How do you even quantify sole stiffness? The shoe manufacturers just assign a number to the stiffness of the sole. But they give no idea what that number even means. Does a Specialized 13 have any relation at all to a Giro 13? Is a 10 twice as stiff as a 5? I’d bet not. And how big a flex is a given number? Is a 10 = 1mm, 10mm, 0.1mm? Who knows. Without any more information these numbers are basically useless. Just get a modern shoe that is comfortable. I have the S-Works 6, S-Works 7, and S-Works Exos. I can tell a big difference in comfort (the Exos are the best by far for me) but I can’t tell any difference in flex. Other fit related stuff but not flex.
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