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Old 08-11-2019, 09:55 AM   #1 (permalink)
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Question Positive effect of roof rack on square-back?

Hi,

I recently talked with a person that said keeping roof racks mounted on 2 square-backs (VW Transporter, Chrysler 300C) actually had positive effect on fuel economy.

The person happens to be a trained pilot and knows a bit about aero (started explaining to me how the back, not front, is important contrary to common belief). When asked how exactly having the roof rack on (specifically the back part) would be able to help, the answer was a shrug - no idea how that would be possible except for unforeseen interactions with the wake; but the numbers are apparently undeniable.

Soo does anyone here know of any way this could actually be true? Or is this just a case of biased observations?

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Old 08-11-2019, 09:57 AM   #2 (permalink)
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What numbers?

Regardless, seems highly HIGHLY unlikely.
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Old 08-12-2019, 03:15 AM   #3 (permalink)
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Quote:
Originally Posted by maczo View Post
..... keeping roof racks mounted on 2 square-backs (VW Transporter, Chrysler 300C) actually had positive effect on fuel economy.
..... does anyone here know of any way this could actually be true?
Can't say about roof racks. But in the 60's, there were "wind guards" with curved surfaces, that were put on the roof top rear of station wagons, to direct air, downward over the rear window that kept the rear window clean.
One extra aerodynamic plus, was the air directed downward, helped kill the vacuum on the end of the car..... & MPG went up.
Such a device would really help kill the vacuum on square-backs. Also, vertical curved surfaces on the rear sides, directing side air into the rear vacuum area would also help lessen the vacuum & raise MPG.

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Old 08-12-2019, 04:05 PM   #4 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
What numbers?

Regardless, seems highly HIGHLY unlikely.
I didn't get any specific numbers, just "mpg is noticeably better".
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Old 08-12-2019, 06:26 PM   #5 (permalink)
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Quote:
Originally Posted by maczo View Post
I didn't get any specific numbers, just "mpg is noticeably better".
That's a problem. How much is "noticeably," and on what grounds does he attribute "noticeably better" MPG to the roof rack and not to some other factor?
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Old 08-14-2019, 07:05 AM   #6 (permalink)
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Given the amount of noise they make, I'd have to have a massive improvement in economy to leave my roof racks on when not needed.

I had my racks on earlier in the week and it sounded like I had my own personal tornado following me at highway speeds.

I have Thule aerobars.
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Old 08-14-2019, 11:36 AM   #7 (permalink)
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roof rack benefit

Quote:
Originally Posted by maczo View Post
Hi,

I recently talked with a person that said keeping roof racks mounted on 2 square-backs (VW Transporter, Chrysler 300C) actually had positive effect on fuel economy.

The person happens to be a trained pilot and knows a bit about aero (started explaining to me how the back, not front, is important contrary to common belief). When asked how exactly having the roof rack on (specifically the back part) would be able to help, the answer was a shrug - no idea how that would be possible except for unforeseen interactions with the wake; but the numbers are apparently undeniable.

Soo does anyone here know of any way this could actually be true? Or is this just a case of biased observations?
I've not seen any evidence of it in 45-years of looking.I've seen a lot of evidence to the contrary.
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Old 08-14-2019, 11:42 AM   #8 (permalink)
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turning vanes

Quote:
Originally Posted by litesong View Post
Can't say about roof racks. But in the 60's, there were "wind guards" with curved surfaces, that were put on the roof top rear of station wagons, to direct air, downward over the rear window that kept the rear window clean.
One extra aerodynamic plus, was the air directed downward, helped kill the vacuum on the end of the car..... & MPG went up.
Such a device would really help kill the vacuum on square-backs. Also, vertical curved surfaces on the rear sides, directing side air into the rear vacuum area would also help lessen the vacuum & raise MPG.
I don't have the most current literature,but Wolf Hucho,who ran the climatic wind tunnel at Volkswagen for a decade couldn't get any of them to reduce drag.
The only thing he actually ever measured a drag reduction with,were either box-cavities,or boat-tails.
The drag behind the vanes always exceeded any potential benefit of 'filling' the wake.

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