Today I took my bicycle over to a hill near my house on Long Island to do a wheel test. It's pretty steep and about a mile long and there was a strong, steady tail wind. I brought three rear wheels, a Renn Disc, a Zipp 404, and a Mavic Helium. All the tires are Verdsteins pumped to 120, the Helium has a Specialized tire. The bicycle; Kestrel Talon with a Reynolds Ouzo Comp Aero fork (my wife got it for me for Christmas) and a Zipp 404 front wheel. Starting from a dead stop and just freewheeling down the hill, here are the results. I made three passes with the disc; 39.5, 43, 41.5 mph (top speed). Three with the Zipp 38, 38.5, 40. One with the Helium; 36. What does it prove? I don't know. What would I recommend? If you are doing an all downhill TT with a tail wind, deffinately choose the disc! My next test will be in the mountains in Upstate NY. I know a shallow uphill that takes around an hour. I'll test the disc against the Zipp on that on. Stay tuned for further results.
I did the same kinds of tests before, but started at a certain place on a hill when I was already going 20 mph. It was eye-opening…sometimes wheels aren’t as fast as they are advertised. BTW, the Hed 3, Hed Deep, and Renn wheels tested very well this way…I did not test the Zipp, but, I did test another “aero” wheel…and this particular wheel on my setup wasn’t any better than a 32 spoke Mavic Open Pro…but it cost about 600.00 MORE. I sent it back! No, I won’t name the slow “aero” wheel…I don’t think that is fair to the company.
Well, interesting, I guess.
I haven’t done a similar test with wheels, but I have with different body positions, and I saw significantly greater differences than what you are describing when comparing VERY minor position changes - for example, a couple of degree difference in my knee orientation results in a larger differential than your test is showing (5 mph or so.)
Unless you are controlling VERY carefully for this, I woudn’t put very much faith in these numbers - and I don’t know how possible it is to control for this effectively on a mile long descent.
The different tire on the helium is also pretty glaring…
Sounds like fun, though.