Slowman wrote:
stipulating that i'm well out of my depth here, do i understand that you tested all of the turbo cotton widths on the same roval CLX64? i ask because i believe i remember specialized making some noise about this wheel being built around the turbo cotton for aerodynamics. so, unless i dreamed that, then it must follow that specialized had a particular width of its turbo cotton in mind and i'd be willing to bet it wasn't the 28mm.
No, if anything, it was designed with the 22C S-Works Turbo in mind (which measures 24mm wide on that rim), not the Turbo Cottons
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but i don't know if i understood your test to be 3 tires on that same wheel. but if so, then i guess i speculate what could be achieved, theoretically, had specialized gone back and designed another roval optimized now to the 28mm tire. now it no longer has that 21mm internal bead diamater but 23mm or 24mm or whatever. is my reasoning sound or am i off on a wild one?
It's actually 4 widths if S-Works Turbo and 2 of the Turbo Cottons, not a mere 3 tires...
There may be something to be gained to designing a rim around a 28mm tire (as compared to the same tire on a current rim, and mostly only at higher yaws) but my speculation is that the overall drag curve is going to be higher as compared to an equivalently matched narrower setup (measured tire width to rim external width) due to similar drag coefficients (Cd) but larger frontal area (A).
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but it could also be that there's something about a 28mm tire on a 21mm internal bead rim that makes it roll faster and if you placed it on a fatter rim maybe the Crr of that wider tire comes down to earth.
anyway, the point of this speculation is to have a good answer for bike makers who're wondering whether they need to make their new frames and forks to fit 23mm, 25mm, 28mm or what?
Thinking about it some more, I think the answer is to tell them to design around being able to fit tires as large as a measured width of 30mm...because, if the course you'll be riding that TT/Tri bike is so rough that it calls for pressures low enough to require tires larger than that, then you probably shouldn't be on a TT/Tri bike for that course anyway ;-)
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