The use of sprockets, or cogs, on a bike allows a cyclist to employ a variety of gear ratios. These ratios are determined by the size of the sprocket in conjunction with that of the chain ring, found attached to the crank arm at the front of the bike. A bike can contain as many as 11 sprockets or as few as one. Old sprockets may need to be replaced, and dirty sprockets occasionally need to be cleaned. Sprocket removal requires little time and is a handy piece of knowledge for the home bicycle mechanic.
Identify the type of lockring removal tool that you're going to need. There is no such thing as a universal design. There are several manufacturers of sprockets (multiple sprockets are known as "cassettes") and almost as many designs. The lock ring removal tool loosens the lockring that holds the sprockets in place. Parktool.com offers a wide selection of these tools and is an excellent place to identify the model that you'll need.
Remove the wheel from the bicycle and place the wheel between your legs, sprocket-side facing away from you. Insert the lock ring removal tool into the lock ring, located in front of the first sprocket. Fit a wrench to the lock ring removal tool. Depending on the design of the lockring removal tool, this may be an adjustable wrench or a drive ratchet.
Attach the chain-end of a chain whip to the sprocket. If your bike has multiple sprockets, attach the chain whip to the largest of the sprockets. The handle of your chain whip will be located at roughly a two o'clock position, your wrench at a 10 o'clock position.
Push the two handles down. This will unlock the ring holding your sprocket(s) in place. Remove sprocket from the bike.