November in the wind tunnel: "semi-aero"


Tunnel testing alloys was actually more exciting for us than it should have been.  While a lot of people choose to buy builds from us with the rims on test, this essentially amounts to a "public good" test, simply because these rims are so widely available.  The excitement mostly came from the fact that the internet was seemingly ready to have an aneurism there was so much demand for us to do this (which is a funny concept when you think about it - no one else spends the money to do this for people, not even the rim makers), and because so many "eyeball wind tunnel" experts had definitively pronounced the results long ago. 

We have to once again stress that directly comparing this test on this day with that test on that day is a fool's errand.  The way that we have designed our testing protocol, we will always be able to make significant back-references, but direct comparisons between different tests are worthless. Relative results of one wheel vesus another from test to test are valuable, which is why we have tied ourselves to the benchmark of the 404 wheel we used in our original test.  We will always keep that wheel as the reference standard. 

We tested these with a 23mm Continental 4000s II tire - one tire used in all tests, as it must be. That is perhaps the most popular tire used with the rims we tested, and is a standard for the wind tunnel.  Significantly, you must take note of the tire width as it is on each wheel when looking at these results.  The same tire will inflate differently on different rims: the wider the interior dimension between the brake tracks, the wider the tire will set up.  Among the rims in this part of the test, the differences are fairly small.  As we get into other rims, this difference will become more significant. 

 The biggest lesson out of this test is how much closer the Kinlin and Pacenti rims come to the performance of carbons than what the A23 did.  The overall delta between worst and first in this test was much smaller than it was the last time.  Tony Martin's not going to read this and think "you know, maybe I ought to use some nice mid-depth alloys in my next time trial," but the gap is much smaller.  Again, we are only able to make this statement because the A23 was tested against our reference wheel using the same tire (although we used a different type of tire last time), and the same reference wheel was used to measure the gap this time.  There is the noise of "maybe that's just a particularly bad tire for the A23" in there, but that's at most a very small part of the story.  Using our back-check method, the Rail 34 is faster than an RFSC38, while the alloys in this test are slightly slower.  

The Kinlin was the faster alloy.  As we will explore later, the tire's increased width on the Pacenti may account for all of the difference between the two.  It is impossible to normalize one tire to the same width on two different rims, and you wouldn't want to anyway.  What you could do is test a narrower tire on the wider rim and interpolate from there, but with these rims we were more interested in seeing if there were any gross differences between the two, and how they stacked up compared to the carbons on test.  We will explore the effect of inflated tire width in more detail in subsequent posts. 


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  • pyf on

    Dave,Sorry you got me wrong.Not trying to discredit anything you do. All the opposite, I even want some Rail 52 for my next powertap build, like your design for its depth/width/weight/nice aero profile compromise. So definitely not a hater ;-) !Just trying to discuss what you measure and discuss what others measure too, didn't mean to be a pain in the * … guess I was :-( .Take care

  • Dave K on

    pyf – You are doing a very good job of helping us to point out every reason not to do what you want us to do. The tunnel shows little (but some) variation in one tire from itself from one run to another. It does a good job of repeating a tire against itself. But the tire they used is not the tire we used. At the tunnel, if you want to test two rims, you have to use the same tire. Not one of the same make and model and size – the same exact tire has to move from one wheel to the other. A good wind tunnel will not validate the results otherwise. What valve stem length was used in all of the tests you show? Was an extender used to inflate, and then the hole taped over, as people who are interested in making a sexy and appealing curve do? Or were they tested with the valve stem length that people are likely to actually use, like we did it? Who besides FLO removed the tare from their results? Do you know that the wind tunnel engineers consider that to absolutely be not correct protocol? But if you want to wow the world with a low number and you can live with yourself for having done it, you do it. The whole reason we invented the convention of comparing against a 404 as a benchmark known, fast wheel was to provide some measure of comparability among wheels. Some have taken it up, others haven't. I am sure that some play the game as straight and narrow as we do, with the customer's best interest in mind, and I know many don't. This conversation isn't entertaining or informative for me. Mike and I have invested years of our lives, our careers, and a frightening amount of money into this mission. In doing so, we are trying to be as absolutely honest and straightforward as we can be and provide outstanding products to people while helping them to understand as much as possible about those products. What you are after encourages the misleading gamesmanship that has so many people tuning out from the whole cycling industry in disbelief. If you have your way, "claimed aero" will become as big of a joke as "claimed weight" is. Good bye. Dave

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