The hidden *real* drag coefficient
 

Most Cd figures are taken at zero yaw. This is why, on road cars, it's not a great idea to look only at that number. This video also has a lot of implications for testing in different wind conditions.

https://www.youtube.com/watch?v=2BVlkNly068

Thanks. I recall driving my parent's Rambler station wagon on the Oregon Coast Highway in an 80mph 90° crosswind.

It wandered a bit? If so, that relates more to Cl and Cy than Cd.

So important in the real world, irrespective of what Aerohead argues here....

I remember being there, but not the details. IIRC I was surprised that the steady crosswind wasn't hard to counter. It was an open road with no gusting.

Yes 'steady' is relatively easy - a constant yaw moment correct by constant steering correction.

Gusty is much harder.

My experience is that moving the lateral centre of pressure backwards (i.e. rear fins) makes an absolutely startling improvement.

Rear fins will return, mark my words...

Excellent video.
Thanks for covering the paper on trains.

When I added some vertical fins to the back of my box cavity, I intially thought of having them at a slant ( think SR-71 Blackbird ) for just this reason ( side wind drag )

They also present less side frontal area at a slant.

I had asked you before about this, but just to reiterate, how did you adress cross wind drag with your Insight fins ?
Also, what purpose do they serve ? I would guess for stability IN crosswinds ?

I added side skirts to my car, but was reluctant to do so due to the possibility of an increase in sidewind drag.

So in an ideal world, freight trucks would have rediused edges not only on the front behind the cab, but on the sides as well ?

Someday, I may take your advice and do a tuft test on a windy day with strong crosswinds. If and when I do so, I will compare the tufts on both sides of the car.
I'm interested to see the variations !

Cd and Julian, I think this paper may be relevant to the side fins.

https://www.researchgate.net/publica...dy_using_flaps

Thank you very much for the link AeroMcAeroface !
👍

Whenever there's a side wind I see my instant fuel economy rise whenever I pass something that blocks it, like a group of trees, buildings or a noise barrier.
This happens even when the wind is supposed to come from behind, so its relative speed in the wind shadow would be higher than out in the open. More friction just from hitting my car at an angle.

Likewise, if I come closer to a semi the turbulence in the wake may increase fuel consumption while on average the wake moves in the right direction; but the messy pattern kills the orderly flow over the car resulting in slightly higher friction.
When following a semi I may not have to adjust my CC settings for miles on end, because the added resistance slows my car a bit when I come within 40 meters or so. In economy mode the CC allows variations up to 3 km/h from the set speed before adjusting the throttle, which makes this possible.
Apparently 50 to 60 meters is the sweet spot where the wake still has some effect but the turbulence isn't strong enough to mess up the aerodynamics.

This suggests the Bertone BATs were on the right track.

Wow, what a paper - on spoiler design on hatchbacks as well as side fins.

It's also the first paper I've seen that supports what I have measured on the road - that fins on a fastback measurably reduce drag. (And another piece of supporting evidence for throttle stop testing.)

The drag reduction they quote on the Ahmed body by the use of the side fins is much higher than I measured on my Insight, but put my measurements together with theirs and I am pretty confident in saying that, at minimum, people should test fins if they have a fastback shaped car.

My aero mods on the Insight are all to improve driving performance, not to reduce drag. So my undertray and diffuser are to give downforce, my rear spoiler is to give downforce - and my rear fins are to provide better straight-line stability.

That said, I have also tried to do aero modifications that are likely to reduce drag - and in the case of the rear spoiler / fins, they measurably do so (at least in light winds).

I haven't tested the rear fins / spoiler for drag in strong winds, so I can't answer the question about drag in these conditions.

Please note I have made a mistake in the video. The graph is not correct for the Calibra, that has a symmetrical increase in Cd with yaw.

Sorry about that.

Fun fact: the graph is actually for the Plymouth Superbird, that does have an asymmetric increase in Cd with yaw.

The asymmetry seems odd.

It's unfortunate that Porsche exploited the vortexes for engine cooling with the Beetle.

