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A cassette can look fairly simple or incredibly complex, yet any great shifting cassette requires extraordinary engineering and practical experience to produce. No bike component company owns as many shifting related patents as Shimano, with thousands of details covered. In developing the XTR CS-M9100 12-Speed Cassette so many aspects where brought together that it can seem like a unicorn among a field of thoroughbreds. Both the borderline ludicrous 10-51 and 10-45 versions are among the most interesting components of the M9100 series line-up, with a plethora of new technologies, striking combination of materials, and in the case of the the 10-51, sheer size.

First things first, the 12-speed CS-M9100 cassettes employ a new freehub body interface, called Micro Spline. Yes, a rear wheel with a Micro Spline freehub is required. The shorter interface features a keyed spline system, similar to a standard HG model in theory, but very different in practice. If you look at the back of the cassette you can see how the spider is built to allow the splined sections of the cassette and freehub interface and that the spider itself is concave, so the freehub goes deeper into the cassette. This one-piece Beam Spider construction features six extensions outwards from the center and carries the cogs. The larger three cogs are machined from aluminum and pinned together and to the spider. The middle five cogs are machined from titanium, and pinned to the spider, while the four smallest cogs are machined from steel. The material requirements are obvious, with the aluminum keeping things light and with the larger cogs, relatively slow to wear per revolution. The smallest cogs are steel because they will wear the fastest per revolution. XTR cassettes have long featured titanium cogs because they are relatively light and wear is moderate, striking the right balance, while also striking the rider's wallet with force as well, almost ensuring that they were mostly used for events. An alloy locking ring is still used, and actually features the same internal notch system as older Shimano cassettes.

As impressive as the construction as, the true performance of the CS-M9100 12-Speed Cassette lives in the Hyperglide+ system that integrates chain and cog performance to deliver better chain engagement, stronger retention, and smoother pedaling to provide greater power and confidence over rapidly changing terrain. This definitive up and down shifting in both directions boosts smooth acceleration and response with a goal towards highly predictable and precise performance that helps riders adapt to drastically changing trail conditions by precisely judging the optimal shifting moment for consistently smooth operation. The shifts and steps among cogs feel more intuitive and natural, no matter what conditions the trail throws at you. Shimano believes that Hyperglide+ helps riders seamlessly maneuver rough trails by reducing pedaling shock and cutting shifting times by a 1/3 versus the current HG system. If you carefully read all of the XTR M9100 information Shimano has published, this emphasis on the mission statement of "Mountain bikers need to be able to seamlessly Perceive the conditions, Judge their options, and Operate their bike in a split second". The is achieved by reducing the time and effort required for shift points through spacing, tooth engineering, crank arm position and chain design, all of which lessens the pedaling shock you feel when shift point is engaged.

The 10-51 version of this cassette is the big deal as Shimano's 1x12 drivetrain seeks to take-on SRAM's Eagle 10-50 system. It is designed only for the 1x12 option and requires the use of the RD-M9100 SGS long cage rear derailleur. The 10-45 can be used with the GS or SGS models, as well as the RD-M9120 SGS trail-oriented 2x set-up. The 10-45 weighs in at 357 grams, while the 10-51 weighs only 10 grams more, at 367g. These are impressive numbers, matching the expected performance. Shifts are quick, precise, intuitive and as seamless as anything we have seen for road or mountain drivetrains.