We periodically have aero threads, but I haven't seen this subject covered.
I know having wheel/tire exposed from a front view is a bad thing and interrupts airflow big time. But I assume there is a point where the fender comes out too far beyond the wheel/tire and creates a vacuum pocket and increased drag.
So what is ideal for lowest drag? wheel/tire face almost flush with the fender lip?
What about using the vacuum generated to suck the front end of the car down (assuming no splashguards and extract air from the engine bay?
Is this for outright speed or road course duty? For outright speed, look at the LSR guys. Moon disks become negligible the farther the wheels are tucked. If I recall correct, there is a lot of positive pressure above and behind a spinning wheel. That's why so many race cars vent the wheel wells.
As an interesting side note, a pilot picked up 2-3 knots from sealing the gap around the opening of a wheel pant with a strip of rubber. It touched the tire slightly providing a tight fit.
Simon McBeath's book touches on this subject a bit. See pages 256-258, where there's some data on different "fence" (tire spat) configurations on the 2005 BTCC Honda. The vertical fence was added to close the gap between the wheel arch and splitter on a "cutaway" air dam configuration. The cutaway air dam exposed the tire in an attempt to tap (with the splitter) the stagnation zone created at the front of the tire. Apparently this doesn't work. But in testing it, they got some pretty good data on the spats, and some configurations which had ramps leading up to the spats. There's too much detail in the configuration and data to cite here, so check out your book if you have it.
There's also some data on Autospeed. See here for one article with tire spat (they call them "deflectors") data: http://www.autospeed.com/cms/article.html?&A=2456
I don't know how far out you can go before you're just increasing drag, but my best educated guess is there's no point in going past the tire, and you may not even have to extend all the way out to the tire to have the effect of diverting air from hitting it.
On most closed-whee l race cars I see, the tires are flush with the fenders. I don't know if this is because they are running as wide of tires as they are allowed to (can't extend beyond the body?) or because this optimizes aero.
ProDarwin wrote:
So what is ideal for lowest drag? wheel/tire face almost flush with the fender lip?
Ideal for lowest drag is where the tire is just inside the low-pressure area of the front fender. Just inside the "slipstream" if you will. More than that and you're just adding drag. Where the fender comes to depends on its shape, if it's angled outwards at all it will be where you can still see some tire sticking out from the front, if not it will be where the fender is about flush with the tire face.
The thing about having them angled outwards is that the low pressure area will change size with speed which isn't good, which is why a lot of race cars have that unattractive front bumper that sticks out to the full width of the car, then turns straight back, flush with the tire face.
ProDarwin: There is some validity in what has been discussed regarding keeping the tire in the wake of the front fenders. However, the flow in this region is definitely not just lateral and longitudinal. There is a fair bit of vertical flow as well. Add to that the entrainment of the air on the tire surface and you get some not quite so expected flows.
The aircraft style wheel pants pictured above do give the lowest drag for a tire that still has to touch the ground at some point, but it is difficult to keep the dynamic seal mentioned when you consider tire deflection from cornering and downforce loads.
For downforce, there is some benefit to keeping the tire tucked in the fender, but there is high pressure in front and low pressure behind the tire at the ground, both causing drag and opposing each other for downforce.
Appleseed wrote:
Is this for outright speed or road course duty? For outright speed, look at the LSR guys. Moon disks become negligible the farther the wheels are tucked. If I recall correct, there is a lot of positive pressure above and behind a spinning wheel. That's why so many race cars vent the wheel wells.
Its actually for MPG, but lets just go ahead and call it outright speed, since the goal for both is lowest possible drag 
My DD is a currently stock 98 Saturn. Like all DDs I have, after a while, I must modify. My stock wheels are E36 M3ty, and I plan to do something about it, so I'm just curious what would produce the best aero benefit. Moon disks are not happening though. I'd like to bump to a 16x7 or 7.5, but physics is pointing to sticking with stock wheels and just cleaning them up real good so they don't look like E36 M3.
From the factory, the wheels are way tucked. The body does taper inward as it gets lower, so it is theoretically impossible to bring the wheel out "flush" unless I wanted to have like +8deg of camber. Or maybe tire spats are a good solution here? I bet it would be pretty easy to fabricate a small one by cutting down a mudflap.
The tire does not fill the wheel well front to rear, so is venting really necessary?
Its also going to get lowered at some point, reducing the huge gap above the wheel inside the fender. This will also cover the front of the tire more, as well as bring the tapered face down more.
Spats are good and will help reduce any need for venting, which is not helpful or could even be harmful for your purposes. Tucked wheels are slightly worse than flush but much better than protrusion. Covered rear wheel wells would help a lot.
Covered wheel wells would help like Gameboy said. Something similar to what this person did (if you haven't seen this car already) http://aerocivic.com/ One of these days I will find a car to try some of that on. The Bonneville is too nice to do it to, plus it is a bit (just a a bit) on the large side.
Covered wheel wells are not something I plan on doing. I've seen the aerocivic thing (he posts frequently on ecomodder), but I'm not interested in going that nuts. I want to make minor changes to the appearance and practicality of the car.
Only reason I am even asking about the wheel well aero is because I have the urge to lower it and change the wheels (due to the completely E36 M3ty stock wheels) and figured if I can make any improvement when I do this, I'll give it a shot.
series8217 - thanks for the link. Good reading there.
don't forget the wheels....
http://grassrootsmotorsports.com/forum/grm/wheel-promoting-brake-cooling/59052/page1/
Make a front air dam. Not those sissy factory thing, but a manly one. Semi-flexible plastic or a thick (1/2 in+) sheet of rubber. get it as close to the pavement as you dare. This will help mileage.
Appleseed wrote:
get it as close to the pavement as you dare.
A great way to do this is to make it a bit too close and then let the pavement file it down for you.
In the latest issue I noticed a couple of 1st Gen Camaros that have a different looking front airdam. Reminds me of a second Gen trans am. I'm thinking the spats must have some effect or those cats wouldn't be running them.
Airdam is on the list. This thread was specifically about the wheel well aero though.
