There are a few types of hubs you could use. One is a track hub, its has 2 sets of threads on the drive side, one for the cog, and another for the lock ring. Another option is a "fixed-free" flip flop hub, which has freewheel threads on one side and cog and lockring threads on the other. Then you have a "fixed-fixed" hub which is threaded for track cogs and lock rings on both sides. This enables you to have 2 different sized rear cogs if you need to change gear ratios for some reason. Another way is to get a drilled pattern cog which will bolt onto a disc brake hub. If you get a flip flop hub make sure its a track style hub. There is a bmx hub which is refereed to as "flip flop" but is something different. ChoppersUS seems to only have disc hubs, no track or flip flop hubs