Originally Posted by
FBinNY
I'll try to explain, though I wish I had some decent photo's which would help.
During a shift the timing of the chain is set by the chainring it's still attached to. So as you shift the chain leads out through the shift gate, and down toward the smaller ring, When it reaches the ring the rollers will always be in a very specific place since it's still attached to the larger ring.
The timing mark ensures that the inner ring's teeth will be oriented to slide into the chain between the rollers for fast smooth engagement, rather than having the teeth hit the rollers and not engage immediately.
Timing only matters if shifts occur at the same place, since with differing numbers of teeth the relative position of the teeth changes andonly repeats in certain places. But with gated outer rings, a properly timed inner puts the "glide" in xxxxxxglide.
OK, now I'll try to explain.
For the big chainring you are correct, the shift gate location, etc is important but the rotational orientation is easy to determine since the chain catching pin is always placed right behind the crank arm. No ambiguity there.
For the inner chainring of a Shimano double, all of the teeth are identical in height and shape and there are no pins, ramps, gates, etc. on these rings. None. Zero, Nada. So, except for having the offset in the proper direction (i.e. engraving facing inward) the rotational orientation doesn't matter.
The middle chainring of a triple crank, definitely does have shaped teeth, pins, etc to assist shifting to and from the granny in the same manner a double has to shift to and from the small ring. These chainring certainly have an alignment index mark.
The granny ring of a triple is also "flat" with no shaped teeth, pins, etc. for the same reason the small ring of a double has none. There is no need to shift down to or back from a smaller ring.