http://www.webmaster10.com/ldr/windchill.html

Here is a pretty cool wind chill chart. Once it gets down in to the 20's there's not much you can do to keep warm. On a motorcycle you can bundle up a little more with heavy leather gear...but wearing chaps isn't very practical for a bike. The hardest things to keep warm are the hands a feet. Another thing to consider when riding in below freezing conditions is ice on the road.