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.

---

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

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.

---

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)

Yep. Patrick O'Brien summed that up nicely in one of the books in the Jack Aubrey / Stephen Marturin series: "Truth was what he could make others believe". Reminds me of some elected officials.

My sense is that there are really two questions here.
First, do riders actually experience high yaw angles? As a guy who spent all his racing years in Texas, I can tell you for sure that those yaw angles do exist in the real world. I have vivid memories of leaning into a side wind so hard that my tires were slipping sideways and I was concerned my pedal would hit.
Second, if one does experience high yaw angles, will drag be reduced similar to what is shown in wind tunnel data? The answer to that is, whatever is true in the wind tunnel will be true in the real world (if rider position is the same). Bikes and riders obey the laws of physics except maybe in races held at Hogwart's.
Cheers,
Jim

As a fellow Texas TT racer, I agree. According to Best Bike Split I've TT'ed in 8-14 degrees (and that at a 56 and change 40 KM time).

Yes, and it's possible to experience that for 100% of the race.

The corollary to "all training is individual" is "all equipment choice is individual."

Flo, et al, did a great service by producing an "average" wind profile just like all the training physiology papers do a great service by trying to find a "population average" physiological response, but any athlete should understand when and how they might be operating near the edge of a bell curve, and the implications.

If you weren't so slow you wouldn't have had that problem.

<ba da bump>

That is where my sail analogy comes in. We did not have a "fastest sail." We had a selection of sails each one optimized for a fairly narrow range of wind speeds (3-4 mph bands at the lower end) and then we could further tweek them for specific conditions by subtly changing their shape some through various aspects of the trim tools we had available. The trimming part will i presume always be banned by rule in cycling but it is not hard to see wind situations where specifically shaped wheels or even frames designed for that specific condition would give race changing marginal gains beyond the binary choice of just aero vs shallow rim.

Hello STP and All,

STP wrote in part: "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 ;-)"

Or ..... to riff on the thoughts of STP above ..... the future might be guys showing up for a triathlon or TT with one set of bicycle wheels rocking articulated aero devices ...... a set of wheels designed and constructed similar to an aircraft wing ........ adaptive bicycle wheels..... bicycle wheels that have articulated aero devices to perform best at a wider range of speeds and crosswinds than current bicycle wheels.

It is interesting to note that the air pressure activated leading edge slats of the German ME-262 (WW II) were copied for the US F-86 (Korean War) and worked very well at increasing the functional speed/lift range of the wing.










Great design (for the era) .... activated by air pressure and gravity .... no external power or controls ........ as speed increased the slat is retracted by air pressure ..... at slower speeds it extends by gravity.

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Cheers, Neal

+1 mph Faster

Some people might already turn up to races with a weather station. ;). When I was racing more, I used to follow the weather forecast to see of it was a 50 or 90mm day, course dependent.

The yaw distribution is what got me started on more serious aero measurement. This is old, unlabelled and from a 40kph rider on typical U.K. road.

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

I did not integrate your graph, but although you say it was a typical UK road, it seems to have been made on a day without any wind, because it seems that 99% of the time was between 0° and 10° yaw.
(Look at post #17:
"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.")

(on-topic:) I saw an interview with the aero-responsible guy at SwissSide, Jean-Paul Ballard, who made a very competent impression to me (he will be known by the insiders at ST), who presented this thing:

https://www.bikerumor.com/...ro-drag-on-the-road/

not yet on the market, which could be very helpful at testing outside (if it measures cda so well as Jean-Paul claims).

Another thing he mentioned in that interview which is relevant to this thread is something I heared already before but which I did never regard as so important:
The insiders know that a continental tt (being a slick) has better rolling properties than a conti 4000s II, but are slightly less puncture-resistant. I for example take the risk and ride tt because I hope to be faster therewith.
The thing Jean-Paul mentioned though is that the 4000s II is better in aerodynamics than the tt (on aerowheels) because the profile on the 4000s II makes the airstream turbulant causing it to stall later from the wheel. The surprising thing to me was that this effect is very important in that they measured 7w difference because of this effect. This effect is by the way only interesting for the front wheel.

