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I recently moved to a city with a lot of steep hills so I'm looking to change my 11-28 cassette to this Shimano Tourney 14-34. I have a Shimano Tourney rear derailleur with the same number of gears (7).

Since the biggest gear (34) in the new cassette is a lot bigger than the one I currently have (a 28 teeth) I was wondering if I could still use my old derailleur.

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    It will depend on the derailleur you have. There are a couple different derailleurs that carry the "Tourney" label so knowing the exact model would be helpful in determining the maximum tooth capacity.
    – Kibbee
    Jun 7, 2017 at 20:14
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    A photo of the rear derailleur from a few angles would really help.
    – RoboKaren
    Jun 8, 2017 at 7:28

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OK, from my research, and the other answers, this are the steps you have to take to change your cassette:

  1. Determine if you have a freewheel or a cassette? This is a distinction that I initially ignored. It is important because you need different tools to remove and install these, so be careful.

  2. Find your derailleur model, it is usually engraved in the back of it (mine is a Shimano Tourney RD-A070) and google the maximum sprocket that it supports. For my derailleur, it was a 28 teeth gear, but I managed to make it work with a 34t. As always, manufacturer specifications are a little conservative.

  3. Adjust the b-screw in your rear derailleur: here is a How-to tutorial

    . Doing this you'll be able to adjust it for a bigger sprocket.

  4. Using the model of your derailleur, google its capacity. The capacity is the difference in teeth from the gear in which the chain is loosest (the smaller chainring and the smaller rear sprocket) to that in which it is tightest (the biggest chainring and the biggest rear sprocket).

If the capacity of your rear derailleur is lower than your actual capacity, I think you can solve this problem by avoiding ever cross-chaining.

This is what I have been able to come up from zero knowledge about this, so, please, correct me if I am wrong in any of the above.

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That depends on a few things. The chain will more than likely need to be longer, so you should plan on changing that. The next question has to do with the length of your cage. A short cage rear derailleur will not support a cassette with 34 teeth. It may be hard to determine if yours is a short cage or mid cage. I would suggest looking at pictures of derailleurs with different cage lengths, as I am not aware of there being any markings on the piece itself.

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    To add: The B screw needs to be adjusted so that there's clearence between the largest sprocket and the deraileur cogs.
    – Alex
    Jun 7, 2017 at 21:14
  • Good point. Since you are working on that end of the bike, might as well fully realign the derailleur as well.
    – CRoberts
    Jun 7, 2017 at 21:17
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    Rule of thumb for a road bike - its a mid sized mech unless you look at it and think "wow thats small" or "wow that's long"
    – Criggie ♦
    Jun 8, 2017 at 0:18
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Derailleur: You don't need to change your derrailleur, it will be just fine.

Chain: You only need to change your chain in two situations:

  1. You've ridden more than 2000 km with the last one. That would mean it will not properly grip to the teeth on the new cassette. The old combination worked because the cassette and chain have the same amount of wear, thus having the right shape of teeth for the old chain.
  2. As pointed out by @CRoberts, there is the chance of your old chain being too short for the new big ring. In this case, just change the cassette and see if it works.

I do have a road bike equipped with Shimano Claris, and recently did basically the same conversion as you want to. Claris rear derailleur is shorter than Tourney, and my new chain (as both cassette and chain were extremely worn) is even shorter than the original. Worked like a charm in the small chainring. In the big chainring I have to be careful to not use the biggest cog, as it makes the derailleur extremely stretched.

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  • Both Claris and Tourney have rear mechs that come in different sizes. More than likely the stock derailleur is a mid length, but there is a chance it is a short. A short cage would not be able to shift into a 34 tooth cog.
    – CRoberts
    Jun 8, 2017 at 16:49
  • TX55 works with 34 teeth, but I could not find about TX35 or TX75, although one reviewer at amazon said it worked with a TX35 RD. I am betting it works, but yeah, a definitive answer would need checking this.
    – Emílio Dolgener Cantú
    Jun 8, 2017 at 17:16
  • Unfortunately with Shimano, at least at the Tourney level, is that there are so many variations. The Tourney line covers the TX as well as TZ rear derailleurs. however, after doing a little more digging, it appears that all of the TX models you mentioned have a max capacity of 34 teeth, so I could be incorrect about needing a derailleur change. I would still recommend a chain swap though, as new cassette and old chain could cause accelerated wear.
    – CRoberts
    Jun 8, 2017 at 17:22

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