45 to 50 is about it for me. I can ride lower, but I don't enjoy it. For some reason the cold just sinks right into me when cycling, unless I bundle up to the point where it gets ridiculous. Would be one thing to do a 10 to 15 mile ride, but if you're shooting for 30 to 40 miles, it kinda sucks. Besides, when you're that cold you're not training, you're surviving. Funny, I can snowshoe at -20 or below and be comfortable. I guess its the wind-chill that gets me.