Is it possible to convert a bike with an old screw-on freewheel (Schwinn Traveler) into a single speed without buying a new rear wheel? Is it possible to get rid of the extra cogs while maintaining the “screw-on”-ability of the freewheel?
I bought a new wheel with flip/flop fixed gear hub for about $90 on ebay. Allows use of single speed free wheel as per your picture on one side and fixed gear (with lock ring) on the other side.
You can probably buy a one speed “BMX” freewheel from your local bike shop. Beware of the BMX “flip-flop” size. Not to be confused with other “Flip-flop” hubs. The BMX flip-flop hub has one side that is a smaller diameter to accommodate a 15 tooth freewheel. This freewheel will not fit a standard threaded hub. Stick with the 16 tooth and up model and you will probably be fine.
I remember doing a conversion for a freewheel to a SS for a buddy’s commuter bike. I used a shimano BMX gear that allowed coasting and was pretty cheap. As for the chainline, the freewheel hub had a bunch of spacers on the axle ends to get the 130mm. I just played around, swapping the spacers from one side to the other. Of course, it requires loosening and retightening the cups and cones everytime.
Wheels Mfg has a bunch of solid axles that I think work better for doing this conversion, and you can use track nuts to hold the wheel in place if on a horizontal drop out. Also used 39/18 gear ratio.