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From that OneUp guide link:
Boost 148 was created to give frame makers more design freedom for tire and front derailleur clearance. From a drivetrain perspective it moves the cassette 3mm away from the frame centerline. The new ideal chainline for Boost bikes is 51-53mm.
To increase the chainline by 3 mm you should buy a BB with spindle 6 mm longer than before. 110 mm before means 116 mm now.
As you have said, the only way to be sure would be to remount the BB onto the old frame, tighten down the crank again and measure chainline. It's a 10 minute exercise including removing everything again. You don't even need to put the left BB cup in. If you find 49 mm then you know you need 52mm chanline for the new bike.
Note: The 68 mm vs 73 mm shell width doesn't make a difference to chainline, only the length of the BB spindle matters.
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Ndeipi
Larmen
Bottom bracket length question
I'd like to move the chainset from this Orbea Team MX 24: https://99spokes.com/bikes/orbea/2019/mx-24-team-usa to an Orbea Laufey 27 H20: https://99spokes.com/en-GB/bikes/orbea/2022/laufey-27-h20
Bottom bracket on the Team MX 24 (9-speed) is 68/110 - removed the crankset to measure today.
Laufey (10-speed) has something that seems to look like a SM-BB52 external bottom bracket that will need swapping to a square taper - and the shell width is 73 mm.
Now to my conundrum - MX24 is 135 mm rear spacing, while the Laufey (the bike getting the chainset) is boost 148 mm.
To get proper chainline I've tried following this OneUp guide but can't for the life of me figure out what spindle width I should purchase for the bottom bracket to go in the Laufey boost bike. Can anyone assist with the maths here?
TLDR; Chainline solve x: 135mm 68/110 = 148mm 73/1xx
EDIT: I do realize I should've measured the chainline when the chainset was still on the small bike - but missed this. If this measurement is essential I can get it but would of course prefer not to remount everything again :)