Q - Road Cassette ???

bunnerscj
bunnerscj Posts: 396
edited February 2009 in Road beginners
Hi all,

I have a Trek 1.7 2008 compact bike with a rear 11-25 cassette.
I want to upgrade the cassette to a 11-28 SRAM cassette.
Can the rear mech take the extra tooth on the SRAM cassette. Think the mech is a short cage shimano 105 version. I have read some reviews that state the short cage mech can take up to 27 but will it cope with 28 teeth ?
Thanks
'We go up we go down, this is bull sh*t yar'

Comments

  • crabstix
    crabstix Posts: 61
    I have a SRAM Rival short cage running cassettes with both 27 and 28 teeth at the top end. That said, I have a compact chainset, so I imagine the limitation will come if you have more than 50 teeth on your big ring.
  • bunnerscj
    bunnerscj Posts: 396
    50 teeth on front big ring, Ultegra short cage rear mech - will the 28 th cassette fit ???
    'We go up we go down, this is bull sh*t yar'
  • alfablue
    alfablue Posts: 8,497
    I fitted a Sram 11-28 to my girlfriends Trek 1.2, it works fine. The mech is a medium cage, but I think the cage length pertains to the triple ring range and it should work fine on a 28 with a short cage. The Shimano specs are rather conservative. B screw and / or Chain length may need to be adjusted to get the jockey wheel to clear the big cog.

    (I subsequently changed it again to a 32 sprocket - she was still struggling, and then I needed to fit an mtb mech).
  • John.T
    John.T Posts: 3,698
    The mech will take a 28 tooth cog with careful 'B' screw adjustment. Your problem will more likely be the total range you are asking the mech to take if you have a 50/34 compact. Shimano say that the short cage mechs will take a total of 29 teeth which is 50/34 and 12/25 or 5036 and 12/27. They will actualy take 50/34 and 12/27 with careful setting up. You will be asking the mech to take up 2 teeth more of chain and I think this will be pushing it too far. A medium cage mech will be fine.
    My Fleet.
  • maddog 2
    maddog 2 Posts: 8,114
    officially, Shimano road rear mechs are only rated up to 27t

    However, the latest DA groupset (7900) has a 11-28 cassette option so maybe this is changing...

    Anyway, in practice, on most bikes you can get a road mech to handle a bigger sprocket by adjusting the B screw on the mech body. This pulls the mechs downwards, away from the sprockets and so gives you more room.

    If you read the various threads, some people have actually got away with a 30t on a road mech - clearly the hanger length or something has an influence here as not everyone can achieve this.

    As for cage length, this will have NO EFFECT on whether the mech can actually run on a 28t sprocket without rubbing it. It is only relevant, as said, to the total range of gears the bikes can handle. Again, in practice, most people don't ever run small sprocket - small ring so it's not going to be a problem. I run 34/48 and a 12-27 on a short cage mech with no problems and I'm pretty sure it would handle a 50t ring too. But then I don't run small-small.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • mostly harmless
    mostly harmless Posts: 162
    It will, most likely, be fine.

    My hack bike uses a 11-32 mtb block and an ancient short cage ultegra mech. I get perfect shifting across all the gears although I do have one ring up front.
  • John.T
    John.T Posts: 3,698
    50/34 with 12/27 is the largest range you can SAFELY use with a Shimano short cage mech. Any more and the chain will be too slack on small/small or worse too tight on large/large. I know you should not use these gears but you never know, you may forget one day. Personally I use them but not to excess, especially the 34/12 as this gear is not available on the 50 ring and at 74" is useful. 50/27 is only used if I missjudge a hill or my legs.
    My Fleet.

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