Under 40F becomes really iffy but more dependent on conditions than a number on the thermometer. The roads or trails must be snow and ice free, I'm not equipped to ride in slippery conditions. I've taken a couple of rides in 30'ish temps and slightly below but it needs to be sunny with no wind and those are short rides just for the sake of getting out. I don't have cold weather cycling clothes and my skiing and ice fishing clothes are too bulky. I don't do either of those sports anymore and confine myself to the indoor trainer when it's 40's F and below. I ride for fun and for me it's just not much fun sub 50's.