Adjusting chain line so can shift to big ring

[Phil W]

I have a square taper bottom bracket mated to a triple chainset.  The Shimsno 105 triple FD I am using does not seem to be able to shift onto the big ring due to reaching its limit (it is not touching the limit screw ) if pushed by hand. There is plenty of extra swing available at the small ring end. I am assuming this is because I have installed too wide a BB.  The 105 derailleur says it is for a 45mm chain line (from what documents I have found).   Would a good assumption be that the chain line as currently setup is more than 45mm and this is causing shifts to the big ring to stall / not happen? So I need a narrower BB by x mm? (Where x is to be determined)

[Brucey]

you can measure the chainline to find out how far out you are.

A reasonable working assumption is that the BB spindle is symmetrical, so if you need 3mm smaller chainline, you also need a 6mm shorter BB spindle. However in shimano UN series BBs, not all spindle lengths are symmetrical so sometimes neighbouring sizes don't give different chainlines in the way you expect.

If you change BB spindle manufacturer, between changes in asymmetry and slight variations in taper type, you may not get the exact chainline you expect.

cheers

[Blade]

I had a similar problem some years ago and sorted it out by removing metal from the tapers on the axle.

As the axle is hardened, you can't just file it, so I used the following method.

Remove chainring side crank from axle.
Fit a grinding stick in the chuck of a power drill.
Remove metal from all four faces of the taper.
Mark taper faces with marker pen.
Re-fit crank and check if the crank has moved sufficiently up the taper.
If the crank needs to move in more, then remove it and take more metal off from where the marker has been rubbed off by the crank (the 'high' spots).
Repeat until the crank has moved in enough.
You may need to remove some metal from the end of the axle if it protrudes far enough to prevent the axle bolt from fully pulling the crank in.

With tapers, you only need to remove a small amount of material from the faces to produce quite a bit of movement up the taper.

[ElyDave]

I have the same issue with a HT2 setup, need to shift one spacer from left to right I think, then re-adjust FD.

Chain line is measured as assymetrical, and there's a lot of chatter in the big ring as well.