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  1. #1
    Must... ride... more... Phil_gretz's Avatar
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    Chain Rub Outer Chairing From Inner Chainring

    Uggh. Sorry I don't have photos available...

    I'm savaging an early 80's Fuji Grand SE that had a worn out Fuji-branded Sugino crankset. The type with a five-fingered swaged outer chainring. I dug out a three-fingered Sugino Maxy (52 x 40) of the same era and mounted it to the original BB spindle, after removing and repacking the BB. The freewheel is 6-speed SunRace (blech), and the chainline appears centered (that is, the large to 3rd highest cog appears straight, as does the small-to-3rd lowest cog).

    The chain is a new KMC Z-series 6/7-speed.

    So here's the problem: When in the inner chainring, the chain rubs against the inside of the larger chainring, when the rear is in the highest (outermost) cog, and when in the neighboring 2nd highest AND 3rd highest cogs.

    The inner chainring is removable, and there are original spacers. I'm thinking that changing the spacers so that there's no rubbing in the 2nd highest - that's the right solution. Meaning that I'll allow the chain to rub the outer chainring when fully cross chained, as a reminder to the future owner not to cross chain.

    I haven't measured yet, but I'm thinking that this might be a move of about 2-3mm inboard for the inner ring. So, I'm creating a gap between chainrings (I'll need to check whether the chain can get trapped between shifts).

    My other option is to swap on a different crankset with smaller difference...

    Thoughts on allowing the crosschain to rub, or should I move the inner chainring further?

    Thanks, PG

  2. #2
    Squeaky Bottom Bracket uluchay's Avatar
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    Seems to me that your problem (presuming you use square tapered BB) lies in the spindle length of the bottom bracket. If you pick a BB with a larger spindle length it will not rub.

  3. #3
    Senior Member
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    It sounds like a chain line issue. it's likely that your crankset is inboard compared to the position of the cassette.

    There are two common causes. Either your spindle is short for the crankset and the crank is too far inboard, or your frame is bent and the rear triangle is shifted to the right.

    Start with a basic diagnostic by placing a straight edge against the face of the outer chainring on a secant, and carrying it back to the freewheel. Allowing for the distance from the outer face, to the center of the space between the rings, your straight edge should meed the freewheel about 7mm or so to the right of center.

    If it doesn't line up that way, find out whether it's the spindle or frame, by checking each, measuring the chainline from the frame, and the frame via the string method (do a search if you don't know how these are done).

    Two other possibilities.

    If the initial crank to freewheel test passed, then your rings are too closely spaced for the chin, ie. a 7s crankset w/ a 6s chain. Or if it failed, but the chainline and frame are correct, then it's possible that the BB shell is twisted with respect to the frame's plane. This is very rare, but not at all unheard of.
    Last edited by FBinNY; 04-09-14 at 11:01 AM.
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    An ounce of diagnosis is worth a pound of cure.

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  4. #4
    Must... ride... more... Phil_gretz's Avatar
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    Gentlemen,
    Thank you for your feedback. So, it seems that my assumptions about correct chainline must be tested and measured. Thank you for the discussion.

    By using the former Sugino spindle, I probably do have the chainrings too far inboard. Measurements will tell. Fortunately, I do have a number of steel spindles, so will likely have one that's a few mms longer, but the measurement will tell me how much more I need. Thanks again.

    I'm familiar with the string method for checking chainstay/dropout symmetry.

    We'll get this right yet! Phil

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