Cleaning or Replacing the Rear Cogs

The rear sprockets, we call them cogs, on your rear wheel are attached to your rear hub in two ways. Some older bikes or low-priced mountain bikes use a freewheel system. You can identify this because the rear wheel has a large thread machined onto the hub. The sprockets are fixed to a ratcheting mechanism called a freewheel. The freewheel threads onto the hub. The freewheel has a clutch or ratchet system inside that allows the sprockets to spin when you are coasting and locks when you are pedaling. When the freewheel is removed, all the sprockets come with it.

The other is a cassette system. A freehub ratcheting system is mounted to the body of the hub. This freehub body, that also acts as a clutch when coasting and locks when pedaling, is splined on its outer shell. The cassette sprockets (cogs) use these splines to properly slide and align on to the freehub body. A lockring threads into the freehub and holds the sprockets.

We will try to explain how to pull the cogs off a cassette system in easy to follow steps. The main reason to remove the cogs is to replace a worn cog. The other reason is for cleaning. Now, while the cassette cogs can be cleaned while attached to the freehub, if we have been through an epic ride where the cassette was exposed to tall grass, mud and sand, we'll take the time to pull the cogs off so we can clean not only the cogs, but the space between them on the freehub. Excessive use of chain lube will also gunk up this area, and that gunk can only be cleaned out by disassembly.

For tools you'll need a chain whip, large crescent wrench, 20-spline lockring tool, a paint marker and a cleaning solvent.
Remove the quick-release skewer or drive-side axle nut. The majority of mountain bike cassettes use a 20-spline lockring to hold the cogs in place. Slide a 20-spline lockring tool into the lockring.

• Slide the quick-release skewer back into the axle, through the lockrin tool and put the skewer nut back on the skewer. Snug the nut and then back it out an eighth of a turn. The skewer and the nut act as a holding device for the lockring tool. This is an optional step. Think of it as added insurance. Use the chain whip to hold one of the middle cogs. Use a large crescent wrench and adjust it to fit the lockring tool. Turn the crescent wrench in a counter-clockwise direction while pushing the chain whip in a clockwise direction. It will take some muscle to loosen this lockring, and you will hear and feel a clicking noise as it starts to loosen.
• Turn the crescent wrench just far enough to break free the lockring. Now remove the skewer nut and skewer completely. You should be able to turn the lockring tool by hand once the skewer and skewer nut are removed. Keep turning the lockring until it comes off the cassette. Careful, the cogs can slide off at this point. Remove the cogs and be careful to record what side of the cog is facing outwards. If in doubt, dab the outside with a paint market. You do not want to flip them around by accident when reinstalling. Also, if there are spacers between the cogs, note their position between the cogs and keep track of the direction they are facing. You want every component to go back on exactly how it was removed. We lay each piece on the workbench exactly in the order they will be reinstalled.
• Clean up the cassette body just to make it easier to slide the cogs back on. You don't need to grease the body. Greasing the cassette body will only attract dirt. Clean all the cogs thoroughly with a solvent and brush. It is difficult to see cog wear. The rule of thumb is, if it wasn't skipping, the cogs are still okay.
• Slide the cogs and spacers back on in the order they were removed. If you don't, you will have one strange-shifting bike. Grease the treads of the 20-spline lockring and thread it onto the cassette, hand snugging it with the special tool. Leave the tool in place and slide the skewer back in and snug up the skewer nut. Again, this is optional, but it helps keep the tool from popping out.
• Use the crescent wrench to turn the lockring until tight. Since you are tightening in a clockwise direction, you do not need to use the chain whip. The cassette is locket in the clockwise direction.