Replacement Cassette Part ethirteen have provided the ability to replace your cassette as it wears rather than replacing the entire cassette in one go. With mountain bikes transitioning to 12 speed drivetrains the price of cassettes has increased with the technology. The construction of ethirteen's replaceable half helps to reduce cost by only replacing the cogs you need to replace rather than the entire unit. The 42 and 50T rings are made from a lighter alloy than the small main body of the cassette, this helps to keep weight down and also the price.