I'm pretty sure I saw a thread on here from someone planning on building and electric flat tracker bicycle. I was impressed. This one on instructibles.com looks pretty cool too:
http://www.instructables.com/id/48V_Electric_Flat_Tracker/
A nice step by step included.
http://www.instructables.com/id/48V_Electric_Flat_Tracker/
A nice step by step included.