What differences would a person notice moving from say a 172.5 to a 170mm crank arm (obviously shorter)? Are there advantages, disadvantages? I'm changing out my crank from a triple to a double (both compact). One crank I'm looking at has 170mm arms. I can get it pretty cheap which is why I'm considering it. Im' not necessarily looking for shorter arms but wondering what differences I would expect or if I should just pass and look elsewhere?