measuer the outside diameter of the steer tube, it will probably be 1 inch or 1 1/8 inch, there isnt any ryme or reason to which size goes where. but it determines what size stem and headset you'll need. i have a lowrider i bough online, its a schwinn krate style imitation with a 1 1/8 inch steer tube and the front bent springer fork is for a full size 26" bike, if i'd have know that i woulda put that front end on a bigger bike along time ago!!
fast eddie outty