Rather than using a flat surface placed roughly perpendicular to air flow, why not use a section of curved plastic, with the convex surface facing the wind?
You could make such curved plastic pieces by cutting a disposable plastic bucket in sections, like cutting a pie. Paint it black or grey.
One of my quests has been to find the "perfect plastic dustpan". Just cut off the handle and zip-tie it under the front wheel of the car. But, most dustpans have something wrong with them in terms of their shape. They are too complex for my needs. Just not right in some way, .
i've currently installed some corugated plastic "revese mudflaps" on my car... i'll take pictures when it's light outside. i'm still looking for a better way to attach them though... curently they have tape at the top, wich i wanted to replace with the mechanical clips from my original mudflaps but this didn't seem to work.
while i was at it i tried to test fit the original mudflaps in reverse but that didn't seem to work.
they where much bigger than the plaps i'd conctructed and wich i feared would be oversized! they're much heavyer and in the end they where designed to fit perfectly in their original location to this made them fir all the worse it a reverse position. the ones that would go behing the front wheels worked best when placed in front of the rear ones because the profile of the car doesn't change much between the wheels but iltimately they'r require a lot of cutting to make them work so i decide to stick with the ones i've made.
i really like the idea of recycling old tings with interesing shapes, but i've had very little luck with them... they never fit just right. imho you're better of with a flat sheet of plastic than with something that almost looks like the thing you need but won't work in the end. than again you might get a lucky find sometimes .
as for the aerodynamic propperys of these flaps... i think they work rather well, i haven't ran trough a tank with them, but the car seems more stable and prehaps coasts a bit further. so if there's a measurable difference, or at least not a big drop in fe i think they'll become a permanent item
This is what the rear wheel air deflector looks like on my car. It is made of a piece of aluminum angle screwed to the underside of the car with a piece of coroplast screwed onto it. I initially made it 2/3 of the distance to the ground and then trimmed it down to where it didn't rub the ground when I hit bumps.
it's mainly held in place with the bracket i gluen on and it's secured over w lip tat portrudes into the wheelwell. the top is a problem, for now its taped on to prevent it from vibrating but i'm looking for a more permanent solution.
unfortunately with last nights rain they got a little sandblasted and that took most of the paint of.... now they look suspicious again
btw why does a slightly dirty car always looks like you took it offroading in a picture?
The rear one which I added a while back seems to have helped me reach more often 27MPG on highway, though it is very hard to be definitive about this since there's so many factors that come into play.
My application is a 2002 CR-V SUV, so very high off the ground. Removing air from under the car is not an option for Cx, so I want it to flow as best as it can.
If you look at modern cars, you'll see the air dams for the rear tires are just tiny square brackets, no angling, so I suspect that they measured it as being the best solution.
I just did the front ones, yet untested:
The front dams are not like mine, they usually are 1/2 tire in size, and I don't know why. Different constructors place them at different locations, often covering the 1/2 inner side of the tire. My CR-V's design is odd, with the wheel mostly uncovered by the underside fender, which is atypical. Maybe it is to improve the off-road passing ability. Maybe it is a different aerodynamic solution. Time will tell.
I fooled around with my scangage, but the bottom line is I think the front deflectors as I made them are either useless or even detrimental. The rear ones seemed to help a little when I had put them in.
I don't know why they don't work. Too far from the wheel? Too big?
The thing is I got poor gas mileage this week-end. I think in part it's because I was not driving but still. I was getting 25~26MPG driving, but my friends were in the 19-22 MPG, and I did not drive highway, seemed to be low MPG.
i never got the deflectors to stay on propperly to test them well enough. they where in a classic rust spot for the car so i didn't want to drill or anything.
the car did feel more stable with them on, but not like that alone would be enough reason to keep them.
i've since changed cars, but have already installed front wheel fairings wich don't seem to hurt as milage keeps going up. the front of the rear weel arch is roughly similar, so attaching anything will be a pain again... i've tried fabricating some clip on brackets but failed, but i definately want to bring them back.