It is of course interesting, considering the discussion in this thread, with which yaw angles these 7w were measured. Anyway, Jean-Paul said that stalling normally happens at 19° yaw, but with a slick already at 7° yaw. What he claimed explicitly is that mounting a tt on a front aerowheel destroys the whole advantage of the aerowheel compaired to a cheap wheel. As I understood this I directly sent one of the tts I just ordered back to exchange it with a 4000s II which I will mount on my front wheel.

Absolutely. There’s was measured airspeed of around 5kph on the bike iirc. Again, this was for one rider fairly fast rider on a hedge lined road. I posted it as an example with caveats, nothing more. It was also one I happened to find when I was looking to 2015 for some other data.

Flo’s data does indeed show that low yaw is where it is most most of the time. The zero/negative drag regions are generally outside this.

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

Speaking of Flo's data, there are several other studies that also agree. Dr. Barry wrote a short summary, which Dan hosts here on ST: http://www.slowtwitch.com/...Yaw_Angles_5844.html

Which raises the question: For general riding and racing (not racing on specific courses, for which wind direction could be predicted more narrowly), how do we normalize the various drag values measured in the wind tunnel across the various yaw angles? Dr. Barry has a good approach: "A New Method for Analysing the Effect of Environmental Wind on Real World Aerodynamic Performance in Cycling" which will be presented at ISEA this year in Brisbane.

Here it is: http://www.mdpi.com/2504-3900/2/6/211

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

This. However, I don't believe what Dr. Barry shows is really a new method. The original source I found when applying wind averaged drag in a study on bicycle wheel aerodynamics (http://fluidsengineering.asmedigitalcollection.asme.org/...px?articleid=2674736), was a study by Cooper (2003) - "Truck Aerodynamics Reborn - Lessons from the Past". SAE Technical Paper (2003-01-3376).
Following this, Brownlie et al. (2010) applied this same wind-averaged drag method to cycling time trial helmets: "The wind-averaged aerodynamic drag of competitive time trial cycling helmets," Procedia Engineering 2(2).

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

Dr. Barry cites both those articles, and indeed his method is similar.

I'm not Dr. Barry, but as I understand it, Cooper prescribes a few specific parameters including a test protocol (particular yaw angles), where Dr. Barry allows more nuanced input data (wind tunnel data with more or fewer yaw angles, etc.). And I never fully understood the specifics of Brownlie's math: does he reveal enough about his method for someone else to apply it to new data?

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

Yep you are right about that. Cooper's can be generalized to arbitrary yaw angles and perhaps Barry has done this. I believe there is some SAE recommended practice in 2012 that might generalize it as well but haven't looked at it in over a year. Agreed on Brownlie as well!

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

Thanks, good discussion, and I appreciate your reply.

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

This is an example of the kind of claim I am skeptical about. Pointing a wheel at 15 degrees into a steady 30 mph wind seems similar to, but not equal to riding a bike at 25 mph through a side wind that produces an effective 15 degree yaw angle, especially when that side wind is not perfectly steady. My guess is that in the real world the amount of time the 4000s II tire gives 7 watts of drag savings will be near zero and that a TT front tire would produce a near identical 40k time. Has anyone tried testing different front tires with similar overall width and rolling resistance in the real world to see if the one with the supposedly better shape really does produce measurable drag savings?

i have run back to back different tires over the same first 4 miles of a TT. results obtained in tunnel correlated pretty freaking well in the field.

I flatted and had to restart. used different tire second run.(tested both in tunnel)

That is the case for me also. Wind is not my friend. But for some a crosswind seems to have a slight benefit.

