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Old 06-20-08, 07:25 AM   #1
T800
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Chain (?) noise using middle ring of GXP triple

Last year, my tourer/commuter gained a 48-36-26 Truvativ trekking chainset (and a GXP bottom bracket). I'm at a loss to explain the noise levels, though, when using the middle ring. The first instinct is to trim the front mech (Shimano 105) a little but when you hear it begin to come into contact with the chain (SRAM 9-speed, SRAM 34-11 cassette) you realise that's a different noise.

The noise does seem to "pulse" a little with each pedal downstroke, almost as though a bit of flex under power was causing the chain to rub the front mech, but, as I said, it's definitely not the same noise as occurs when I deliberately adjust the front mech to allow it to begin to rub the chain.

Any ideas what might be causing the problem?

To be honest, though the noise levels are less on the outer ring, and there's no pulsing nature to the sound of the transmission on that ring, I do think the bike is quite a bit noisier than it used to be. It originally came with a square taper Shimano BB and unbranded road triple.
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Old 06-20-08, 12:23 PM   #2
T800
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Chris Juden has offered two thoughts:

1) It could be the chainline is wider on this chainset than the old one, see my comments on the new XT groupset in the current mag. I know that should make the outer ring even worse, but people don't tend to use the outer except with the top few outermost sprockets. Whereas middle gets used across the whole cassette, and is pedalled hardest when the innermost hill-climbing gears are engaged. And the trend to wider front chainlines has worsened the alignment of those very gears.

2) The middle ring will have more tooth modifications to facilitate shifts in both directions, up and down. These modifications reduce the amount of chain tooth contact when straightforwardly driving, which accordingly feels rougher.

I'm inclined to go with number 2, because extreme chainlines (if you can have them on the middle ring of a triple) do not seem to alter the noise level appreciably.
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