Friday, October 11, 2013

Yaw angle measured by Trek engineers on Ironman courses

I came across the following interesting tidbit on the CyclingNews review of the Trek Speed Concept 9:

To tweak the original Speed Concept, Trek engineers studied real-world wind conditions on Ironman courses, and settled on optimizing the bike for yaw angles between zero and 12.5 degrees. (They found 3-5 degree average yaw in Arizona, up to a 13 degree average yaw in the notoriously windy Kona worlds course.)

I came up with my own ad hoc calculation, including the Hellman model for ground shear, back in 2011. I published that here:

So the end result of all of this is that, given the assumption of an average 10 mph wind at 10 meter elevation (a stiff wind), a Maxwellan distribution of the ratio of wind speed to rider speed, an average rider speed of 11 m/sec (40 kph), and a Hellman coefficient of 0.34 (associated with urbanized areas), I get the average yaw angle magnitude is 6.8 degrees, with yaw probabilities extending from -20 degrees to +20 degrees. The yaw of greatest probability is, surprisingly, 0 degrees.

I've not seen any experimental data which contradicts this conclusion. But I look forward to seeing what comes across the forums, as this subject has gotten plenty of attention as wind tunnel testing becomes more popular.

This calculation, which drew some criticism, is standing up fairly well. The key here is "urbanized areas". The Kona course is rather extreme. Still, the point remains that those who argue rather extreme yaws on the order of 20 degrees are most important don't have much factual support.


ian spivack said...

here is the entire paper:

Its interesting that you just posted this as I just got a 9.9 speed concept. The engineers did a great job of really making the cockpit very simple and easy to setup.

djconnel said...

Thanks for the link! What people often neglect is the ground shear effect: that when the weather service says the wind is, for example, 5-10 mph, that's not the speed of the wind affecting the bicycle.