Get back to basics with a fixed gear. You'll improve your form and your conditioning dramatically.
Understand that you will need a bike with horizontal dropouts. Many road bikes from the 1980s and earlier have these dropouts, so they are excellent candidates for fixed gear bikes.
Remove your derailleurs, derailleur cables, shifters, etc. Hey, your bike just lost about two pounds!
Convert your cranks. Remove your big chainring and bolt the small chainring back on with single or 'shorty' chainring bolts.
Convert your rear wheel. If you have a freewheel hub, you can remove the freewheel and thread on a cog. You may want to consider using a lockring to hold the cog in place, and you may want to switch to a solid axle. Otherwise, it is best to buy a track hub or a flip flop hub and build up a new wheel.
Adjust the chain tension by moving the wheel back and forth in the dropouts. You want the chain fairly tight, but it shouldn't bind in any position.
Check your chainline. Your front chainring must line up with the cog in back.
Keep your brakes. Some people remove both brakes or the rear brake to get a cleaner look. But in the real world, it's nice to have the stopping power of two brakes.
Tips & Warnings
- Many fixed gear riders prefer 42 x 16 or 17 gear, but experiment to see what works for you.
- Of course, you can always just buy a track bike. But keep in mind that track bikes have pretty steep angles and somewhat twitchy handling. They also have no brakes. For road riding, it is best to convert a road bike.
- Consider shorter cranks. Remember, you're going to have to pedal through all corners.
- Riding a fixed gear bike is fairly dangerous. You are forced to pedal through corners, and when you go to brake, you must remember to keep your legs moving. It is also fairly tough on the knees.