You're right in saying that 15 degrees in a 30 mph hour wind vs. a 25 mph wind is similar but not equal. For a steady state, one could compare the Reynolds number (Re) for each part in question and determine how similar the flow might be. It could be very similar or very different - the issue with aerodynamics in cycling is that the Re and yaw angles tend to be near "break points". For example, everyone adds flow trips (the "rough" curve) to reduce flow separation at lower Re but the effect happens at a very specific point and if the relative air velocity is too low, you may never reach this point to take advantage:


If someone tests in the wind tunnel on "one side" of the curve and rides in the same regime, then their results are likely to be very close, but if not, they can differ significantly.

One other thing that most people forget when thinking about "sailing" in the wind is that gusts that change the yaw angle significantly can cause flow separation that remains even after the gusts go away due to flow separation hysteresis. Basically you can not have separation at say 15 degrees at first, but if a gust comes and changes the angle to 20 degrees and causes separation, the separation bubble will remain even after returning to 15 degrees. This may be why in the real-world people don't experience significant sailing and why real-world results *can* be very different then what is experienced in the tunnel. And for most wings, this effect is around 12-15 degrees (lift on top, drag on bottom):

Unless the difference is due to the very real departure between the WT and the real world. Things like wind gradients with height on the road, turbulence in the airstream, etc.

Plus... I'm going to ask a dumb question, but are those WT graphs of drag vs yaw always corrected? A bike turned at an angle is not simulating a 30mph bike speed + crosswind. It will be less.

Take a look at the figure from Martin et al 97 posted by Tom Anhalt earlier in this thread. During those trials the conditions were about as challenging as they could possibly be; the wind was high, gusty, and constantly changing directions. We used just the average wind speed and direction for each trial. The model still accounted for 97% of the variability with a standard error of less than 3 watts. I reckon if we had continuous values for wind speed and direction we would have cleaned up a lot of that final 3%.
Can't comment on the yaw angle graphs.
Cheers,
Jim

In my experience, yes. I learned in the late 1990s some tunnel techs call it the "cosine beta squared" correction. At the time I worked out the trig to convince myself, but don't ask me to do it again today, ha ha!

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

But I don't believe you test at 15 degrees in the tunnel? ;)

There seems to be two questions in this thread. One is whether we actually experience the yaw angles that marketing departments are happy to tell us about, the other is whether wind tunnel results apply to the "real world". The latter is genuinely believed (and proven) to be true (with small caveats like the ones rruff is mentioning). The first is a little more open for debate, but the general concensus is that yaw angles outside ~10 degrees are usually only seen if you are either riding decently slow (<35 kph) and/or on specific courses/days (high wind, open fields or coast lines, riding perpendicular to the wind direction).

Note for Damon: The last point is why we would still like drag vs. yaw graphs and not just a "dumbed down" weighted average drag so you have no idea about in which conditions specific products excel and vice versa ;)

i prefer 180deg :)

For the smooth vs rough, I understand what you are trying to say, but the effect is also strongly a function of the nature of the roughness, and placement of the trip wire.

For a point of reference, a 2 inch (25.4cm) diameter sphere in a cross-flow of 40 kph wind, would correspond to a Reynolds number of about 40,000 (hope my math is right). This is the onset of drag crisis (big drop in drag) for a dimpled sphere. I agree 100% with the hysteresis. It is certainly overlooked, but for low Reynolds number cases doesn't it rely on the flow actually being laminar at the separation point? This is something that would depend largely on the nature of the incoming flow and surface roughness too.

