Saw link posted on Lcool - great video from the 1930's but makes it simple to understand your differentials and how they work.
skip to 1:30 to miss intro...
http://www.youtube.com/watch?v=K4JhruinbWc
skip to 1:30 to miss intro...
http://www.youtube.com/watch?v=K4JhruinbWc