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I have a fork with a 12 mm axle. As it happens, the flat part inside the dropout that buts up against the hub end-cap is large enough to accommodate a hub designed for a 15 mm axle (using a reducing shim between the axle and the hub). The hub's end-caps have an outside diameter of 21 mm.

Is this fork just unusual, or would other 12 mm forks be able to accommodate this hub in the same way?

Put another way, what is the standard width of the dropout at the point that the hub end-caps contact it? (I assume there is such a standard, but I have no idea where to find the details.)

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    I feel like I've seen both Shimano and SRAM release documents that basically have this info, but I'm coming up empty finding them. There's some data in the SRAM/Rockshox "Frame Fit" documents. Commented Aug 24, 2022 at 20:23
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    @NathanKnutson Thanks, SRAM's "MTB frame fit specifications" document suggests that a Maxle-compatible fork should have a flat area in the dropout of at least 23 mm diameter, regardless of whether it's designed for 12 or 15 mm axle (page 130 in this PDF). However, there's a comment on the same page that "Customer is responsible for ensuring hub, frame, and axle compatibility." – I'm not sure what to make of that. Also not clear whether this spec is Maxle-specific. Commented Aug 25, 2022 at 7:01

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Having Googled around on the topic of 12/15mm hub-fork compatibility, it seems there is no established standard governing the outer diameter of the hub end-cap for thru-axle forks.

When discussing one such adapter, Radhaus notes that:

Please note that the critical dimensions for compatibility are the inside diameter (ID) of the fork dropout pocket and the outside diameter (OD) of the hub end cap. In practice there are no standards in use for either dimension.

Meanwhile, Halo Wheels states that both forks and hubs can be designed for/with end-caps of between 19 mm or 21 mm outer diameter, which can lead to incompatibilities:

Some 12mm thru axle forks are only compatible with hubs which have end cones with a maximum outer diameter of 19mm. Several thru axle hubs have end cone outer diameters larger than 19mm (typically 21mm) and will not fit into the recessed area under the hub location lip.

To check the compatibility of your 12mm axle fork with any thru axle hub, measure distance ‘A’ in the diagram.

If ‘A’ is less than 4.5mm you can only use a hub that has a maximum outer diameter end cone size of 19mm. If ‘A’ is 4.5mm the fork should accept a hub using 21mm end cones.

enter image description here

I am happy to be contradicted if someone can find a more authoritative source than these!

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