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

If we assume an equal probability that wind can come from any direction, then I obtain the following graph which represents the % of wind angles for which yaw is greater than 10 degrees.
Code is here: Feel free to implement and check the math.
clear all;
V = [5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 40 42.5 45 47.5 50 52.5 55 57.5 60];
w = [0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 40];


for j = 1:length(w)
for k = 1:length(V)
count = 0;
for i = 1:360
wan(i) = i*pi/180;
VR(i,j,k) = V(k)*sqrt(1+2*(w(j)/V(k))*cos(wan(i))+(w(j)/V(k))*(w(j)/V(k)));
A(i,j,k) = (w(j)/V(k))*sin(wan(i))/(1+(w(j)/V(k))*cos(wan(i)));
psi(i) = 180*atan(A(i,j,k))/pi;
if psi(i)>10 || psi(i)<-10
count = count +1;
end
end
P(j,k) = 100*count/360;
end
end


surf(V,w,P);
hold on;
xlabel('Ground Speed [kph]');
ylabel('Wind [kph]');
zlabel('% Above 10 Deg Yaw');

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

Noted with gusto. That was the same response I had: even a very nice average does not replace the drag-yaw data! (I even considered putting more than one exclamation point on there!!!)

The nice average helps us decide which design is faster in general. We still need the drag-yaw data to optimize equipment selection for specific events.

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

Add to that - equipment is not the only thing we can select for specific conditions.

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http://www.cyclecoach.com
http://www.aerocoach.com.au

How silly of me. I coded to only look for wind angles greater than 10 degrees, but did not include angles less than -10 degrees. So the results simply double (I changed the plot in the above post to reflect this).
Keep in mind, this plot is based on equal probability of wind occurring from any direction (reasonable assumption as any I suppose).
Some reference line plots are shown for different wind speeds that commonly occur. With a ground speed of 30kph, and wind of 5kph, there is no possible way to obtain a yaw angle above 10 degrees. That same ground speed with a 10kph wind, produces a yaw angle greater than 10 degrees at >65% of all possible wind angles. If on average, wind is experienced in all directions with equal probability, it would seem that the % values can be stated at time values. More than 65% of the time when you are traveling at 30kph with a wind of 10kph, your wind angle experienced will be greater than 10 degrees (as an example).

---

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

Interesting discussion. I've been thinking about this for a long time, well before this post from 2012. And I spent about a zillion hours working with this sucker (a calculator where you can enter your speed, the wind speed, and your true angle to the wind and get your apparent wind angle - or yaw as it's called in bike world) when we really started looking into aero wheels. I have a let's say much more than incidental sailing background so I'd used that calculator a ton before when planning sail selection and likelihood matrices for offshore races.

A couple of points to think about:
1. Bontrager did a huge study whose results substantially agree with Flo's results. The link I had for it is dead but if you Google "Trek Aeolus White Paper" the .pdf will come right up
2. The cluster of incidence at low yaw angles is startling, but the distribution once you get past about 7.5* is also notable. You're basically as likely to experience 45* as you are 20*. What's optimized for 20* can't be optimized for 45* and vice versa.
3. The one premier triathlete with whom I've discussed this at length (he's been 2nd in Kona) had no interest in testing beyond 10* when I talked to him about it, which coincidentally was just before he went to test all of his stuff for the upcoming year. We've never tested past 20* and I've never seen anyone else do it. I don't think A2 or any of the other prominent facilities even go out past 20*. So what any piece of equipment will do way out there in the angles is anyone's guess, right?

Does this account for the vertical wind gradient?

At rider COM height wind velocity is ~ half of the wind station velocity (taken at 10m).

Wind velocity at wheel axle height is ~ 70% of that at rider COM height (about 36% of wind velocity at 10m).

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http://www.cyclecoach.com
http://www.aerocoach.com.au

This would be under an assumption of zero wind gradient. The way I am thinking about it, is with the use of an on-board aero sensor measuring the wind. Indeed, the station wind will over-predict the true wind experienced by the rider, but that is not what I am focused on here. If the rider experiences (say using an on-board aero sensor) a 5kph or 10kph wind, what does that mean for the possible yaw angles assuming equal probability of wind coming from any direction.
As for wind velocity at COM vs wheel axle, I never really characterized that so I am not sure about the 70%. I personally focus on the velocity at the frontal area centroid, and whether that velocity is a good estimate for the majority of the frontal area. I have found over 90% of the frontal area exists above the wheel axle, so the wind measurement from a sensor at or close to the frontal area centroid is a very good estimate of that experienced by the rider.

