"Bumps" on OEM Subaru Impreza roof racks supposedly improve MPG

[CigaR007]

Stumbled upon this video of the 2012 Subaru Impreza. At the 30 seconds mark, the guy talks about the roof rack and its design. The added "tab" at the end of the roofrails added 1 MPG to the highway fuel economy as per their aerodynamic testing. Apparently it smooths the airflow. I find that aerodynamic tweak amazing given its small size and its correlating impact on fuel economy.

http://www.youtube.com/watch?v=rpJxL8ZDsZw

[aerohead]

Originally Posted by CigaR007

Stumbled upon this video of the 2012 Subaru Impreza. At the 30 seconds mark, the guy talks about the roof rack and its design. The added "tab" at the end of the roofrails added 1 MPG to the highway fuel economy as per their aerodynamic testing. Apparently it smooths the airflow. I find that aerodynamic tweak amazing given its small size and its correlating impact on fuel economy.

The tail end of the rack ends just where attached longitudinal vortices might form off the roof.
The tail of the rack sections are just like a Mitsubishi Lancer roofline (very steep!) and the 'bump' should act just like a vortex generator to help the flow follow that steep incline.
Also,EPA Mobile Sources certification testing includes faster speeds ( up to 70 or 75 mph) than in past decades,so drag is more important than ever before and 'small' differences would matter.

[instarx]

1 mpg? I find that hard to believe.

[BamZipPow]

Originally Posted by instarx

1 mpg? I find that hard to believe.

Keep in mind that engineers probably gave the marketing people the numbers and the marketing people rounded up. So it might have been some fractional gain...

[MetroMPG]

I'm skeptical about their claim too. Needs some fact checking!

[kach22i]

I agree that it might be a vortex generator of some kind which alters the flow over the car, or creates it's own small vortex which affects the larger one which would normally form.

I did some studies on pick-up truck roof cabs which intended to do something similar. However I am far away from doing something about it (spring project), and even farther away from proving how or why it may work.

Anything which lowers the air pressure at a critical point (aft) will result in sucking the air above it (at higher pressure) down. In the end, it will appear as though the overall flow is better.

[Quasimoto]

Anyone know of any bridges for sale in Brooklyn?

[kach22i]

Originally Posted by Quasimoto

Anyone know of any bridges for sale in Brooklyn?

Remember, there is typically a little truth in every lie.

The truth might be that a conventional roof rack costs 1-mpg more than this one.

This rack combined with the aerial acting as yet another vortex generator may indeed net a gain of .66666 mpg, rounded up to 1-mpg.

What do the Mitsubishi vortex generators claim to increase mpg by (or is it just a downforce clam)?
http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

And old Ecomodder thread:
Where to buy vortex generator fins?

Picture of similar product:
APR Mitsubishi Lancer Evolution | Evo | 10 | Vortex Generator

[CigaR007]

What are the chances of Subaru releasing the test results in order to prove the 1 MPG-benefit claim ?

[kach22i]

This is what they used to do, looks pretty good according to what I know of the topic. I'm just thinking maybe they are not all BS.







Just maybe vortex control is more important than drawing the air downward.