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The big news with the release of Shimano's 12-speed XTR M9100 series groupset is a major change in the rear freehub design. The move from the standard Hyperglide (HG) freehub that has been a Shimano feature for years, to the Micro Spline design was crucial for the engineers to develop an efficient, reliable, and long wearing freehub that could support the requirements of a 10T small cog with 11 others on the same design. Some of the space requirements were solved through the concave rear Beam Spider of the M9100 cassette, which is why the Micro Spline freehub is shorter than you'd expect. As usual with Shimano, total integration of engineering, materials and technology allowed for a true XTR-level, world class rear hub design for 12-speed mountain use. While Micro Spline has made this all come together, it is hardly the only technology or feature used in the 12x142mm XTR FH-M9111 Rear Hub.

As Shimano is only offering disc brake mountain groups, the hub is clearly disc and employs their Center Lock spline rotor mounting system. The 12mm thru-axle hub is not adaptable, for reasons discussed below, yet the stout but lightweight aluminum axle features cone wrench flats for adjustment and easy service access. Shimano makes the machined aluminum hub shells in the appropriate widths for the the actual axle width, such as standard 142mm or Boost 148mm. One of the hub-based rationales behind Boost spacing was that it allowed bearings to be moved wider with the increased shell width, offering greater stiffness, wider spoke flange placement, and increasing wheel build strength and stability. If you only add longer axle end caps to the standard shell width (which admittedly increases potential dropout compatibility), you aren't gaining the technical and performance advantages. This is XTR, so you know Shimano is going to maximize premium performance over compatibility and the 142mm hub shell is indeed wider than then older 135mm standard.

The machining and construction of the shell and hub internals is impressive, yet also built for the best balance of top-level performance, light weight and durability. The driveside spoke flange of the j-bend M9110 rear hub is sculpted and strong, while the standard XTR-level hub axle internals: stainless steel ball bearings, grinded cups, CBN cones are safely ensconced behind a dual Labyrinth and Contact seal system that offers great shielding from the elements without adding significant drag. Micro Spline has it's own internal system, with Contact seals protecting the fast ratchet engagement system, which offers max engagement of 7 degrees. The quicker engagement and higher rigidity of the more compact freehub structure generate a more direct pedaling feeling with more rapid acceleration.

Shimano refers to the thru-axle aspect as their E-Thru Axle design, but the key is that it is simply a 12mm thru-axle system. Thru-axles themselves are frame specific due to actual external dropout dimensions and the thread pitch used, so those units are generally supplied with the frameset. The 142mm distance is the inside measurement of the dropout, hence it is the true hub axle distance. Shimano's Center Lock disc rotor interface is simple and easy to use, with a meshing spline system that is secured by a low profile lockring. The standard allows for great consistency in proper alignment and positioning, making wheel changes faster with better rotor to caliper set-up. One note on the Micro Spline freehub body and cassette lockring interface. Despite the tight confines, Shimano was able to maintain the same 12 internal notch design so standard HG cassette lockring tools will remove and install the new lockring design.

Shimano is also offering the M9100 series rear hubs in Boost and Boost straight pull flange designs. A non-series, XTR-level MT900 Micro Spline hub option is also available in a black finish with full XTR internals, but with a slight weight penalty.