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Mountain Hubs

Shimano FH-MT901-B Rear Hub

Shimano released several non-series components with the XTR M9100 line-up. These MT900 series options are considered XTR quality and have all the same features and materials, but as in the case of this Boost spacing rear hub, less machining has been done on the hub shell, and a black finish has been used, with the end result being the Shimano HB-MT901-B Rear Hub weighs in at 258 grams versus the M9100-B version at 237 grams. And it doesn' say XTR. That's all, and you save some money in the tradeoff for grams and a loss of logo, albeit the most prestigious tag in mountain biking.

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 Boost frame-ready FH-MT901-B 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, as the 148 Boost shell is wider than that of the 142mm.

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 MT901-B 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 148mm 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.

If you need reliability, top-level hub performance and plan to build your rear wheel up with forgiving and widely available j-bend spokes, keep it simple and pure with the HB-MT901-B Rear Hub.


  • Innovative Boost hub/freehub design for Shimano 12-speed XTR M9100 series
  • Premium grade XTR materials, technology and design
  • Lightweight aluminum freehub body to reduce weight, allows for 10T cassette cog
  • For use with j-bend spokes, Boost 12/148mm thru-axle, Center Lock disc rotor
  • Stout machined and anodized aluminum hub shell, oversized aluminum axle
  • Features simple and easily serviced cup, cone, and ball bearing design
  • Internals heavily protected by proven Labyrinth and Contact seal system
  • Axle ends feature cone wrench flats for service/adjustment
  • Premium grade materials with direct, simple, and utterly reliable technologies
  • Micro Spline freehub with fast ratchet system, with 7 degrees of engagement
  • Not silent, but minimal noise and friction when coasting
  • Micro Spline only works with 12-spd Micro Spline cassettes, with HG lockring tool
  • Wheelbuilder specifications
    • Flange diameter: 68/70mm
    • Flange distance: 62.26mm
    • O.L.D.: 148mm
    • Offset: 6.63mm
    • P.C.D.: 59/61mm
  • Center Lock disc brake rotor interface; Anodized aluminum lockring included
  • Options (TA, hole count): 12/148mm 28 hole, 12/148mm 32 hole
  • Finish: MT900 series Black
  • Weight: 258 grams