On Jul 16, 7:30*am, John B. Slocomb wrote:
"Cross shifting", i.e., Large chain Ring/large cassette cog or small
chain ring/small cog is generally taken as a mortal sin. But with a
triple chain ring what limits are made to the shift pattern when on
the center ring? Assuming that the chain line through the center line
of the 5th cog, on a 9 speed cassette, and the center line of the
center chain ring?

It appears to me that considering the actual distance between the
three chain rings versus the actual distances on the cassette that the
center chain ring should be able to shift to any cog without problems.

Sheldon Brown has a gear "visualizer", and there's another one called
"Gerz" that makes charting pretty easy.

It seems to me that with the 30/42/52 triple I have, run against 8sp
and 9sp cassettes with 13-23 or 14-26t cog spreads, you wind up with
gear ratios that favor running the two or three cogs most in line with
both the inner and outer rings, while the middle ring works over four
or five cogs, overlapping so to speak with the inner and outer rings
on one cog. It's been a few years, maybe I'm misremembering but I
think this "question" pretty much takes care of itself in practice if
you will.

Meaning, you're keeping the chain fairly straight while you use "same
ratios" (or close duplicates) of gears you might find by "crossing"
farther on any of the ring/cog combinations. Not to say you shouldn't
"be able" to use any cog/chainring combination without either drooping
the chain (which invites unshipping and/or dragging in the dirt) or
binding/breaking the rear derailleur due to too short a chain, but why
not use the chain in happier combinations?
--D-y