What size rear mech

carbon337 Posts: 414
edited June 2011 in Workshop
Think I'm gona get a new rear mech - Tiagra one possibly but SS or GS?

For an 8 spd 12-25 cassette

And a triple front chainset 30-42-52 (i think - not going out to shed in the rain to check either)

I presume a 9spd rear mech will still work ok with an 8spd cassette and 8spd shifters as the indexing distances are controlled by the shifter and not the mech.

Thanks for the advice ...


  • Chris James
    Chris James Posts: 1,040
    I am pretty sure that the Shimano tech docs will tell you that you need to have the long cage derailleur (forget whether this is SS or GS).

    However, my 52/42/30 12-25 triple came specced with a short cage 105 rear derailleur and it it works fine. There is obviously a risk that bad cross chaining could explode the mech but I have never found it a problem!
  • bompington
    bompington Posts: 7,674
    About this bigyeah_that_big.jpg
  • Capacity required = (52-30)+(25-12)=35

    Without checking I think SS=29, GS=37

    So GS it is.
  • carbon337
    carbon337 Posts: 414
    Thanks Rothers i never understood the capacity number.

    GS it is then.