Chain line for 1 chainring conversion

[pfm401]

My son bought a Calibre Bossnut recently and I was checking out the after market kit put on it which is pretty good (Deore, Deore XT, SLX, Hope Rotors and pedals). Apart from a Snail 32T chainring which looks pretty cheap! Which opened up a can of worms on what to replace it with (eventually!)

It’s on an M617 crank which looks like Deore 2 speed and 104BCD so struggle to get Shimano 11 speed, presumably why the Snail is on there. So I thought “put a Shimano 11 speed eg Deore Fc-M5100 and everyone’s a winner…

Then asking around I get into chain line… I’ve learned it’s a non boost bike and best run on 49mm chainline, whereas FCM5100 is 52mm ie Boost. But there’s a 2.5mm spacer in the standard BB52 bottom bracket.

Am I being really stupid or can you just fit the BB without the spacer to align the chainline to 49.5mm? And does it really matter?? Presumably biggest cogs are more “diagonal” chain line than ideal due to chain line being 2-3mm to the right but shifting seems fine?

Any views?

 

[wafter]

Not quite sure what you're getting at, but a few things that may or may not be relevant..

If you're going 1x the speed designation of the chain doesn't matter much, since the internal width of all 9-11sp chains is the same so the chainrings should be too.

IIRC you can get 1x narrow-wide rings in 4x104 format.

Shimano typically do multiple versions of any one crank to suit different chainlines. Apparently the M5100 is available in 49mm (FC-M5100-2) and 52mm (FCM5100-B2) chainlines.

Note that on a double the chainline is measured from the midpoint between the two rings; with each being offset from this centreline / the stated chainline by around 4mm. As such sticking a single ring on a 2x crank will not result in the same actual chainline as stated for the crank with two rings fitted.

Cranks with different chainlines have different length axles, so messing with the spacers to alter the chainline will result in pedals that are spaced unevenly from the bike's centreline. Ultimately this could lead to your son's legs going on the wonk, causing him to walk in circles.

If you want a 1x Deore crankset why are you considering basing the upgrade around a 2x crankset?

Have you measured the chainline of the existing setup?