When I'm not on my bike, I often ride my Kawasaki Ninja 600 (a four cylinder "crotch rocket"). My motorcycle helmit is full face with a visor, and would be way too heavy and hot to wear while riding a bicycle. It has small vents that work well at 60mph but would be useless at bicycle speeds. However if there was a warm bicycle-specific full-face helmit I might wear it when cold enough outside. I do, however, wear my snowboard/ski helmit when it's cold. They are designed for winter activities and are warmer than a bike helmit, but light unlike the motorcycle helmit. My boarding helmit has vents which can be closed in the winter and opened in the spring (on the fly) as well as fleece ear flaps which are removable.