Originally Posted by
Mithrandir
All the info above is correct, but there *is* one exception. Most Fatbikes run a 170mm hub spacing in both front and back, allowing you to swap wheels out. I know a guy with a single-speed Pugsley and he put a bailout gear on his front wheel so that if he ever gets stuck anywhere he can swap the wheels and have an easier gear.
But generally speaking this is uncommon.
except the early fatbikes that ran 135mm front and rear... I ran a 1/2 fat 29er with a 135mm rear SS hub on
but to the OP... yes the rear standard has changed over the years and based on what sort of bike it is (track, road, mtb, tandem)...
this is a good bit of info to learn
http://sheldonbrown.com/frame-spacing.html