Removing and Replacing the Cogs

The cogs must be removed to replace spokes on the rear wheel. Or you may need to get a specific cog free of the rest to straighten out a bent tooth. Eventually, the teeth become worn and the entire cassette must be replaced. (Keeping your chain clean, and replacing it every 1000 miles, helps your cogs last longer.)

To remove the cogs, you need a freewheel tool and a chain section. The freewheel tool has a toothed ring on one end that fits into the locking wheel of the cogs. The other end is designed to fit a wrench.
Freewheel tool. The bumps on the side of the wheel fit into the corresponding sl...
Freewheel tool. The bumps on the side of the wheel fit into the corresponding slots on the lockring. A wrench attaches to the end the fingers are holding in the photo.
Using a chain whip and freewheel tool.
Using a chain whip and freewheel tool.
If you have a cassette immobilizing tool (a "chain whip," which is like a wrench with a section of chain on the end), wrap it around the cogs. The bar should be on the clockwise end of the curve of the chain. The small loop of chain sits against the teeth to anchor the handle of the tool, while the loose piece of chain wraps around the cassette.

After inserting the freewheel tool into the lockring, use a wrench to turn it counter-clockwise to remove the lockring.

If you don't have a chain whip, just use a section of chain to immobilize the freewheel. You can just take an old broken chain, drape it over the top of the big cog, then stand on the other end with your foot. Works great!
Using an old chain to hold the freewheel.
Using an old chain to hold the freewheel.
Removing the old cassette.
Removing the old cassette.
Lift the cogs off. Each cog must fit on its neighbor exactly, and the spacing rings between the smaller cogs must not be mixed up. It's best to keep the whole mess together as a unit.

If you need to replace or work on a single cog, lift off the cogs down to the damaged cog, keeping everything else together.

Sliding a new cassette onto the freewheel.
Sliding a new cassette onto the freewheel.
If you're straightening a rock-bashed tooth on a cog, note the trend of the teeth before and after the bent tooth. The teeth on the cog are NOT exactly straight. They have a tilting pattern that helps the teeth pick the chain up from the adjacent cog. Note where the damaged tooth fits into this pattern, and bend it to the proper position.

Replace the cassette. The center of the cassette, and the smaller freestanding cogs, has square teeth that fit onto the shaft of the freewheel at one, and only one, position. Turn the cogs until these inner teeth line up with the slots on the freewheel shaft.

Using the freewheel tool, replace the lockring. You don't need a chain whip for this. As you turn the lockring clockwise, the freewheel mechanism engages the wheel. Just keep the wheel still as you rotate the lockring.