What I don't understand is why we're adding and subtracting these displacements only once, rather than twice, once to each side.
A very good point, Andre, and one with direct Impact (sorry) on your application.
I made the adjustments only once -- because I assumed for calculations' sake the bottom bracket would be conventionally asymmetrical, with the left-side extension holding fairly constant and the length adjustments being made on just the right. If the left side is held constant, then with an asymmetrical BB, any length adjustments must take place on only the one (right) side to get the needed chainline.
If you value symmetry in your tread -- or, as in your case, your frame requires additional offset on the left so that arm and your left heel will clear the wide chainstays needed to accommodate the wider balloon tires you're running -- then a second adjustment must be made for the other side (I was in a similar situation with the full-sus Folder I'm building from scratch; the folded rear triangle rests lying forward across the BB, requiring a spindle wider on both sides to clear it).
In that case, to get everything to clear properly, then -- yes! -- two
adjustments will need to be made, and you will have to calculate for the left
side as well. Given all components are JIS-dimensioned, I would measure the left offset of your present BB, take the difference between it and the one Stonglight is using for reference, and add that figure to the overall length we came up with, making sure the right side stays where needed (it will likely be listed as a "symmetrical" BB). That should do it. To recap: I think you'll need ~120mm to get the single inside-mounted chainring centerline mounted to a 54mm chainline with an asymmetrical BB. Add to that whatever length you need to get the left arm out far enough to miss the chainstay (which implies a symmetrical BB, most likely a 124mm*) and you'll be fine. You once mentioned you wanted a wide "tread" or Q-factor, so it might be worth punching out the left side for that reason alone.
Or, you could just use a chart.
Sheldon Brown's site has one here that shows symmetrical equivalents ( http://sheldonbrown.com/bbsize.html
), but please note
: Sheldon's chart lists interval measurements for traditional cup and cone
BBs. Those interval measurements will be off for cartridge units but the overall length recommendations and distinctions between symmetrical and asymmetrical types should still hold
. Sheldon also rightly cautions,
Substituting the 'symmetrical equivalent' bottom bracket will preserve the position of the right crank and chainwheel, but the left side will often wind up farther out...These spindles are getting harder and harder to find.
I suggest, if the proper symmetrical BB cannot be found, go with a longer right-offset and then shift the cartridge leftward using Phil Wood rings or place spacers under the left retaining cup. It is a bit of a bodge, but works if the needed difference is <4mm or so.
[Later edit] *Confirmed! Looking at Sheldon's chart, Andre, he lists a 120mm asymmetrical in the "D-Length Overall" column. The symmetrical equivalent is 124mm, as I suggested above. Remember, Sheldon's measurements are for cup 'n' cone units, but the overall length and proportions should be okay. You can still identify a symmetrical cartridge BB; just ensure the spindle is symmetrical by checking that the "A" and "C" measurements past the cartridge bearings are equal (or nearly so). 'Might pay to first measure what you already have. By happy accident, you might achieve the same same chainline with the new crank on the old BB.
I hope this helps, Andre. I love this stuff, and am happy to talk about it further if you'd like.
if you suddenly see an Irishman with a French name pedalling a highly polished crank reading "Dan Wood"...
That means I'd finally see Lovely Ireland in some way!
Thanks for the kind words, Andre!
All the best,