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

On a side note, is there a particular wind gradient model that's considered best for the close-to-ground levels that riders experience, as well as the types of "tarmac?"

My wiki-fu says that the "log wind profile" model is best for the "lowest 10-20m of the planetary boundary layer." And I assume a "roughness length" of around ~0.001m for typical asphalt roads?

Or is some other model used for vertical wind gradient in cycling?

On the other hand it appeared to be just one set of conditions in one location. And one surface material. And pretty far away from any structures or vegetation. That's not precisely "real world" conditions, compared to, say, the Flo experiments which used actual race courses.

Maybe that doesn't matter to the end result, though.

1. One set of conditions, but by chance rather extreme. If you wanted to expand that envelope, you would have to be able to control wind speed and direction... which requires a wind tunnel!

2. The exposed nature of the taxiway means that the subjects were fully exposed to any shifts in wind speed and/or direction. IOW, this means it was a more, not less, extreme test.

3. The road surface has nothing to do with things.

I should clarify. I was looking at the "surface roughness" parameter used in vertical wind profile gradient models. If seems that if you're riding in any area with many small obstacles on the ground (maybe riding next to a curb) the elevation at which the wind can assumed to be effectively zero raises from 0.0m to something higher than that. Now for most purposes that would probably be in the noise, particularly since most rider-bike surface area would be much higher. But it might fall into that 1-3% error range in some cases (e.g. riding next to a curb).

Some people (and their setups) "sail" better than others ;-)

For example, look at the shapes of the curves for Cervelo bikes with and without "Foam Dave". Mr. Z apparently "sails" really well.

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http://bikeblather.blogspot.com/

A row of hedges, houses, trees, cars parked along the side of the street - anything that blocks the path of the wind will divert it to its path of least resistance, which is often up. Beyond that, whatever obstacle to the wind doesn't even have to be between you and the wind. Go stand on the windward side of a building on a windy day. It'll be windy, but nowhere near as windy as it is if you walk 100m away from that building. Not only are the reporting stations 10m up, they have a good clear unobstructed path for the wind in 360*.

And gradient is a big variable. Cold surfaces create massive boundary layers. Sailors talk all the time about "wind weight" and how "mixed" the wind is. In the spring, with a sea breeze, you can look out on the water and it's glassy. A sailboat with a 15m tall mast might be bobbing like a cork in no breeze at all, while a boat with a 25m mast will be moving right along (and it's no fun when you're the boat with the 15m mast and you're racing against the boat with the 25m mast). That's because the cold water keeps the wind from mixing down to the surface.

Wind direction is also not even close to stable. Sure, a weather station might say it's been blowing north all day, but that could be anywhere between 345* and 015* - a HUGE range. Here in New England and on the east coast in general, any wind from the west is going to be pretty squirrely, northwest directions being the worst. Sea breezes are more stable directionally, but they still oscillate throughout the day and they veer (meaning they move to the right) as they get established. And they are building until they peak and then they start dying, which of course changes their contribution to your yaw angle vector.

Good stuff, thanks.

Thanks Jim!

But I don't know if that paper answers the question in the title of this thread. Your measured drag coefficient in the tunnel was:

0deg, .269
5deg, .265
10deg, .265
15deg, .255

With about +-.008 uncertainty. There is a general drop in CdA with yaw (nominally ~5%), but not nearly as dramatic is other sources. For instance in the Cervelo test with the DZ mannequin, there was a 15-25% drop from 0 to 15 deg yaw. I think the OP is trying to ascertain if these large reductions in CdA with yaw can be replicated on the road.

