Chain Line Issues

[wazzaa]

Hey

So i am upgrading the drive train on my GF's Giant Yukon from 8sp to a 9sp groupset that i have acquired. both run Octalink BBs

The original BB is a 68mm x 126mm (shimano ES25), and i assumed that i would be able to run this BB with the 9sp groupset

So after installing and adjusting new groupset, there are issues with shifting into the lowest sprockets on the rear, when in the second and third front chainrings (the chain drags noisily on the derailleur cage). This appears to be a chain line issue to me, which suggests that the 126mm BB is too wide and i need a smaller width. Would this be accurate? If yes, any thoughts on which width would be optimal? If not, is there something else i've missed that may be causing this problem?

The crankset is this or similar:

http://www.chainreactioncycles.com/au/en/shimano-acera-m361-triple-chainset-octalink/rp-prod106890

Cheers

Waz

 

[The Duckmeister]

Did you change cranks or keep the old one? Different cranks have different offsets of the chainrings relative to the butt of the crank arm, so require different length bottom bracket spindles. 8 & 9-sp. cassettes are the same overall width, so that won't be affecting chainline.

 

[wazzaa]

yeah i changed cranksets, and im pretty sure thats the problem - i envisage that i need a shorter BB spindle width. any ideas on how i get this info for this particular crankset? shimano tech data sheet didnt shed any light on the situation....

 

[The Duckmeister]

Is that M361 crank the old one or the new one? According to the archival info I have, 126mm is the correct spindle length for the Octalink M361 (M361-8 to be correct). If that's the old crank, what's the new one?

 

[wazzaa]

OK so the new crankset is actually this one (FC-M431):

http://www.bike-discount.de/en/buy/shimano-crankset-fc-m431-octalink-black-175mm-82063

In most cases, shimano seems to specify that this is a square taper bb crankset (well the tech data sheet does), whereas in fact the one i have is def octalink

Any thoughts on the spindle width?

thanks

 

[The Duckmeister]

M431 is square taper, the Octalink version is the subtlely renumbered M431-8. The tech archive says that the 126mm BB is correct for that too. However, if it's M430-8 that you have, you need a 118mm ES-series Octalink BB.