There is another reason which does not seem to be mentioned - the cassette gear uses a ratchet which grabs the wheel in one direction and spins free in the other.

There can be two or more of these spring loaded wings that grab on teeth inside the cassette, this is where the clicking sound comes from. In some cases defective parts or bad assembly the ratchet may not be working (stuck spring or missing one of the ratchet gears). You will get a skipping action on the chain but the chain is not jumping over teeth or trying to change gears. In fact you wont see the chain jump at all. The skipping will happen more when you reverse directions of the pedaling. The skipping is the chain cassette gear moving all at once until the ratchet grabs on.

There is another reason which has not been mentioned - the rear hub uses a ratchet mechanism which grabs the wheel in one direction and lets it spin free (coast) in the other.

There will be two or more of these spring loaded "wings," called pawls, that grab teeth inside the freehub body or freewheel, this is where the clicking sound comes from. In some cases, because of defective parts or bad assembly the ratchet may not be working (e.g., a stuck spring, a missing or broken pawl, or broken ratchet teeth). Then you will get a skipping action in the drivetrain but the chain itself is not jumping over teeth or trying to change gears. In fact you wont see the chain jump at all. The skipping is the pawls moving all at once until they catch again on the ratchet.

there is another reason which does not seem to be mentioned the cassette gear uses a ratchet which grabs the wheel in one direction and spins free in the other. there can be two or more of these spring loaded wings that grab on teeth inside the cassette, this is where the clicking sound comes from. in some cases defective parts or bad assembly the ratchet may not be working (stuck spring or missing one of the ratchet gears). you will get a skipping action on the chain but the chain is not jumping over teeth or trying to change gears. In fact you wont see the chain jump at all. The skipping will happen more when you reverse directions of the peddling. The skipping is the chain cassette gear moving all at once untill the ratchet grabs on.

There is another reason which does not seem to be mentioned - the cassette gear uses a ratchet which grabs the wheel in one direction and spins free in the other.

There can be two or more of these spring loaded wings that grab on teeth inside the cassette, this is where the clicking sound comes from. In some cases defective parts or bad assembly the ratchet may not be working (stuck spring or missing one of the ratchet gears). You will get a skipping action on the chain but the chain is not jumping over teeth or trying to change gears. In fact you wont see the chain jump at all. The skipping will happen more when you reverse directions of the pedaling. The skipping is the chain cassette gear moving all at once until the ratchet grabs on.