The collection of cogs on the side of a bicycle rear wheel is known as a "cassette." The cassette cogs are each a different size, and each offers a particular gear ratio. Cyclists use gears to allow for more efficient pedalling to help overcome challenges such as hills and headwinds. Cassettes are removable and may be cleaned of grease and grime, which can sometimes hinder the movement of the chain between the cogs.
Work the rear gear shifter into top gear, until the chain is positioned on the smallest cassette cog.
Open the release lever on the side of the rear brake. If the brake lacks a release, as may be the case on some models of mountain bike brakes, squeeze each side of the brake and separate the portion of the brake cable that attaches to the left side of the brake.
Open the release lever at the centre of the side of the wheel opposite the cassette. Unscrew the nut by hand at the centre of the cassette. The lever and nut are part of a device known as a "quick release," a skewer that enables tool-free removal of the wheel.
Raise the rear wheel off the ground. Force the wheel down and away from the bicycle. Grabbing the rear of the wheel, remove it from the bicycle.
Insert the cog remover into the side of the cassette, engaging the notches on the remover with the notches in the side of the cassette.
Reinstall the quick-release nut onto the end of the skewer. Tighten the nut against the side of the cog remover, holding the cog remover in place.
Wrap the chain whip across one of the cogs in a clockwise direction. The chain whip prevents the cassette from rotating during removal of the cassette lock ring.
Grip the end of the cog remover with an adjustable-type wrench. Holding the chain whip steady with one hand, turn the adjustable-type wrench counterclockwise, toward the chain whip. The action loosens the lock ring, a threaded disc around the centre of the cassette that holds the cassette in place.
Remove the quick-release nut once again. Set the cog remover and other tools aside.
Unscrew the lock ring the remainder of the way by hand. Place both sets of fingers behind the cassette and lift the cassette away from the wheel.