What is the effect of crosswinds?

[puddleglum]

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.

[Frank Lee]

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

Single front wiper crx?

[RobertSmalls]

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.

[3-Wheeler]

Originally Posted by RobertSmalls

...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....

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.

[Bicycle Bob]

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.

[3-Wheeler]

Originally Posted by Bicycle Bob

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.

[Bicycle Bob]

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

[Peter7307]

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.

[Thymeclock]

Originally Posted by Bicycle Bob

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.

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...

[puddleglum]

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?