cassette change

adam62
adam62 Posts: 4
edited January 2018 in Workshop
my first post so please be gentle with me , i have only just started cycling and have only had a bike for 3 months. it is a specalized allez 2013? with thumb shifters . front sprocket is 50/34 and the chain is kmc z51. cassette is 12/25 . i have no reason to believe the bike is less than standerd. My question is , can i change the cassette to a 30 without changing anything else or is it not that simple . I am really struggling uphill. Your thoughts please .
Adam

Comments

  • Mad_Malx
    Mad_Malx Posts: 5,001
    You Will probably need a longer chain. It’s usually a good idea to swap chain if you are changing the cassette anyway.

    The other issue might be the derailleur, but this will be ok if it’s medium cage length (GS for shimano, which it probably is). It’s outside the short cage range but even then some people say it’s ok.
    You may have to adjust the b screw on the derailleur so it clears the big cog.
  • Mad_Malx
    Mad_Malx Posts: 5,001
    And welcome to the forum!
  • Mad_Malx
    Mad_Malx Posts: 5,001
    Also, (apologies if obvious) you have to keep the same number of gears (or change the shifters too).
  • milemuncher1
    milemuncher1 Posts: 1,472
    There are a couple of parameters to do with the gearing set up you have to bear in mind. One is maximum sprocket size, the other is what’s called the capacity. Different derailleurs will have differing allowable limits.

    https://www.mantel.com/blog/en/deraille ... echcenter/

    Have a read of this, it explains it.
  • Mad_Malx
    Mad_Malx Posts: 5,001
    ^The key point is that the Shimano GS derailleur (medium cage) can cope with a total gear range of 37 teeth. 50-34=16 at front plus 30-12=18 at back gives a total difference of 34, so it's fine.
    The short SS cage allows a difference of 33; you are slightly past this, but it will probably still work.
  • svetty
    svetty Posts: 1,904
    If the bike is from 2013 it will likely limit you to a 28t sprocket unless it's a long caged rear mech - and as it currently has a 12-25 I'd guess that the existing mech is an SS (short) not a GS (long).
    FFS! Harden up and grow a pair :D
  • adam62
    adam62 Posts: 4
    thanks all for your coments .
    Adam
  • pilot_pete
    pilot_pete Posts: 2,120
    A 28 tooth lowest gear will be plenty with a compact chainset, unless you are
    1. Extremely heavy and ride in very hilly terrain or
    2. Don’t plan to MTFU, ride more and get stronger/ fitter!

    All advice offered in a sarcastic, yet tinged in reality manner...! ;-)

    PP
  • tsarouxaz
    tsarouxaz Posts: 70
    wellcome to the forum. Here is the deal. is it a 10 sp, 11sp, 9sp, 8sp bike? with a 12-25 cassete, you prpbably have a short cage deraileur, so you cant' go 11-30 easily.please state on how many gears you have on your cassete and then we will see.
  • adam62
    adam62 Posts: 4
    latest up date is don't read , learn to count properly!!! Chainset was 52/39 and eight speed cassette of 12/25. This is why i have been really struggling . I have now changed the chainset for a 105 50/34 so i will see how this feels before i go changing anything else .Thanks all .
    Adam

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