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I have problems with adjusting front shifting on my MTB so I suspect something is wrong elsewhere.

This is a 27.5" Merida Big Nine 100 XC MTB. Its chainset has been replaced with a Shimano FC-MT101 by its previous owner. Original chainset is SR Suntour XCM 40-30-22 CG (BSA) according to 99spokes.com. Bottom bracket shell is 72mm wide but I don't know the model of the bottom bracket so I don't know its width.

Do you think the space between the frame and chainring is too wide? It's about 1 cm. I suspect that's why I can't shift from smallest chain ring to the middle without tightening the shifting cable too much via the adjuster barrel on the shifter.

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After I got the advise from @Nathan Knutson to find the chain line of the bike I measured it as 57-58mm. enter image description here

However the front derailleur's chain line compatibility is 50mm. So the answer is they are incompatible I guess.

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    I recommend taking a chainline measurement, which takes no disassembly and can be done with various tools, at least when the main goal is too see if it's way off. One of the things in question here is whether the new cranks wound up with the correct spindle length, either by replacing the BB along with them or lucking into the old one working. Judging it visually as suggested is not helpful. Is the FD bottoming against the frame? Have you eliminated FD positioning or limit screw error as a cause? Jul 21, 2023 at 18:17
  • Is the front derailleur able to move to the big chainring? (to make sure: Turn the limit screw all the way out and move the derailleur by hand so you are not limited by the limit screw or insufficient cable tension)
    – Michael
    Jul 23, 2023 at 6:19
  • Yes, it was able to move but the chain was rubbing on 1st, 2nd and 3rd sprocket. I had to push the shifter's arm higher up when it was already on 3rd gear to avoid rubbing. Then I took off the cable and try to find if something wrong elsewhere.
    – Ender
    Jul 23, 2023 at 16:25

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If you're suspecting the crank is out too far, the information needed to see if there's a chainline/spindle length problem is public at si.shimano.com. Nominal chainline for this crank is 50mm with a 123mm BB. Measure the chainline. You can then compute the current spindle length without taking anything apart. You can also compare it to the FD specs on the same site to know whether the FD you have should be playing nice with either 50mm or whatever the chainline is now.

It's possible that someone put a new crank on without re-adjusting the limit screws and cable anchor position from scratch, which in most cases will not work unless it's a like for like crank replacement.

If the chainline is good or really if it's anywhere between about 47mm and 52mm, and if by tightening the cable "too much" you mean to say that the problem you're running into is literally running out of barrel adjuster adjuster, then the first thing to do is loosen the cable anchor, put the barrel adjuster back in all the way, confirm the FD position and limit screw setup is good and correct it if it's not, then anchor the cable and re-adjust the FD. The way the question is written, it's unclear whether these normal FD setup steps have been taken or not - if not, the best thing in any case would be do these setup steps from scratch and seeing if it works before looking for a deeper problem, unless the chainline is way off.

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  • I didn't know what the chainline is. I've seen a video and need to take measurements now. Probably something is wrong there. Thank you for your advice. I'd seen many videos about adjusting the gears but none of them mentioned this.
    – Ender
    Jul 23, 2023 at 16:19
  • I took the measures and have edited my question. You may check.
    – Ender
    Jul 23, 2023 at 19:27

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