The Shadow+ system will be well received by all sorts of riders, not just the trail riders the 985 Trail name suggests. The one way friction cam adds to the spring tension to keep your chain from slapping about through rough, rocky sections of trail. This creates a smoother, quieter ride and reduces the chance of dropping your chain. Just like the Shadow version of the 970 and 980, the Shadow+ keeps a smaller profile, tucked up closer to the dropout to reduce the chance of damage from getting caught on something.
At this stage, Shadow+ is only available on XTR so you get the best technology that Shimano has to offer. The M985 Shadow+ derailleurs feature a cage with a carbon outer plate and aluminium inner plate to save weight where possible while maintaining stiffness and strength for great shifting performance. The pulley wheels spin on top quality sealed bearings and the medium cage (GS) will suit 2x10 applications while the long cage (SGS) can handle 3x10. Both will work with 36T cassettes.
Tired of your chain bouncing up and down, beating up the finish on your chainstay? This Shimano XTR Shadow Plus Rear Derailleur RD-M985 uses an innovative selectable damping system to reduce the cage movement in response to the chain bouncing as you clatter over obstacles on the trail. In essence it maintains consistent tension on the chain. For all you full suspension riders out there, worry not. If you have the Shadow Plus turned "on" it will not harm or be harmed as the suspension cycles normally through its travel.
Aside from this feature, this is just a second generation Shadow XTR rear derailleur. As such, it benefits from the shadow design in the same way as the 970 did. The low-profile geometry keeps it closer to the dropout and safer from impacts with roots and rocks. Both pulleys have sealed bearings, and the pulley cage blends a carbon fiber outer with an aluminum inner plate for a just-right combination of lightweight, strength, and stiffness. Shimano engineers also changed the return spring so that it works better with full-length cable housing. With the new spring, the XTR Shadow Plus Rear Derailleur RD-M985 snaps onto smaller cogs as quickly as you can make the shifts.
You'll have the choice of a medium (GS) or long cage (SGS). The differing lengths of the cages translate into different chainwrap capacities for each version. The GS derailleur is best paired with a 2x10 XTR Crankset. The long cage SGS model has a greater chainwrap capacity, so it will handle the XTR Trail Triple Crank.
The Shimano XTR Shadow Plus Rear Derailleur RD-M985 has a top normal spring configuration, which means that it returns to the top gear when the cable tension is released (which is normal). It's designed to work in harmony with Shimano Dyna-Sys 10-speed Rapidfire shifters and 10-speed cassettes with a maximum 36t large cog.