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Old 05-04-2014, 11:22 AM   #1 (permalink)
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Yaw angles and aerodynamics

Prevailing winds here have been mainly W-E this past week, while my commute and an extra trip I took this weekend were N-S, which got me thinking about yaw angles and car aerodynamics.

Manufacturers of time trial/triathlon bikes test their bikes in wind tunnels, usually releasing drag figures not as Cd measurements, like cars, but as total grams of drag (I was traveling to/from a duathlon a couple hours away, so my bike was in the back of the car, which was the instigation of my thoughts). It's quite easy to find tests by various companies of their own bikes and the competition, with or without riders (or dummies), water bottles, cables, etc., and all of unknown veracity (since they're used as marketing tools, and generally show whatever company conducted the test in the best light). But these tests universally demonstrate something interesting--drag decreases from 0 degrees wind angle out to around 10-15 degrees, and then begins to increase. Here's one test by Cervelo (with a dummy rider) of several different bikes:



(Not surprisingly, the solid red line is their current range-topping model, the P5).

Another, this one by Trek:



(Again, here the Trek SC "wins").

So, I wonder: is this effect seen on cars as well, a reduction in total drag at low yaw angles? Do car manufacturers consider yaw angles in their designs and testing? Is there any way to improve or tailor an existing car's aerodynamics to better handle higher or lower yaw angles, depending on expected conditions?

Discuss.

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Old 05-04-2014, 12:08 PM   #2 (permalink)
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Quote:
Originally Posted by Vman455 View Post

So, I wonder: is this effect seen on cars as well, a reduction in total drag at low yaw angles? Do car manufacturers consider yaw angles in their designs and testing? Is there any way to improve or tailor an existing car's aerodynamics to better handle higher or lower yaw angles, depending on expected conditions?

Discuss.

The Yaw would effect the frontal area, a smaller frontal area is superior.
Both should be harmonious. If the Yaw was to be adjusted to achieve a smaller frontal area it would have a effect (by the smaller frontal area)

Thats my stab at your question
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Old 05-04-2014, 01:43 PM   #3 (permalink)
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The Yaw would effect the frontal area, a smaller frontal area is superior.
Both should be harmonious. If the Yaw was to be adjusted to achieve a smaller frontal area it would have a effect (by the smaller frontal area)

Thats my stab at your question
See, I would think the same thing too, but a bike+rider presents more frontal area at higher yaw, yet achieves lower drag. I'm not sure why this is. Maybe because the profile of the object has changed and air is moving across not the original arc form but a chord with a lower effective angle from horizontal?
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Old 05-04-2014, 02:59 PM   #4 (permalink)
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I find myself wondering why the curves are asymmetrical... I would think they'd be symmetrical around zero yaw.

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Old 05-05-2014, 08:26 AM   #5 (permalink)
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I think that's because of the drivetrain components being all on one side. Additionally, many frames these days are asymmetrical.
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Old 05-05-2014, 03:46 PM   #6 (permalink)
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Yaw

If I remember correctly,Class-6,7 and 8 trucks are tested up to 12-degrees of yaw as per some SAE Standard Practices.
Ford Motor Company reports wind averaged Cds for their cars,which represent yaw angles up to 8-degrees.
Baine Dayman Jr. of Cal Tech's Jet Propulsion Laboratory published an SAE Paper a few decades back which addressed crosswind effects to Cds.
Hucho dedicates a section in his books to annual statistical wind speed vs wind direction as pertaining to drag.
DOE/EPA has also published mpg effects of wind.
Each vehicle would have to be taken on a case-specific basis.There wouldn't be any patent expectation for yaw effects.
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Old 05-06-2014, 10:28 AM   #7 (permalink)
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It makes sense to me.


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