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-   -   Positive effect of roof rack on square-back? (https://ecomodder.com/forum/showthread.php/positive-effect-roof-rack-square-back-37752.html)

maczo 08-11-2019 10:55 AM

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?

Frank Lee 08-11-2019 10:57 AM

What numbers?

Regardless, seems highly HIGHLY unlikely.

litesong 08-12-2019 04:15 AM

Quote:

Originally Posted by maczo (Post 604318)
..... 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.

maczo 08-12-2019 05:05 PM

Quote:

Originally Posted by Frank Lee (Post 604319)
What numbers?

Regardless, seems highly HIGHLY unlikely.

I didn't get any specific numbers, just "mpg is noticeably better".

Vman455 08-12-2019 07:26 PM

Quote:

Originally Posted by maczo (Post 604410)
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?

oldtamiyaphile 08-14-2019 08:05 AM

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.

aerohead 08-14-2019 12:36 PM

roof rack benefit
 
Quote:

Originally Posted by maczo (Post 604318)
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.

aerohead 08-14-2019 12:42 PM

turning vanes
 
Quote:

Originally Posted by litesong (Post 604360)
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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