Advice - Double chainset to triple

s.woody
s.woody Posts: 181
edited May 2009 in Workshop
Evening all. I'm looking for some advice regarding the changing of a double chainset to a triple. At the minute I'm running a 10 speed 105 compact double chainset but like I say, thinking of changing to a triple.
If I did make the change, what parts would also need changing? Would I need a longer rear mech to handle the greater variation in gears? Also what about the front mech, would that need changing too?

Any advice would be greatly appreciated. Cheers.

Comments

  • geoff_ss
    geoff_ss Posts: 1,201
    You need to work out the range of teeth in your new set up and see if your rear mech will accommodate it. IIRC Shimanos web site provides the capacity of all their rear mechs.

    In my case I use 44/32/22 chainset and a 13/25 cassette. So the rear mech needs to be capable of coping with 22 tooth difference at the front plus a 12 tooth differnce at the cassette ie 32 teeth in all. In my case a Shimano 105 RD-5600-GS rear mech more than copes with its total capacity of 37 teeth. You also need to check that the it will also cater for the biggest and smallest sprocket on the cassette.

    You'll also need to check that both your front derailleur and the gear change lever are triple compatible. Also that the ring sizes are appropriator to the front mech. You can check on the Shimano web site here http://bike.shimano.com/publish/content ... index.html

    I suspect your deraileurs will be OK if you're not going to extremes. *sigh* it was a lot easier before all this new-fangled indexed gear shifting. :lol:

    Geoff
    Old cyclists never die; they just fit smaller chainrings ... and pedal faster
  • andrew_s
    andrew_s Posts: 2,511
    You have to bear in mind that Shimano won't say something is OK unless it works almost perfectly. If you are willing to put up with "generally works OK" or to avoid using the small/small combination, you can get away with stuff Shimano won't recommend.

    a) New chainset.
    There do exist replacement middle chainrings that convert a standard double to triple (Thorn, Stronglight), but I'm not aware of any that would fit a compact double.

    b) Shifters. Is your front shifter triple compatible?
    If it isn't you'll need new shifters. Triple compatible STI will have 3 clicks (giving 4 positions for inner, middle/large cogs, middle/small cogs, outer).

    c) Front mech. (this will have to be a road mech - MTB ones are no good)
    Triple mechs have a deep inner plate to lift the chain off the granny, and a longer cage for a chain running closer the the chainstay. You may find that staying on the large half of the cassette keeps the chain high enough not to drag across the end of the cage. I'd be inclined to try the existing mech out before spending money, but this may depend how handy your bike shop is if it doesn't work.

    d) Rear mech
    Road mechs don't work on sprockets larger than 30T, and 30 may take a bit of fiddling.
    Other than that it's a question of capacity. You need a chain long enough to fit on big/big, but if you avoid using the small end of the cassette with the small chainring you can push the capacity quite a way.

    e) BB
    If you've got an old fashioned square taper, Isis or Octalink BB, you'd ideally want a slightly longer BB. I'd probably only bother if the small ring was too close to the chainstay.
    Your new chainset may also impose new BB bearings.


    Note for anyone thinking about whether to get a double or triple on a new bike:-
    It's much cheaper to convert triple to double than it is to go from double to triple. All you have to do is unbolt the small ring, and possibly new middle and outer.

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