Double Sprocket Freewheel from White Industries
This double freewheel fits in the same space a single freewheel would normally fit in, allowing you to change gears on your singlespeed just by loosening the axle and moving the chain, without any need to reverse the wheel as with a flip-flop hub.
These are made in U.S.A., and their quality puts all others to shame. Most singlespeed freewheels have a 20 tooth ratchet, with only two pawls to take the load. White Industries freewheels have a 36 tooth ratchet for minimum slack, and 4 pawls for reliability and durability.
Also feature O-ring seals to keep the innards clean even in nasty muck!
Sizes 16-18 or 17-19 teeth.
A 9 speed chain is recommended. The flexibility of the 9 speed chain will help compensate for minor chainline issues.