Originally Posted by
TejanoTrackie
Fixed/Fixed is the most versatile. You can set it up as:
1) Fixed/Fixed
2) Fixed/Free
3) Free/Free
And, yes, it's ok to mount a single freewheel on fixed cog threads, even though the freewheel threads are wider than the hub threads.
As to BMX (metric), the only "advantage" is that you can mount a freewheel with less than 16 teeth, however, that's mainly because BMX bikes have smaller wheels, which effectively lowers the gearing. You should not need anything less than a 16T frrewheel for your son, and the fixed cogs go all the way from 12T to 22T
Beat me to it.
The 'fixed' thread has the smaller, left handed thread for the locking ring. The fixed/free hubs have one side with the fixed set up but only the right handed thread on the other.
As my esteemed colleague above mentioned, you can run a bmx cog on the fixed threading - there are enough threads there to do so, though if you're absolutely definite that you'll always have a freewheel cog on your bike, a fixed/free hub would be better because it's the correct part for the job.
However, if you get fixed/fixed, you can run anything you like.
You can run a track cog on a freewheel thread - you simply do it up really really tight and use a bb locking ring against it. These are called 'suicide hubs' because the locking ring, being on the same thread, isn't overly effective as a locking ring and excessive back pressure can spin the cog off (it's fun ... not btdt).
My tip?
Buy fixed/fixed - it's what's going on my Hillbrick ... but if you can't get it, you won't be sad with fixed/free because not many people flip the wheel around to use the other cog anyway.
Richard