Square Taper Bottom Bracket

john2002 Posts: 158
edited October 2013 in MTB buying advice
I need a new bb, I've seen this (link below). I need a 68mm shell but not sure what spindle length I need.
what is the difference in spindle length? do some frames need longer length so the chain set clears the frame or is it to do with aligning the chain set with the rear cassette? or other reasion?

GT Avalanche 1.0 Disc 2011, Fixie, frankenbike