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Chain Line Question

Old 07-06-20, 09:35 AM
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Sjtaylor
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Chain Line Question

I just finished installing an outboard bearing bottom bracket spindle, the one with three spacer washers. It’s on the Trek 720 frame I’ve been posting about. It has a 68mm long bottom bracket.

You're supposed to put two of the washers on the drive side. I notice that the chain line at the chain rings is outboard of the chain line at the cassette. I also notice that crank arms are not the same distance from the chain stays. The drive side crank arm is further away from its chain stay.

The chain rings seem to be plenty far away from the frame so why shouldn’t I move one or two of the bottom bracket spacer washers to the non drive side to improve the alignment? I realize that with the longish chain stays that excessive cross chaining probably isn’t a concern. Mostly just like to take advantage of a learning opportunity.

Thanks.
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Old 07-06-20, 09:52 AM
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frankenmike 
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I absolutely would do(and have done) a spacer shuffle to improve chainline.
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Old 07-06-20, 10:17 AM
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Do you know that it is different than before? Though the middle of the rear stack of cogs seems most natural, bike makers might move it over some for various reasons. One of which is what cogs do they think will be used most of the time.

Chainline on the front is based on the mid point between the outer most chain wheels on the crankset. If the extension of that plane to the back is not even hitting the cog stack in the back, then yes you need to do something. But if it's only a few mm from the mid point of that stack, then it might be okay. Particularly if that is the side of the stack you use the most.


https://www.parktool.com/blog/repair...nline-concepts

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Old 07-06-20, 11:33 AM
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Sjtaylor
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I watched the Park Tool video on Chain Line. The original bottom bracket had loose ball bearings, a 1984 bottom bracket. I wish I had measured the chain line before I modernized the bottom bracket. Right now the front chainrings are inline with the smaller cogs on the cassette and the drive side crank is more outboard than the other. I’ll be touring this bike and I guess I’ll be hugging the larger cogs on the cassette.

im getting pretty good at removing and installing bottom brackets so I’ll move some washers around. I installed the used bottom bracket from the donor bike and it’s a bit crunchy. When I get the new bottom bracket I’ll do it.
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Old 07-06-20, 12:58 PM
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Are you installing a MTB crank on a bike with road bike chainline in the rear? In that case it'd make sense. In truth you'll probably be OK mismatching the chainline, but you certainly could rearrange the spacers also.
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Old 07-06-20, 02:19 PM
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Iride01 
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Some rear hubs can be moved over by changing out the spacers on the axle too. So a combination of spacers on your crank and rear axle might get you something better. My new bike seems to have one spacer on each side cut to the specific length, so it leaves the mix and match aspect of moving the hub over a little more difficult.

Still I'd try out what it is currently and see if it's an issue for real use. We can armchair all sorts of chainline horrors till proven otherwise by actual use.
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Old 07-06-20, 04:28 PM
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It would help if you gave more details with numbers. What is:
  • offset between the left and right crank arms
  • chainline at the midpoint between your double chainrings or the middle chainring of a triple set
  • dropout width
  • cassette speed
  • cassette chainline
  • Do you have a mountain or road crank/spindle? I assume you put on a mountain BB since you used spacers on a 68mm shell? So I assume you put on a mountain crank?

If you have 130mm dropout, then the standard road chainring chainline for double is 43.5mm and triple is 45.0mm. If you have a 135mm dropout, then the standard MTB chainline for triple is 50mm, however if you look at the Shimano framebuilder manual, the middle chainring is actually at 49mm. Also, if you compare road to MTB, the extra 5mm in dropout width would be 2.5mm to the drive side, so theoretically you could also use 45.0+2.5 = 47.5mm chainline for a MTB triple if you want to reduce your Q-factor a little bit.

Supposedly an 8 speed cassette has a chainline somewhere near 43.5mm on 130mm dropout (but this would vary by the hub manufacturer and model), but as you increase the number of cogs, the chainline of the cassette will move further inward since the cassette gets wider.

Front and rear chainline doesn't have to match exactly. A couple of millimetres probably doesn't matter unless you have very short chainstay.

If you want symmetric crank arms, then you can try removing a spacer with the BB, but check your spline engagement at the left crank arm to make sure it doesn't bottom out, and also that the compression bolt doesn't bottom out too.
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