We first saw double-ring mountain cranks when Tom Ritchey and Frischy experimented with it about a decade ago. The idea never got traction until SRAM re-introduced the concept with XX. The reason for SRAMs success and the first attempts failure was due to cassette and gearing options. If you are going to run 2x10, you need a larger selection of lower geared cassettes, which SRAM was able to provide.
Now Shimano has throw their hat in the ring to come up with, not only their first double mountain crankset, but possibly the most advanced crankset design we have seen to date.
Like the M970 series we see Hollowtech II hollow and cold forged arms that receive the dual finish, anodized and polishing treatment. Perhaps what is most impressive with the M985 crankset is the shifting. The HG-X profiled teeth and tight Dyna-Sys gearing are designed to work specifically with the CN-M980 HG-X chain. Shimano has a achieved the quickest and most accurate shifting on a mountain crankset - shifts are light, quick and telepathic. The chain moves back and forth between the rings with ease. One other design feature which adds to the shifting prowess and durability is the 3D shaped outer ring. It is super stiff and virtually flex-free. Dumping the inner ring, decreased the Q-factor by 3mm per side.
Take all of this technology and engineering and wrap it into one package and we have the best performing and highest quality crankset ever produced. Shimano includes an XTR bottom bracket with shims to work with 68mm or 73mm bottom bracket shells