Does anybody here actually have XTR Di2?

*crickets*

If you're running a single chainring, its size is irrelevant to whether your rear derailleur has enough travel to handle the cassette. Rather, it just dictates how long of a chain you need. However, a 10-42t cassette would definitely require a long-cage rear derailleur. Right now, Shimano already makes a mid-cage Di2 11-speed derailleur: the Ultegra RD-6870-GS. It lets you use 11-32t cogs with a double, but would *probably* work with the 10-42t cassette with a single ring up front. Otherwise, if you'd be content with, say, 11-32t and 50t up front (I am), you can run the standard RD and a single ring. I have that on my P2 right now, and it's rad. The other benefit of single ring is it simplifies your shifting decisions to a simple "up or down" and therefore eliminates the need to ever look back at your drivetrain.

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TriRig.com

the new XTR GS RD (M9000 / mechanical) will work with the 11-40 cassette; i'm using one. a friend has used it with a 10-42 cassette, too.

capacity of the RD-M9000-GS is stated as 37T by shimano (typically conservative), so no issue with 10-42 in a 1x setup there. the max cog size is listed as 40T....but again shimano is conservative; to a degree this depends on geometry.

the RD-6870-GS has a capacity of 37T, too, but only handles a stated maximum of a 32T cog -- so although conservative i doubt 11-40 or 10-42 would work.

GS is typically referred to as "medium cage" with SGS as "long cage", but some people use "GS" to mean "long cage" and "SGS" as "super-long cage".

in other words, a GS / medium cage should work for just about any 1x setup in terms of capacity....but the geometry of the road derailleurs is a bit different from the MTB derailleurs. as short a cage as possible is desirable for a few reasons.

i know of one person who has an RD-M9050, but he's very well connected and supply even to those in the industry is VERY tight right now. probably a few months before we start seeing them show up for consumers.

it will work regardless of the size of the front ring, but it's a matter of getting your hands on RD-M9050-GS. it'll be a while before you can (easily) do so--probably looking at january.

as a bonus, it is a clutch RD, so chain retention will be a bit better--you may get away without an inner catcher or outer guard. with my 1x MTB setup (XTR mechanical), i do not use any guard/catcher, but i do have a clutch RD and a narrow-wide front ring. with my 1x cross setup, i use a road RD (RD-9070-SS)...along with a catcher and guard.

a TT bike, in theory, will be subject to less bouncing around and less chance to drop a chain.

if you're going 1x, you may strongly want to consider a narrow-wide ring. not sure if you can find them in >=50T yet; i haven't checked. you'll need an even # of teeth, regardless.

you want the shortest cage you can possibly run; it will reduce chainslap and improve shift quality, esp with a wide-range cassette.

Wolf Tooth just released a 50t narrow/wide ring. 5 bolt, so won't work with the new Shimano crank.

50 is a little too tall for me. Also seems like a big selling point of di2 is the front shifting. I have SRAM 1x10 on my CX/commuter with a 44t front ring. 11-28 is ALMOST enough for everything. For real climbs near me (Old la Honda, King's Mtn) I would want 11-32.

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/kj

http://kjmcawesome.tumblr.com/

The 50t x 10t is overkill but a 1 x 11s road bike is certainly an option if you're willing to give up reasonable gaps between shifts (bad idea for road/steady state riding) or limit you're willing to change the chainring to suit the ride of the day.

I think a 52t with 11-23t would be fine for most of the races I do but on my daily rides I use a 52/36t and 12-27t and use every gear combination on just about every ride.
http://www.specialized.com/...nge-lunch-race#specs


-SD

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https://www.kickstarter.com/...bike-for-the-new-era

So I've got the 6870 on my new road bike with an 11-32. Admittedly that's a good amount of gearing but there were some spots in Italy last month when the 32 might not have been enough (15+ percent sections). Where does one find an 11 speed 37T cassette? Next year I'll be in Spain with some more super steep climbing.

Look for a 11-36 mtb cassette and switch you rear derailleur also

The wide range cassette would make that 50t pretty easy in 50/42, and the 50/10 would be the same as a 55/11. Nice top end and low end if the ratios are there in the middle too. I think the 2t gaps are the biggest question for me, how does the commuter ride with the wider range in the middle of the cassette?

Subscribing considering I can get the same ratio with a 50/40 as I can with a 34/28. Would love to get rid of that front deraileur sail!