How to Convert a Bike to a Single Speed

What you'll need:
Donor bicycle
*Single-speed freewheel cog
*Chain Tensioner
*Spacers
5mm BMX-style stack bolts

*Parts usually found in a single-speed conversion kit

So you're interested in this whole single-speed bike phenomena, but don't like the idea of riding a fixed gear? A single-speed bicycle is similar to a fixed gear, but you've got the added bonus of being able to coast.
The first step in building a single-speed bicycle is to choose a frame. If you have an old bike laying around, such as a '90s mountain bike, you can use almost all the parts to build a single-speed. If the frame has horizontal dropouts, you have the added bonus that you don't need to a chain tensioner and you can, if you wish, easily convert it into a fixed gear down the road.

Dropouts are found in the rear of the bicycle where the wheel axle connects to the frame. Horizontal dropouts allow the wheel to move back and forth. Most bikes built in the last 15 years do not have horizontal dropouts.

If your frame has vertical dropouts, like most recent bikes do, you can still convert it into a single speed, you'll just need to buy a chain tensioner. A chain tensioner replaces the rear derailleur and keeps the chain tight. Bikes with horizontal dropouts don't need the chain tensioner because you can move the rear wheel back and forth in the dropout to adjust the chain tension.

Once you have your frame picked out, remove the front and rear derailleurs, shifters (unless they are integrated with the brake levers), chain and chainrings. Don't remove the brakes, or you won't be able to stop! You can sell or recycle the derailleurs and shifters, but hold onto the chainrings.

You'll now need to decide what gear ratio you would like to run. Since most road bikes come with a 42-tooth chainring, a 42x16 combination is popular. The 16 refers to a 16-tooth cog on the rear wheel. On a Mountain Bike, you might want to use the 32-tooth front chainring paired with a 16-tooth cog in the rear. If you decide to use the middle (or smaller) chainring on either a mountain or road bike, you'll need to remove the larger ring and then buy some smaller stack bolts to attach the smaller ring to the crank arms. Shorter 5mm BMX-style stack bolts will do the job.

The next step is to decide what to do about the rear wheel. If you have a freewheel on the rear wheel, you can remove that and install a single-cog freewheel. If you have a frame with horizontal dropouts, you might want to consider buying a flip-flop hub. This type of hub allows you to run a fixed cog on one side and a freewheel cog on the other.

Once you have your cog figured out, you might need a new chain, as most cogs are 1/8 inch. Luckily, a 1/8 inch chain will also work on a 3/32nd inch chainring. Although, you can also find single-cog freewheels in 3/32 inch sizes. When you install the freewheel, you may need to install some spacers to maintain a good chainline. The chain should be straight between the front ring and rear cog. If it's at an angle, install spacers to correct it. You'll want to make the chain as tight as possible while still allowing the wheel to turn.

If your bike has vertical dropouts, you'll need to install a chain tensioner, like the Surly Singleator. The chain tensioner mounts to the rear derailleur hanger and provides the proper chain tension. Another option is to purchase chain half-links, which you can install in your chain to get proper tension. With half-links, you might be able to get by without a tensioner.

Many bike stores and online shops carry single-speed conversion kits. These kits typically come with a chain tensioner, different sized cogs spacers, and instructions on how to install it all.

Put everything back together and go for a ride!