Hi Ian!
I feel your pain...getting the right replacement bottom bracket for the ideal chainline with a new crankset is sometimes a bit of a black art; so much depends on the offset of the spider from the crank arm itself -- and the individual bike and all the parameters related to it -- tube diameter, the width of the chainstays behind the BB, knobby tire clearance; the factors are myriad.
It sure isn't any fun ordering multiple bottom brackets in the hope of getting it right.
I usually do one of two things:
• If I already have a crankset and the chainline is correct -- and I wish to replace that crankset and bottom bracket with a new one of different design -- then I measure the BB length I have, take the difference with the new (recommended BB/crankset combo) and then I know the offset of the crank arm/spider compared to what I have and can use that information to choose the right BB spindle length.
• If I am going for new and have never fitted a BB or crankset before, then things get iffy in a hurry. Sometimes, as a double-check, I'll rob one of the crank/bb sets I own and test fit it, checking chainline to see how much it is off and how much shorter or longer the BB would need to be, then do the subtraction route to determine the offset on a new crank and hone in on what I need from there.
As for measuring chainline, it sure helps to have a wheel of the proper axle spacing ready to hand with the necessary cassette or freewheel already mounted. That takes care of the rear and you can choose which cogs you wish to optimize for chainline. I dearly love(d) half-step and granny gearing (say, 50/45/24 and 5-speeds or 48/45/24 and 7-speeds), and found it optimal to set things up so
the split between middle and high chainrings on my triple crank hit the dead-center of my cogset. The innermost ("granny gear") would only be used with the bottom 3 or so cogs anyway, so it was pointless to set the triple crankset up so things aligned with the middle ring. With careful selection, my most-used gearing combinations all had near-perfect chainlines with little deflection and so were very long-lived. It also meant most of the dealer, online vendor, and factory chainline recommendations were out the window for my needs.
To measure chainline the easy way, just measure from the right side of the seat tube to the desired point on the crank, then add half the seat tube diameter to get to the "center". It sure helps when one doesn't have a bolt placed dead-center for reference (lucky you!
I was never that fortunate
).
Where things get really wooly is the way manufacturers report their BB spindle lengths. Some spindles are of equal length on each side, while others vary left-to-right...but only overall length is reported. This can really upset the apple-cart. I always check the manufacturer's site first and if the information is not there, then the email contacts begin so I can be assured of getting the right length. Most manufacturers are very pleased to help and even seem a little flattered by the interest. It is to their benefit to get a good fit and happy customer as well.
I guess a longer axle might have allowed the crank to clear the cage at the expense of a less than optimum chain line...I sorted my wife's bike by judicious use of Molegrips.
This may have been the best you could hope for with this particular derailleur/crank/BB combo, Ian, so a "well done" from me. Adding length to the right side of the spindle likely wouldn't have helped, as the cage would still have to center over the large 'ring and the space between the spider and back side of the crankarm is sparse. This was he bane of my existence when I tried to pair old T/A Cyclotouriste and Sronglight 49D crankarms with the newer wide-cage front mechs that came out on the 1980s. Sometimes, I had to mill a small groove on the backside of the arm to pick up the millimeter or two of needed clearance.
And...sometimes, a person can rotate the front mech around the seat tube a smidge and come out alright as well.
Best,
Dan. (...who used to shorten and re-weld front mech cages when he made his own compact 38/34/16, 13-34 drivetrains in the late-'70s/early-'80s using freewheel cogs as innermost chainrings)
