[ahobie17]

I'm looking at using FC-M8120 XT crank which is a 2x crank, I see the group is offered with 10-45T cassette and the 2x rear derailleur (RD-M8120-SGS) but I want to us e the 2X crank and go with the 10-51T rear cassette and the 1x rear derailleur (RD-M8100-SGS). as I want the extra wide gearing from the 10-51T cassette.

I understand why I must use the 2x derailleur with the 10-45T cassette and know the 2x derailleur won't work with the 10-51T cassette, but is there any reason the 1x derailleur won't work with a 2x crank?? I just don't see how it would be a problem.

any advice would be greatly appreciated.

 

[AndrewHardtail]

The 1x derailleur (and the 2x for that matter) does not have enough capacity (max is 45) to handle two chain rings and the 10-51 range in the back. If you’re not familiar with derailleur capacity, it’s worth reading about it.

You can get away with being beyond the listed capacity if you are very careful about not using the big chainring with the bigger cogs (not enough chain) or the small chainring with the smaller cogs (too much chain), but it’s really not a good idea. Too easy to make a mistake and cause some damage.

 

[ahobie17]

so the jist is not enough chain length for the 2x and large 51T on the rear..rear derailleur cage isn't long enough.

I read about the rear derailleur capacity the 1 x having a max of 51T just wasn't sure how the 2x crank figured in the equation.

 

[AndrewHardtail]

Pretty much. If you made the chain long enough to handle the big-big combo, you’d have too much slack in the small-small for the DR to take up.

The capacity is different from the max cog size. It’s a measure of the ability to take up slack. It’s defined as the sum of the difference between the chainrings (probably 10t for you) and the difference between the largest and smallest cog (51-10=41). So you would need a capacity of 10+41=51. But the 1x derailleur only has 45 capacity.