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« Wind Tunnel Testing the Al33, XR31T(FSW3), and others, Part 2 | Main | BYO Hubs in RFSW3 »

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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Reader Comments (19)

"Watts of drag"? At what air speed and air density?

February 9, 2017 | Unregistered CommenterTom Anhalt

I've never rushed any blog out this quickly - if we don't post by 5p it doesn't go to the mailing list, and wanted to hit that. You know that we're going to follow this up with all the details, but this is 30mph at standardized tunnel day. Also remember that your preferred means of seeing this stuff is WAY PAST what most people want and are best served by.

February 9, 2017 | Registered CommenterDave Kirkpatrick

No worries...I figured a more thorough explanation was coming :-)

February 9, 2017 | Unregistered CommenterTom Anhalt

Tom - Check my comment on your site - Dave

February 9, 2017 | Registered CommenterDave Kirkpatrick

So how does a 10mph crosswind compute into angle of attack? Either 90' or a 3/4 headwind. It's been 35 years since college physics and I'm a jr high history teacher now..

February 9, 2017 | Unregistered CommenterDarrell Edwards

Darrell -

We're living in the age of "let's let the internet do that works for us," so here you go: Your wind speed and angle go in as "True" inputs and you're going to look at the Apparent Wind Angle in the outputs section. Don't worry about units of speed being in knots - a knot is 1.15 statute miles, but that doesn't matter so long as vehicle speed and wind speed are both in the same units.

I spent a long day running through about a million different combinations in the summer of 2012 as we were working on the Rail 52.

Editorially speaking, a 10mph 90* crosswind computes "rarely" into your angle of attack. Daytime average windspeed in the US is only like 8mph, and that's 30m above ground at airport towers, in unobstructed wind planes. Closer to ground level, with trees and bushes and hills and houses and blah blah blah, there isn't that much wind. 1' above the ground, in the boundary layer, where your hubs are, there's even less. Not to say 10mph doesn't happen, but it's not the norm.


February 10, 2017 | Unregistered Commenterdave

10mph wind ground speed is a serious wind. You would look out the window and think twice about riding that day. Wind speed tends to be greatly overestimated by amateurs.

February 11, 2017 | Unregistered CommenterRobertW

Apparently Dave doesn't race in farm country. Winds are frequently 10-15(at the ground) with gusts of 20mph.

February 11, 2017 | Unregistered Commenterscott


You're right, I don't. But the book on our methodology is extremely wide open, and I don't see any other wheels builders giving you a hot link to a calculator where you can do the math on what your angles are going to be. We're giving you all the useful information we can. It's up to you what you want to do with it or make of it, seriously.

And even if it is windy, low angles of attack happen, and every wheel except SUPER deep ones and discs stall by about 15* or so (Al33 is actually a rock star for keeping it together as much as it does out that far, given its depth). So you're really only talking about a 7 to 10* window where the deeper wheels are notably better speed wise. Which also comes at the cost of handling. The Rail 52 demolished other deep (and also shallower) wheels at handling, and living in a windy coastal place (in fact I live where I do BECAUSE it's windy - I sail and windsurf) I know that even 52s get tough in big breeze and cost you energy. So the handling tradeoff probably ain't worth it anyhow.

Then if you do a geographical overlay of racer density on a map, the racer density in farm country is way less. Human density is way less. Which means that statistically a smaller number of people race in farm country, and a smaller number of racer days happen in farm country. I don't have an answer for you on that one - you're getting screwed for being in a small constituency there. But it's not us who's doing the screwing. I mean Trek IS in farm country, and look at what they say.

I know or at least hope that that was ribbing, but in all seriousness we have considered this thing from every freaking angle - see what I did there?


February 11, 2017 | Unregistered Commenterdave


This is sort of an odd one, but its been so long since I have read the data on it. Where would an original Rail52 fit on this plot, just so I could use it as a frame of reference. If I remember correctly, they were around 24 watts of drag at a 2.5* AOA, so is this chart saying that at 2.5* AOA the XR31T and AL33 are faster than the rail?

February 12, 2017 | Unregistered CommenterKevin

No, don't make that comparison. First Rail test was a different tire - some kind of Vittoria we loved riding on at the time. It was before the whole testing paradigm had gelled around the Conti GP4000. The best comparison I can do is that the Tour Magazin test from last fall (2016) gives a 3w edge to the 404 from the 303. When we do the final calc from the whole distribution and make the "one number of seconds gained or lost in the mythical 40k TT" it will show the 303 ahead of the other two by some fraction of a second. So if 3w = 9 seconds, and we called the 404 2 seconds faster than the 52, you're at the Rail being 7 seconds faster than this group, and about 13 seconds faster than a Belgium+.

When things go kind of not the way you expected like this, one of the challenges is learning exactly what it is you've learned. And I'm going to repackage a sentiment expressed by Tour in their test here - once you have a competent wheel as a minimum, your room for gains over that is limited. You might even call it... marginal. But of course as always it matters what problem you are solving, and there are as many of those as hairs on my head (which fortunately still means that there are lots of them).

February 12, 2017 | Unregistered Commenterdave

Thanks for all the info. Now I have to work through my analysis paralysis to decide where I want my wheels to land in the spectrum of weight, aerodynamics, and cost. :)

I see the RFSW is now available for pre-order, but the Custom Al33 is not. Is that intentional?

February 13, 2017 | Unregistered CommenterGraham

Custom is going up momentarily, and don't get paralisized too badly - we sold out half of our first order in 3 days. It will surprise us none if this round is gone by Friday or sooner.

February 13, 2017 | Registered CommenterDave Kirkpatrick

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?

February 14, 2017 | Unregistered Commenterjoe

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?"


February 14, 2017 | Unregistered Commenterdave

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

February 22, 2017 | Unregistered CommenterJosé

You guys are the fucking best. And you continue to set the standard for general wheel tech bloggage content. Thanks.


February 24, 2017 | Unregistered CommenterJerry Chabot

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.

March 9, 2017 | Unregistered CommenterChris K

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.

March 18, 2017 | Unregistered Commenterdave

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