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Old 01-19-2010, 07:24 PM   #7 (permalink)
EcoModding Lurker
Join Date: Nov 2008
Location: Portland, OR
Posts: 53

Daq Civic - '92 Honda Civic DX sedan
90 day: 42.54 mpg (US)

The Hardbody - '91 Nissan D21 (pickup)

The Hardbody - '91 Nissan D21 (pickup)
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Tire deflectors are something I've been studying even since I noticed how many cars have them. The are almost always positioned inward of the tire centerline, and for a long time I wondered why. I believe it was here I read in some discussion that the front underside is a high pressure zone, which means the air wants to divert outward along the sides of the car. That was a "duh" moment for me. There is some talk about the wheel wells being a major source of drag and a complex problem in this post of an interview with a BMW aerodynamicist. For more of my thoughts on deflectors and wheel wells, see this thread.

I've paid particular attention to BMW and Mercedes designs. The deflectors often wrap around and curve inward along the arc of the wheel well. Usually the height of the deflectors also tapers as you move inward. A good example is the E-class, which in coupe form has a cD of .24.

My understanding is that they do this to both divert air around the front tire surface and across the void of the wheel well. The large difference in pressure fore/aft of the deflector also makes the wheel well a prime location to extract hot air from the engine bay. This makes more sense than extracting it at the center, where the slow exiting air breaks up the clean and fast flow underneath the otherwise smooth underbody. The airflow near the sides of the car is already somewhat dirty, especially rear of the front wheels, so extracting air there doesn't hurt anything and may even help by equalizing pressure.
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