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  1. #1
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    New question here. chainline confusion

    I always thought bikes were designed to provide for a perfectly straight chainline
    between the middle cassette cog and middle chainring. I just assumed this to be the case.
    So, while tinkering with my bike over the last few days I decided to actually measure
    the respective cassette cogs and chainring chainlines. This is what I found.

    chainrings: 28-36-46
    28: 43.5 mm
    36: 49.5 mm
    46: 55.5 mm

    cassette: 12-34
    12 61.36 mm
    14 57.02 mm
    16 52.68 mm
    18 48.34 mm
    20 44.00 mm
    23 39.66 mm
    26 35.32 mm
    30 30.98 mm
    34 26.64 mm

    WTF:
    The 36t chainring falls between the 16t and 18t cassette cog.
    The 28t chainring is almost a perfect fit for the 20t cassette cog.
    I have measure this again and again. The numbers don't lie.
    I'm pretty sure this applies to all modern mountain bikes.

    Anybody care to elaborate.

    The reason I bring this up is that I went 1x9, believing the middle chainring provided
    a perfect chainline for accessing all cassette cogs. Such is not the case.

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  2. #2
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    it will take a while,but I will measure mine up.

    I eyeballed mine and it looks like it is one cog smaller than middle, to the middle chain ring.

  3. #3
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    just supposing you measured correctly from the center

    of the middle chainring to the middle of the bottom bracket, what is the spindle length of the bottom bracket and what length does the frame maker suggest. The common chainline I believe is 47.5 so it should be close.

  4. #4
    1946:2006:2066
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    Quote Originally Posted by ol-crank
    of the middle chainring to the middle of the bottom bracket, what is the spindle length of the bottom bracket and what length does the frame maker suggest. The common chainline I believe is 47.5 so it should be close.
    It all depends...
    The bottom bracket is ISIS 73x113 with Race Face Turbine cranks.
    The chainline is as I measured above. ~50mm.
    A 68x108 would give you a ~47.5 chainline but wont fit.

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  5. #5
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    FireDog 46, just curious, how did you make your measurents down to the hundredth of millimeter and what method did you use for front and rear?

    I don't think you can make a case for all mountain bike chainlines based on your single example in any case; the goal is still a straight chainline between middle ring and cog for most. Parts spec can play a role, especially with the newer external type cranksets. What are the frame and crankset details in your case?

    ol-crank, fyi the frame maker doesn't determine the spindle length to use, the crankset and chainline do (if you see a spec for a spindle length in a manufacturer's spec, it should be considered only with the crankset those specs provide). Mostly it's 47.5mm with the older cranksets, 50mm with the newer external types but it varies somewhat.

    Good info here about chainline www.sheldonbrown.com/chainline.html
    "...the people get the government they deserve..."
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  6. #6
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    my question was worded wrongly, yes it is the frame

    [QUOTE=Bikinfoolferlife]ol-crank, fyi the frame maker doesn't determine the spindle length to use, the crankset and chainline do (if you see a spec for a spindle length in a manufacturer's spec, it should be considered only with the crankset those specs provide).

    maker that designs the frame with a certain chainline in mind, but it is the BB & crank that determines the chainline. I was just asking if the frame maker specd's a 47.5 or a 50 mm chainline.
    I do wonder though about the dimension to the center cog. Shouldn't it match up with the spec'd chainline?

  7. #7
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    While it might seem ideal to have a straight chain between your middle chainring and middle of the cogset, it may not be the right chainline for you or your bike. If the bike is used mostly in the granny and middle chainrings, then perhaps a wider chainline up front is more advantageous for you in the gears you run the most (in that the chain will be straighter in the gears mostly used). You may simply need a wider chainline up front to give your front derailleur clearance to work properly. Even if you follow the frame spec for a chainline, were the cranks the same ones in mind at the time of design? The rear cog positions aren't adjustable in most cases, either, so it's always a compromise with chainline measured only at the chainrings...
    "...the people get the government they deserve..."
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  8. #8
    1946:2006:2066
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    Quote Originally Posted by Bikinfoolferlife
    FireDog 46, just curious, how did you make your measurents down to the hundredth of millimeter and what method did you use for front and rear?

