Freehub & Driver Bodies: HG, XD, MicroSpline & Freewheels

The freehub is the ratcheting mechanism the cassette locks onto

The freehub (or driver) body is the splined or threaded piece on a rear hub that a cassette slides onto and locks to, and that does the actual ratcheting work of letting the wheel coast while the cranks stay still. Three standards dominate modern mountain bikes: Shimano's long-established HG (HyperGlide), SRAM's XD/XDR, and Shimano's newer MicroSpline — each with its own spline or thread shape, and none of them physically interchangeable with the others. A cassette built for one standard simply will not seat on a hub built for a different one; see the Cassette, Freehub & Derailleur Compatibility guide for exactly how the compatibility checker treats this as a build rule.

The driver body itself is usually a swappable part

It's worth knowing that on most modern hubs, the freehub body isn't a permanent, built-in feature of the hub the way it might seem — it's a separate, individually replaceable piece, and major hub makers sell conversion kits letting you swap a hub between HG, XD, and MicroSpline. So a wheel you already own often isn't permanently locked to whichever standard it originally shipped with; buying the correct replacement driver body can convert it to a different standard entirely. A catalog can't know which driver is physically installed on a specific wheel you already own right now, which is why a compatibility tool working from a fixed parts list treats each wheel listing's freehub spec as fixed rather than assuming a swap — but in the real world, a "wrong freehub" mismatch is very often solvable with a driver-body swap rather than a whole new wheel.

The one documented exception: XD cassettes fit an XDR driver with a spacer

SRAM's XDR driver is 1.85mm longer than its original XD driver, and SRAM documents that any XD-labeled cassette fits an XDR driver once a thin 1.85mm spacer is added behind the cassette — a simple, everyday adapter fix, not a wheel-building job. The reverse direction (an XDR-specific cassette on a plain XD driver) doesn't have the same documented fix, and every other freehub mismatch beyond this one specific case is a genuine, unfixable non-fit.

Cassette vs. freewheel: they look similar and aren't the same system

Everything above assumes a cassette-and-freehub system, but a genuinely different, older design still shows up on lower-end and some older bikes: a threaded freewheel, where the cogs and the ratcheting mechanism are one single threaded unit that screws directly onto plain hub threads — no separate freehub body at all. Unlike a cassette, a worn freewheel's cogs aren't individually replaceable; the whole unit gets swapped. There's a simple field test to tell the two apart: pull the wheel and spin the cogs backward by hand. If the splined driver body spins along with the cogs, it's a freewheel. If the driver stays still while the cogs spin freely against it, it's a cassette on a freehub. Knowing which system you're looking at matters before you shop for replacement parts — a cassette and a freewheel are not interchangeable, and neither is the tool that removes them.

Cassette installation basics

A cassette's splines include one deliberately wider gap that only matches the freehub's correspondingly wide spline, so a cassette stack physically can't be installed backward or in the wrong rotational position — there's no orientation to get wrong. The lockring that holds the stack on torques to roughly 40 Nm. A chain whip tool is needed to hold the cogs still while removing that lockring (since the lockring's removal direction is opposite the freehub's own drive direction), but it isn't needed for installation — the freehub's ratcheting pawls resist the tightening direction on their own.

How BuildMyMTB checks this

The cassette's freehub spec is checked as an exact match against the rear wheel's freehub spec, with one adapter-tier exception: an XD cassette on an XDR-driver wheel is a warning naming SRAM's 1.85mm spacer fix, rather than a hard error. Every other freehub mismatch — including the reverse XDR-on-XD direction — stays a hard error. Because the same physical hub is commonly sold across multiple freehub configurations, the catalog lists a wheel model once per freehub variant it's genuinely offered in, so picking the freehub-matching version of a wheel you like is usually the fix for a red freehub conflict, not evidence the wheel itself is wrong for your build.

Practical takeaways

• Match your cassette's freehub spec to your wheel's freehub spec exactly, with the one exception being an XD cassette on an XDR driver, which just needs a cheap spacer.
• Remember most modern hubs can have their driver body swapped to a different standard — a "wrong freehub" flag often just means "buy the other freehub version of this wheel," not "this wheel is off the table."
• Before shopping for cassette removal tools, confirm whether you actually have a cassette-and-freehub system or an older threaded freewheel — spin the cogs backward by hand to tell them apart.
• A cassette only installs one way — if it isn't seating, don't force it; recheck the spline orientation.