Dropbell

MTB drivetrain compatibility, minus the confusion

The goal: when you see a cassette or derailleur in a sale, you know in ten seconds whether it fits your bike.

Modern mountain bike drivetrains are mostly 1x11 or 1x12, and the two big ecosystems — Shimano and SRAM — don't mix freely. The three things that decide compatibility are the freehub body on your rear wheel, the speed count, and (for the most part) staying within one brand's shifting system.

1. Freehub body: the thing people get wrong in sales

The cassette mounts on the freehub body of your rear hub, and there are three common standards on modern MTBs:

  • Shimano HG (splined) — the long-running standard. Takes Shimano 11-speed MTB cassettes and most third-party 11-speed cassettes. Shimano's own 12-speed MTB cassettes do not fit HG.
  • Shimano Micro Spline — required for Shimano 12-speed MTB cassettes (Deore through XTR, which start at a 10-tooth cog). Needs a Micro Spline freehub body; many hub brands sell one for existing wheels.
  • SRAM XD — required for SRAM's 10–50/10–52t Eagle cassettes. Again, usually a swappable freehub body rather than a new wheel.

Before buying any discounted cassette: check which freehub body your rear wheel has, and whether a replacement body is available for your hub. A £90-off cassette that needs a £45 freehub body is a different deal — and for some older or budget hubs no body exists at all.

2. Speed count and brand: keep the shifting family together

Shifter, derailleur and cassette need to agree on speed count, and in practice on brand too — Shimano and SRAM use different cable pull and cog spacing, so a Shimano shifter won't index a SRAM derailleur properly, and vice versa. The reliable rule:

  • Shifter + derailleur + cassette: same brand, same speed count.
  • Chains are the flexible part — a chain of the right speed count generally works across brands (a 12-speed chain on a 12-speed drivetrain), with SRAM's Flattop chains for its AXS road groups being the notable exception to keep separate.
  • Cranksets/chainrings care about chainline and chain width, not brand — a 12-speed-compatible narrow-wide ring works with either ecosystem.

3. Mixing generations within a brand

Within Shimano's 12-speed MTB range (Deore M6100 up to XTR M9100) the parts interchange: a Deore cassette with an XT derailleur is a well-trodden, sensible combination — that's where sale-hunting pays. Same for SRAM Eagle: mechanical Eagle parts (NX through X01) mix, with the caveat that NX cassettes use HG freehubs while GX-and-up use XD, so "upgrading" an NX bike to a GX cassette also means a freehub swap. SRAM's AXS electronic parts shift among themselves and don't mix with mechanical shifters.

The quick checklist before you click buy

  • What freehub body is on my rear wheel? (HG / Micro Spline / XD)
  • How many speeds is my current drivetrain, and which brand?
  • Cassette, shifter, derailleur: same brand and speed count?
  • Chain: right speed count? (Brand flexible.)
  • If the deal needs a freehub swap: is the body available for my hub, and does the total still make sense?

Edge cases exist (aftermarket cassettes, conversion kits, older 10-speed setups). When in doubt, the manufacturer's compatibility documents are the source of truth — this guide is the 95% case, stated plainly.