Cassette, Freehub & Derailleur Compatibility

Three freehub standards, and why the driver matters

The freehub (or freehub body) is the ratcheting mechanism on the rear hub that the cassette slides onto and locks to. Three standards dominate modern mountain bikes:

• Shimano HG / HyperGlide — the long-established splined standard.
• SRAM XD / XD-R — a threaded driver body designed to let the smallest cog get down to very low tooth counts.
• Shimano MicroSpline — Shimano's newer splined standard, also designed to support smaller minimum cogs than classic HG.

A cassette built for one of these standards physically will not mount on a hub with a different driver body. The cassette's freehub spec and the rear wheel/hub's freehub spec need to be an exact match.

The freehub body is swappable — but the tool doesn't assume that for you

In the real world, freehub bodies on most hubs are actually a swappable part: many hub makers sell the same hub with interchangeable HG, XD, or MicroSpline driver bodies, so a wheel you already own can sometimes be converted to a different standard by buying the correct replacement driver. A compatibility checker working from a fixed catalog of parts can't know which driver body is currently installed on your specific hub, so it treats freehub standard as fixed per wheel listing rather than something that can be swapped on the fly. The practical fix for this in a catalog context is to list the same wheel model more than once — once per freehub configuration it's actually offered in — so you can pick whichever version matches (or will match, after a driver swap) your intended cassette.

A special case: integrated-cassette wheels

A small number of wheels take this a step further and ship with a built-in cassette rather than a separate freehub body at all — the e*thirteen LG1r DH wheel is the notable example, where the driver itself is a built-in 7-speed cassette. On a wheel like this, there's no separate cassette to pick at all; picking any cassette alongside that wheel is a straightforward mismatch, since the cassette functionality is already built into the hub.

Cassette range vs. derailleur capacity

Separately from the freehub interface, a derailleur is rated for a maximum cog size (capacity) it can wrap around. Pairing a cassette whose largest cog exceeds the derailleur's rated maximum — for example, a 10-52-tooth cassette on a derailleur rated for a 50-tooth max — is a straightforward mismatch: the derailleur's cage and mechanism simply aren't built to wrap that large a cog.

The reverse isn't automatically a problem, though. A derailleur rated for a larger maximum (say, 52T-capable) paired with a smaller-range cassette (say, a 10-50 cassette) is generally fine — manufacturers document that a wider-capacity derailleur is backward-compatible with a narrower-range cassette, so there's no need to flag that combination.

How BuildMyMTB checks this

• Cassette range vs. derailleur capacity: if the cassette's largest cog exceeds the derailleur's maximum rated cog, that's flagged as an error. A cassette with a smaller max cog than the derailleur is rated for stays silent, since that's a documented backward-compatible pairing.
• Cassette freehub vs. rear wheel: the cassette's freehub standard (XD, MicroSpline, or HG) must exactly match the rear wheel/hub's freehub standard.
• Integrated-cassette wheels: picking any cassette alongside a wheel whose driver is itself a built-in cassette (like the e*thirteen LG1r DH) is flagged clearly, and the cassette slot isn't required to be filled at all for that wheel to count as a complete build, since the cassette function is already built in.

Because real-world freehub bodies are swappable, remember that a "mismatch" flagged by the tool often just means: buy this wheel's other freehub variant, or swap the driver body, rather than that the wheel is fundamentally incompatible with your drivetrain.

Practical takeaways

• Match cassette freehub standard (XD, MicroSpline, HG) to your rear hub before buying — or plan on a driver-body swap if your hub supports it.
• Check a derailleur's maximum rated cog size against your cassette's largest cog, not just the overall speed count.
• Remember a bigger-capacity derailleur than you strictly need is not a problem — the backward compatibility runs that direction.
• If you're eyeing an integrated-cassette wheel like the LG1r DH, understand you won't be buying (or swapping) a separate cassette for it at all.