Campagnolo Set of 2 bicycle derailleur rollers

Rear derailleur hubs play an essential role in the bike's transmission. There are two rear derailleur pulleys that provide tension and guidance for the chain during gear changes. The top roller is called the "puley" and moves the chain, while the bottom roller, called the "string", tensions the chain. The derailleur rollers must be in perfect condition to ensure precise and smooth gear changes.