[lunarhighway]

jugeing from the volvo supplied data in the autospeed article they have both the potential to generate lift or downforce.
but if they're mounted to a flat panel than a high pressure pocket will build up in front of them and that's going to give lift. than again, how much of an issue is that in a mormal road going car?

one thing to remember is that a the frontal face (the part with the thread) of the lower part of the wheel (below the axle) moves slower than the rest of the car in relation to the ground, and therefore also the air. the bottom of the tire touches the ground, so at that point the speed relative to the ground is 0 . so below the axle the face of the wheel will begin to slow down. (the top of the tire actually moves at twise the speed of the car in relation to the ground) a fairing in front of it will move at the same speed as the car no mater how far down it is, so even if it creates a shape with a lower drag coeficient than that of an unfaired wheel it will move with a higher speed! as drag increases with speed. a slower moveing with a high Cd might still have less overall drag than a low Cd but very fast moveing object! anyway i'm not good enough with these matters to fully understand what the impact of this would be on the aerodynamics, but i conlude that a flat fairing that is to far down might actually create more drag than no fairing!

so i think the right dimentions are very important.

also the air behind them is as important as the air before. when there's no fairing an airflow moveing at the speed of the car will hit the spinning wheel... if there's a fairing no air will hit the spinning wheel...

it's one of the reasons why a lot of windtunnels have instalations that allow the cars wheels to spin...
unfortunately without a windtunnel it's quite a gamble.

so i don't think it's a good idea to start with something thats much different than what's seen on todays production cars... unless you can test if quite well to see the effect.

i suppose these things are a tradeoff that lower drag, are cheap and east to install, don't add crosswind instability or lift, and that don't lower ground clearance and risk being damaged, like most slightly lowered airdams on production cars do

the gurney flap seems an interesting idea as well, makes me wonder if something similar wouldn't work on top of the vehicle, but thats a different thing