Nothing to do with temperature for me. Best cycling is when it doesn't rain. After that the absence of wind would be icing on the cake. 10C isn't really cold in my experience. -15C and I can tell it's cold out and change the way I dress. 30 no problem either. I even like 40 C. But, almost all my rides have a duration of one hour, so I might feel differently if I was touring or looking for a ride duration of 5-6 hours.
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Longbikes Slipstream