wheel airdam - deflector - strake? design questions - Fuel Economy, Hypermiling, EcoModding News and Forum

lunarhighway · 05-04-2008, 05:59 PM

i intent to fit some small dams in front of the wheels of my vectra. it's got a pretty sweet 0.29Cd but there's room for improvement.
I'm trying to better understand why exactly these little dams are installed on production cars and how they function aerodynamically so i could trancelate that to my speciffic situation rather then just copy an existing design... of course these might have resulted from trial and error wind tunnel tests, but still some info might be extrapolated from the increasing number of existing designs. BMW for example seems to have the bigges ones i've see so far, but almost all their cars also have relative short overhangs before the wheels.

Ford seems to use a split design on the new focus and mondeo models with a straight section in front of the wheels and a second backwards angled part in front of the suspention sepparated by a slit (wich i guess is to allow cooling air to the breaks and perhaps to allow flexing when an obstacle is encountered).

my vectra already has small dams in front of the wheels at the bumper edge, but nothing directly in front of the wheels. In fact the inner wheelwell pannels end about 5cm above the lowest part of the bumper... extending these below this line would make very normal looking "wheel dams" as they exist today.

i'm wondering what dimentions i might aim for an if break cooling would have to be accounted for?

any insight on the matter would be appreaciated!

kafer65 · 05-05-2008, 02:10 PM

Best I can tell is that they help steer the air around the tires in a less blunt manner than just being naked in the wind. The most dramatic ones I've seen are on late model hondas and the Toytoa Prias.

donee · 05-05-2008, 08:47 PM

Hi All,

I have the wheel air-dams on my Prius. But recently I was up close to a brand new LS-460, and it had the same thing. The Prius and the LS-460 have the same CD. The trunk lid of the 460 had a very unusual center depressed shape to it. The 460 rear window was depressed into the sheet metal, just like the Prius is too. This makes for some strakes on side of the rear windows.

Arminius · 05-05-2008, 09:29 PM

Quote:

Originally Posted by donee

Hi All,

I have the wheel air-dams on my Prius. But recently I was up close to a brand new LS-460, and it had the same thing. The Prius and the LS-460 have the same CD. The trunk lid of the 460 had a very unusual center depressed shape to it. The 460 rear window was depressed into the sheet metal, just like the Prius is too. This makes for some strakes on side of the rear windows.

Hey, I never noticed those air-dams before. Thanks for mentioning that. Are the ones on your Prius seperate pieces fastened to the front under-cover, or are they part of it? I ask because I would use them on my car if I can buy them.

trikkonceptz · 05-05-2008, 09:33 PM

Check out the ones the Honda s-2000 has also, they are larger and you may be able to manipulate them more to fit your front bumper ...

Arminius · 05-05-2008, 10:21 PM

Quote:

Originally Posted by trikkonceptz

Check out the ones the Honda s-2000 has also, they are larger and you may be able to manipulate them more to fit your front bumper ...

Yeah, thanks. Tonight's project is to search the web for performance mods.

lunarhighway · 05-06-2008, 09:41 AM

i'm not sure if it's worth looking for existing fairings unless they're designed for your car

often these are part of the wheel well inner wall or the attachements where desined for a speciffic car.

find a big flat piece of plastic and perhap some L shaped brackets and you should be able to cut something that fits perfectly and that's setup to aid the airflow over the speciffic car structure. you could probably get a fairly large sheet of black abs plastic that you could make several different models form to test.

this autospeed article has some info that might be usefull but it's not that complete that it can be used as an exact guide

http://autospeed.com/cms/A_2456/article.html

there's little further i can find on them on the web, not even a general name.

so i suppose i'll have to keep sticking my head under parked cars when i seen these things a look at wich parts of the wheels and suspention it covers and perhaps measure the distance to the ground and the center of the weel etc...

Arminius · 05-06-2008, 07:13 PM

Yeah, I'm looking under parked cars too. The issue for me is that it needs to look like it belongs on the car. Modding an existing piece should be easy. However, I found some trash cans that can be cut a particular way and look like they belong there (believe it or not). I just need to study and figure out where to place them and how big they should be. My car actually has something on the underbelly that looks like air-dams, but their size and placement is rather odd.

OfficeLinebacker · 05-07-2008, 04:35 AM

I have seen them on just about every car from a big Audi SUV to Corollas.

They all just look like an inverted Gurney flap to me. Which makes me wonder if they generate lift.

I have been thinking about just how big I could make them without scraping, and I just had one of those thingers where a light bulb turns on.

Contrive some sort of contraption that is mounted to the suspension/steering knuckle that is exactly the size of the wheel!

If made of ABS, I can't imagine it would add too much unsprung weight.

lunarhighway · 05-07-2008, 07:22 AM

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