I went to my university's bicycle co-op today to help out and was shown an old 27-inch wheel where it had five sprockets on it, but no freewheel. Apparently it was from a very old bike which had the freewheel mechanism in the crank instead of on the rear hub. So effectively the rear hub is "fixed gear." The crank had been dumped, the frame repurposed, but the wheel remained.
I found this to be a curious idea; while thinking about it, I realized that means that the chain is moving even when coasting, and you could probably shift while coasting too. pretty neato, though definitely has all the downsides of a fixed gear for things getting caught in the drivetrain.
I was busy helping people so I didn't really get time to do more than just glance it over, but next week I'm definitely going back to see what I can do with it. So now, to the questions. Could sprockets be removed from this wheel but one, and have it be used to build a fixed gear bike? Would there be any huge caveats (like requiring a lockring removal tool which doesn't exist anymore, or not using standard threading) to doing this? Do these wheels have a name? Because I really had no idea what to search for on google; I just do not know the correct terminology for a threaded freewheel minus the freewheel.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Shimano had a FFS (Front Freewheel System) years ago which, as you surmises, had the freewheeling mechanism in the crank and had the "advantage" of allowing shifting while coasting.
AFAIK, the wheel has standard freewheel threads so if you remove the "fixed" cluster you could substitute a standard freewheel. There is no lockring and using it as a fixed wheel will have the same problems as using any regular freewheel hub i.e. there are no reversed threads for a lockring.
The FFS system does have a freewheel in the sprocket. It just doesn't freewheel as easily as the front one. Are you sure you haven't found a wheel with a frozen up freewheel?
I am just going on what I was told, that it was taken off a japanese FFS bike. It could be that it is frozen up as well. If that's the case, it sounds pretty useless for a conversion. Guess it's time to take the dive on a singlespeed or flip-flop wheelset then.