3

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.

Improve this question

2 Answers 2

Reset to default
5

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.

Improve this answer
1
  • Many thanks for pointing out those BBs. Considering the price I will start with cheaper BB, but you are almost spot on with what I am doing -- I am trying to fit new triple into touring oriented gravel bike with 135mm rear axle.
    – greenoldman
    Dec 14, 2020 at 14:13
3

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.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.