What is the effect of crosswinds?
 

I've been reading a lot on this site about determining Cd by coastdowns and the importance of "no wind" to be accurate. Also, there is lots of discussion on reducing frontal drag. I haven't been able to find much on how crosswinds effect drag and fuel economy. My experience is that crosswind seems to drop my mileage just as much as a headwind. I found one article relating to soapbox derby cars that said that a crosswind changes the effective frontal area as it moves the angle of the headwind around to the side of the car and changes the way the air flows over the body. (sorry if I'm not expaining this very well) If I understand the article correctly, a 25mph crosswind could double the effective frontal area of my car at 60mph. I would like to understand this better and would appreciate if some of you experts could explain this better or direct me to the info if this has been discussed already. The wind blows a lot here and I'm wondering if crosswind should be taken into account when thinking about reducing aero drag.

Aero in yaw seems to be my province for some reason.

http://ecomodder.com/forum/showthrea...rx-7115-3.html

Stand in front of your car and look at how aerodynamic it is, and note the frontal area. Now step to the side so you're looking at it from a 20° angle, like a crosswind does. Its frontal area is much larger, and chances are the side of your roof is now a large part of the trailing end of your car.

Don't judge a car solely by its CdA with 0° yaw, look also at its crosswind sensitivity. Rounded sides are nice to have, as is a short overall length.

As an exercise, picture a tractor trailer, bus, or Club Cab longbed pickup in a crosswind. Longer vehicles suffer the worst in crosswinds.

I'm not much of a sailor but...

Several friends that are avid sailors pointed out that, with the right shape, a good side-wind can actually make the car more slippery due to the sail effect.

A good airfoil shape also has this tendency up to the point of stall. So, in this case making the tail longer actually helps....

And again, the above only works for low angles of yaw. These designs are always a compromise.

If the wind is coming at you at 90°, then you have more losses due to the larger surface area.

Jim.

Sail cars, like ice boats, really take off when hit with a good side gust. Getting the angles right takes you to what is in effect a negative frontal area, by sneaking the usual low-pressure zones up the side and over part of the nose. With cars, it is usually more a matter of minimizing negative effects, and maintaining stability. It is a big topic, one best kept in mind as a frequent complication while learning the other basics.

Hi Bob,

So from the above, can a boat tail help or hurt in a car application, regarding the change in CdA and side winds?

Certainly, MetroMPG's big gains in efficiency is nothing to sneeze at after his boat tail was added. And I'm sure he tested on calm days.

Thanks, Jim.

Yes, a boat tail can help or hurt. YMMV.
Test Team Test! Go Coast Coast!

A good question Puddelgum and one which I am sure is not considered much if at all by many discussing fuel economy and aerodynamics.

Cross winds have two effects in the main.
The first being to expose more of the vehicle surface area to the wind , and the second being to change the (ideally) smooth flowing air into a torrent of turbulence on the side opposite the side being attacked by the cross winds.
How much this second factor damages aero varies a huge amount and I am not sure there are any numbers out there to help even get into the ball park.

Pete.

Anyone who has ever ridden a racing bike (or even done hiking) knows this: if the wind is at your back, it's a gift. If you are headed into the wind or blown sideways by it, it's a drag... literally.

Now, maybe if our cars had adjustable sails... :rolleyes: :D

Thank you for all your replies. I'm guessing that mods like removing roof rails will be of much more benefit in a crosswind situation than in calm air. What about belly pans vs air dam and side skirts. I know they both are proven to work but would a belly pan be affected less by crosswind than sideskirts?

In crosswinds, I think the odds are that a belly pan would show slightly more improvement relative to skirts. But aero on anything but pure shapes is full of surprises.

"Anyone who has ever ridden a racing bike (or even done hiking) knows this: if the wind is at your back, it's a gift. If you are headed into the wind or blown sideways by it, it's a drag... literally.

Now, maybe if our cars had adjustable sails... "

Most rigid-wing land sailers don't adjust for wind direction, they just work. Life is different in an airfol than a human body.

Epa
 

In the late 1970s the EPA published a table showing the effect of an 18-mph wind as a tailwind,crosswind,and tailwind.
I've meant to post it.I do think the data is in the seminars though.
SAE criteria for coastdown tests limit crosswind testing to fairly low windspeed because of the difficulty of reducing the data.
If memory serves me,if any run has a standard deviation of 2 % or greater ( and crosswind will effect this ),that data must be discarded.They're very stringent.
Dead flat course and zero wind is almost absolutely necessary when you see the mathematics necessary to compensate for more than one variable.
If you've got no plans for the rest of your life,then I'd say go for it.

Stability in Cross Wind
 

I am drawing up "boat tail" plans for my new project car 96 Geo Metro.
I am concerned about the side area increase by elongating the rear.
Can someone who drives a "boat tail" comment on the stability or instability feeling while driving in cross wind?
Thanks

If it's properly boattailed, the side area doesn't necessarily increase. Depends on speed vs windspeed and angle, but plan for the most common condition in your area.

Here's how I visualize it (with sort of off-the-cuff numbers): Look at the vehicle from the front, offset at, oh, 10 degrees. That is what the x-wind sees (pic 1):

http://i146.photobucket.com/albums/r...cher/clipe.jpg

http://i146.photobucket.com/albums/r...her/clipe3.jpg

http://i146.photobucket.com/albums/r...her/clipe2.jpg

Looks like proper tapering in plan view added exactly ZERO to yawed "frontal" area and thus shouldn't adversely effect stability.

Perhaps this is 5 degrees yaw?

http://i146.photobucket.com/albums/r...0212009001.jpg

Perhaps this is why outside mirrors don't hurt drag much in the real world? (not adding much "frontal area" at all)

My car is a little more sensitive to crosswinds than before the boattail was installed, but nowhere near as sensitive as my old VW bug used to be.

comment
 

My VW bus saw nothing funny.The CRX is rock solid.The T-100 is rock solid,and the full boat tail trailer pulled with no surprises in 15-mph crosswinds.

3-wheeler
 

Frank,I just wanted to thank you for the pics of the red 3-wheeler.That vehicle should be in a thread by itself.It's aft-body looks dead on! Thanks mucho!

I heart that trike but unfortunately have zero info on it- just those three pics.

Crosswinds are almost always mor

I came across this link yesterday that illustrates crosswinds effect on mileage quite well. I'm guessing the hit would be even greater on a larger car with a worse Cd.

Crabbing the whole car into the wind is an interesting concept, but I have to wonder if it would lead to more accidents- would crabbing down the road disorient drivers?

I was'nt thinking much about the crabsteering idea when I posted this link, but I think it could be a safety risk.

...head-on, the wind speed is a direct "drag"

...but, oblique wind direction will be basically a cosine function (I think), with a MAJOR increase in tire rolling resistance (Trr) as YOU keep compensating for the body's "windvane" effects by slightly cranking in steering offset.

puddleglum -

Such a cool picture. I tried to make a meaningful plot out of it :

http://ecomodder.com/forum/member-cf...sswind-mpg.jpg

The "Angle" is the ambient wind direction, in degrees.

The 90 degrees is (obviously) the point where the crosswind is orthogonal to the direction of travel. It looks like two different equations on each side of 90 degrees to me.

I wonder what sailboaters would have to say about this?

CarloSW2