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Old 05-11-2007, 11:30 AM   #1
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Optimal shape of wheel deflectors - convex/concave?

Viewed in profile, most shaped (ie not spat type) wheel deflectors on production cars seem to have a concave shape to them:


eg: Honda Insight

I suspect this is for practical reasons (overhang clearance), rather than optimal aerodynamics.

In a perfect world with no parking berms or high driveway curbs, would the deflectors be a different shape? I suspect they'd be convex - roughly hemispheric in plan & profile view.

Thoughts?
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Old 05-11-2007, 12:55 PM   #2
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I have noticed lately that a lot of cars just have flat pieces of plastic there, sticking nearly straight down, and they extend inboard of the tire nearly to the A-arm attachment point.

My guess is that the goal is to keep air out of the whole wheelwell, and not just to send air around the tire.

I am going to add some of these to my air dam. I think that maybe my air dam has increased my Cd, unfortunately. I have a whole long question that I want to ask about that, though.
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Old 05-11-2007, 09:17 PM   #3
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Remember my thread on the subject? http://www.gassavers.org/showthread.php?t=2381

I had a chance to examine a 2007 Prius a couple weeks ago. I noticed it had front tire deflectors similar to the Insight pictured above. Then I got down on my knees and groped the underside of the front bumper and felt a rounded bulge located in front of and offset slightly inboard of the deflector. It was very similar to the fairing I constructed for my car except it was smaller; it had more length than width, and it was thicker at the back.

Maybe the deflector is meant to start the air flowing downward? This might help the air flowing under the bumper mix with the air coming out of the wheel well.

When it was raining here last winter I had a chance to observe the way rain mist moved around the front tires and wheel well of cars on the freeway. For the most part, there was a turbulent mist below the bumper area in front of the tires. This mist billowed out to the sides of the tires, and extended upward into the wheel well. On most sedans the mist would extend up to around the 10 or 11 O'clock position, assuming 9 O'clock is the leading edge of the wheel, and a typical fender extends down around 8 O'clock. Sometimes the border between mist and clean air coming up from the back of the wheel was distinct. On SUV's the mist would carry up farther and have a more turbulent border. At the area where mist/ clean air met the airflow would exit the side of the wheel well and mix with air coming off the hood.

Have a friend drive you around on a rainy day on the freeway and watch. Better yet, have a friend drive your car while you ride in another car with a video camera while it's raining.
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Old 05-13-2007, 07:36 AM   #4
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Interesting: in the latest blog entry on Tesla's site, aerodynamics are brought up by the engineer whose focus is the range of the car.

There's a wind tunnel photo showing the "reverse mud flaps" affixed ahead of the front wheels. Attached with what looks like duct tape



Edit: here's a link to a large photo that shows the front wheel deflector on one of the prototype cars: http://stadium.weblogsinc.com/autobl...s/IMG_6866.jpg
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Old 12-31-2007, 12:06 PM   #5
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Apparently, OEM concave mudguards are available for the `98-02 Corolla, so I've been considering getting two sets for my car and attaching them to the leading and trailing edges of the well openings.

If they fit on the leading edges, the rears would be a great platform for flat rear skirts.

Thoughts?
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Old 12-31-2007, 01:59 PM   #6
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Good point about concave shape because I have rubbed a curb with the convex hubcaps on the Geo and have been very careful with the convex xB "mag" hubcaps. I would tend to think that convex would smooth out the tire shape better and reduce air pressure (higher velocity lower pressure) over the center of the wheel drawing air out of the center to cool the brakes. Air flow in a wheel well is going to be screwy no matter what the cap looks like but it would be interesting to try to improve the air flow somehow. I always thought that a cap or mag wheel with fins pumping the air out from under the car would work great but then there would have to be a left and right side to them.
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Old 01-01-2008, 05:07 AM   #7
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Quote:
Originally Posted by MetroMPG View Post
Interesting: in the latest blog entry on Tesla's site, aerodynamics are brought up by the engineer whose focus is the range of the car.
Has anyone calculated the range and efficiency of a similar car (same aero and weight and battery pack) designed with a motor capable of allowing the same performance as with a small car like the Saturn or Civic?

I'd like to see the difference.
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Old 01-01-2008, 08:10 AM   #8
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The most optimally shaped wheel deflectors are those found on the Daihatsu UFEIII and the Loremo, since the main purpose of the deflector is to split the air flow around the sides of the tire. But the deflectors on these 2 cars would be very easy to scrub on the pavement on sharp approach angles. The front deflectors on my car are straight rather than being concave or convex, mainly for ease of manufacture.
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Old 01-01-2008, 09:29 AM   #9
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Quote:
Originally Posted by theclencher View Post
op not talking about caps...
OOPS! though you were talking about the caps - concave makes the air flow downwards in a blast extending the effect of the deflected air to the ground where as convex would not. I would like to see something that could blow nails out of the way so I would not get so many flat tires.
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