good day maam/sir, i would like to ask for some ideas if i can use the new Deore XT Crankset 1x12 (36t) to my 9 Speedchain + 9 speed cassette? If not, how about 10 speed chains + 11-42 10 speed Cassette?
Thank you in advance.
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Seems to be two options here.
going from ?x9 to a 1x9 using a 12 speed single chainring (m8100 groupset?)
going from ?x9 to ?x10 by replacing the cassette and chain but not the front?
The first option should work okay - you're removing all the chainring/s and fitting a replacement crankset. This forgoes all the small chainrings, so the low-low gears will be gone completely.
Your front derailleur might go, or you may need to keep it as a a chain-keeper. The left-hand shifter is redundant.
Your biggest problem there is having the right bottom bracket to fit the new cranks, which appear to be Hollowtech II and require something like a Shimano BB52. If your frame takes something else, then some mix and match will be required.
Option 2 would replace your existing 9 speed cassette with a 10 speed. This should simply slide onto the existing hub.
However you'll need a new rear derailleur and shifter with an additional position.
The derailleur would have to be 10 speed, like a M786. Note BOTH the SGS (long cage) and GS (medium cage) are only rated for a 36 tooth low gear, so a 42 tooth is pushing it a bit too far.
The new 10speed chain should run well-enough on the existing chainrings, assuming they're not worn.
Your other option is to look at a GRX 11 speed, which can go to 42 tooth.
Also consider the cost of all these parts, and whether a new bike works out cheaper.
Typically one can fit a front drive designed for one speed with the rear drive designed for another speed if the difference is +/- one speed. It's best to go with the chain that's correct for the rear drive, 9 speed chain in this case. However, i question the compatibility of a nine speed chain with the 12 speed crankset, though i cannot say what symptoms will result from such use other than less than ideal chain retention possibly. The internal spacing of chains doesn't change with increasing speed rating, but the overall width of a chain decreases as you move up to more speeds. This may infer that a 9 speed chain should run fine on a 1x 12 speed chainwheel, but i can find no anecdotal information regarding this. Using an 11 or 12 speed chain on a 9 speed cassette will present problems with smooth shifting and the big danger would be risk of severe chain jam should the thinner profile chain of 11 or 12 speeds get stuck between cogs of a 9 speed cassette which are spaced wider than an 11 or 12 speed cassette's cogs. These types of jams can be severe, going in under power jams the chain quite tight which then has implications for the involvement of the derailleur. It is not something one wants to encounter mid-ride and have to deal with at the trail side as there will almost certainly be damage to the chain at least.
If i had to make this situation work, i would opt for a 10 speed chain, preserving good rear drive compatibility. Hopefully you have a front chain guide in place to aid in chain retention. There are few if any models of 9 speed derailleurs with a clutch system, which, when combined with a narrow-wide chainring, are the two most important aspects of a 1x system to keep the chain on the drivetrain.