Testing Some Assumptions


We've talked a few times (maybe more than a few) about wind tunnel data around here, and it's something we continue to think about quite a bit.  The basic quandary is that on the one hand, until you test something, you kind of don't know much.  On the other hand, how to apply what you've tested for is a real issue. 

Apparent wind angle, or what bike companies insist on referring to as "yaw" angle, is a vector sum of the bike's speed, the wind's speed, and the wind's angle relative to the bike's direction of travel.  Here is a page worth looking at and bookmarking.  Look at the charts, play with the calculator at the bottom, go to town with it.  But what you will see is that the yaw angles that a bike will experience are REALLY dependent on the bike's speed.  If you go fast, you won't get to very high yaw angles.  If you go less fast, you will see some higher yaw angles.  If your bike is going 30 mph and the wind is blowing 9 mph at a 90 degree angle to you, your yaw angle will be about 16.5 degrees.  If you slow down to 20 mph, the same wind conditions will give you a yaw angle of nearly 25 degrees. 

Critically, the bike that's going 30 will "feel" 31 mph of wind, while the bike that's going only 20 will "feel" only 21 mph of wind.  Since air force increases with the square of the velocity increase, a 31 mph wind is about 2.25 times as forceful as a 21 mph wind.  And since power required to overcome air force increases with the cube of the air speed, it will take 3.375 times as much energy to overcome a 31 rather than a 21 mph wind. 

For those of you who have power meters, next time you have a seemingly windless day, ride along at 20 mph on a flat straight road.  If this takes you 200 watts, dial your speed up to 25 without changing your body position.  It's going to take you 312.5 watts to go 25.  If you want to go 30, it's going to take you 450 watts.  So as we go faster and faster, any amount of aerodynamic savings becomes more important.  And as we go faster and faster, we are going to be more and more concerned with aerodynamics at lower and lower yaw angles.  During the Tour de France, we looked at some of the gear that the top guys had used and saw that Wiggins used a front wheel that is VERY efficient at low yaw angles.  Given his parameters (he is fast, averaging a shade over 30 for the tt, and it was not a windy day), he made a great choice.  For an Ironman competitor, riding at relatively much slower speeds at a reasonably consistent angle relative to a predictable, strong tradewind, the same wheel would be the wrong choice.

So the whole ball of wax gets pretty confusing.  Manufacturer A and Manufacturer B might each test their wheels against one another and come up with different numbers for each wheel in both the relative and the absolute - from memory I think HED's data shows that Zipp's wheels are better than Zipp says their wheels are, but HED of course shows their wheels as being even more better still.  Then you have the environmental assumptions that dictate what yaw angles will be relevant at a given bike speed.  Then you have the factor that rider speed is the biggest variable relative to both what yaw angles you are going to experience, and how critical a role aerodynamics in general plays.  It's floating point math, and you just have to peg some parameters onto something in order to get anything that resembles something that makes sense.

This is a topic that neither Mike nor I can get out of our heads.  Stay tuned for more. 

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  • Kent Wenger on

    Actually, power doesn't go as the cube of airspeed; it goes as the square of airspeed, times the ground speed. (Of course, if the wind 0 they're the same thing.) If you're in doubt, think about this: if there's a wind, how much power does it take to go 0 mph ground speed? Still zero…A small point, but something to keep in mind.

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