I have a Giant TCX cyclocross bike with shimano 105 components, all except a bontrager ISIS drive cross crankset. The bottom bracket I have is a 113mm, and I am having a problem where I have my front derailleur set all the way to allow it to extend to the furthest limit, but I still need to shove the shifter very hard in order to make the shift to the outer ring (where it makes a sluggish shift and sometimes rubs on the cage). I noticed that there is also a commonly available 108mm bottom bracket... would putting in a shorter BB help with my shifting issues? Or would the 5mm less not make a difference?
I'm not familiar with your crankset, is it a double or triple. A chainline of 43.5 mm would be correct for a double. I think the Sheldon Brown website has a paragraph on chainlines and how to measure them.
The crankset is a standard road double fitted with cyclocross rings. I measured the way from the sheldon brown article and came up with about 47.5-48mm, but according to him the correct chainline should be 43.5 like you said... I also noticed that when in the large ring, the chain only appears to be straight when in the outermost three rear cogs.
Should I assume from this that my chainline is "wrong" and that I need the shorter BB? If this is the case could it be the root cause of my front shifting isssues?