Wind Tunnel Testing the Al33, XR31T(FSW3), and others, Part 2

Yesterday's was a quick post to show the basic results. Today's post will explain the Angle of Attack distribution that we've used, and fill in some of the finer details on the test. 

The test occurred at A2 Wind Tunnel in North Carolina. A2, along with the San Diego Low Speed Wind Tunnel, is one of the two default standard, publicly accessible wind tunnels in the US. If you want credible aerodynamics data, A2 is a an outsanding place to get it. No one from November was at A2, we Skyped in during the test. The test is a very standard one - 30mph test wind speed, temperature and pressure are normalized. We used a 23c sized (Continental's chosen descriptor) Continental GP4000sII tire, inflated to the wind tunnel standard 100psi. The alloy wheels used a tube with a 48mm valve stem while the Zipp used a 60mm valve stem, so valve stem protrusion was normalized as closely as possible. Other details of the builds have been detailed before

The standard test sweep is to go from 0* to 20* angle of attack, or yaw angle, in 2.5* increments. This is enough resolution to give an accurate representation of how the wheels perform through any statistically significant wind situation you will encounter. Wheels were tested alone, front wheel only. There are already some rattles of "this is irrelevant because it doesn't account for frame and fork" pushback on this. Simply, testing wheels standalone has decisively proven to have outstanding transfer to their performance in a bike system, and it's impossible to test with the range of bikes/forks/situations to satisfy everyone. The validity of this test's scope is established legislation, which you are free to relitigate as you wish, but it's not something we'll engage in arguing. 

Al33 in the tunnel at A2

Wind speed is 30mph. This is the standard test speed as it's been established to give the cleanest data. You can scale with software to produce results for more or less air speed, but the shape of the curves doesn't change - a wheel that's a laggard at 30mph doesn't become a star at 20, it stays a laggard. But since the effects of air resistance increase so quickly with air speed, the differences between wheels get compressed at lower air speeds.If you want to do the quick and dirty calculations on watts versus grams of drag versus time in the mythical 40k TT, here's the teacher's edition: using the 30mph parameter, 10g of drag roughly equals 1 watt, and 1 watt roughly equals 3 seconds in the mythical 40k. Not good enough for real science, but good enough to become a hyper-aware wheel consumer. 

Something we included in yesterday's chart that we've previously omitted are the blue bars showing the amount of time you're likely to spend encountering any given wind angle. How we arrived at this distribution needs explaining.

It had been an established convention that 10* was THE heavyweight angle, likely because Zipp's collateral always placed such heavy importance on 10* in particular, and the 10* to 20* range in general. When they give a time savings figure, it is computed at 10* as the only angle. However, the world has long since moved past "because I said so" as an acceptable premise, and Trek and Flo have both gone to the trouble of doing actual data collection in real world situations, and come up with distributions that show that lower angles are actually vastly more prevalent. In fact 10* is shown to be largely irrelevant.

We've linked a few hours of quality technical reading there, but the abstract is this: at real world riding speeds and in real world conditions, this is what you see. The Flo data is very plainly presented in percentages, and you'll notice that if you add the percentages up they don't equal 100% - that's because 0 to 20* doesn't encompass every situation encountered. You have a bunch of small data points going out from >20*. The Trek data is harder to break down, but break it down we did. 

The frequencies we show are a straight average of those three data sets - Flo's gathered data, Trek's data from the Ironman AZ course, and Trek's data from the quite windy Ironman Hawaii course. Anyone can argue with our methodology on this, but we think that this is the most robust, relevant, defensible distribution available. And for what it's worth, Hawaii distributed a bit differently, being very exposed and windy, but the Flo and Arizona data matched very well, and Hawaii really wasn't that different.

We are not asking you to believe anything on faith. Every bit of what's been done and how it's been done are available to you here and in the links. As none of these rims is our own or available exclusively through us, we derive no benefit from the data leaning one way or another, or shading the data in any way. This entire exercise to to provide the wheel market with good information to become educated about wheel aerodyamics. 

More in subsequent posts, but I've run long as is, so that's it for now. 

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The one number to rule them all*:Zipp, Kinlin, Al33 have weighted average of 20.6, 20.7, and 20.7 watts of drag, respectively; i.e., a tie.Flo and Hed have weighted average of 22.7 and 22.7 watts of drag, respectively; a tie, but worse than Zipp/Kinlin/Al33 category.*I had to manually extract numbers from the graph, so possible my estimations are off in places, and/or I made a typo.


Dave, did you guys get any time to do 25c tire testing? I'd speculate that the uber wide rims (>24mm wide) might get a slight performance benefit with the wider tires.


Hello, this is really cool stuff. As a guy who doesn't want to spend $3500 on a set of wheels, it is interesting to know that a 404 firecrest may only save ~20sec or less over a TT assuming the time is spent at those shallower wind angles over one of these rims. That is a lot of money, and that is also assuming I am at 30mph, which few riders can really maintain for a long time. I assume at 25mph it is even less.A couple questions-It appears the xr31T has the lowest drag up until about 6 degrees or so, and then still holds its own against the others for quite a while. How do you like it verses the new AL33 rim? You haven't listed a price for rim only on the AL33, but the XR31T is quite affordable. I wasn't sure if looks aside, if you would just go with the 31T.Were you able to compare these results to the test you did in 2014 with the pacenti SL23 and XC279? I have built with both of those rims and like them quite a bit. Just wondering if the benefits of the XR31T or AL33 would be enough to justify building a set of those. :) Thanks!


Sorry! :) I was working on some graphs for work today and I couldn't resist. But, looking forward to your vetted numbers!


Erik – You're stepping on our lines! And yes you are exceedingly close to what we came up with.COOSKULL – We got a good run with the 303 and a 25, and a run with the 25 on the Al33. The error margin on the Al33 run was high, and the run gave some crazy low drag values out at high yaw. Figuring that out with A2, which perhaps means a rerun if it's warranted, is one of tomorrow's priorities. Generally speaking, going to the wider tire coast you a couple of watts and causes an earlier stall – very closely following what we documented here: Also bear in mind that a "25mm" GP4000sII is over 28mm on every rim in this test – they're huge tires.


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