Now this is something nobody will disagree with

The values we reported are for CdA along the axis of the bike. To get CdA, one must account for the velocity of the air along that axis, which is less than the velocity of air in the tunnel. The drag numbers in that figure you showed may or may not be corrected for air velocity along the axis of the bike. If not, they reflect both yaw and reduced wind speed. Consequently, the reductions would be inflated.
Perhaps whoever took those data could comment.
Cheers,
Jim

Like I mentioned above, DZ apparently "sails" really well...whether or not the reductions in drag at yaw can be realized depends on how well the rider also "sails".

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http://bikeblather.blogspot.com/

So, they aren't in terms of "CxA"?

For modeling purposes, it's much easier to use CxA (since it can be multiplied directly by the apparent wind speed ^2) to get the retarding force in the direction of travel.

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http://bikeblather.blogspot.com/

Not sure what you mean by CxA Tom. CdA is Coefficient of Drag x Area. So, if that's what you mean, then yes, that's what we report. And, yes, that is what gives force when multiplied by 1/2 x air density x air speed^2.
What I was saying is that those drag force values in the figure might or might not be corrected for the wind velocity relative to the axis of the bike. With greater yaw angle, the component of the wind tunnel air velocity along the bikes front to back axis is reduced. If not corrected they will represent a combination of actual sail effect and reduced velocity. The effect of reduced velocity will be further exaggerated because force is related to v^2. I'm sure you know all this so we may just have a terminology issue.
Cheers,
Jim

Although pendants sometimes insist on CxA, most people studying the aerodynamics of ground-based vehicles still refer to it as just CdA.

Yeah...I think we're talking the same thing.

Also, seeing as how that data is from the San Diego LSWT, I'm fairly certain the "beta yaw correction" HAS been applied. Their standard reporting form us set up to automatically apply it, IIRC.

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http://bikeblather.blogspot.com/

Well, maybe that's the reason for this whole thread. That seems like a LOT of sail effect. I'd trust it if I could have the raw data and run the calculations myself.

IIRC, from 0 to 15 deg my CdA fell by ~15% when measured at TAMU on my Hooker.

I have also always seemed to do better relative to my (typically stockier*) competition when it is breezy instead of dead calm.

*You were the first to point out to me this apparent relationship.

So a narrower rider is more likely to experience high yaw drag reduction... interesting.

Anyone else care to chime in on their WT data, and how well they sail?

There was one year at Moriarty since I moved to NM (11 years ago) when there was a strong 90deg crosswind. It was for most people a pretty fast year (I missed that one), but I looked at the data and compared times for people who are regulars. The impression I got was that it was a better year for the fast guys than the slower ones. At the time I thought better equipment might be the reason. I hadn't considered body shape.

That's every year. :)

I've put a lot of time into testing standard (very fast) bikes vs. beam designs and the relative changes in CdA seen by a compact power rider vs. a long legged, thinner rider.

What I have observed on every beam style frame that I have gotten my hands on - Dimond, Softride Rocket TT, Softride PowerWing, Zipp 2001, and a Pearson - is that I benefit more going from a thin traditional frame to a beam frame than a shorter rider with wider (and shorter), more muscular legs. I obviously do not have enough data to say whether it might be specifically a leg length issue, or leg width.
In my experience, these frames also sail better than anything else. The Zipp especially was incredible in certain conditions...like a North/South out and back run over 15 miles doing 25mph on 190W.

I would love to just sit around and study this for days and days, but job.

...until I talk Nike into building a wind tunnel with a treadmill in the floor for testing the aerodynamics of shoes.

I tend to think that my ‘muscular’ legs are a limiting factor in more than one way. :)

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

I was there for that Cervelo data, and Tom is right: it includes the cosine beta squared correction. Dave Kennedy was the one who asked them to code it into their results spreadsheet.

Sorry Jim, I don't have the raw data. :-(

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

Dave Kennedy knows a thing or two. I reckon I trust it if he was involved.

A bunch of half-assed assumptions and speculation follow:


Coming back to this, it seems that the laminar flow is more likely to become detached in the areas near to the rider's legs, which create dirty air.

