Triple to compact...

Redhog14
Redhog14 Posts: 1,377
edited August 2011 in Workshop
Having inherited a road bike with a triple I feel the need to go to a compact double as I never use the granny ring, the set up is forever needing adjusted and I think the teeth are a wee bit worn.

Question - Sora triple to Shimano Sora compatable compact double - is there anything else involved or can I just do the swap and trim the front derailleur?

Comments

  • Your sprockets are maybe too big then. Fit something like an 11-23 cassette and you might use your granny ring.
  • Assuming that you actually want to have only 2 rings at the front and not 3 then you will need to change your bottom bracket to move the chainline closer to the frame.

    Although your left shifter is capable of running a triple you can limit it to 2 by adjusting the front mech limit screws. If there are gear indicators they might show something different than before but if it changes gear okay then just ignore the indicators.

    I too had a bike with a triple 53/42/30 and never used the granny ring so instead of swapping to a compact double I just went for a 53/42 double when I built up my next frame. I do have a 50/34 on another bike but I prefer the closeness of the gears on the 53/42 more.
  • Redhog14
    Redhog14 Posts: 1,377
    Thanks - am looking to simplyfy things by only having a double at the front, so could you clarify what you mean by changing the chainline?
  • With a triple the centre chainring would line up with the centre rear cog so the length of the bottom bracket axle keeps it spaced correctly.

    When you only have 2 chainrings the length of the bottom bracket axle needs to be shorter otherwise the chainrings are pushed out away from the centreline.
  • Redhog14
    Redhog14 Posts: 1,377
    With a triple the centre chainring would line up with the centre rear cog so the length of the bottom bracket axle keeps it spaced correctly.

    When you only have 2 chainrings the length of the bottom bracket axle needs to be shorter otherwise the chainrings are pushed out away from the centreline.

    Ok thanks so does the BB axle - assuming I am using a 2 piece chainset - do this for you? Or are there spacers involved, the BB is and external unit
    My experiece on this to date is having changed the BB bearing SRAM GXP external cups on my CX bike.
  • keef66
    keef66 Posts: 13,123
    With the external bearing / 2 piece chainset there's no issue with the BB axle length.
    A Shimano road double with the corresponding BB cups will fit a 68mm bottom bracket shell with no spacers.
    (The equivalent triple would have a longer axle and so has a spacer each side)
  • Yes it does, because the cranks go onto the axle as far as they can so the longer the axle the further out the cranks are and so too are the chainrings.

    I don't know the exact figures but looking on CRC they range between 107mm and 127mm with the lower ones being for doubles and the higher for triples.

    I have never yet had to change one with BB30 so maybe they are different because the axle comes with the chainset (I believe?) and presumably they are then the right length. But if your bb is square taper or octalink or similar then you need to get the right sized one.
  • John.T
    John.T Posts: 3,698
    Question - Sora triple to Shimano Sora compatable compact double - is there anything else involved or can I just do the swap and trim the front derailleur?
    Basically yes but you may need to change the front mech as triple ones sometimes do not work well on a double. Not sure why but they just don't.