Vexing front mech prob

clanton
clanton Posts: 1,289
edited August 2008 in Workshop
Having an issue with my 105 front mech. It started on a ride whilst shifting down from big ring to little ring (Compact 50/34 chainset) when I heard a loud crack. The chain froze momentarily then dropped into small ring and seemed fine but I could then not shift into the large ring again. I assumed it was a slipped cable or the mech itself shifted and as I was near home finished the ride.
However the cable seemed firmly attached and the mech alignment fine. After a lot of fiddling and increasing cable tension and loosening off the outer limit screw I managed to get it to shift up, whereupon it threw the chain outside the big ring! I corrected this with the limit screw but then found it wouldn't shift down!
After further adjustments I ended up undoing the lower limit screw completely AND literally taking the cable off totally and it still won't shift down unless I push the mech across. Its as though the range of motion of the mech has become impaired somehow. I have checked mech height and the alignment and this seems fine.
So - anything else I can do? Or is it LBS time or even new mech time?

Comments

  • MIsterGoof
    MIsterGoof Posts: 128
    could be the ratchet in the STI is FUBAR. I had it happen on my rear STI
  • clanton
    clanton Posts: 1,289
    The mechanism certainly is stiff - I thought this was due to the mech but you may be right.
  • peanut
    peanut Posts: 1,373
    I'd dissconnect the front mech cable and try the lever function seperately with no load.
    Also check that the front mech return spring hasn't unwound a turn which would reduce the strength of the front mech return to the inner ring position . That could account for the crack you heard :roll:
  • clanton
    clanton Posts: 1,289
    peanut wrote:
    I'd dissconnect the front mech cable and try the lever function seperately with no load.
    Also check that the front mech return spring hasn't unwound a turn which would reduce the strength of the front mech return to the inner ring position . That could account for the crack you heard :roll:

    Thanks will try this. Having thought about it I think the prob is with the mech as when I took the cable off it didn't spring down to the little ring. How do I check the return spring?
  • peanut
    peanut Posts: 1,373
    clanton wrote:

    Thanks will try this. Having thought about it I think the prob is with the mech as when I took the cable off it didn't spring down to the little ring. How do I check the return spring?

    Its a very strong spring almost too strong to operate with your fingers and you can see it by taking the mech off and looking underneath. If the spring has gone you'll need another mech I'm afraid as the mechs are riveted together and non -servicable generally.
  • clanton
    clanton Posts: 1,289
    Thank. Had a feeling it was a new mech job :-(
  • peanut
    peanut Posts: 1,373
    clanton wrote:
    Thank. Had a feeling it was a new mech job :-(

    If there is no spring resistance when operating the mech then I'm afraid it looks that way.
    Although it is possible to replace the spring on some mechs its hardly worth it because of the faffing around and delay. new front mechs are surprisingly cheap actually. Have a look on Ribble Cycles or Sprockets

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