I'm inclined to agree.
Even assuming magical clean air, the fatal flaw in yaw is that it assumes the yaw angle is constant with height, but no matter what the yaw angle is a bar height the yaw angle at the tarmac will be zero, and in between those two points is some non straight line yaw gradient.
In theory it wouldn't be beyond the wit of man to chop a bit out of a deep section rim, stick a wireless differential pressure sensor in the rim and compare pressure readings of a rotating wheel in a wind tunnel compared to real roads.Whether it would yield anything interesting useful is another question!

If only someone would validate wind tunnel measurements vs. "real world" outside power (in relatively large cross-winds)... :-/

http://cdmbuntu.lib.utah.edu/...39/filename/5200.pdf

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http://bikeblather.blogspot.com/

I think you'd want to just have two pitot tubes, one at the bars, one at the skewer or something vs. one rotating open-air sensor. Would be hard to factor out the pressure caused by the rotation of the wheel vs. the pressure caused by wind.

It sounds to me like you have some good intuition. I have yet to been convinced that all riders in all positions on all bikes will show Reynolds number invariance (same CdA at a variety of flow speeds for a given wind yaw for example). The fact is, laminar flow separation (stall) is extremely sensitive to free stream turbulence levels, surface roughness and curvature, as well as Reynolds number. That said, your reported 230grams vs 40grams difference at those two tunnel velocities does seem significant enough to consider all possibilities. There could be a Reynolds number effect, but it is also possible that when the tunnel is run at a lower (non-ideal velocity) the turbulence and flow uniformity are altered. I would need more information on the test conditions that day to get a proper assessment.
Yes, we have wind guests, passing cars, other cyclists. The question is, what did you want to do about it? There is no viable way to design a "faster bike" based on wind gusts as there are presently no simple way to model the length scale and time scale of those gusts in a controlled wind tunnel environment. Bikes and the like are designed assuming steady flow conditions. It could be steady yaw at 10 degrees, or steady yaw at 1 degree, etc. This is what wind tunnels do - they produce a nice steady wind. How good of an approximation are steady wind conditions? The answer is that it depends on the environment you are riding in. In some locations, it is very common to have a consistent wind direction and wind magnitude with very modest gusts. In other cases, such as what you have described (trees, cars, cyclists), the gusts are more significant.
At the present moment, I am thinking that stability with wind gusts is probably of more importance in engineering design than the aerodynamics. Stability is a different topic, so I will not open that one up for discussion here.

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Chris Morton, PhD
Associate Professor, Mechanical Engineering
co-Founder and inventor of AeroLab Tech
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I believe it. I’ve ridden with power for about a year now before and after deeper wheels and nice GP 4000’s.

The cross winds hurting from the front are much more manageable. You get used to what to expect on a pan flat stretch for a wind condition and speed. The help just basic 50mm wheels with GP4000’s was in a hurting cross wind was a bit shocking. Better than lower profile wheels in pushing you sideways.

They claim that yaw wind help with 4000’s.

I’d be curious how a much newer wheel is with that tire.

Beat me to it!

To try to help put things in context: I would rate the winds that day as a 6 on the Beafort scale:

https://en.m.wikipedia.org/wiki/Beaufort_scale

In particular, I remember the sounds of the flag snapping in the wind and the lanyard clanging against the metal flagpole...

Just realized my post almost implies that drag at 30mph should be the same at 20mph. The drag at 20mph (for the same CdA) should be 4/9 of the drag at 30mph (ratio of the squares of the velocities). Just wanted to make sure that was clear.

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Chris Morton, PhD
Associate Professor, Mechanical Engineering
co-Founder and inventor of AeroLab Tech
For updates see Instagram

Aerotech's response is pretty solid IMO. Also, as TomA has pointed out, there has been some real world validation. With that said, you'll notice the fast amateurs that geek out about this sort of stuff (many of whom are engineers) don't really give a hoot about performance at yaw. They'll test at zero and ten degrees and that's it.

That's the first thing I looked at as well...and it turns out the Premier Tactical data pointed to above shows exactly that at zero yaw. It's above ~7.5 deg. that it apparently starts falling down.

(Hmmm... the load cell rotates with the unit under test, right?...maybe the loads on the cells in their axis directions is getting too low to be able to resolve the vector sums accurately? Maybe that result is just showing why it's a good idea to test at 30 mph ;-)

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http://bikeblather.blogspot.com/

I want to give a different more practical answer based on experience in the field. I test a lot and also ride a fair number of outdoor TTs, many in windy conditions, and I estimate my CdA from every race (using the VE/Chung method so windy conditions will tend to bias the estimates upward). I've only had one race where I experienced the "sailing" for a sufficiently long time that it appeared to substantially lower my CdA. That race was an out and back with consistent strong crosswind and the whole race I had to lean the bike into the wind, kind of like a sailboat. In every other race I've found that more wind appears to make me less aero not more. Keep in mind there's the bias issue mentioned above. However, if crosswinds were having a large effect on my CdA in practice, then even with an upward bias I should find lower CdAs sometimes and I'm not really ever. I'm finding the opposite: more wind generally means higher CdA, even crosswinds. My least aero race last year was in a very gusty crosswind. Now, all that doesn't mean it's not happening sometimes. I think it's just not happening enough to make a big difference to anything.

I *think* I experience it fairly often at my local district TT course (Lake Los Angeles).

It's a rectangular course with an often fairly strong crosswind that often cuts about a 45-degree angle with the rectangle (a different angle of attack every time you make a turn, obviously). I never thought about it until I ran Best Bike Split on it, and it predicted that I'd see between 8-12deg for essentially the entire TT (97% of it). At around 29MPH. If *feels* like sailing at times, and my Aerolab validation of BBS *seemed* to be somewhat on point, though it was a bit messy. And "feelings" can be deceptive.

