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I have a square taper (JIS) Shimano UN-55 bottom bracket with dimensions 68mm (shell width) and 110mm (spindle length) on my single speed (chainline 42-44mm).

Can a set of Stronglight JIS track crank arms (107mm axle length) be fitted to my BB (110mm spindle length), or must the axle length of a crank arm match the spindle length of the bottom bracket?

If the cranks can be fitted, how is the chainline affected?

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Axle length and spindle length are the same thing.

Three-piece cranks typically specify a required axle/spindle length to achieve a specific chain line, I.e. how far away the chain rings are from the center plane of the frame.

An axle/spindle longer or shorter by x millimeters will move the cranks x/2 millimeters outboard or inboard respectively.

Some manufacturers are very good about specifying axle/spindle length for individual crank models. Some provide a recommended bottom bracket model from which the length can be found, others provide no information at all.

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Can a set of Stronglight JIS track crank arms (107mm axle length) be fitted to my BB (110mm spindle length), or must the axle length of a crank arm match the spindle length of the bottom bracket?

Possibly. If both the bottom bracket and the cranks are JIS, the difference is only 3 millimeters. It will move the cranks 1.5 millimeters outwards. Thus, because the cranks move outwards and not inwards, there would ordinarily not be much danger of the cranks or chainrings touching the chainstays.

However, since the Stronglight JIS crank arms are "track cranks" and your existing cranks may not be "track cranks", it is very possible the Stronglight crank arms have a narrower "Q-factor", i.e. the pedals are closer together. Thus you may possibly run into clearance issues. This depends on whether your frame has been designed for track cranks.

It is the cranks or chainrings touching the chainstays that will ultimately determine whether or not a particular crank fits.

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  • OK, so the chainline specified for the Stronglight track cranks is 42mm on a 107mm spindle. My BB spindle is 3mm wider, giving a chainline of 43.5mm (+1.5mm both sides). My current measured chainline is between 42 and ~44mm (I haven't been able to measure it more accurately). The inner side of the end of the crank arms (Q-factor minus width of crank arm at that point) will always be outside the chainline, so the chainstays will likely not be hit? What is wrong with my reasoning? – 52d6c6af Sep 19 '20 at 13:25

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