Power To Spin

No, I'm not talking about the power I will undoubtedly have after this morning's smoothie - a spinach, beet, beet green, avocado, walnut, pumpkin seed, tart cherry juice, cherry and blueberry extravaganza that will undoubtedly leave the shop's toilet in smoking ruins before the day is out. A nagging hip injury and some other recent corporeal mishaps led me to discover that I am, in fact, old, which led to a vast nutritional rethink. Coffee is replaced by tea, nutrient density is the new watchword, and though the beer budget still exists, it is a mere shadow of its former self. All of which has nothing to do with today's topic.

The aerodynamic properties of a wheel contains two drag components: the power it takes to push that spinning wheel through the air, and the power it takes to spin the wheel at the speed at which it's spinning. The former, called translational drag, is the headliner. The second, "power to spin," is something of an afterthought. It's an important afterthought, though, if only because whenever a discussion of spoke type or spoke count or really anything about wheel aerodynamics comes up, power to spin gets called up. The test is soon called useless for not having included power to spin. The counterpoints are made that most wind tunnels don't provide a measurement of it, that without a reference for comparison against other wheels having that number for any particular wheel is useless, etc etc etc. If I could bet on the arc of these conversations, I'd be rich.

Someone with the quick Google skills will call up this document (the link is actually a Google search - pick the first result, download it, remove the ".bak" extension from the end, and open it as a .pdf), from a test that Zipp did a while ago, and let me tell you that's a smoking gun, eh? Huge gain in power to spin for CX Rays over both bladed spokes and more so over round ones. There are a few problems with this test, the most notable of which is that they never actually compare two otherwise identical wheels, with the same rim and spoke count and only the spoke shape is different. They talk about doing such, but it's not shown. The other of course is that they don't clarify if they've normalized for tires, and if anyone was unaware of tire importance before, it's now a hot topic since this came out

But if you reference this sheet, you see that the Zipp 101 is only shown to gain 42 seconds, or a savings of 14 watts, in the mythical wind tunnel 40k TT versus "the industry's benchmark aluminum wheelset." The industry's benchmark aluminum wheelset doesn't come with CX Rays. Said benchmark is usually a 32 spoke Open Pro-based build, which would, as an industry standard, come with 14/15 gauge spokes (or possibly straight gauge spokes, which would make it even worse per the power to spin test). Given the differences implied by the power to spin test data, one would expect that the shallower industry standard wheel would suffer more than a 14 watt beat down just from longer, less aerodynamic spokes alone. So why not add that power to spin edge to the translational drag edge, and show a really huge gain?

My simple guess is that they found out that there wasn't that much "there" there with regard to power to spin. One year ago today, we posted on the blog and on our FB page about the tests that we did in collaboration with Velonews, which were published in their January 2015 issue. Per the discussion arc I described above, power to spin came up. No less an authority than Dr Andrew Coggan got involved in the comments in the FB post, and directly called power to spin a red herring, saying that he'd measured a worst-to-first change of 2w across wheels when doing wind tunnel calibration work. A great wheel takes 3w to spin, the worst takes 5. Interpolating that info is a slippery slope, but if a super deep wheel with 20 CX Rays or DT Aerolites takes 3w to spin, and the least aerodynamic wheel you can get your hands on with 32 round spokes takes 5w to spin, we're pretty sure that changing from CX Rays to Lasers in an otherwise identical wheel doesn't cost 10, 5, or even 1 watt. 

It should go without saying that this post isn't a bag on Zipp. They had a test, it showed data, and even if that test was incomplete or wrong, the internet doesn't forget. I hope that in 20 years people are as aware of us as they are of Zipp now, and are retrospectively discussing our old tests. Plus, we can absolutely categorically agree with the last bit of Zipp's power to spin thing: "many wheel companies avoid the CX-Ray due to its extremely high price (roughly 4 times the cost of butted round spokes), but when ultimate performance is desired, there is no other logical option." There's a spoke for every purpose. 

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1 comment

Thanks for this post and others like it. They are very informative.


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