Single speed drive trains are said to be more efficient than derailluer systems. I think it's true, but I'd like to know exactly why. Commonly listed reasons include: fewer moving parts = less friction, shorter chains = less chain mass and less friction, straight chainline = less friction(?), less angular momentum change since they don't have to zigzag through the derailleur (?), ,,, Must be lots of opinions about this. List the reasons in rank order.