Cross wind drag and side skirts / 'tails
 

A few years ago, I added side skirts to my Civic.
I removed them since they add several feet of frontal area to the car when hit by a crosswind. *

What kind of FE hit does a car get from increased side frontal area ?

On a similar thought, what about boattails and the increase in frontal area that they produce ?

* Where I live ( the 'hill country' ) , there is a lot of wind that blows in from the sides of the car.


:confused:

I wonder how crosswinds affect semis regarding FE.
I know that the semi becomes less stable, but is there also a big FE hit ?

skirts/tails
 

Skirts should not project below the "bottom" of the vehicle and consequently should not aggravate it's side area.
I can't answer the question about drag vs increased side area.I've never seen anything published on it.There is a table published by EPA on wind/mpg effects.
One benefit of skirts in crosswind,is that they help cancel roll moments above the C.G.,actually increasing stability and safety to an 18-wheeler.
I've only run "short" tails,so I can only address those,and my experience so far is that they offer no degradation of stability,thanks to Professor Morelli's work at Pininfarina.

I can only imagine underside airflow at a yaw angle is extremely complex. I don't think side skirts have a negative impact on drag even in crosswinds. Remember, the flow vector is very rarely gonna be perpendicular.

Got any piccies of yer vehicle with the side skirts on? ;)

I knew that was coming ...............I'll have to find them. :D

( Not sure if I deleted them. )


EDIT : I found this picture, but it has some quick and sloppy Photoshop work . The skirts and wheel gap filler are real, but the diffuser thingee etc are Photoshop additions.

http://farm3.static.flickr.com/2693/...f659861234.jpg

Phil : When you say that the skirts shouldn't project below the 'bottom' of your vehicle, what do you mean ? Do you mean the floorboard ??
( surely not )


Tas : Can you simplify what you said about 'flow vectors' going perpendicular ?
( Imagine that you are trying to explain what you just said to someone with the mental capacity of a child ...... or a very smart monkey . )

Well, if you get a crosswind on a sailboat, and increase the sail area, it goes faster. Most of the rigid-wing sailing cars have not had a way to adjust for right or left crosswinds. I saw Mana La get hit by a gust, and the acceleration was obvious. Second Inning: Mana La Solar Car Put On Display At Peterson Automotive Museum - Green Diary YMMV

I'll show you mine...
http://i677.photobucket.com/albums/v...0/IMG_5979.jpg

http://http://www.bentleypublishers....ery-768-8.html

Frank Lee posted this link in response to a question I had on crosswind.

http://http://207.242.75.40/derbtech/windeff.htm

Here is another one that I found. Both have good explanations of crosswind vectors.

Fixed yer links...

Say you're going 60 mph and you're getting a 60 mph crosswind at 90 degrees. What I called the flow vector is going to be 45 degrees.

Thanks. What did I do wrong?

I think you clicked on the "Insert Link" button and when you pasted yer link, you had copied the "http://" part which the link button already added in. ;)

wind: EPA & SAE
 

EPA published a chart in 1970s.
An 18-mph crosswind reduces mpg by 2%.
----------------------------------------------------------------------------
From Bain Dayman,Jr.,Jet Propulsion Lab,Cal Tech Univ.,SAE Paper 780337:

Annual mean wind spectra effect on zero-yaw Cd

Annual Mean Wind Speed,mph

5.8 10 15

Highway + 4.5% +12.0% +33.0%

This is not targeted specifically to crosswind,but will give you a feel to effect on Cd.


With respect to skirts,I've never come across any research which suggested that a skirt should be any lower than the bottom of the floorboard.

For a ground-effect racing car there would be a very carefully designed intake at front,then the rest of the car would be sealed all the way around until the very rear,where the low-pressure wake could communicate under the car, inducing downforce.

then its front and rear strakes aka tire boattails. instead for me,, the rocker panels are even with my belly pan already.

strakes/boattails
 

Yes,the leading and trailing fairings really help for the wheel drag.
Ford found that a trailing fairing after the front tires was not necessary on it's Probe-IV.The MG EX-181,Sunraycer,and Honda Dream-2 all employed complete fore and aft fairings for their tires.GM did delete theirs' for the trans-Australian Solar Challenge so they could survive 40-mph crosswinds,a strategy which allowed them to out run storms and get to sunshine.

I have Hucho's "bible" here with some data on side skirts and yaw in semi trailers and they improve the CD. At a yaw of 25˚ the cd is .15 better with fully skirted wheels.

CD @ 25˚ yaw faired: 1.0
CD @ 25˚yaw open wheeled: 1.15

Amen.