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I have a Zee crankset on a 83mm bottom bracket (downhill bike). The Shimano manual says to use two 2.5mm spacers on the drive side, and one 2.5mm spacer on the non-drive side. However, this presumes the rear hub has a 150mm spacing, typical for downhill bikes. My bike has a 142x10 hub, when I use the spacers as specified, the chainline is completely off, I can't even get the chain on the largest cog. Since I'm not using a 150mm hub I obviously need to change the spacers, but I have no idea how to figure out how many spacers of what size I need on each side?

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2 Answers 2

83mm bottom brackets were designed specifically for use with a 150mm rear dropout width. You would be able to shim the bottom bracket across 5mm to bring it to where it would sit on a 73mm bb to align the chain line but you would be pushing your non drive out 10mm which would have an anatomical impact on the rider. This is a significant amount of q-factor imbalance and could quick become an uncomfortable bike to ride.

The simplest solution is to swap the crankset for a 73mm version.

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It's possible to use a 73mm crankset with a 83mm bb? Don't know if the imbalanced q-factor would be noticable on a DH bike, where there isn't any uphill pedalling? –  Nicolas Mommaerts Jun 30 at 7:07
    
Sorry didn't pick up that it was a 83mm bb shell, thought you were asking about 83mm crank shimmed out on a 73mm shell. Whats the bike? I've not heard of anything with a 83mm shell having a 142mm dropout? Any off balance that much could throw balance off. Its a bodge anyway, not ideal. –  DWGKNZ Jun 30 at 8:09
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Chainline difference is going to be 7.5mm between a 83mm 1x and a 68/73 mm 1x. 7.5mm is quite large, what about swapping 1 of the 2 spacers on the drive side to the non-drive side to reduce it to 5mm and try that? –  DWGKNZ Jun 30 at 8:20
    
It's a Banshee Darkside with swappable dropouts, I choose the 142x10 because those are the wheels I had. Didn't know I was getting myself into trouble :) I'd figure they would know how to build it but the designer didn't seem to know the correct answer himself (forums.mtbr.com/banshee-bikes/…). Indeed, I'll try swapping –  Nicolas Mommaerts Jun 30 at 9:00
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Nice bike, hope you get it sorted. –  DWGKNZ Jun 30 at 9:18

The best solution I found so far are chainring spacers: http://sheldonbrown.com/chainline.html

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