of the time it will have the typical 2 length wise rails (like on a sienna but 3-5" tall) offering a tiedown when unexpectedly needed, for a modest drag penalty. The other 1% of the time, for a long highway speed outing (ie: vacation), it will entrain cargo in a "shell" which covers the remaining 5 sides (the roof being the 6th side). This is when cross bars are installed and something like the air deflector similar to that seen on an nissan xterra is used. This frontal deflector extends the same angle as the front windshield. This seemed to be the right way to go, but then I had doubts after observing many "bulbous" shuttle vans, camper vans, truck campers and handicap vans, on the streets. Was there something about the aerodynamics I overlooked? Was it possible, that for a high frontal area vehicle, it is better to push some of the wind load off to the sides, rather than channel all of it up and over the top?
I realized a ship's bulbous bow made use of differing densities between air and water so that was not the reason for the shape on a vehicle. I searched for a cfd image of a shuttle van with no success. Granted, there may be utilitarian reasons for especally the truck camper shape (sleeping quarters, must be invariant w/respect to truck brand, cab length, model year variations etc), or design requirements for a shuttle van (very front forward entry doors). But as the shuttle van and camper van are customized and don't have the limitations of a universal truck camper, they could dictate what shape was best(?). Is there an aerodynamic advantage to the bulbous shape on a high frontal area vehicle (especially at speed)?

-curious john

(as a new member I can't post links yet so please bear with the incovience)

xterra w/rack and deflector:
goto wikipedia "xterra"

discussion and pic of camper over extended cab:
this board discussion "4-mpg-gain-mod-slide-camper-pickup-truck-15450"

"bulbous" shuttle van
from google images: piercetrasit.org/shuttle.h t m l
"non-bulbous" shuttle van
from google images: premierluxuryrentals.net/passenger_shuttle_bus. p h p
(or search "shuttle bus" and "shuttle van")

various camper vans some bulbous, some not:
goto wikipedia "camper van"

ship's bulbous bow:
goto wikipedia "bulbous bow"

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luggage rack, roof rack, shuttle van

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