    I don't think you can make a case for all mountain bike chainlines based on your single example in any case; the goal is still a straight chainline between middle ring and cog for most. Parts spec can play a role, especially with the newer external type cranksets. What are the frame and crankset details in your case?

    ol-crank, fyi the frame maker doesn't determine the spindle length to use, the crankset and chainline do (if you see a spec for a spindle length in a manufacturer's spec, it should be considered only with the crankset those specs provide). Mostly it's 47.5mm with the older cranksets, 50mm with the newer external types but it varies somewhat.

    Good info here about chainline www.sheldonbrown.com/chainline.html
    Chainring chainline was easy...measure from the left side of the seat tube to the
    centre of the teeth of the middle chainring. Subtract half the diameter of the seat tube.

    Cassette cog chainline was problematic. Measure from the centre of the teeth of the
    middle cog to the inside of the axle dropout. Subtract from 67.5, ie: 135 divided by 2.
    Gave me 44.0 mm for the middle cassette cog. The rest are +4.34 or -4.34.
    No way could I measure to hundredths of an inch. It's simple math.

    The frame is an RM ETSX. The bb is 73x113 with RF Turbine LP 110 BCD cranks.

    The new 50mm standard aint so new.

    And yes...three cheers for Sheldon Brown...great site.

    edit:
    and after doing all this I stumbled across this
    http://www.parktool.com/repair/readhowto.asp?id=74

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  9. #9
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    So are you having shifting problems?

    Quote Originally Posted by FireDog46
    Chainring chainline was easy...measure from the left side of the seat tube to the
    centre of the teeth of the middle chainring. Subtract half the diameter of the seat tube.

    Cassette cog chainline was problematic. Measure from the centre of the teeth of the
    middle cog to the inside of the axle dropout. Subtract from 67.5, ie: 135 divided by 2.
    Gave me 44.0 mm for the middle cassette cog. The rest are +4.34 or -4.34.
    No way could I measure to hundredths of an inch. It's simple math.

    The frame is an RM ETSX. The bb is 73x113 with RF Turbine LP 110 BCD cranks.

    The new 50mm standard aint so new.

    And yes...three cheers for Sheldon Brown...great site.

    edit:
    and after doing all this I stumbled across this
    http://www.parktool.com/repair/readhowto.asp?id=74

    michael
    That doesn't sound all that bad. A 5mm difference between the front and back is not that big, the difference in chain angle for a given ratio as compared to an absolute perfect chainline will be tiny. Chainline isn't really something that is investigated until problems can be traced to it or a frame has undergone re-alignment. If your shifting is working fine, then you are simply searching for a solution to a problem that doesn't exist.

    -R

  10. #10
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    Quote Originally Posted by Unemployed_mechanic
    That doesn't sound all that bad. A 5mm difference between the front and back is not that big, the difference in chain angle for a given ratio as compared to an absolute perfect chainline will be tiny. Chainline isn't really something that is investigated until problems can be traced to it or a frame has undergone re-alignment. If your shifting is working fine, then you are simply searching for a solution to a problem that doesn't exist.

    -R
    No problems shifting...sometimes it seems a little klunky in the largest 3 or 4 cogs.
    This was merely an exercise satisifying my curiosity. I know...it killed the cat.
    The thing is, you never hear about cassette chainline. So what's the effect of
    45, 47.5, 50 for the front? And it seems everyone assumes the middle cog
    lines up with the middle chainring. Which is not the case for most mtb bikes.

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  11. #11
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    Quote Originally Posted by FireDog46
    No problems shifting...sometimes it seems a little klunky in the largest 3 or 4 cogs.
    This was merely an exercise satisifying my curiosity. I know...it killed the cat.
    The thing is, you never hear about cassette chainline. So what's the effect of
    45, 47.5, 50 for the front? And it seems everyone assumes the middle cog
    lines up with the middle chainring. Which is not the case for most mtb bikes.

    michael
    I think some people also assume that everything must be perfect to work well. Most MTBs work best with a 47.5-50mm chainline; that's just the target range when putting on a given crankset and BB. BB shells often aren't perfectly 68mm or 73mm, or symmetrical and a certain crank may press slightly further onto a BB spindle then another one. There's always a little room for error; modern bushingless chains are much more flexible laterally than older bushing chains, so alot of chainline error can be absorbed by the chain itself.

    If Shimano or Sram made the frame, hub, cassette, BB, crankset and the chain, then maybe you would see a perfect chainline to within a fraction of millimeter. With all the brands of components and frames, there's just too many variables to get everything that accurate and, as you can see, largely unnecessary.

    -Ryan

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