The 1936 BMW snow machine Schneekrad, for Schnee-Snow and krad-caterpillars.

I found these photos before, but never could find the top one to make them a pair in a post, because in the top photo, you can't tell that there is a sidecar, and I didn't know it was a BMW

Top photo from http://motorcycle-74.blogspot.com

Update, Jan 2014, George made a full post about this bike, http://www.dwrenched.com/2014/01/dwrenched-special.html