Bicycles Asked on April 15, 2021
I would like to buy new crankset which is described as having 54 mm chainline for 110mm bottom bracket (this is square taped BB). Since I need 50mm chanline I calculated that I would need 102mm spindle, luckily I found such bottom bracket.
My worry is whether there is some other unspoken factor here, I mean can I just use this BB with shorter spindle or do I have to check something more? If more checks are needed what are they?
I currently run 44/32/22 chainrings and those are exactly the same as in the one I would like to by. The only differences are BB type (I will convert from HTII to square taped, same width, 68mm, both BSA) and crank length.
You are taking the right approach to figuring your chainline per a given spindle, but yes, there can be other limiting factors.
If you go too far, it is possible for the crank body near the spindle interface to smack into the bottom bracket shell or the cartridge body, or be low-clearance enough that flex and grit wear a groove into the crank. Low-profile cranks increase the likelihood of this. Using something like a Phil BB that has zero protrusion from the shell can avoid the problem where a Shimano or similar will run into it, which can be useful if you absolutely need a certain spindle length.
The above is not super common, but you can get it to happen in some situations. I've seen it mostly when putting 135mm-oriented era touring triples, i.e. the Sugino XD600 and XD300, on bikes with older chainline numbers or when trying to get the Q minimized. I think I've seen it at least once when someone was configuring a road crank for track chainline with the ring in the clean-looking outer position.
Interference between the ring(s) or spider and the frame is also possible.
Correct answer by Nathan Knutson on April 15, 2021
Your calculation is correct. I think all you need to do is double check your current chainline.
Given you will be using the same size chainrings you should not have an issue with interference between the chainrings and chain stay.
Answered by Argenti Apparatus on April 15, 2021
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