Originally Posted by
cyclingd
Per Sheldon Brown:
The cheapest way to convert a multi-speed bicycle into a fixed gear is to use the original rear hub, assuming that it is made for a conventional threaded freewheel. A fixed sprocket will thread right on, but there is no provision for a left threaded lock ring.
So I thought it would be possible - just the instructions weren't too clear.
Yes, it is possible -- just search on "suicide hub" on these fora.
That said, and as Sheldon notes, there is no provision for a lockring on a standard freewheel hub. This means that using your legs as brakes -- skidding, for example -- will tend to unscrew the cog from the hub. Once this happens you lose the ability to brake with your legs, which can be worrisome in an urgent situation.
If you decide to install a track cog onto your freewheel hub, avoid using your legs to brake and install at least a front caliper brake (preferably front *AND* rear) as well to control your speed. Otherwise you're asking for trouble.