Can I use a new chainring (or some other spacer) to make my non-boost Fc-m785 xt crank work with my new boost frame? I want to use a 30t 12sp chainring if that makes any difference.
Probably, at worst you’ll need to fit the chainring on the outside of the mounting tabs and grind down the granny ring mount.
Edit, just seen they don’t use standard chainring mounts and you can’t mount on the outside.
In short, no.
With the offset and your desire to run a 30t chainring, which I suspect won't fit on the 'big ring' tabs,I would say it isn't gonna happen.
I'm going to say yes . I've just done the same with the same crank but 11s not 12s . I was told that a 30t ring will work but 32 prob not. Fitting on a cotic frame
So that's one yes, one no and one maybe...
Have a 30t 11 speed non-boost ring on there at the moment but it can't go on the outside of the tabs.Can I add another space at the BB or is that dumb?
Lots of folk (including me) are running non-Boost cranks on Boost frames. As long as your chainring doesn't foul the chainstay it'll probably work just fine.
Do you have a chain ring to try?
A few of us run normal cranks on boost hubs and the chain line is fine. You won't know until you test it. No point wasting money if it won't work.
I'll give my current non-boost stuff a try and see how it goes
It’ll be fine as is. The 785 has a wider chainline on a 1x set up than the 780. The quoted chainlines confuse things as on the 780 they quote middle ring chainline (average of the triple) but on 785 it is the mid-point between the 2 rings (average of the double), but the chainline of the outer position on the double is wider than that of the middle of a triple.
Clear as mud?
On bikes in the past I've deliberately spaced the chainring inboard to get a better chainline than standard, you would get a similar effect using a non boost crank with boost rear spacing. I found that as you're putting max torque through the pedals when climbing, rather than cruising in the highest gear, getting a straighter chainline in the low gear works better (less noise and potential wear I guess).
So as long as he chainring doesn't hit the stay you'll be fine without any spacers
Assuming you’re running Shimano 12spd it will be fine without additional spacers - much more tolerant of wide chainlines than 11spd.
What is the frame?
I'm assuming it's the double crank which I'm running on my boosted Intense Spider frame with no issues at all. One Up have a handy guide and basically the outer position on a double crank is aligned fine for boost as it's further out than the middle ring would be.
I think with a 30t ring it offsets it slightly further inboard for clearance issues, I haven't tried that but I don't think it should cause you any trouble. From playing with chainline on a non boost frame, being slightly further inboard is much preferable to being too far out.