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TLDR: How much leeway there is, if any, in selecting the bottom bracket axle length for octalink or square taper cranks in order to reduce chainline?

Background: I'm doing some minor modifications to my old bike to make it suitable for bike touring. Probably the main change is moving from 48-38-28 cranks to 44-32-22 cranks. Also I want to do it cheap – it's an old bike and the point is if it gets stolen or broken during transport I would not lose that much. However, the solutions have either unknown or larger chainline. The shifting is buttery smooth on my bike and I do not want to lose that.

At the moment I have Shimano FC-M171 cranks. The service manual says the chainline of the cranks is 47.5mm which I also measured on the bike. The BB is square taper Shimano BB-UN26 68x122.5mm. I calculated the rear chainline using the instructions on the Parktool's site to be 45.5mm.

There is about 1.5 cm clearance between the largest chainring and the chainstays and about 1.8 cm clearance between the crank arm and the chainstays. The new crankset has a 7mm higher Q-factor than the current cranks, so it's possible that it will help with the clearance, too.

I'm considering two options:

One is to get Shimano FC-T4010 cranks and replace BB with an octalink one. The advantage is that I can get these cranks at 170mm length which is slightly better for me. The problem is that according to specs it has 50mm chainline and a 126mm axle should be used. Sheldon Brown mentions possibility to use shorter BB to reduce chainline. However, it doesn't seem to be a common solution. In fact I found mentions that you absolutely have to use the axle length recommended by the manufacturer. Can I go with 5mm shorter (ie. 121mm) octalink bottom bracket to get the same chainline with these cranks?

The other option is getting Sunrace FCM914 for the square taper bottom bracket. However, there is no information on the chanline and the recommended axle length. I found some unofficial info on one of the shops selling it that the chainline is 50mm, but not information about the axle length. Does it mean the cranks will fit any square taper BB and only the chainline will change?

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  • Do you want to reduce the chainline or is that just a consequence of the parts you're considering for purchase?
    – Paul H
    Commented Apr 16 at 18:36
  • I would like to keep the chainline as-is because it works well. I'm afraid increasing the front chainline could introduce issues such as chain rub because the difference from the rear chainline would be too large. The problem is that pretty much all 3x cranks nowadays have larger chainline than I have. So I'm thinking whether it's possible to buy new ones anyway and somehow reduce their chainline.
    – stativ
    Commented Apr 16 at 20:09
  • I run a BB with a 2.5mm too short chainline for my crankset. Works fine, but I get slight chainrub when mid front ring and smallest cog in rear. So minor deviations of 2.5mm might be workable.
    – WornChain
    Commented Apr 16 at 23:10
  • If the front chainline was 50mm, the difference from rear would be 4.5mm.
    – stativ
    Commented Apr 17 at 7:22

1 Answer 1

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The bottom bracket in terms of type and spindle length is dependent mostly on the needs of the crankset. So we can ignore the square taper's 113mm length because it doesn't relate to the proposed new crankset, FC-T4010. As you've noted this proposed crankset requires an Octalink bottom bracket with a 126mm spindle to achieve a 50mm chain line. This requires also, the bike has a 68mm bottom bracket shell width. They do not make an Octalink BB for a 73mm shell with this length of spindle.

If you want to continue to run a 47.5mm chain line up front, the next Octalink spindle size down is 121mm (also only in a 68mm shell version). The reduction of the spindle by 5mm brings things in 2.5mm per side, and thus, should achieve the 47.5mm chain line you desire.

Issues may arise with clearance. The crankarms may come too close to the chain stays. The large sprocket may come too close to the chainstay as well, but I doubt this will be an issue for your bike because of the 4 tooth reduction in the large chain wheel.

In short, to use the T4010 crankset, you'll need a bike with a 68mm BB shell. The Octalink options are either the called for 126mm spindle or the next size down, 121mm. Given appropriate clearances to the frame are maintained, either of these options should work.

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  • So I'm in luck that I've got the 68mm bottom bracket shell. I checked the clearance. There is about 1.5 cm clearance between the largest chainring and the chainstays and about 1.8 cm clearance between the crank arm and the chainstays. The new crankset has a 7mm higher Q-factor than the current cranks, so it's possible that it will help with the clearance, too.
    – stativ
    Commented Apr 17 at 15:22
  • That sounds encouraging. Enjoy the project coming together.
    – Jeff
    Commented Apr 17 at 23:39

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