Viewing 19 posts - 1 through 19 (of 19 total)
  • Road mech (rear) help
  • muddydwarf
    Free Member

    I’m possibly buying a road bike (again…) on Saturday and i’m going for a sit-upon this time.
    Bike comes with a short-cage mech and an 11-25 cassette, i would prefer an 11-28 and have been told it will need a medium cage mech to go with it. Bike has a 50/34 compact chainset as well.
    Just checking mechs on CRC and the short cage mech will apparently take a 30T cassette, but the bit that is puzzling me is the bit about the maximum front difference being 16T,maximum 34T.

    What does that mean?

    benji
    Free Member

    Maximum front difference is the number of teeth difference from the big ring to the little ring so on a 34/50 compact that would be 16 teeth.

    The maximum is the maximum total of teeth front and rear, as the rear mech has to keep a certain tension on the chain the whole time and there is only so much swing it has.

    So 11/28 on the rear is 17 teeth, and 50/34 on the front is 16, gives you a total of 33 teeth difference, so that is within specification.

    Hope this helps.

    muddydwarf
    Free Member

    Cool, thanks for that!
    The shop are telling me i need a new mech if i switch up to an 11-28, seems like the current shortcage mech will suffice then?

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    davidtaylforth
    Free Member

    Yes, although you will have to run a very short chain and the big/big combo might not be wise

    simondbarnes
    Full Member

    It depends on the mech. AFAIK, Shimano short cage rear mechs have been designed to work with a 28t cassette since the 6700/5700 series. Relatively recently, they have been redesigned to work with a 30t.

    benji
    Free Member

    Big big should work as long as you have the right length chain, it shouldn’t be used but it definitely needs to be acheivable as that is a common way to break mech hangers.

    If you are running shimano, there tech sheets for all their current products are on line, and some of there obsolete ones as well.

    muddydwarf
    Free Member

    5700 series i think, might be better to use a medium cage mech then after all.
    Am i correct in thinking that, given Shimano STI units have a trim function then running a big ring/big sprocket combination is OK so long as you have the correct mech?

    geordiepaul
    Free Member

    I’ve got a short cage mech with 11-28 and it works fine. Came like that from new.

    davidtaylforth
    Free Member

    Cross chaining looks awful, I cant imagine its good for your gears. But if you want to run big/big more comfortably, then I reckon the chain will be too long for small/small

    benji
    Free Member

    Big Big should be avoided to use as it puts undue strain on the chain, and the same gear is available by going small to mid.

    Short will work based on the numbers you have provided, but would leave you with no real futher options, but if you can’t get up it on 34/28 it might be time to think about putting it on your shoudler and running.

    muddydwarf
    Free Member

    I don’t like cross-gearing on the mtb, so was surprised to find the trim function. Not something I’d consider really but thought it must have been designed to work that way, why else the trim function?
    Anyway, it was the mech/cassette ratio i was puzzled about, think that will save me £50!

    TiRed
    Full Member

    Mine came with 12/28 and 50/34 on a short cage 6700 Ultegra. I run 23,25 and 28 and haven’t changed chain length. I also treat the compact as a 1×10 with a bailout, and to be honest, that 25 with the 34 is all you need for most climbs.

    What have you found and is it in the size we suggested?

    mboy
    Free Member

    Shimano Tiagra/105/Ultegra short cage mechs from the 4600/5700/6700 groupsets (10spd) are all fine with up to 30T cassettes. You will not need a longer cage mech. Previous generation short cage mechs were OK up to 27T, but would need a medium cage for anything larger.

    What happened to the Colnago?

    muddydwarf
    Free Member

    The colnago wasn’t a good enough fit i’m afraid. Got a shorter stem and reduced the reach to where i wanted it but then found i couldn’t get the bars low enough.
    Found a Colnago sizing guide that was markedly different from CRC’s and spoke to CRC about it, they agreed to take it back. Annoyed about that because it was such a lovely bike 🙁

    EDIT:
    It’s a MEKK Potenza SL 5.0 from EBC for £1099, down from £1399 and in a 44cm size. Not too sure about it TBH because it has almost the same TT as the Colnago but we’ll see how it fits.

    FuzzyWuzzy
    Full Member

    Yeah sounds like the bike shop is working off 2 year old info, short cage will handle a 28 fine now. As has been said, avoid big/big due to the additional wear/strain it puts on the drive train not because it might rub the front mech cage.

    TiRed
    Full Member

    I find 50×28 rubs, even with trimming. 50×25 and 50×23 are fine, hence use as 1×10 with a bail out gear option.

    MD those mekks look big. The 44 has an ETT of 52.7 cm. My youngest son rides an XS Defy with an ETT of 51cm. He’s 5’1″. You are looking At the equivalent of a Small Defy.

    mboy
    Free Member

    By the looks of things, if you’ve got the extra £300, CRC have got the Ultegra version of the bike you just sent back in a 45cm frame…

    muddydwarf
    Free Member

    WHAT?????

    mboy
    Free Member
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