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Originally Posted by mpg_numbers_guy
The rear wheel spats in the original picture look like they would increase the size of your wake by shooting air out the sides instead of directing it around.
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Properly shaped, a spat will force air down (like a wickerbill) to channel it toward the contact patch of the tire. Keeping it out of the wheel well. The side skirt could run out front inner front to outer rear, like a semi-trailer.
There is always a jet of air that squirts out either side of the tire right at the surface, where it is wedged together. Above that everything enters the ambient flow and is swept away. (Maybe a localized pressure increase. It's all about trading pressure and velocity.)
Squirting sideways would be like trying to hold your car door open at speed.