It's seriously made me double-think my bike upgrade. Thought about P5, but would seriously consider something like the IA which is a great high-yaw bike (have a DA now). And also made me put a mental asterisk on the "fast people don't need to care about high yaw" mantra.

my guess though would be that while frames and wheels might be able to generate a sail effect, the bulk of CdA is from your body which i (without any actual facts or knowledge) highly doubt is able to generate a sail effect. so yes, in practice there will be minimal % drag change in the system at yaw. i can imagine a disk might be able to generate enough lift to be noticeable as part of the system but nothing else. thats not to say that other components are not helping, just that a 1% decrease in total drag is not going to be noticeable - i'll still happily take it though :)

i suspect the OP may be right though that you need a reasonably consistent wind to generate worthwhile lift - i've spent some time sailing and experienced the difficulty of getting good lift in shifty conditions, though it does still work

you need to have quite a good range of yaws at which the lift is generated too as the moment you get some lift, you accelerate so your yaw decreases and the lift disappears on you, or the wind increases further so the yaw jumps and you stall... hence slowing you and increasing yaw even more! it all seems too complex to be able to give significant benefit for significant periods of time

My observation here is that you really don’t see angles above 10° most of the time. The fast guys really don’t see much beyond 5°. This is course dependent as hedges/fences line the wind up with the road to an extent on a lot of roads over here.

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Developing aero, fit and other fun stuff at Red is Faster

Why guess, rather than read the paper to which Tom A. linked?

Cuz we live in a world where reading is hard and opinions trump evidence and data.

The guys at Flo wheels put measuring devices on bikes and rode a bunch of miles in different conditions and found that 80% of the time the yaw angle was between 0 and 10 degrees. http://www.flocycling.com/aero.php So, they claim to have designed their wheels to work best in that angle range and I'd assume since that data is easy to gather, everyone else has too.

Of course, that is average data. If you ride every day on a coast road and you are going to have a 90 degree cross wind 90% of the time, you are not going to benefit from something designed based on averages ;-) Everyone does still have to tailor their equipment to their actual riding.

I spent a lot of time racing sail boats in my youth and one to keep in mind is that yaw angle is the same as "apparent' wind angle used by sailors and it is not the same thing as just comparing the angle of travel and the raw angle of the wind. Anything moving is going to have an apparent wind angle that is different than the actual angle of the wind vs angle of travel. And as you go faster, the angle gets closer to zero. And, whether it is a sail, or a wheel, or an airplane wing, the only thing that matters is the apparent angle because that is that the thing is actually experiencing. Go fast enough, and that 90 degree cross wind will become a 10 degree yaw angle ;-) My surmise is that is how they ended up with the 80% of the time the yaw is 0-10 degrees data. There is a reason it would tend to move toward a smallish number.

It is not that hard to come up with a shape that will generate lift at certain angles and that lift can be transferred into a usable force. Sailors have been doing it for centuries, aeronautical engineers for about 140 years. Bike/wheel folk are actually kind of late to that game ;-)

Thank you for your insight. In 2006 I did a small duathlon on the east coast. It had a long, straight, flat out and back and there was a strong steady quartering headwind on the way out and quartering tailwind on the way back. I was riding a Guru CronAlu (only aero feature being a shaped downtube), a 58mm deep front wheel, and a flat disc in the back. Like you I had to lean into the wind in order to avoid being blown off my line. I did not have a power meter. I was the only one riding a disc that day and opened up a 6 minute lead on the bike leg. It felt like I was sailing. I have never again experienced this, even on supposedly much more aero frames and deep wheels. I was wondering whether there is something about a disc (large surface area? behind the rider?) that allows it to reduce drag even in less than perfect conditions.

At Galveston last year we had a constant 20mph side wind almost the entire bike. From the left on the way out and from the right on the way back. The yaw was so high that best bike splits aero analyzer didn't know how to handle it. So it does happen.

On the "sailing" thing, that is real. And it could have some untapped potential, at least for time trials.

The boat I raced on had 10 sails each designed for a very narrow range of wind speeds and there a couple different basic types of sails for grossly different wind angles. (We would have had more, but our class limited the number of sails on board to keep costs down)

The future of bike wheels might have you all showing up at a triathlon with multiple sets of carbon wheels of subtly different shapes and pre race warm ups will include groups of guys standing in the parking lot with anemometers and professional weather forecasts trying to predict what the wind is going to be doing one hour into the bike leg ;-)

"The guys at Flo wheels put measuring devices on bikes and rode a bunch of miles in different conditions and found that 80% of the time the yaw angle was between 0 and 10 degrees"

The corollary is that 20% of the time the yaw angle is higher than 10 degrees, and that will vary by region. A lot of bikes seem to perform fairly similarly at low yaw, but diverge at higher yaw angles, so in some cases it would make sense to go for the gear that performs slightly worse at low yaw but substantially better at higher yaws.

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I’m not saying it doesn’t. It’s just not that common, certainly over here.

90° 20mph wind at rider height would some people a lot of trouble.

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Developing aero, fit and other fun stuff at Red is Faster

In bike racing that 20% of the time is when selections happen, and a bike that handles high yaw can be wildly advantageous. You have to love when the bike hits the wind perfect and it feels like it is just floating right through a heavy cross wind.

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Denying or simply ignoring science is the new way to show your intelligence and enlightenment! Never let facts get in the way of your opinion.

If you say something often enough and with enough conviction, you can convince people, even yourself!

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)