The wear item nobody looks at
Pulley wheels — jockey wheels — are the two small cogs in the rear derailleur cage that guide the chain. They spin constantly, they collect every bit of grit the chain picks up, and their teeth wear into hooks long before anyone thinks to check them.
A drivetrain with worn pulleys is noisy, feels slightly draggy and shifts less crisply, and the usual reaction is to blame the cable or the chain. If everything else is new and it still sounds rough, look here.
They are speed and model specific, so match them to your derailleur rather than the cassette. Part of the wider SRAM drivetrain range.
