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I will have to replace my crankset to switch to a 26-36-48 one.

It is on a BSA 68 mm aluminium frame. I have a chain casing on this city bike (MTB converted to city).

Is it a problem to use a Hollowtech BB with the chain casing bracket ? The bracket used to be tighted between the frame and the BB (octalink or squared).

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There is an issue with putting anything between a Hollowtech bottom bracket cup and the frame.

A cartridge bottom bracket is not very sensitive to its position being changed relative to the frame shell a little by the chain case mounting bracket. The axle just gets moved slightly towards the drive side.

The hollowtech cups are more sensitive to their position being changed, as they need to be the correct distance apart to match the length of the axle on the crank. If you put something between the cup and frame the cups may be spaces too far apart and you will not be able to fit the non-drive side crank.

However, mountain bike cranks are typically made for 73mm frame shells and MTB bottom brackets are installed on 68mm shells with two 2.5mm spacers. You may find you can omit the drive side spacer to make room for the chain case mounting bracket.

If the bracket it less that 2.5mm in thickness you may need to find thinner spacers to make up the difference and get the correct cup spacing.

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  • OK I will have to measure the thickness of my chain casing bracket. I could find that we can purchase spacers with different thicknesses. Thank you. I only have to find the thickness as close as possible to 2.5 - e, where e is the thickness of the chain casing bracket. – Arnaud VILLANOVA Oct 26 '20 at 12:07
  • @ArnaudVILLANOVA Correct. The frame shell+casing+spacers width does not have to be exactly 73mm, there is some leeway in mounting the crank arm. However, make sure the cups are wide enough apart so that the crank arm does not bottom out in the axle, or you will not be able to preload the bearings. – Argenti Apparatus Oct 26 '20 at 12:53
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I measured the bracket thickness at 1.3 mm. I could find a spacer with just the adequate thickness. I only made one mistake : I put first the casing bracket, then the spacer and finally the BB cup. I should have done it the other way around : first spacer, ther casing bracket, and finally (of course) BB cup. It was a little bit difficult to find the right position for the casing around the crankset, as the big ring is 48T, the maximum accepted by the casing. Now it works. I am not sure if it is a good idea to remove all that just to change the part order. In french, we say "le mieux est l'ennemi du bien". I could try to translate : "better is good's enemy"

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  • 1
    "Don't let the perfect be the enemy of the good" is likely to be equivalent or very close in English. – Weiwen Ng Dec 3 '20 at 15:14
  • Well, I like it, thank you for this much more idiomatic quasi-translation – Arnaud VILLANOVA Dec 4 '20 at 10:41

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