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 for straight pull spokes, 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-BS Rear Hub weighs in at 254 grams versus the M9100-B version at 241 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.

Handbuilt mountain bike wheels offer customization for rider weight, style, and forgiveness, and we have seen an increase in riders asking for builds with straight spokes and hubs. There are real advantages to straight pull: better spoke angles to the rim, less likelihood of spoke contact when built, higher spoke tension, and potentially a spoke that slides a bit on impact which reduces pressure on the rim and puts less stress on the spoke head in the process. Shimano recognizes the need and as they have built mountain and road wheels with straight pull spikes for years, clearly advocates for the practice. The FH-MT901-BS Rear Hub is a Boost - Straight pull (hence the BS designation) specific Center Lock Disc front hub released for custom builds with the XTR M9100 series component line. Straight pull has some definite advantages and the spokes rarely break at the head like a j-bend, with the major drawback being that straight pull spokes are harder to find at local shops around the world, so our advice is definitely have your wheel builder provide you with extra spokes with your wheel.

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 12x148mm FH-MT901-BS 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 cleanly machined straight pull flanges are clearly built for precision and strength, 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 resilient straight pull spokes, keep it simple and pure with the HB-MT901-BS Rear Hub.