Originally Posted by
HillRider
Shimano sells their double chainrings in matched pairs, for example 52/42, 53/39, etc. The outer ring's shaped teeth, pins, etc are supposed to be specific to the specific inner ring. The inner ring's teeth are all the same and I can't see how a rotational orientation would matter. Aligning them a specific way can't hurt but I don't see the benefit. ....
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.
BTW- The same thing happens on the cassette, where the shift gates are timed to be in the right place so the shift happens smoothly, rather than skipping before engaging. That's why all cassettes with gated shifting have a "king" spline to ensure all the sprockets are timed correctly to their neighbors.