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Indexed OK but stil changing by itself?

chuckcorkchuckcork Posts: 1,471
edited March 2009 in Workshop
I've just put a new chain, chainrings and cassette on the bike, and on the first ride out found that the chain wasn't wanting to stay on the cassette where it should, changing up/down on its own.

Knowing of course that this means indexing, I've set it up on the stand at home, and adjusted to where I think it should be spot on, but taking it out yesterday it was for some reason still doing that? I stopped at one point and looked at the alignment and it looked dead on, got back on the bike and 20 metres or so later it had shifted by itself, and the alignment was halfway across now to the next (larger) gear?

How does this happen? It also seems to occur at a different gear, depending which of the two chainrings I'm on. It doesn't even happen consistently when I'm on the road, just intermittently so trying to work out whether to further loosen the cable via the barrel adjuster, or if there is some other issue, is what I can't figure out.

I know the last thing I want to be accelerating hard and the having it jump all over the place!
'Twas Mulga Bill, from Eaglehawk, that caught the cycling craze....


  • maddog 2maddog 2 Posts: 8,114
    sticky cable or perhaps as you turn the bars it is stretching the cable outer and changing the tension. Check the outer isn't bent too tightly or too short.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • chuckcorkchuckcork Posts: 1,471
    Um, stretch the bars? What does this mean?
    'Twas Mulga Bill, from Eaglehawk, that caught the cycling craze....
  • chriskemptonchriskempton Posts: 1,245
    If your cable runs from the bars to the gear cable stops are too tight, then turning the bars can cause the inner to pe pulled through the outer cable and the gears could change as a result.

    If it's not that, check that your gear hanger is fastened securely with the bolts (if it's replaceable). I faffed with my gears for ages before I spotted that one....

    If it's not that, do you have inline gear cable adjusters (on the cables between STIs and frame, rather than on the frame stops like on most bikes)? Those things are a nightmare if there's not enough friction to keep them tight. Loctite required.
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