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Old 12-20-2020, 11:58 AM   #6 (permalink)
Aero Wannabe
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Join Date: Dec 2007
Location: NW Colo
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TDi - '04 VW Golf
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Originally Posted by Taylor95 View Post
I let part of the spat go outside the wheel well. Let me know what you think about that. I think that it would create an air curtain effect.
It’s complicated. I have added spats in front of my rear tires but they do not extend out beyond the body of the car. My coasting distances and tank averages are better than before I started my mods but it is very difficult to tell if the spats are helping or hurting my mpg by themselves. (I have a full under tray that may be helping the most).

It is my understanding that we want the air to stay attached down the sides of the car and detach cleanly at the back end. From the photo, it looks to me like you have added a “Gurney Flap” which is like a small spoiler that may kick the air out around the spinning wheel. I cannot say if this will help reduce aero drag without testing. It may be a good attachment point for a future fender skirt to help clear the lower edge of the tire. I hear you about the spats being difficult to remove for testing. Sometimes a compact ratcheting right angle screwdriver works. I found one cheap at a discount tool chain. You could try some tuft tests to see how turbulent the airflow is. I have seen where Julian will add a tuft taped to the very center of the wheel to see how the air flows across the wheel face.

If there is a high mpg or all electric version of your model of car it is usually safe to copy what the factory has done. FWIW some of the factory spats I have seen are about the size of a playing card, usually much smaller than the size of the front of the tire.

The “air curtains” we discuss here generally have a duct to speed the airflow across something like the front wheels so the flow is more attached down the sides after the wheel opening. Good luck. I hope his helps.
60 mpg hwy highest, 50+mpg lifetime
TDi=fast frugal fun

Originally Posted by freebeard View Post
The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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