How critical is crankset chainline?
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  1. #1
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    Question How critical is crankset chainline?

    Only recently did I find out that the chainline of my 2x sram crankset is 47.5mm. Its the wider S952 crankset built for disc frames, but I have it installed on a rim brake frame (with everything else being shimano tiagra 4700).

    Long story short, all gears shift fine, but in extreme cross chaining — only in big-to-big, it makes some noise due to the more extreme angles.

    I wanted some of your opinions on whether or not I should go fix this. I have spacers on hand, but fixing it would mean tearing most of the drive train out and re-tweaking a lot of stuff.

    How bad is it gonna be for the components if I leave it be? I never really use a big-to-big chainline anyways.

    Thanks


    EDIT: PS: the correct chainline for 2x road setups according to sheldon brown is 43.5mm. So I'd have to move the chainrings 4mm inwards. Sounds like a lot. Will it have a big effect on symmetry and riding comfort?

  2. #2
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    You say you have no shifting problems or front derailleur rub? And there's only noise in the extreme cross-chain gears? Then for all practical terms, it's not an issue. I'd still fix it if for no other reason you may be missing some other benefit that's not immediately tangible. And yes, I think 4mm is a lot.

  3. #3
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    How sensitive are you to Q-factor? Moving the chainrings 4mm inboard also moves the crankarm inboard those 4mm's while also moving the left 4mm outboard and only you know how your knees and hips are going to feel about this.

    Myself, I'd rather have everything centered 'specially considering that there are no shifting problems, unless the spindle can be replaced with a shorter one keeping everything centered I'd leave it as is.

    Like Peter P said 4mm is a lot, would you rather your transmission deal with it or your knees?
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  4. #4
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    I'd fix it, You may have noise with extreme gears, but another issue that may be overlooked is the chance on tearing off your rear derailleur. With chain line issues often the rear derailleur is really stretched, and a lot of stress on it.

  5. #5
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    I would solve this problem in the simplest way which is don't use big-big. You don't need it. Why make this more difficult than it is?
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

    “Statistics are like bikinis. What they reveal is suggestive, but what they conceal is vital.” -- Aaron Levenstein



  6. #6
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    Quote Originally Posted by oberwil View Post
    I'd fix it, You may have noise with extreme gears, but another issue that may be overlooked is the chance on tearing off your rear derailleur. With chain line issues often the rear derailleur is really stretched, and a lot of stress on it.
    Thanks for the reply + advice. I certainly see validity in your concerns and appreciate the advice, but in the end I've ultimately decided to just let it be for 3 main reasons:

    1. Everything shifts fine when I need it to.
    2. I never really use big-to-big in the first place, and due to the fact that the chainrings are too far out, I have less crosschaining when running big-to-small (and even small-to-small), while at the same time having no issues with small-to-big either.
    3. Being an industrial design student, Ive learned a lot about the importance of small ergonomic balances in the long run. The bike cost a little over 1500 USD, which is cheap for a carbon bike, and I would much rather lose the bike than my articular cartilage lol..

    Thanks!

    PS: If one is really anal about the derailleur hanger in the first place, I believe one is supposed to replace it every year or so in the first place.

  7. #7
    'brifter' is a lame word.
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    Quote Originally Posted by hihihi8 View Post
    PS: If one is really anal about the derailleur hanger in the first place, I believe one is supposed to replace it every year or so in the first place.
    I'm interested why you think this.
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  8. #8
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    Quote Originally Posted by cxwrench View Post
    I'm interested why you think this.
    I've read that metal fatigue weakens the aluminum hanger over time, in worst cases leading to the RD just snapping off while riding. Dont have a source tho

  9. #9
    'brifter' is a lame word.
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    Quote Originally Posted by hihihi8 View Post
    I've read that metal fatigue weakens the aluminum hanger over time, in worst cases leading to the RD just snapping off while riding. Dont have a source tho
    Aluminum does fatigue but it generally will involve being bent and the number of flex cycles. If that doesn't happen there is no fatigue. There is zero stress on a derailleur hanger in normal use.
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  10. #10
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    Quote Originally Posted by cxwrench View Post
    Aluminum does fatigue but it generally will involve being bent and the number of flex cycles. If that doesn't happen there is no fatigue. There is zero stress on a derailleur hanger in normal use.
    Exactly. Generally derailleur hangers can be bent a couple of times before they fatigue. Mine has been bent back at least three times. However, I did know someone who bent their hanger and it snapped when their shop tried to bend it back. It was the first time it was bent. I guess it depends on the hanger.

    There are some budget bikes, usually older, that don't have a hanger. The derailleur is directly attached to the frame. I have a friend who bent their frame when it fell over. The first shop she took it to told her it was unsafe to bend it back and would not do it. She took it to a different shop who bent it back for her, but told her if it happens again, it wouldn't be safe to bend it back a second time.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

    “Statistics are like bikinis. What they reveal is suggestive, but what they conceal is vital.” -- Aaron Levenstein



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