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. 

Back to blog


Dave, looks like the test went well and the data is definitely interesting. I'm surprised the 303 isn't further ahead at higher yaw but I guess it is only a 45 mm deep rim which isn't that much more than the Al33.Erik – I replicated Flo's method for producing the one number that they call the 'Net Drag Reduction Value' in an excel spreadsheet using their yaw weightings. It needs a little more information than has been presented here so far because you need the actual drag numbers at each yaw sweep increment. I guess you could read the values off the plot but it would reduce the error to have the values used to generate the plot.From that data it's pretty straightforward. You calculate the slope of the drag curve between each sweep increment and interpolate it to the weighted yaw bins (Flo gives their weights in 1 degree increments vs. 2.5 degrees for the tunnel sweeps). Then weight that interpolated drag by the percent time spent at the given yaw angle to get the drag contribution of that yaw angle, then sum all the yaw angles to get your total drag.


Oh, you'll see a benefit!! Your integrity alone deserves much more than a second look…

Scott Booth

Dave,As usual, some really good information here. I was really surprised by the Flo30 data considering that they based their whole design off of the data they collected that showed riders spend the vast majority of their time at shallower wind angles. Can you confirm that it was the full carbon Flo30 and not the model with the aluminum brake track? I assumed it was the full carbon version but didn't see it stated definitively. I discovered your website and blog about 9 months ago and poured through most of it. I remember reading your decision to discontinue the Rail34 was in large part due to the fact that there really wasn't a significant aero gain in shallower carbon rim designs over aluminum options at the time. Especially when you factor in most amateur riders don't spend much, if any time riding at the standard wind tunnel test of 30mph. The AI33 wasn't an option at that time and I don't recall if the Kinlin XR31T was a factor in the decision to discontinue the Rail34.I think this test probably only confirms your decision to discontinue the Rail34. I guess my question is what kind of rider really benefits from the AI33 or XR31T? How fast does a group ride or crit race have to be before you would say that these two options are a worthy investment over a set of some of your other shallower, aluminum offerings? And at whatever point that is, would that type of rider be even better off with a set of deeper carbon rims similar to the Rail52?Thanks!

Matt Weaver

Oh I'm completely with you! Don't sweat and don't take mine as antagonistic to you. Kona data has points out to like 60*, which anyone can go see (again, we love that anyone can go look at what we used and judge it for his/her own).You're better off knowing something than nothing, right? And even better if you can see why you're being told something. As the contracted project managers of this test, we tried to solve for "what is going to answer the customer concerns and questions that will arise, as credibly as possible." That's the yaw angle for which we hoped to optimize.


Hi Matt,Good questions.First, the FLO30 is their alloy clincher wheel. It was not part of their redesign. It is still a current product, though. 45mm is the shallowest carbon wheel they do. The reason we chose the FLO30 is that it is a 30mm wheel advertised as having and popularly purchased for aerodynamic benefit. Had we not included it, it would have been fishy. The Rail34 decision pre-dates our working with the XR31T by a bit. The XC279 (slightly shallower and narrower Kinlin rim) was one of the rims that initiated the Rail34 decision, and honestly the whole line of argument/inquiry that we've been on for 2.5 years now. We intuitively felt confident that XR31 would be an aero improvement over XC279 but stated it just that way – "we're confident that is is faster." We're very exact about what claims can be made and what can't, and we're proud of how we approach that especially in context of the standard bike industry hype.Our general take has been that unless you're really chasing top level TT performances, wheel aerodynamics don't make a substantive difference to the average rider or racer. This test galvanized that philosophy and backed it with more and harder evidence. The FSW3 wheel set is the least expensive set of wheels we sell – barring a custom build of same hubs and rims with round spokes, which is $40 less – so there's no investment to be made there versus any other choice. I mean, literally, it's 1/4 the price of much of what's billed at as aero wonder wheels, it's hand built by us, it asks you to make zero compromise in terms of spending money or being able to brake or whatever else. Our enthusiasm for it rose exponentially we were able to source them in a unique to us matte finish, so at this point we don't think it asks for an aesthetic compromise either.The Al33 is the best looking alloy rim we've ever seen, maybe the best looking rim we've seen. You've got carbon that gives you the "I've got carbon" look and introduces brake compromises, for which you get to pay a lot extra. Any carbon within shouting distance of Al33's price is going to come with zero confirmation of aero performance, braking parameters, anything. So to me that decision looks like "hmm, for $715 I can get Al33 with very good hubs, great spokes, hand built. For the same dough I can get some random carbon off of an anonymous internet seller. For $1200-ish I can switch to carbon wheels from a company I know about, with similar hubs and spokes to the $715 build. And for $1600 to $3500 I can get expensive carbon." If you gave me that decision and $3500 and said "keep what do don't spend" I'd buy the Al33 without a second though. If you gave me $3500 and said "give me back what you don't spend" I'd still get the Al33 without a second thought. Without poring through all of our other builds, the Al33 is a bit more than an Easton build. I love the Easton rims, would happily use them. The price difference isn't significant to me – get the ones you prefer. Same exact story with HED builds. You your last question, Zipp's own collateral gives an 8 or so seconds difference in the mythical 40k TT to a 404 over a 303, and that's for a Firecrest 404 versus a pre-Firecrest 303. Given the compromises involved, I don't think anyone legitimately benefits from a "middle ground" carbon clincher. Dave


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