Wind Tunnel Testing the Al33, XR31T(FSW3), and other alloys

This blog, and series, will be a way more difficult story to tell than I'd thought it would be. 

What we'd expected was that the Zipp 303 reference wheel would be that shade faster at the heavily prevalent and thus more important narrow angles of attack (aka yaw angles), and then extend that lead out into the angles that occur with much less frequency. What actually happened was that the Al33 (our RFSW3 wheelset's rim) and the Kinlin XR31T (that we use in the FSW3) both performed better than the 303 at the most prevalent low yaw angles, starting to cede a bit at around 7.5* and going on from there.


When you do a test like this, you get a lot of data, and it takes a while to chew and digest it. What we present here is just a first, very broad, pass at things. 

The blue vertical bars that you see in the graph are the amount of time the average cyclist is likely to spend encountering each wind angle during a ride. We will offer a very complete explanation of that in the next blog. 

For now, we're just trying to wrap our heads around this, and make good on all the teasing that we've done. Sorry for that, hope it was worth it. 


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I am very pleased with the information in this article, note 10!Good article! Very nice!


Hi Joe,Simple answers all around. We didn't compare it to a 34 because the 34 isn't available anymore, so it's an irrelevant comparison. It's also not that well known. We didn't choose a 202 to compare it to, because no one thinks of a 202 as a fast aero wheel. So we thought that people would think "who really cares how your aerodynamics compare against a wheel that no one considers for its aerodynamics?" Dave


kudos for doing the aero tests. always an interesting read and I appreciate your hard work for the benefit of all who are willing to read it. I'm just curious though, why didn't you test against the Zipp 202 FC Clincher? I get why you'd like to test against the 303 FC, so as to compare against an industry standard powerhouse and see if the rim can "punch above its weight", etc… but the 202 FC Clincher is much more comparable to your RFSW3 than the 303 FC is (from an aero and build perspective; weight-wise, the 303 FC is more comparable). or even compare it against the Rail 34 for that matter?


Hi Chris – Sorry I missed this comment. Most of your questions are addressed in the next post, and to get the complete story you have to read the source material behind the wind distributions. Decide for yourself what's relevant to you once you've taken all that onboard. Slower riders are more likely to experience wider angles, you are correct, but air resistance increases/decreases exponentially so when you slow down, air resistance becomes a lesser part of your total drag package.


It seems a big part of interpreting these results is how it's determined what the distribution of yaw angles are. What kind of speeds are these for, what locations, etc…information on the sampling to determine the distribution of yaw angles. If it's for a high speed, it'll skew the distribution to lower yaw angles. Also, if this is for a given windspeed at that given yaw angle, which means that the actual headon windspeed will be lower as the yaw angle increases? How much of the "reduced" drag is due to the headon windspeed being lower (as a result of coming from the side) vs the aerodynamics of the wheel?Ironically, it seems a slower rider, who will experience a higher distribution in the higher yaw angles, will actually benefit from the Zipps? Obviously that's offset by the fact that slower speeds have exponentially less aero drag.

Chris K

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