triple to double

fatfreddy
fatfreddy Posts: 332
edited August 2010 in Workshop
Hi
my son-in-law has a Tiagra triple (50,39,30) on his road bike. He wants to fit a standard double (53/39). Can he use the triple front mech or will he need to get a double?

Thanks

FF

Comments

  • sungod
    sungod Posts: 17,583
    use a double front mech - a triple is much taller, it'd foul on the 39 ring
    my bike - faster than god's and twice as shiny
  • keef66
    keef66 Posts: 13,123
    How come? It currently works with a 39 ring.
  • Mister W
    Mister W Posts: 791
    I was about to write some waffle about front mech capacities but then I checked the Shimano website. The problem you're going to have is with the size of the big chainring. The triple front mech will only cope with a ring up to 50T.
  • sungod
    sungod Posts: 17,583
    keef66 wrote:
    How come? It currently works with a 39 ring.

    the triple mech is designed with contact areas for the triple rings, with the triple's spacing

    to use it on a double you'd need to use it on the bits for big and middle - always asuming it doesn't simply choke on the 53t as mister w points out

    the rings on a triple are closer than on a double...

    going big to middle the indexing of the shifter will move the mech enough for a triple, not enough for a double (fit a double front mech and it now takes two strokes of the shifter to change rings, call one of them 'trim')

    so the mech contact areas are closer, and the shifter will move them either not far enough, or too far

    grind, crunch etc. etc.

    maybe you could play games with the limit screws to force the correct indexing, but the narrower mech spacing will still cause it to rub i think

    it's just too horrible to contemplate
    my bike - faster than god's and twice as shiny
  • sungod
    sungod Posts: 17,583
    fatfreddy wrote:
    Hi
    my son-in-law has a Tiagra triple (50,39,30) on his road bike. He wants to fit a standard double (53/39). Can he use the triple front mech or will he need to get a double?

    Thanks

    FF

    btw i should have mentioned...

    it will also need a new chain if the old one is too tight on the big ring - for shimano rear mechs, with chain on 53 ring and smallest sprocket, the axes of the jockey wheels should be vertically aligned

    now, if the chain is very worn it'll have worn the cassette too, which would then skip with a new chain, then it'd need a new cassette as well

    for how to measure chain wear, go here and scroll down a bit...

    http://sheldonbrown.com/chains.html#wear
    my bike - faster than god's and twice as shiny
  • fatfreddy
    fatfreddy Posts: 332
    thanks all for your help. I'll tell him to get a new bike...

    FF
  • sungod
    sungod Posts: 17,583
    fatfreddy wrote:
    thanks all for your help. I'll tell him to get a new bike...

    FF

    it isn't that big a job even if the other bits need changing, but on the other hand you can never have enough new bikes!
    my bike - faster than god's and twice as shiny
  • Cleat Eastwood
    Cleat Eastwood Posts: 7,508
    I did the same with my allez, triple to double, the trple mech works fine. just fanny about with the H/L limt screws.
    The dissenter is every human being at those moments of his life when he resigns
    momentarily from the herd and thinks for himself.
  • keef66
    keef66 Posts: 13,123
    It was just the "it'd foul on the 39 ring" I was puzzled by, since it's already plopping the chain on / off a 39 ring.

    I can believe that a 53 chainring might be out of spec for the mech according to Shimano, but moving the mech up a bit should give you something that works acceptably well. Since the LH shifters can be set up for triple or double, I thought the cable pull and hence the ring spacing must be the same. Getting the chainline right could well be the most difficult bit.

    So all in all, the new bike seems like the safest bet.
  • fatfreddy
    fatfreddy Posts: 332
    Thanks Guys.

    I found a Sora double on CRC for £13.

    Chainline is easy - it's a Tiagra external BB, so just need to remove the LH spacer that is fitted for triples.

    He got a very nice S/H Trek 1.5 triple - (he's come to cycling from rugby) - having watched the TdF with me, the bike is now not fast enough for him. Still - he'll learn some basic fettling.

    FF
  • John.T
    John.T Posts: 3,698
    The geometry of the triple mech is different to the double. It is to do with cable pull and the different position of the rings. These differences are not great and it will work, just not as well and it needs more careful setting up. Front mechs are not expensive though.
    As above you may need to fit a longer chain. If it will change easily to the big/big combination you should be OK. The chain must bend considerably round the jockey pulleys.
    My Fleet.

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