I've got an RD-6870 GS and for 90% of what I do its perfect. Occasionally I do some more climby cycles and I'd love to be able to swap out the chain, cassette and cage.
Is it possible to swap out the cage on a 6870 with something like the SGS cage from the RD-M8000?
In the old days you probably could have swapped a road mech cage for an MTB one. But now I would assume they're using fundamentally different pivots one accomodating a clutch, the other not.
The obvious answer would be to look at a GRX mech, same cable pull as your 6800 but with more capacity, just turn the clutch off.
The 6870 is DI2 ...
Was kinda hoping not to spend "quite" the amount required to buy a bigger mech 😉
Conventional wisdom is that Shimano are always very conservative in stating mech capacity so you can probably go bigger than they claim with no issues. Otherwise, there are a few companies that make little doofers to mount in between the mech and the hanger to allow bigger cassettes. Still might end up without being able to use small-small comfortably, but, well, just don't cross chain then.
Here's one example:
Roadlink
GRX di2 should talk to your Ultegra shifters and have lots of capacity, the price difference between what someone will pay for a used 6870 mech on eBay (they sell way over the odds IMO) and what a new GRX one costs is probably not too different from what a longer cage will cost you.
The major challenge/problem you face is finding a longer cage guaranteed to fit, most modern MTB mechs are probably substantially different due to the addition of clutches...
It's not been done that much because most people don't want to gut their nice mech, they tend to replace the whole unit or, as suggested above, just try a roadlink.
TBH the roadlink is the most sensible suggestion, cheap, effective and easily reversible...
A longer cage won't give you any extra clearance for bigger sprockets. You will still be limited by the arc the mech moves through. That roadlink doofer is the best option.
Fair enough. If I can fit an 11-34t at the mo ... what range would i be able to get with one of those extenders?
Could i push it to an 11 to 42t?
Oh well .. ordered an 11 to 42t and an extender .. will let you know how it goes 😄