I have a FG with a flip-flop wheel that now only has a fixed cog on one side, and I am looking to get a freewheel cog on the other side but I have a few questions first. Am I safe in assuming the freewheel mechanisim is in the cog itself and not in the hub? Is a lock ring required? Are they only one size, the threads I mean? Will a BMX freewheel my hub now? Thanks.