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Old 10-20-09, 05:05 PM   #1
CCrew
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47.5 versus 50mm chainline

Looking to replace a cheapo Shimano M191 crankset that spec's at a 47.5mm chainline with a Shimano LX that spec's at a 50mm chainline. The M191 uses a JIS BB versus the Hollowtech II on the LX, so I really don't have the option of different BB widths. Is that 2.5 mm of chainline going to cause me issues and if so what should I anticipate?

I'm swapping from 8 speed to 9 speed including shifters (LX Dual Control) and Front (XT) and rear (Ultegra) derailers at the same time if that matters.

Thanks..
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Old 10-20-09, 06:21 PM   #2
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See "Chainline Measurement" : http://sheldonbrown.com/gloss_ch.html#chainline

MTB Triple is generally 47.5mm, but may be larger at 50.0mm if necessary to clear oversized seat tubes.

According to Sheldon, 47.5 is preferred if the frame can handle it. In this case, Shimano has mandated a 50.0 chainline; I assume so it can be more universally compatible.

Every other bike that has this--and similar--crank and bottom bracket combo work fine, so I imagine yours will, too. I doubt you'll notice anything.

All that will happen is that each chainring will now be moved out just over one-half of a cog's worth (the center-to-center spacing between 9-speed cogs is 4.34mm) http://sheldonbrown.com/k7.html#spacing
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Old 10-21-09, 05:47 AM   #3
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Originally Posted by CCrew View Post
Looking to replace a cheapo Shimano M191 crankset that spec's at a 47.5mm chainline with a Shimano LX that spec's at a 50mm chainline. The M191 uses a JIS BB versus the Hollowtech II on the LX, so I really don't have the option of different BB widths. Is that 2.5 mm of chainline going to cause me issues and if so what should I anticipate?

I'm swapping from 8 speed to 9 speed including shifters (LX Dual Control) and Front (XT) and rear (Ultegra) derailers at the same time if that matters.

Thanks..


The only real issue ..... Depending on which rings you ride on most, a 50mm chainline may be better, or worse for drivetrain wear due to the angles of the chain.
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Old 10-21-09, 06:16 AM   #4
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Because the external LX BB is made for both 68mm and 73mm, you can play with the washers to set the chainline to either 50mm or 47.5mm. If you have measuring calipers it's not hard to figure it out the correct combo.
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Old 10-21-09, 05:53 PM   #5
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Because the external LX BB is made for both 68mm and 73mm, you can play with the washers to set the chainline to either 50mm or 47.5mm. If you have measuring calipers it's not hard to figure it out the correct combo.
Thanks. I found the Shimano Techdocs, looks like I do have some wiggle room. And yeah, have a nice set of Mitutoyo calipers
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Old 10-21-09, 06:46 PM   #6
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Because the external LX BB is made for both 68mm and 73mm, you can play with the washers to set the chainline to either 50mm or 47.5mm. If you have measuring calipers it's not hard to figure it out the correct combo.
Bringing the right crank in will push the left crank out an equal amount, right? Is Hollowtech II centered already?

If re-arranging washers does make the whole assembly off-center, I'd want to weigh that against getting the desired chainline.
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Old 10-21-09, 07:25 PM   #7
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I just installed a Truvative Stylo on my Bergamont, the chainline required me to tighten the fd cable and adjust the stops a little to compensate, but it works excellent. I did not want the cranks offset to one side.
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Old 10-22-09, 07:47 AM   #8
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With a 2.5mm shift/offset and a seat height of 30" above the pedal, what's the angluar difference?...Using HS trig I get a angular difference of 0.4 degrees (87* on non-drive, 87.4* on drive side). I don't have an issues with high Q cranks, I suppose this is why the 0.4* isn't noticed at all by me (it is less than 0.5% difference).
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