Remember that if you have a breakdown you can't bike to generate heat. It's often very difficult to do bike repairs because your hands get cold quickly. This is why I always carry my half finger bike gloves as well, for doing repairs, and carry some bit of extra clothing, like a fleece vest. I've had to run with my bike on my shoulder before to stay warm, for almost an hour. Not pleasant. My method is to wear kway nylon shell pants and a gortex cycling jacket as air pockets that I warm from biking. Of course I'm wearing pants and sweater and whatnot, but once I stop producing that heat the system breaks down. When it's really cold I switch to my german army surplus winter coat and thick snow pants and have no worries of being cold with a breakdown. This is also what I wear when I bike to the trails to do snowshoeing.