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. ... Read More