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On my 29" wheels MTB, I currently have a 3x7 setup in which the front 3-speed crankset is a Prowheel 42-34-24 teeth unit, which got damaged in a crash. I look forward to upgrading to the Shimano FC-TY501 crankset which has 48-38-28 teeth.

Will there be any problem in installing a higher tooth crankset? Will I be needing to do any mods to the front derailleur settings or increase chain length, etc?

Other info on my bicycle's setup:

  1. Rear Derailleur: Microshift M21 REAR DERAILLEUR 6/7 SPEED LONG CAGE (https://www.microshift.com/models/rd-m21l/ )
Max Cog: 28-30T
Min Cog: 13-14T 
Capacity: 43T
  1. Front Derailleur: MEZZO FRONT DERAILLEUR 3×7/8 (https://www.microshift.com/models/fd-m20/)
Chainring Count: 3x Triple 
Mount Options: Band Clamp 34.9 (31.8 / 28.6 adaptor) 
Cable Route: Dual Pull 
Chainstay Angle: 66-69° 
Capacity: 18T
Top Gear Teeth: 42T 
Crankset Compatibility: 42-34-24T 
Chain Line: 47.5 mm, 50 mm
  1. Freewheel: SHIMANO TOURNEY TZ Multiple Freewheel 3x7-speed (https://bike.shimano.com/en-EU/product/component/tourney/MF-TZ500-7.html)

  2. New Crankset: SHIMANO TOURNEY FC TY501 (https://bike.shimano.com/en-EU/product/component/tourney/FC-TY501.html)

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    You will need a longer chain (unless you have enough "reserve" in your existing one) and your front derailleur needs to move up to fit the new set. I'm not sure if you will need a new front derailleur or if you can keep the old one, though.
    – Burki
    May 16, 2023 at 10:37
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    Rear derailleurs have a max capacity which is the difference of teeth between biggest and smallest chain ring plus the difference between biggest an smallest cog at the rear. The rear derailleur needs to have enough movement to keep the chain tight both in case big-big and small-small is used. With your new crankset the difference at the front is increased by 2. In my experience such a small amount is never a problem even if you exceed the limit given by the manufacturer, but the tolerance on chain length might be tighter. Make sure to check it before riding the first time to prevent damage.
    – linac
    May 16, 2023 at 11:34
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    Do you climb steep hills at all ? You're loosing your lowest gears.
    – Criggie
    May 16, 2023 at 23:15
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    @Criggie no, I do not use it much for climbing, this is just my daily commute to office 🙂
    – Akay
    May 18, 2023 at 3:27
  • @linac per the specs of my rear derailer, it has a supported capacity of up to 43T (I have updated the info on the question), and per the formula you mentioned the capacity after the crankset upgrade will be ((48-28)+(28-14)) = 34, so it under the supported capacity of 43T. So it should work, right?
    – Akay
    May 18, 2023 at 3:36

1 Answer 1

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As mentioned in the comments - chain length will likely need to be increased, but new chain rings I would be installing new chain and cassette anyway. Check rear derailleur capacity is not being exceeded. There is some wriggle room here if you avoid cross chaining and take responsibility for consequences of doing so.

The front derailleur has a maximum chainring size. If the derailleur is specified for 48tooth, you will need to adjust it. If this is possible depends on the mounting system used, and may require a new cable. Typically pull ratio for a mountain front is different o pull for a road front, and MTB derailleurs have a max chainring around 44tooth, with road being needed for 48 tooth.

The frame clearance may not be enough for the larger chain ring. if this happens you really have no choice, you cannot do it.

Without knowing bike make/model, drive train components (and therefore specs), its impossible to say if it is a bolt in swap or following the garden path of incompatibility hell. Most manufacturers are conservative with their component speciation's, so it is often possible to go a couple of teeth more (or less) without too much ill effect. You can end up with a finicky drive train that is hard to tune.

Often the best option is to learn to spin the pedals faster.

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    Does rear derailleur have any effect here? ...thinking it through, only the chainrings are changing, so the overall capacity of rear mech doesn't change. OP's just adding ~4 more teeth to both max and min chainring, and the middle one is between them so not relevant. Thoughts ?
    – Criggie
    May 16, 2023 at 23:27
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    42-24 to 48-28 means he is adding 2 teeth to required capacity.
    – mattnz
    May 16, 2023 at 23:35
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    Unlikely it won't work with 2 tooth difference. Might get some rubbing in crossed gears etc. Check max chainring spec of FD. then look at other issues - main one being chain ring - chain stay clearance.
    – mattnz
    May 18, 2023 at 3:54
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    Can't tell you. It depends on chain stay length and derailleur cage size along with how much slack was in the chain.
    – mattnz
    May 21, 2023 at 4:06
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    @mattnz alright so I didn't had to change the chain length, the long cage rear derailleur took care of that easily even with a 48T chainring. But I had to switch to a new front derailleur as the old one supported only 42T max and made noise on different combinations. Now I upgraded the FD to SHIMANO FD-TX800-TS6.
    – Akay
    May 24, 2023 at 12:27

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