Rear Derailleurs
Used MTB rear derailleurs from UK sellers, Shimano Deore through XTR and SRAM NX through XX1. The rear mech handles shifting across your cassette, so match the speed count, cage length and brand to your drivetrain before you buy.
What does a rear derailleur actually do?
The rear derailleur positions the chain across the cassette and takes up chain slack. Modern MTB mechs add clutch mechanisms, shadow geometry and far more cassette capacity than a decade ago, but the buying decision still comes down to three things: speed count, cage length and hanger type — the same fit-first check that applies across the rest of your MTB components.
Why does the clutch matter?
A clutch is a friction mechanism in the pivots that prevents the cage bouncing on rough terrain. It keeps chain tension consistent, reduces chain slap on the chainstay, and prevents ghost shifts and chain drops. Every quality MTB derailleur now has one — a rear derailleur without a clutch is either budget-tier or dates from around 2012. The clutch is the single most important feature; everything else is refinement.
Shimano or SRAM?
Shimano Deore M6100 (12-speed) is the value king, with the same internals as XT in a heavier body. SLX M7100 adds a quieter clutch and lighter weight, XT M8100 is the sweet spot for refined shifting and long-term reliability, and XTR M9100 shaves weight with carbon and titanium. On the SRAM side, NX Eagle is the budget 12-speed option — heavier and less refined but it works — GX Eagle is the SRAM sweet spot, and X01 and XX1 use carbon cages for weight savings only. Brand has to match your drivetrain here, which matters far more than it does with pedals or handlebars.
Which cage length do you need?
Cage length determines chain capacity — how much chain slack the derailleur can take up. SRAM Eagle derailleurs are all a single cage length. Shimano offers medium and long options, and a 1x12 setup with a 10-51T cassette needs the long cage (SGS). Any 1x setup with a wide-range cassette needs a long cage; running the wrong cage length causes poor shifting in extreme gears and potential chain drop.
Will it fit your hanger?
SRAM's Universal Derailleur Hanger (UDH) standardises the mounting point, so if your frame has UDH, SRAM Transmission derailleurs bolt directly to the frame without a separate hanger. Older frames use brand-specific hangers that vary by model and year. A bent derailleur hanger is the most common cause of poor shifting, so carry a spare — and if shifting suddenly goes wrong after a crash, replace the hanger before blaming the derailleur.