Build a fixie for pure riding enjoyment and a decent workout at the same time. A fixie’s beauty is in its simplicity.
Building a fixie from a former road bike is the least expensive route. Look for a road bike from the 80s or 90s,often an inexpensive acquisition. Old road bikes usually have road dropouts that allow the wheel to be moved back and forth in the dropouts for proper chain tension.
Once you have the bike for fixie conversion, remove the unnecessary parts. Remove the chain, all derailleurs, shifters, and rear brake. Of course you can also change the handlebars to a bullhorn style or straight style as desired.
You will only need one chainring on the crankset and will typically use the smallest. Most often the small chainring is placed on the outside position and bolted in place. To determine the number of teeth required for the rear cog, the gear ratio must be determined. Count the teeth on the remaining front chainring and go here to calculate a bicycle gear ratio. If you want to keep up with urban traffic, something in the low 80s should be sufficient. Something in the low to mid sixties would give you plenty of gear for puttering around. Normal track bike chain is 1/8. Road bike chain is 3/32 and to keep costs low, a 3/32 cog is the best way to go.
Chances are the original road bike wheel has a built-in gear assembly. In this case, a different wheel will be necessary. It can be a road bike wheel with a threaded hub or a special track setup. If the rear wheel is a standard road wheel, you may need to add freewheel/cog spacers to the hub before installing the cog to achieve the proper chain alignment (front to back).
Once you have the chain alignment figured out, you can determine the chain length. You want a little bit of slack in the chain when the wheel is located properly in the dropouts.
That should do it! You have removed the rear brake, shifters, and derailleurs. One of the chain rings from the crankset has been removed. You determined the rear cog size by calculating the desired gear ratio and shortened the chain and adjusted the tension. Enjoy your fixie!