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With SRAM chain sets that use a spider less ring (in my case 3 bolt), is it the chainring offset that makes the chain set "boost"?
Yes - for Sram they use the offset of the chainring to determine the chainline. The mounting face is always in the same place on the cranks.
You can see that some SRAM type direct mount chainrings are flat and some have a dished shape.
On Shimano, the cranks are all different but the chainrings are the same - flat.
Sram direct mount chainrings can be had in 0mm, 3mm and 6mm offsets.
Some of the newer cranks are available in two widths, but I'm pretty certain that's the newer 8-bolt, T-type, stuff.
Also, if you're using a DUB bb, then you can tweak chainline with the bb spacers. Obviously, that moves the cranks over a bit too!
This might help:
Thanks folks - good ole GXP on this chain set.
Now to find a 3mm Boost ring.....
Absolute black, Unite, Woolftooth. They all make compatable chainrings. Theres loads of other brands too.