[abbike18]

i recently went to a single ring up front. when im in my lowest gear, its pretty obvious how much the chain is actually going sideways to make it all the way in to that rear cog. so i got to measuring and found out that a 50mm chain line is stupid. if you know otherwise, please let me know.

this is all referring to the below diagram. as you can see, the front ring is 50mm from the middle of the bike (aka, a 50mm chainline). a standard rear end is 135mm between the droputs, meaning that each drop out is 135/2 = 67.5mm from the center of the bicycle. if you draw the ideal chainline (perfectly straight) back from the front ring, you find that this intersects the rear at 67.5-50 = 17.5mm from the rear dropout. it would make sense that this "perfect chainline" would be when you are in the middle cog on the cassette. a standard 9 speed cassette has its middle cog about 23mm from the dropout, meaning that a 50mm chain line is about 5mm too far out.

So i want to fix this on my bike. shimano has you put in a 2.5mm spacer between the bb cup and the bb shell. can i move this spacer to the non-drive side to move my chainline in 2.5mm? anyone have any experience with this or any wisdom to add?

 

[crohnsy]

Go for it. it will work. The only thing you should be aware of is you non drive arm being 2.5 mm further out.

Don't think you would notice that difference but some might

 

[Lelandjt]

I use the BB spacers to get my cranks centered. It would bother me if one crank was closer to the centerline of the frame than the other.
Is the chainring on the middle position? Could you put spacers between it and the crank tabs to move it inward?

 

[PhilipW]

Good reading from an official source:
http://www.parktool.com/repair/readhowto.asp?id=28

This should answer most questions.

Of course, "chainline" for a crankset is entirely dependant on design, manufacturing tolerances, etc etc.

 

[lew s lugnut]

Shimano specs say the chain line should be 47.5-50mm. This is purely designed around multiple chain rings, a derailleur and a 68 or 73 bottom bracket shell. The tolerance of 2.5 mm is to allow for 68 or 73 bb and slight manufacturing differences from different companies. Obviously to have enough room (for chainstays, f. der. movement, multiple chain rings, tire clearance, crank clearance with chain stays, etc....) they need to have some offset from being centered with the cassette.

I would try the chain ring spacers first, to keep the crank arms even and centered.