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First introduced just about a year ago, Shimano’s XTR Shadow Plus rear derailleur represents one of the first major advances in rear derailleurs that the mountain bike market has embraced in a very long time. Starting with their low-profile XTR rear mech, Shimano have added (Plus!) a one-way friction clutch to the cage pivot. By making the derailleur much more willing to take up chain slack than give it, the chain slap is reduced. Less chain slap means less noise, less frame damage, and (most importantly) reduces the likelihood of the chain being dropped. After extensive XC and trail use and on the eve of the release of SLX and XT versions, has the added complexity and 30g been worth carrying? Hit the jump to find out!
First mounted on our Project 24.2 race bike, the long-cage Shadow Plus rear derailleur has been on two very different bikes and ridden in countless settings. 
The unidirectional carbon fiber cage is subtly sexy while the rest of the derailleur makes use of Shimano’s extensive aluminum forging experience. In my opinion, Shimano have managed a nice balance between organic and machine aesthetics with their latest flagship group.
Because the clutch makes the cage extremely reluctant to rotate counter-clockwise, Shimano have added a small switch that disengages the “Plus,” allowing for quick normal wheel removal. The gold switch and bulky pivot are the only indications that anything unusual is going on inside- installation and adjustment are easy for anyone who’s installed other rear derailleurs.
Paired with 10s XT shifters and XT and XTR cassettes, the Shadow Plus shifts very well. Current groups don’t shift with the silkiness that was once Shimano’s trademark- but that’s a response to the market rather than a failure on their part and shifts are better defined as a result. 
With the Plus disengaged, shifting is ever-so-slightly smoother, but for me the system’s benefits outweigh slightly clunky shifting.
And the benefits are clear. The only dropped chain I’ve suffered in the past six months was when the bike slammed into a rut- and the gold lever was disengaged. More satisfying is the way in which the Shadow Plus keeps my bikes quiet. Appealing to both sides of my personality, reduced chain noise makes coming up on hikers much more civil while increasing the psychological impact of race course passes. As a bonus, the derailleur should reduce chain-inflicted damage on carbon chainstays. Win, win, and… win.
Under a small cover, the Plus clutch is adjustable using the elfin wrench stored there. Seeing as I like smooth cables and was one of four people who embraced Shimano’s velvety <del>Rapid Rise</del> Low Normal system, I actually backed the clutch off 1/4 turn (1/2 turn was too much), which improved shifting while maintaining the system’s benefits on all but the roughest trails. 
After having had six months to adjust, the only time that shifting feels balky is after riding an older XTR-equipped bike. The rest of the time, it feels just fine- fresh cables and housing make much more of a difference to lever feel than the Plus system.
The best that Shimano can do has never come cheap (or, to be fair, cheaply made). In keeping with this precedent, Shimano is asking $250 for the RD-M985 XTR Shadow Plus rear derailleur. That’s a lot of dosh for the most exposed part of a mountain bike- but this summer’s SLX and XT Shadow Plus releases will be considerably more accessible. My sample does exhibit a surprising amount of lateral play when wiggled side-to-side in the stand. It doesn’t seem to impact performance- but Shimano have proved that they can do better.
Is it worth it? A reasonably careful rider, I still have 2-generation old XTR derailleurs in my personal fleet. The anodizing has faded from pewter to silver, but I can’t see replacing something that still works as well as they do. XTR gear may be pricey, but it does last. While I can’t recommend throwing a perfectly good mech on the shelf, on a new build or when a stick eats your XTR rear derailleur a Shadow Plus replacement should be a no-brainer. The added weight is negligible and even the weight-weeniest racers can benefit from fewer dropped chains. The benefits are clear and the word is out- I simply don’t see any reason to run non-Plus derailleurs on the trail once the XT and SLX models hit.
marc
