I used to ride down to 0F, but for the last few years my cold temp cut-off was 23F. But last year I went back down to 12F, and this year I'm gonna shoot for 10F. Three lesson I learned are to underdress slightly. If I can be stand to be chilly for the first 10 minutes of the ride, I'll be warm for the rest of it. Cover your skin to protect it from the cold, but not with heavy material. I use a light balaclava, and light tights. And finally, leave your shoes and gloves loose. Donning extra socks and squeezing into shoes restricts blood flow, dimminishing its warming ability. Same for gloves and fingers. The more you sweat the colder you'll be. At least this has been my experience; and I haven't done sub 12F in 17 years.