Win Tunnel Playtime - Part 1

As I mentioned in my last post, thanks to the generosity of Specialized, Chris Yu, and Cam Piper (both pictured above), I recently had the opportunity to spend a day in Morgan Hill, CA at Specialized's Win Tunnel facility. You're probably asking "How did THAT happen?" (something I asked myself repeatedly)...and well, it's a long story. I had met Chris Yu a few years back when I had the opportunity to observe the wind tunnel testing of my friend and professional triathlete, Jordan Rapp. At the time, Specialized had recently opened their "Win Tunnel" facility and I had a great time watching the proceedings and trying not to be too annoying in peppering Chris and Mark Cote with questions. Since that time, Chris and I had "conversed" on various forums, and even swapped a few emails.

Earlier this year, Chris had sent me an email asking if I was interested in participating in one of their videos that they occasionally produce. I jumped at the opportunity and said "Sure!" and we planned on doing something in the July/August time frame. Well...one thing led to another, and the planned purpose of the video was changed...and then it turned out the video team wasn't going to be available on the dates we had planned. No worries though. As Chris explained to me, the tunnel time was already blocked out, and it turns out that like most forward thinking companies based in the SF Bay area (i.e. Google, Apple, etc.) Specialized allows their engineers time to "play", or pursue subjects that may not have an immediate application...on the thought that this "playtime" may spark some unexpected innovation. Chris told me we could brainstorm and come up with some things to look at, and like most tests, we'll most likely come up with some answers, but also some good additional questions to pursue. Sweet. How could I say no?

Due to the VERY large amount of data collected, what you'll see here in Part 1 is mostly the results of the morning of testing on that day in the Specialized Win Tunnel. I came up with the idea of trying a host of wheel and tire combinations, and then following it up with some of those same wheels and tires in bare bike tests...and then finally, I was hoping to get into the tunnel on a bike myself for a few runs. Shown below are the wheel/tire results. Part 2 (coming later) will show the remainder of the data. The idea was to see what sort of info could be gleaned about how wheel and tire combos are affected by the tire mounted (especially width) and if the differences observed "carried through" to both bare bike testing and testing with a rider. It was an "ad hoc" plan and group of equipment, but I figured at a minimum I would be getting a crash course in wind tunnel testing and the difficulties of doing so.

What you see above is the test matrix I put together for the wheel and tire testing. The wheels listed in the column on the left are the ones I would had available to me, and run the gamut from shallow to very deep. Across the top are listed the tires. Knowing that there was a limited time for the wheel/tire runs, I decided to go for a mix of tires on the wheels, with the one I was most interested in seeing was the one in the first column, the new S-Works Turbo 22C model. The greyed cells are the combinations tested, with the number in front of the hyphen the order in which they were to be tested. I wanted to make sure we weren't wasting time waiting for a tire to be swapped for a run.  The number after the hyphen is the measured width, as mounted.

So, let's get to the data...but, before we do that, I want to point out how difficult it is to get "clean" data using equipment this sensitive. Seemingly small things can throw some of the results off...which is why it's good to have guys with tons of experience running the show. For example, when we ran the first runs using a "known" wheel/tire combo (the Roval CLX64 with S-Works Turbo 22C), Cam immediately noticed that the positive yaw values seemed "off"...and it was traced to simply an end cap on the wheel fixture not being fully seated. Anyway, after 11 runs, here's how the data looked as a whole.

The big takeaway there is that the Roval CLX64 w/S-Works Turbo 22C truly is the "benchmark" for this grouping of wheels and tires tested. The Jet 6+, also with the S-Works Turbo 22C tire, basically matches it, although at the positive 15 deg point there appears to be an asymmetry (which should probably be investigated - Is it the wheel? Fixture? Tire?).

Looking closer at just the Jet 6+ runs, here's how they looked:

Another thing to note about the above data is that the Turbo Cotton 24C tire tested was my own personal tire with ~700 miles of front wheel use at that point.  As can be seen, the wider 24C tire gives up some drag, not only at zero yaw, but especially so at the higher yaw angles. However, don't forget that the Turbo Cotton tires have VERY good Crr properties, so when we look at this data in an "overall speed" context, the differences may not be as large. THAT analysis will be done in a later blog post.

Another wheel tested was my personal Zipp 101 wheel. For this one, I wanted to see the effects of tire width for such a shallow wheel, so the comparison was between the S-Works Turbo tires in both the 22C and 24C sizes. As you can see, the 2mm wider tires results in a fairly fixed offset across the range of yaw angles tested:

One of the things I wanted to check out in the wheel/tire testing was how well the venerable Specialized Trispoke/Hed H3 wheel performs vs. more modern wheels...plus, I wanted to see how well it worked with a VERY narrow tire, like it was designed around. As such, I took a brand new "out of print" Bontrager Aero TT 19C tire and compared it to the 20C Veloflex Record (that the wheel owner used as a tire) and also a 20C Continental Supersonic. As can be seen below, as compared to the Roval "benchmark", the older wheel leaves a bit to be desired, especially at yaw angles above 5 degrees. To be fair, this data doesn't include "power to rotate", which some claim can be a significant advantage for the Trispoke/H3. In any case, I think it's fair to say that of the tires tried, the 20C Continental Supersonic is probably the best combination of aerodynamics and Crr for that particular wheel.

Lastly, we looked at my personal Flo 90 front wheel, comparing the aero performance between a Continental Attack 22C, a Continental SuperSonic 23C, and an S-Works Turbo 22C tire. The surprising result there (for me, at least) was how well the SuperSonic tire performed out to 10 degrees of yaw as compared to the other slightly narrower tires. Combine that aero performance with the excellent Crr of that tire, and it looks to be a tough combo to beat as a front wheel application (I'll have more on the aero+Crr combos in a later blog post). It's also important to note how well the Roval CLX64 wheel performed vs. a wheel 26mm deeper!

That's about it for now for this blog post.  There's a ton of data I'd like to go through, and I figured it was high time I started sharing some of this stuff.  I wanted to throw this stuff out there first to generate discussion. I'll have more analysis later.

For those interested, the entire data set can be found in this spreadsheet here: Wheel Aero Data