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Dear collective wisdom of the hive-mind…
I’m currently running shimano m8100 175mm 1x12 cranks on a boost hardtail (cheapo Chinese one). I’ve noticed that the chainring-chainstay clearance is minimal and it’s starting to… shall we say… “brush” the chainstay. I think there’s some rear end flex/compliance, so I think it will start to chew its way through the chainstay at some point.
I’ve realised that I must have fitted the wrong chainset, as I think it must be for non-boost frames. I can’t work out what I should be buying to replace it, should I be looking for:
M8120 - 55mm chain line, 178mm Q-factor
Or go to an XTR model which seems to offer wider chain lines with a narrower Q-factor..
M9130 - 56.5mm, 171mm q-factor, or
M9125 - 55mm, 174mm q-factor
Finally, are the chainrings all cross-compatible. I presume the chain line is set by the crankset and not the chainring offset. In which case I could source any 0mm offset chainring? Or reuse my existing one?
Thanks in advance.
Can you reverse the chainring, or does that move it too far out?
You could try putting a spacer behind the drive-side BB cup - 1 or 2mm should be enough. As it’s a Shimano crank, there should be enough axial tolerance for it to work.
There's no such thing as a non-boost M8100 crank so you've not gone wrong, its just got low clearance. As above you could find a spacer for the Axle (RF do them on their axles @24mm) as long as your BB is not already wide vs. what it should be. I wouldn't go more than 1mm though.
This chaps gives a pretty clear explanation
Finally, are the chainrings all cross-compatible. I presume the chain line is set by the crankset and not the chainring offset
My understanding is that SRAM use the chainring to change offset; they offer 0mm (55mm boost) 3mm (52mm boost), and 6mm (non boost) offset chainrings, I think Shimano use the crankset, so you'd need to buy a different set of cranks - unless you can get after-market chainrings that will fit Shimano direct mount and have the offset you need?
Thanks for the suggestions. So, should I find a spacer for the axle or the BSA bb shell? I might have a Bottom bracket spacer in the “general stores”… I’ll go and look now.
At the risk of being a numpty, could someone explain what frames are the chain line variances are for? I thought I had bought the correct chainset at the time, only to realise that Shimano say that the M8120 has a 55mm chain line, for 148mm OLD. I’m sure this wasn’t an option at the time of buying mine.
The M8100 has a 52mm chain line, which I understand to be boost, but Shimano don’t specify what OLD it’s compatible with.
I’m confused! And I don’t like it!
Newer frames are starting to use a 55mm chainline Boost set up, as opposed to the previous 52mm Boost chainline. Depends how new your frame is I guess. Both are compatible with 148 spacing, but you might find (as you've found) the clearance on the chainstay's quite tight.
Spacer goes between BB cup and frame - your LBS should have them
At the risk of being a numpty, could someone explain what frames are the chain line variances are for? I thought I had bought the correct chainset at the time, only to realise that Shimano say that the M8120 has a 55mm chain line, for 148mm OLD. I’m sure this wasn’t an option at the time of buying mine.
The M8100 has a 52mm chain line, which I understand to be boost, but Shimano don’t specify what OLD it’s compatible with
Chainline sets the angle that the chain hits the cassette, in order to give the best combination of tyre/chainstay clearance, wear on the chain/rings and clearance for the chain as it runs past the smaller cogs. As the cassette position changes with hub shell width* you need a wider or narrower chainline to match.
* 55mm (56.5 also) is super boost and/or MTB wide/'Boost' 55mm sometimes one chainline for both, sometimes separate. But it's not the 'boost' chainline even though it can be for 148 hubs, hence breaking my rule of thumb above. There's a difference between the two; MTB wide runs a cassette that is spaced wider on 148 hub, a la transmission, whereas Superboost runs a wider 157 hub with 'normal' cassette spacing. They have the same effect on cassette location more or less, but different ways of doing it.
If you have 55mm cranks on a regularly spaced, 148 hub, you're essentially just running the wrong chain line. Some brands of frame might force you to do this due to tyre/frame clearance issues, but that doesn't make it right.
Shimano calls 55mm boost because Shimano has never followed any kind of standard when it comes to chain lines, so why start now? They finally made their chain-lines match everyone else's but couldn't bring themselves to go the whole way and name them correctly too.