I have always built my own bikes, but the one thing I have never really understood is how you go about getting the correct chain alignment between the rear casette and the front rings. The only option I am aware of for adjusting alignment is to add the shim between the bb and the frame so as to push the chainset away from the frame.

My question regards how do you know what the correct alignment should be ?

Thanks

I wondered the same. Never sure if I’ve pushed the spindle in far enough.

middle gear front and rear perfectly in line would make some sense

50mm from centre line of bb shell to middle chainring (on an mtb)
Sheldon knows ALL.

What if double chainrings front does 2 or3mm really matter

given that most of the time you won’t be in middle / middle, I think as long as the chainline isn’t massively compromised it won’t matter anyway. Barely worth shimming out axles or BB’s to compennsate. Different on a s/s where you don’t have a deraileur to straighten things up again.

As long as it isn’t so far off that you can’t use the extremes without severe derailleur rubbing I don’t fuss too much on it.