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Old 10-31-2010, 09:45 AM   #1 (permalink)
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Under-Car and Wheel Well Airflow Interaction

When I look through the wheel spokes of my Prius, front and rear, I can see
all the way across under the car. This is not the case when doing the same
on other cars. The sight line is blocked by the inner wheel well. The Prius
hardly has any vertical inner wheel well.

* As understand it, from an aero perspective, it is best that once air gets
under the car it pass all the way to the rear.
* Air flow under the car is not strictly front to back. It angles out to the
sides, more so at the front.
* From my observations, the Prius shows less than average water droplets
being driven out of the wheel wells on rainy days/wet roadways.
* The Prius has the air dams in front of both the front and rear wheels.

Could the induced air flow around the tires be enough to keep under-car air
from going through the wheel wells?

There seems to be contradictions here. Since the Prius has more open wheel
wells, you would think that there would be more air entering the wells from
under the car which would show more air/water being blown out.

I'm wondering if adding something like Coroplast to drop the inner wheel well
would be beneficial from an aero/FE perspective.

I find all this very confusing... but then I can't pretend to understand what
makes for good aero in relation to under-car air flow and the wheel wells.

Last edited by Rokeby; 10-31-2010 at 11:31 AM..
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Old 10-31-2010, 10:42 AM   #2 (permalink)
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You're definitely on to something here...

We know from numerous discussions on car-body/road-surface interactions, that the air flow under the car does not go straight from the front to the back.

And like your observation, the air flow tries to squeeze out sideways due to pressure-expansion when entering the under-body space.

Someone could spend a great deal of time trying to understand exactly what the air flow looks like, and then design parts to work with that flow.

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Old 10-31-2010, 12:29 PM   #3 (permalink)
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The automatve industry is too focused on getting the XYZMobile aerodynamic on top. Even if they reach the point where they can add underpaneling to redirect airflow, it still has to compensate for the braking system and the exhaust.

Now instead of being cynical, I'm going to be constructive.

From what I have seen on the highway during wet weather, the air tends to flow out from under the car about 6-8 inches before the rear wheel well, and quite a bit is pumped out from thefront and back wheel wells themselves. Since it pumps out near the back wheel well it would be somewhat reasonable to assume that putting some spats and skirting around both the outside and inside of the rear wheels.

The interior skirts could help to prevent air from entering wheel well, and to direct the flow out to the rear of the car. The exterior preventing the same thing, but directing the flow around the body of the car.
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Current Mods: Deleted mudflaps, Denso TT plugs, Synthetic blend motor oil

Future: Aero rim covers, air dam, grill block, full synthetic oil, synthetic transmission oil, reas wheel skirts, partial camback
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Old 10-31-2010, 02:55 PM   #4 (permalink)
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I saw similar differences in outflow from various wheel designs when about 30 of us with Porsche 944 cars did a 500 mile ratrace through British Columbia. This was a long line of otherwise identical cars over the same wet roadways at the same speed: Open-spoke car wheels threw out a lateral column of misty air several feet into the slipstream, whereas cars with smoother, more closed shapes more like pizza pan covers did not throw out such lateral mist into the slipstream. I believe the latter type wheels mean effectively less frontal area for those cars, less disruption of the slipstream, and surely better fuel economy. Smooth wheel covers would also make less drag than spokes, which also may act as crude turbines.

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