On a Sturmey Archer AW 3 speed hub, what is the smallest tooth count possible for a cog? Did they even make a selection of differently sized cogs for these hubs, or were they all of one size? Any help would be appreciated... I've got an SA Three speed AW wheel that I would really like to snap a smaller cog onto if possible (I didn't count the teeth on the current cog, but it looked to have about 18 or 19 teeth?).