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 :)