ok all you areo buffs, do side skirts like this actually give any aero benefits or are they all looks?

 

there suppose to be performance. i see this thread turning into the rear diffusser one

 

I don't think so...

the where the front of the skirts meet with the front fender... i think too much is going on there (curves and joints). So simpler the better. Joints cause the flow to bounce around and cause resistance. I guess you can say, "you want to direct the air flow." But with that joint where fender and skirt meet doesn't really look like directing the air too well. (It looks like fender is though)

But I also think the skirt is there to flatten out the bottom of the car to flow the air underneath FASTer, to cause down force. (but you need the WHOLE car to be flat to really feel the difference)

I hope i didn't put any bull **** up. if i did...let me know so i can delete it.

-joe

 

what rear diffuser one?
I know for sure that certain rear difusers do help smooth rear air flow, some are more for looks.

 

Well i'm obviously no expert but i would say just like all aero parts that some are just for looks and do little to improve aerodynamics, while others actually are functional. I would imagine that having side skirts would keep air from going in or out from under the car laterally. This would keep the air under the car running more steadilly out through the back of the car wether it has a smooth underbody or diffusers or not. And the faster and more directly air moves under the car, the more of a low pressure area their is, sucking the car to the pavement. So, I guess in short IMHO they would help to at least a small extent at high speeds, but I could be wrong.

 

Rear diffusers aren't really THAT useful.

Anyways, sideskirts are beneficial, the goal is to increase the velocity of air underneath the car. Velocity increasing == less pressure. Less pressure on bottom than pressure on top == negative lift.

 

Faster air underneath the car doesn't add downforce, it does exactly the opposite and creates lift. Fortunately the angle of attack of the bottom surface is usually negative and the other stuff under the car produces enough turbulence to prevent our cars from flying.

 

This is not the case. The venturi effect doesn't apply here; if the bottom surface is very flat the higher velocity air under the car will increase pressure - that is why we have a front lip, to provide a stagnation point at the front of the car and prevent high velocity air from going under the car so that the pressure on the upper surface of the body forces the car into the road. High-velocity air under the car is certainly NOT what you want.

The ground-effect tunnel racing cars of the '80s relied on that principle by using a ground hugging front lip and two big fans to create a vacuum under the car and stick it to the tarmac.

 

so all that and still no answer on if sideskirts help with aero?

 

That's against the basic Bernoulli principle. Can you explain your point? (Otherwise, I have no choice but to conclude you're talking out your ***)

 

Side skirts give some added aerodynamic performance enhancment. Infact the entire mazdaspeed GTC kit was built and tested in a wind tunel. I have pictures somewhere from the testing. At higher speeds sideskirts being lower to the ground than stock it helps channel air from going under the car causing extra turbulance under the rear. Also rear diffusers add better high speed stability by propperly channeling the air from under the rear and keeping air from being caught up in the pockets of the rear. This is all stuff I noticed by watching wind tunel results from mazda and car companies.

 

These sideskirts and add-ons make me wonder - Mazda obviously spent good $$ making the aerodynamic design of this car. Wind tunnel testing at least. Kinda like the 'I could **** in a box and label it guaranteed' thing.

Do any of the aftermarket mfrs have any claim of backup up their 'improvements' with wind-tunnel or CFD experiments?

Dave

 

Exactly, the slow moving air causes lift because of the low pressure under the car at high speeds. Adding rear diffusers, and even side skirts channels it so its a more dirrect path for the air to follow thus causing faster moving air under the car.

 

I guess the reason these things don't come stock is because they reduce clearance too low for the mass public?

 

yep, i think its like 4inches clearance they have to meet.

 

Yes it does.

Faster air underneath (compared to above) the car = lower pressure zone (compared to above).

 

Egg-******* 'xactly he is.

 

Wow broken93, you have your concepts reversed. The Bernoulli principle states that for any flow, the sum of the static pressure (P) and the dynamic pressure (0.5*rho*v^2) is constant. Obviously, dynamic pressure is a function of velocity, so as the velocity increases, the static pressure decreases, causing a pressure drop. That is why cars naturally produce lift. The air generally has to take a longer path over the top of the car, making the velocity generally higher above the body and the static pressure lower.

That is the simple explanation. When the viscous effects from boundary layer drag and the ground effect are factored in, its impossible to make a reliable gut prediction about the net pressure distribution. To answer the initial post, it really depends on the car and the side skirt. I doubt that many body kit manufacturers do analytical work or aerodynamic testing of their designs.

 

so....i'm ok...

 

Actually, yes, I do have my concepts reversed. After I thought about it for a minute, and realized that I am an EE and not a MAE, and pulled the large chunk of stupidity out of my ***, I came to the conclusion that the high velocity air under the car is the source of the pressure drop; channeled air SHOULD create negative lift, and I am a moron. Thank you all for the clarification while I go and get out my aero book, and I apologize for the misinformation.

 

C-west did wind tunnel testing on there newest body kit for the FD. those kits are used by Super Taikyu teams.

-joe

 

It's all good. I respect you more for acknowledging the error anyways.

Now if you were jimlab, this would have turned into a 20 page thread (hi Jim! :P).

 

A lot of the higher end Japanese companys (Feed, RE, Panspeed, Mugen (Honda i know)) wind tunnel test their stuff which wonderfully adds to the cost of their parts.

 

Jeez, don't beat up on yourself.

(I'm an ME who works with some EE stuff and I get muc worse things mixed up regularly)

 

well, lets seperate a few things out here, one is how much air we let under the car, and two, how fast we want the air that does go under to move

we defiently want less air goin under the car, right? we can all agree on that, less air going under will help keep the car to the ground since we know that there is a lot of air going over the car, less going under means less for the top air to fight against.

now, for the air that does go under, we do want it to be smooth, and since we do not want a high pressure area there i would assume we want it to flow fast too