Where does the hub end, and the freehub begins?

Question

When shopping for a Shimano SLX M7100 Groupset, 1x12 one reads

⚠ For use with Shimano MICROSPLINE freehub bodies

Cassette NOT compatible with standard HG
(8,9,10 or 11 speed), XDR or XD freehubs

I'm confused about where the hub ends and the freehub begins. The hub is the "block with holes", where the spokes are attached.

The freehub is the "block with pawls", the ones producing the buzzing sound (loud or not), and the one enabling coasting.

I understand that the connection from cassette to microspline freehub is unique. The main cassette block and the individual cogs (if any) fit in exactly one way.

I also understand that microspline was required to accommodate a 10t smallest cog, which, oddly and in retrospect, seems like an invitation by Shimano for users to upgrade to SRAM Eagle 1x12 NX/GX (even if maybe SX is less than desirable) because although SRAM's smallest cog is still 10t, their cassettes fit on the old non-micro splines.

But what is the connection between freehub and hub? Can the buzzing, pawl-carrying freehub be replaced without replacing the hub (and ultimately rebuilding the wheel)?

If/when the pawls fail? Can the the "pawl-block" be replaced without rebuilding/replacing the wheel?

If you answer with a picture showing what remains after the microspline freehub is removed, that would be ideal.

As an aside

Shimano is careful about branding compatibility interfaces. Yet the very warning above refers to "standard HG". Is that really the best way to refer to non-micro splines? (I understand that hyperglide, or HG, itself refers to the ramps on cogs).

Answer 1

According to Wikipedia, a freehub is a kind of hub that includes a ratchet mechanism, so freehub is a subset of hubs. What you describe as the hub is in fact just the hub body.

The part of the hub on which the cassette is mounted is the freehub body. Depending on the brands, you can mount only one kind of freehub body, or several ones.

Shimano obviously considers their products as "families" and only make variations of their components to fit within the specs of the product family. The M7100 family only uses cassettes with Microspline freehub bodies, so is only offered with Microspline.

Specialized wheel manufacturers (Mavic, Fulcrum, DT Swiss,...) are usually more flexible and design their freehubs so that one can fit different kinds of freehub bodies. You can typically buy the wheel with HG or XD, and change afterwards.

You'll find an example here of the exploded view of a Mavic freehub, illustrated with HG and XDR bodies. It is a hub with sealed bearings. So everything can be removed from the hub body, if a part needs to be replaced. That's also true for cup-and-cones hubs, with one nuance: the bearing cup is a part of the hub body. So if the cup is worn/damaged, the hub body needs to be replaced.

Then the question of the freehub bodies is mentioned on several answers, so I'll only describe it shortly, for reference:

• HG/Hyperglide/Shimano: most common one, up to 10-speed road cassettes, or 12-speed MTB ones (there's an overhang of the large sprocket in that case)
• HG11, HG Road: similar to HG, but a bit longer, to accomodate 11-speed+ road cassettes (or MTB with spacer).
• XD (from SRAM): first freehub body that can accomodate 10-teeth cassettes (and even 9-teeth).
• XDR: long version of the XD, for 11/12-speed road cassettes (or MTB with spacer).
• Microspline: Shimano's answer to XD.
• Campagnolo ED: basic Campagnolo body (9-12-speed cassettes)
• Campagnolo N3W: Campagnolo's answer to XDR

Answer 2

Freehub bodies can be removed and reinstalled, but that generally requires a compatible freehub body. I don't believe that Shimano hubs for example would have cross-compatible freehub bodies, so that if you have a Shimano hub for hyperglide, that you would be able to find some micro-spline freehub body for that hub. Generally there is only one compatible replacement. If the hub is micro-spline, that's micro-spline; if the hub is hyperglide, that's hyperglide. Note that road 11-speed freehub bodies are wider.

Other manufacturers may have cross-compatibility, but you need to check this for the specific hub you have.

Here's an example: https://dassets.shimano.com/content/dam/global/cg1SHICCycling/final/ev/ev/EV-FH-R7070-4334A.pdf (not posting a picture but a link to one due to copyright considerations).

The part (7) is freehub body. The rest is hub. Note that between the freehub body and hub axle there are bearings -- the right-side bearings ride on the freehub body.

On regular QR hubs, the bolt was so small you could use a standard sized Allen wrench and turn it with torque capable of achieving with your hands to remove the freehub body. On newer thru-axle hubs, the bolt has to be larger, so you use a 15mm Allen wrench (which Shimano sells for a horrible price), and you need to mount the 15mm Allen wrench in a bench vise since the 15mm Allen wrench isn't long enough to achieve the needed torque which on a 15mm bolt is obviously larger.