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I have a 42.5 rear chainline (12 speed sram eagle).
In the front, I purchased a eewings-mountain crank. According to the docs, the offset is (55-chainringoffset). If I get a 6mm offset chain ring, it means the difference will be 6.5 which is quiet a lot, I believe.

I was wondering - if I "exchange" a 2.5 spacer on the bottom bracket, moving from the drive side to the non-drive side, I pretty much think it will result in an additional 2.5mm decrease in chainline, which will get the difference to be 4.

Perhaps than reasonable? I tried that in the following photo. Basically you can see there are two 2.5 spacers on non-drive, with 2.5mm spacer on drive side. It was the opposite previously. What do you think? enter image description here

2 Answers 2

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by moving the crank spindle spacers you move the crack arms to one side or the other more you could come close to hitting the chain stays with the crank arms and be off balance a little while peddling.

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  • Yes, in terms of chain stays clearance it is actually going to be fine. The frame can handle that. However, I am wondering, whether this change of spacers orientation could result in some kind of "weaker" crank arms
    – yuvalon
    Jan 12 at 17:42
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I was not aware chainline was measured for cassettes as well, as I was reasoning in terms of boost/regular: the position of the cassette should be the same for all hubs of the same category. Then I would take the matching boost/non-boost crank. For reference, non-boost is 142mm thru-axle/135mm quick release, boost is 148mm thru-axle/141mm quick release.

Standard MTB crank chainlines for non-boost/boost are 49mm/52mm respectively (for SRAM,for Shimano it's 48.8mm/51.8mm). Assuming the chainline you computed for the cassette is correct (conditional because this metric is a novelty for me), a 6.5mm difference would be exactly within specs for a non-boost rear hub (142mm thru-axle).

With a boost rear hub, the chainline difference would be too small (but if you have 68mm BB shell, it's unlikely that you have boost hub - a potential exception being monster gravel bikes).

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  • Hey @renaud thanks for the comment. As far as I am aware, the position of the casette on the hub should be the same for all XD hubs. But it doesn't mean that it will yield the same chainline. That's because the casette itself can also vary in how wide it is. 12 speed is wider than 11 speed, at least for the sram eagle case.
    – yuvalon
    Jan 12 at 17:39
  • @yuvalon the position of the sprockets relative to the end cap is the same for all hubs/cassettes (XD or not in fact, you can interchange wheels with HG and XD hubs/cassettes), for a given number of speeds. My point is that the chainline at the level of the cassette is probably not critical: in current bikes, there are only 2 formats for TA hubs: 142mm and 148mm, so you can't go wrong there. In the the front, you just have to ensure that chainline matches the format of the rear hub, as there are only two possible values for MTB groupsets (for a given brand).
    – Rеnаud
    Jan 13 at 9:18
  • (of course, if you consider the road/gravel groupsets, there are additional possibilities, but given you've chosen an MTB transmission, you can ignore them),
    – Rеnаud
    Jan 13 at 9:21

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