I've been lurking on these forums for a while, but this is my first post! I am so appreciative of the fund of knowledge to which you all contribute here. Before buying my first adulthood bike in 2015, and every other bike related purchase and decision I've made, I've researched it here; you all have helped me immeasurably.
After a couple thousand miles on my 2015 Trek FX 7.4 WSD, I am getting to the point of considering how to upgrade the Shimano Acera triple crankset it came with. So far I am favoring a Stronglight Impact Triple crankset, which is apparently a rebranded Sugino XD (BTW is this considered a touring crankset? Mtb? XXcycle.com seems to think it's a road set); and I'd like to stick with a 9-spd mtb cassette. I want to replace the big ring on the Stronglight with a chainguard (with my current riding style and urban riding environment, I simply don't need a big ring), hopefully thereby circumventing any potential compatibility issues with my mtb front derailleur, as long as I adjust the inner/outer limit screws properly. (Please advise if my thinking on this is totally incorrect!)
What causes me the most uncertainty is the length of the bottom bracket to achieve the correct chainline with this crankset. According to Shimano specs, the UN26 square taper BB on my FX has a 122m spindle length, giving me a 50mm chainline. Sheldon Brown's site says that a 115mm BB with the Stronglight triple yields a chainline of 45mm. Treefort Bikes recommends a 113mm BB length for this crankset on mountain frames, for a 47.5mm chainline; and from reading various other threads and vendor websites, I have gathered that other recommended BB lengths include 110mm and 115mm. Also, some folk report that 118mm is wider than anyone needs.
Hence my confusion. If the desired chainline on my bike is 150mm, and a 115mm length BB with the Stronglight gives me a 45mm chainline, as Sheldon says, wouldn't I want a BB length of at least 125mm? I am assuming that any BB I get (probably a Shimano UN55) will be symmetrical, and thus increase distance equally on both sides, which may be incorrect? It seems to me that in order to work with a mtb cassette, the front chainline should be as least as great as it is now (with current mtb crankset and cassette set-up). Not only that, but removing the big ring in front, as I plan to do, effectively moves the front chainline inward (i.e., results in reduction) about 2.5mm, according to Sheldon. I do want to be able use the granny ring a little more than I do now, and potentially use it with the first (biggest) 5 cogs at least, since I will have only 2 rings. Does this mean I should be looking for a 127mm BB at least and/or use bracket spacers?
115mm (to get 45mm chainline) + 10mm (to get 50mm chainline) + 5mm (to compensate for removing outer ring) = 130mm.
On the other hand, if TreeFort's info, not Sheldon's, is right for my bike, then it seems the BB I have now would just about work.
113mm (to get 47.5mm chainline)+ 5mm (to get 50mm chainline) + 5mm (to compensate for removing outer ring) = 123mm
Which info is correct? And what am I missing?
Please forgive my ignorance if I am asking all the wrong questions and making the wrong assumptions/decisions, as I am still feeling pretty new to all of this (just changing tubes and tires by myself last year was a major spiritual and physical victory for me
). I am trying to research this as well as possible ahead of time, rather than just buying different length BBs and experimenting (as was frequently suggested), because I will be attempting the installation myself at my local bike co-op; and even with their help, this is a job I'd rather not do several times. I did find several threads re: this crankset, but in none of those cases did they seem to be installing them on a bike just like mine. PLEASE feel free to advise me in any aspect of this upgrade I am contemplating. Many thanks for your patience and your help!