If we hold this assumption, then a foil at the seat-tube is less likely to be effective, and it may actually be more important to create clean air in this area and allow it to reattach around the rear wheel or disc.

The other implication of this is that the rider's legs are crucial, and them being shaved will help over non-shaved, and that surface treatments or clothing that allows air to remain attached might be advantageous over bare legs.

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'It never gets easier, you just get crazier.'

Legs are a big deal. More drag than the rest of the body for a good position. The smart guys who focus on TTs have learned some tricks to deal with this, that doesn't rely on clothing (though that is used when allowed). There isn't a leg clothing restriction for Tri is there?

Surface treatments; to the skin? Maybe some strategically placed scars. Wonder how long it would take for the UCI to ban that practice...

Hello rruff and All,

Bicycle racer doing aero preparation of shoulder and neck area prior to race.

(Strategically placed bee sting welts)



Some question if the swelling will outweigh aero boundary layer gains.



If it were me I do not think I would like the group of bees forming near my eye.

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Cheers, Neal

+1 mph Faster

I appreciate everyone's thoughtful responses. What I was trying to get at is whether achieving low drag for a bicycle or wheel at 15 degrees in the wind tunnel depends upon the wind being perfectly steady (allowing good laminar flow?) Would a slight change in the direction or speed of the wind cause the bicycle or wheel to stall (such as someone walks across the front of the bike while in the wind tunnel). The relatively minor changes to rim shape, tire shape, and frame shape that produce decently different results at 15 degrees yaw suggests that keeping the air attached and preventing stalling is difficult and that the frames and wheels are operating on an aerodynamic edge. Earlier posts indicate that riding in windy conditions that should produce some time riding at 10 to 15 degrees yaw does not result in drag savings over no wind unless the roads are pretty open and straight and the wind is steady. Related to this would be the question of whether some designs would be less sensitive to stalling. For example, would a disc be less likely to stall than a well designed 90 mm deep wheel that has a similar drag curve? Would the particularly deep tube shapes of a Felt IA or P5x allow them to avoid stalling in shifting winds that would stall an aerodynamically good 3:1 tubed bike?

I can see how riding down a straight, open road into a steady 15 mph 10 to 30 degree headwind could reproduce the drag savings seen in the wind tunnel. What would happen if we slowed the wind tunnel down to 15 mph, turned the bike and rider 90 degrees, and then measured the force to drag the bike and rider through that wind? Would the force required to drag the bike and rider at 25 mph through the 15 mph wind be less than the force to drag the bike and rider at 25 mph through still air?

1. Numerous studies have demonstrated that cyclists/bicycles do not undergo any significant flow transition in the range of speeds at which people normally ride. IOW, CdA is independent of speed, such that testing at, say, 30 mph (as is routinely done to maximize precision) provides data equally valid at slower speeds.

2. Testing at realistic yaw angles is also routinely performed. Whether CdA stays the same, increases, or decreases depends upon the specific equipment, individual, and/or position being used.

No, they do not. During our field tests, the wind was both strong and gusting. Furthermore, although the venue was wide open (which would influence the relationship between wind at 10 m as normally reported and wind at ground level), wind speed (and direction) were measured at the cyclists' height.

Your paper states: "Drag area tended to decrease slightly with increasing yaw angle, but ANOVA indicated that the differences were not significant." Were the measured differences of drag force in the wind tunnel at different yaw angles significant? For the aerodymic drag part of the equation, did the model predict SRM power based upon the drag area measured in the tunnel, the drag force measured in the tunnel, or both?

I appreciate your taking the time to discuss this.

Would you be able to expand on this at all? We are now seeing lots of clothing with features designed to modulate the flow transition e.g. Endura's new skinsuit data (http://road.cc/...el.png?itok=HV_olFxz) which even reports that different skinsuits have different relative behaviours at different speeds.

Does this not mean this is no longer true, for the case of the cyclist even if not the bicycle?