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Old 07-15-2009, 01:59 PM   #1 (permalink)
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Wheel skirts slowing me down?

Awhile back I added some wheel skirts.

http://ecomodder.com/forum/showthrea...tml#post113666

A week before the skirts we were getting warm weather and I was getting 37 mpg to work and back every day.

After that it was crappy for awhile and mpg went down. Then it got nice for awhile last week, but I was still only getting 34 mpg to work and 35-36 home. So Monday I took off the skirts. Tuesday morning was a cold 30F and I still got 35, and going home was warm and I got 37. This morning was 45-50F and rain and I got 35.5. Better than I could get any nice warm morning with the skirts on. So I'm leaving them off and reaping the benefits.

It seemed like they could only help. There is a pretty good size area for wind to get trapped in the wheelwell. I extended them past the wheelwell on the bottom to keep the transition shallow. Only thing I can think of is they are about 15" high, and are flush at the top but stick out 2" at the midpoint of the bottom (they are slightly curved to keep the flow smooth). So between both sides that's 30" square inches of frontal area added. Maybe that's what was slowing me down?

I'm thinking of trying something like a half skirt, that only goes down far enough to where it doesn't have to stick out at all to clear the wheel, so no increase in frontal area. However there is also the possibility of a bigger pocket to trap air than with no skirts.

Anybody ever had any similar results with wheel skirts?

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Old 07-15-2009, 02:43 PM   #2 (permalink)
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I'm not sure about this at all, but try this theory out. The flow down the side of the car is probably better, but the flow underneath the car has been inhibited. Flow under the car may have normally ejected at the rear wheel well but is now getting caught parachute-style. I'm guessing here, but it you added rear wheel spats and perhaps a front air dam, the parachute effect would disappear and you'd see a net gain.
Having no experience with the skirts yet you probably want somebody else with some expertise to chime in.
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Old 07-15-2009, 03:54 PM   #3 (permalink)
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I tried skirts. My fe varies too much tank-to-tank to draw conclusions after only a few tanks. I left the skirts on almost a year. Still couldn't draw any fe conclusions. Took 'em off cuz they looked so stupid.

You'd think they must help but it ain't always much.
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Old 07-15-2009, 06:11 PM   #4 (permalink)
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I'm guessing that the effect wheel skirts have is going to vary dramatically from vehicle to vehicle. Perhaps there is an interaction with something upstream on the car? I'd be curious to know who all has done ABA tests. MetroMPG reported 2.9%.
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Old 07-15-2009, 06:15 PM   #5 (permalink)
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Add deflectors in front of the rear wheels. Remember that airflow under the car is diagonal towards the sides so place the deflectors accordingly, place them a bit inward under the car. You'll get better results.
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Old 07-15-2009, 06:26 PM   #6 (permalink)
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frontal area

I tossed your numbers around on Al's calculator.30-inches of added frontal area to a car with say,20-square feet of frontal area would raise the drag by 1-%.The mpg penalty at 55-mph could not be over 0.5%.Since a jump between 35 and 37 is on the order of 5% mpg,the wheel skirts should not be able to alter the mpg to this magnitude.I think there is more going on than just the skirts.Just my guess.
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Old 07-16-2009, 02:02 PM   #7 (permalink)
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36.5 going home yesterday with mild headwind on the 8 mile south leg of the trip, 60-70F, and 36.4 this morning with mild headwind on the 12 mile west leg of the trip, 50-60F. This is comparable to what I was getting on similar weather days before the skirts. Warmer days at 70-80F I was getting the 37-37.5 mpg. I had several similar days, even a bit warmer, with the skirts on and still couldn't get any of these numbers. Nothing different going on, didn't have to stop for any more cars or deer or anything. I didn't get new gas when I installed or removed the skirts. As you can see I finally removed the mudflaps, but I did that something like 2 weeks before the skirts. I would think if the skirts were giving negligible effects then I would be getting the same numbers with the skirts on when driving conditions were the same.

Ok so that makes sense, frontal area is not going to make that much of a difference, especially since that extra area was very tapered and smooth and not a blunt flat area.

I did put on an airdam last year. Maybe the diagonal airflow under the car normally exits partially through the rear wheelwells and I was blocking that. Or maybe they were creating turbulence of the air trapped in there, rather than letting it sling out. Who knows. The skirts were definitely not rubbing, they were backed by a welded steel frame of flat bar that would keep them rigid and away from the tire by 1".

The deflectors would be a good thing to try. Just something with a half round front profile to simulate the leading edge of a teardrop?

I'm also thinking of trying a bolt-on extension to the partial boattail. Since that did make a positive difference, I would think an extension would make it better, even if just a tiny bit.
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Old 07-16-2009, 04:57 PM   #8 (permalink)
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How much did the tail do?
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Old 07-17-2009, 01:38 PM   #9 (permalink)
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Quote:
How much did the tail do?
Hard to say exactly but 6% would be a good estimate. Seems to be a tad over 5%, but definitely below 10%. Still decent since it didn't cost that much. The screws cost the most at $13. $10 for paint. $5 total for 4 taillights. $6 for the shower liner. All the steel was free.

Still doing well on mpg. 37.1 going home yesterday, it was 70F and dry. 36.2 this morning, 60F but rain.

Hopefully downstate this weekend will bring in 40+ again. Only thing is I never get a 40 mpg tank average because I can go so much farther on a tank now. So if I fill up downstate where it is cheaper, I can drive home then still go to work and back for a week and a half. That was one of my goals, summertime I go downstate every 3 weeks, and I wanted to make it for 3 weeks with only one fillup locally.
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Last edited by wagonman76; 07-17-2009 at 01:49 PM..
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Old 07-18-2009, 02:11 PM   #10 (permalink)
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deflectors

Quote:
Originally Posted by wagonman76 View Post
36.5 going home yesterday with mild headwind on the 8 mile south leg of the trip, 60-70F, and 36.4 this morning with mild headwind on the 12 mile west leg of the trip, 50-60F. This is comparable to what I was getting on similar weather days before the skirts. Warmer days at 70-80F I was getting the 37-37.5 mpg. I had several similar days, even a bit warmer, with the skirts on and still couldn't get any of these numbers. Nothing different going on, didn't have to stop for any more cars or deer or anything. I didn't get new gas when I installed or removed the skirts. As you can see I finally removed the mudflaps, but I did that something like 2 weeks before the skirts. I would think if the skirts were giving negligible effects then I would be getting the same numbers with the skirts on when driving conditions were the same.

Ok so that makes sense, frontal area is not going to make that much of a difference, especially since that extra area was very tapered and smooth and not a blunt flat area.

I did put on an airdam last year. Maybe the diagonal airflow under the car normally exits partially through the rear wheelwells and I was blocking that. Or maybe they were creating turbulence of the air trapped in there, rather than letting it sling out. Who knows. The skirts were definitely not rubbing, they were backed by a welded steel frame of flat bar that would keep them rigid and away from the tire by 1".

The deflectors would be a good thing to try. Just something with a half round front profile to simulate the leading edge of a teardrop?

I'm also thinking of trying a bolt-on extension to the partial boattail. Since that did make a positive difference, I would think an extension would make it better, even if just a tiny bit.
If you can locate or get a hold of an image of Subaru's XT from the 1980s,they have an example of scientific deflectors,developed painstakingly in the windtunnel.A few millimeters apparently makes quite a difference.I tried to respect their parameters when fabricating the rear wheel deflectors for T-100.

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