Confused about RD capacity

[sfscott]

I am running a 2x Di2 XT crank on my MTB. There is a 10-tooth gap b/w big and small.

I currently have an 11-40 cassette and wondered if the 11-42 would work, assuming proper chain length.

Shimano's site says that the RD has a max capacity of 35, but even with the 11-40 (which shifts perfectly) the capacity (40-11)+ (34-24) is still greater than 35.

Yo no comprende.

The double-asterisk note says system limits cross chaining to prevent slack. So does this mean the 42 would work but you won't get certain front-rear combinations?

If so, do you end up with lower gearing using a bigger (38-28 or 36-26) chainring up front?

[oldpotatoe]

Originally Posted by sfscott

I am running a 2x Di2 XT crank on my MTB. There is a 10-tooth gap b/w big and small.

I currently have an 11-40 cassette and wondered if the 11-42 would work, assuming proper chain length.

Shimano's site says that the RD has a max capacity of 35, but even with the 11-40 (which shifts perfectly) the capacity (40-11)+ (34-24) is still greater than 35.

Yo no comprende.

The double-asterisk note says system limits cross chaining to prevent slack. So does this mean the 42 would work but you won't get certain front-rear combinations?

If so, do you end up with lower gearing using a bigger (38-28 or 36-26) chainring up front?

That 'formula' is really just a place to start. As you've seen, it really isn't accurate for your setup. It depends on if you have enough chain now with the 40t to go big-big and no droopy chain in small-small and no pulley to cog dragging in lowest gear(b limit screw all the way in?). If you have a bit of an 'S' shape of chain thru rear der when big-big, chances are a 42t will be fine.

[tctyres]

Originally Posted by oldpotatoe

That 'formula' is really just a place to start. As you've seen, it really isn't accurate for your setup. It depends on if you have enough chain now with the 40t to go big-big and no droopy chain in small-small and no pulley to cog dragging in lowest gear. If you have a bit of an 'S' shape of chain thru rear der when big-big, chances are a 42t will be fine.

What Peter says here ... the only other problem would be the b-screw. The rd has to clear the biggest cog in the rear.

[speedevil]

Originally Posted by tctyres

What Peter says here ... the only other problem would be the b-screw. The rd has to clear the biggest cog in the rear.

There are specs for the proper distance between the upper jockey wheel and the big cog, but there is some fudge-factor to this adjustment. This is where the RD and big cog limitations are most obvious. Chain wrap capacity is the other - but it's usually less important. You don't generally ride in the small-small cogs, that's usually cross-chaining at it's worst - so if the chain sags a little in that combination it doesn't usually affect you in normal gear selections.

[clyde the point]

There is a little more b screw adjustment by completely removing it, and then a black washer that sits on the end. Sometimes this is enough to give clearance, with the additional couple mm's available for pushing in the b screw.