I don't get involved in the throttle stop argument, probably not highly accurate but even if the numbers are out by +/-10% it is still a useful technique.

What about sailing, cars can be designed to get some thrust from crosswinds.
https://core.ac.uk/download/pdf/32425613.pdf figure 12
https://en.wikipedia.org/wiki/Midnig...olar_Race_Team
Is it ridiculous to envision some pop up roof sail or wheel fairings that reduce the effect of the crosswind? Or maybe a wheel fairing like the Daihatsu UFE, maybe road cars are too big, with too wide wheels etc.

most
 

I was under the assumption that, all, reported, contemporary drag coefficients, are crosswind-averaged, as per E.P.A. certification protocols, and have been for decades.:confused:

I don't think so, and nothing I can find today in a quick search supports that. But happy to look at any evidence you can cite that supports that.

supports
 

I'll look. Seems like it was Ford Motor, or SAE that mentioned it. They understood that driving in the REAL world involved a statistically-averaged, annual, 7-mph crosswind component and the wanted manufacturers to reflect that in their testing results submitted for new car certification.

Please do. I'm being challenged for using that number.

edit: ....incorectly.

I keep thinking that rear-wheel steering could be used to turn a car a couple of degrees into the wind, but I wonder how comfortable it is driving a car that is a bit off.

If that is not under driver control it will feel janky.

I've had a couple of cars with four-wheel steer, and I read a really excellent article on this the other day.

I think yawing down the road would upset all the visual / mental cues that a driver knows to control the car eg if it is skidding.

It's always funny to see a vehicle with an obviously tweaked frame yawing down the road sideways, sometimes called crabbing.

It seems to me that old pickup trucks are bad about that.

I always wonder what their tire life is.

I had a VW superbug that had some interesting collision damage that "dogwalked" and got 60,000 out of the tires.

Wow...60,000 miles out of a doglegged VW yawing down the road. I wonder how the alignment industry feels about that as they labor to get things to within the last ten-thousandths.

AFAIK, alignment is relative to wheel direction. So if all 4 tires are set correctly where the body is pointing (to the tires) is inconsequential. When I had the bug altered to drive straight, the alignment shop had to adjust centerlines of the rear axles for equal distance and change some shims from one side to the other side

That's a good point about the wheel direction v the body direction. It hadn't occurred to me that the wheels could be so aligned. It's probably easier to get the wheels aligned than to "un-tweak" a frame.

Oh BTW: a bug is semi independent suspension such that the rear axle (s) are allowed to move about on their own. The dogwalk I had would not be repairable on a typical differential axle unless it was a mounting position error

My Notchback was the best handling car I've had. Because I had a streetrod shop do a four-wheel alignment and they got the toe-in on the rear wheels right.

I must have missed the above mentioned mods being analysed originally for handling/DF improvements, and not for drag reduction. I'll have to go back and review.

I would also like to research the detailed explanation for the use of slanted fins for mainly drag reduction for effectively a ground based vehicle.

If maybe when one turns their head to adjust the radio, or chat, and promptly changes lanes or runs off the road, I would think a fixed yaw would matter little once the driver made the adjustment for cross wind drag benefits.

'evidence that supports'
 

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I'm not finished searching my archive, but I believe that the SAE transitioned into crosswind-averaged drag coefficients with
SAE Paper# 780337, 'Realistic Effects of Winds on the Aerodynamic Resistance of Automobiles,' by Bain Dayman, Jr., Jet Propulsion Laboratory, California Institute of Technology ( CALTECH ), Pasadena, California, USA, February 27-March 3, 1978; which looked at yaw conditions from, negative 5-degrees, up to positive-30-degrees yaw, computer modeling a 'wind-averaged' wind spectra, at 360-degrees effect on CITY, HIGHWAY, COMBINED cycle fuel economy.
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SAE Paper# 881874,from Texas Tech University, is one of the first documents I collected which presents coefficients of aerodynamic drag as a function of degrees yaw, at zero, 2-degrees, 5-degrees, 10-degrees, and 'wind averaged.'