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  1. #1
    Dirt Bomb sknhgy's Avatar
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    How do I find out the chainline measurement for a bike?

    The Park website has a section on chainline maintenance. In it, they tell how to measure the chainline on your bike. But how do you find out what the chainline measurement should be? The bike I'm working on is a 2011 Rockhopper.
    In other words, I can find out what the chainline actually measures, but how do I find out what it should measure? The measurement I'm talking about is the distance from the centerline of the bike to the center of the cassette. And the measurement from the center of the bike to the center of the midddle chainring. The bike I'm working on has a 3 ring system.
    Actually, I think I just answered my own question. If I measure the chainline distance at the cassette, that measurement should match the chainline measurement at the chainrings. I can change the measement at the chainrings - by changing BB spindle length - to match the measurement at the cassette.
    Last edited by sknhgy; 10-14-11 at 09:43 PM.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  2. #2
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    The "correct" chainline will put the center line of the middle ring on a triple crank exactly in line with the center of the cassette which is the middle cog of an odd number (7 or 9-speed) and between the cogs spanning the center on an even number (6 or 8-speed) cassette or freewheel.

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    I'm curious- Why do you need the chainline measurement? Do you think it's changed? Are you having shifting or noise issues? Are you changing some components and trying to get the right combination of parts?

    Following Hillrider's explanation... if you have this- you have the correct chainline.

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    You find the chainline by looking up the manufacturer's information at their website. Any recently made crank will have the service instructions posted.

    A triple's chainline is measured to the tip of a tooth on the middle ring. Use a precision rule and measure from the side of the seat tube, then add half the seat tube diameter. For a road bike, that's usually in the 45-47.5mm range.

    There should not be any need to measure the chainline on such a new bike.

  5. #5
    coprolite fietsbob's Avatar
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    Measurement base, is the centerline thru the bike frame.

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    Quote Originally Posted by fietsbob View Post
    Measurement base, is the centerline thru the bike frame.
    True but establishing the centerline of the seattube is not easy by inspection. DaveSSS's procedure uses an easy to determine hard point for the measurement.

  7. #7
    Dirt Bomb sknhgy's Avatar
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    Quote Originally Posted by reddog3 View Post
    I'm curious- Why do you need the chainline measurement? Do you think it's changed? Are you having shifting or noise issues? Are you changing some components and trying to get the right combination of parts?
    I need the chainline measurement because the LBS upgraded the chainrings due to a problem the bike was having. In doing so they changed the chainline. The shop is not close by and they are rather a pain to deal with so I'm fixing it myself.


    You find the chainline by looking up the manufacturer's information at their website. Any recently made crank will have the service instructions posted.
    The LBS put a Shimano Octalink on the bike. Do you know where I can find the manufacturer's information?
    Do I look up information on the bike or the Shimano crank?

    Thanks for your patience.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  8. #8
    Dirt Bomb sknhgy's Avatar
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    I measured the chainline to be 44.45mm. I'll be changing the stock 68 x 118mm bb cartridge for a 68 x 126. That should move the chainline out 4mm, closer to the 50mm standard for mountain bikes.
    I hope this works.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  9. #9
    Happy go lucky trevor_ash's Avatar
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    Edit: I just saw you posted that you figured things out (didn't know if you were road or mtn or what). Glad you got it sorted out! Here's my post anyway.

    I understand why you're asking the questions, they're very intelligent questions to ask. But with modern bikes it all comes down to standards. Your bottom bracket shell is most surely 68mm. You mention a Shimano Octalink crank (and thus BB). For doubles the BB has an axle length of 109.5. For triples it has an axle length of 118. The intended chainline for a double is 43.5mm. The intended chainline for a triple is 45mm.

    If you try to take it any further than this you're just going to make your head explode. The entire system is designed such that the chainline is whatever it's supposed to be per the spec. Don't try to guess what it will be, because if it's not then somethings not right

    Note: Chainlines can be different with campy/shimano/whatever.

    Now that you know what it SHOULD be, and using DaveSSS excellent technique for measuring the chainline, measure it. See how close you are. If you're within +/- 1mm you're rock solid. If you're off, make sure the BB, cranks, and chainrings are all correctly installed. If you're still off, you could have the wrong BB.

    I'm honestly not sure how close is close enough. I suppose those with more experience working on multitudes of bikes has a good number to share.

  10. #10
    Dirt Bomb sknhgy's Avatar
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    The mechanic at the bike shop switched the chainrings. He admitted that the Shimano Octalink put the chainline closer to the frame, but he said he adjusted it so that it worked acceptably. I don't entirely agree with him but I got tired of going back there. The mechanic said that he and the shop owner discussed putting in a spacer or a longer bb, but for some reason they did not. The reasons I want to move the chainline out are:
    1) I can't adjust the low limit screw enough to keep the chain from rubbing on the derailler cage when in low gear.
    2) I can't get a smooth shift when going from the small to the middle chainring. I have to shift way past the middle indent in order to get the chain to go on the middle ring.
    I've heard two figures for the intended chainline for a triple on a mountain bike - 45mm and 50mm.
    My "experiment" with putting on the 68 x 126mm bb will put the chainline out close to 50mm.
    It should also allow me to properly adjust the low limit on the FD.
    I figure for the cost of the cartridge, about $16, it's worth a try.
    If nothing else this will be a learning experience.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  11. #11
    Low car diet JiveTurkey's Avatar
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    It sounds like the entire "crank" was replaced, not just the chainrings. The crank manufacturer will determine the proper BB length. Get whatever BB you're supposed to. I'm surprised the LBS wouldn't have suggested selling you this extra item.
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  12. #12
    Dirt Bomb sknhgy's Avatar
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    Quote Originally Posted by JiveTurkey View Post
    It sounds like the entire "crank" was replaced, not just the chainrings. The crank manufacturer will determine the proper BB length. Get whatever BB you're supposed to. I'm surprised the LBS wouldn't have suggested selling you this extra item.
    Yes, I'm pretty sure the entire crank was replaced. I am still learning about this stuff, so bear with me.
    All the work was done under warranty. The lbs said the bike works, even though an inexperienced person like myself knows the chain shouldn't rub - and rub hard - the derailler cage.
    The mechanic admitted that the chainline should probably be moved out.
    It all comes down to the old saying; If you want something done right, then do it yourself.
    And in all reality I do value the learning experience, although I feel the product I was sold is sub-standard.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  13. #13
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    Just for some testing. Measure your existing chain rings and see if you have one at the 50 mm chain line spacing. If you do shift to this ring and then shift your back gears and see if you have any rubbing on the front derailer. This should give you some good information about where the new chain line needs to be located.

    Replacing the BB may be tough unless you have a vice or the right tools to take out the BB and put the new one in. I think that these BB's take a good bit of torque.

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