Rich's Tongfamily (Richtong.net add-on)

Uncle Al. A great newsletter that I get and some good advice too. This one I haven’t seen covered, but what do you do with the zillions of gears. I haven’t doing it right for a while.
DEAR UNCLE AL: I just had a 9-speed drivetrain put on my bike. I’m wondering how many cogs at either end I should avoid when in the opposite chainring. For example, when I’m on the
big ring and shifting towards larger cogs, where should I stop? I’ve heard different advice. — Dan D.
UNCLE AL FIRES BACK: Cross-chaining, like cross-dressing, happens all the time, Dan-O, but it’s not really recommended behavior. One involves metal, gears and chains; the other, chiffon, lace, shaved legs and spiked heels. (I’m just guessing here. No, really!) Neither should be attempted without adult supervision.
If you are running double chainrings with that new 9-speed cogset, and you are on the big ring, it’s kosher to run up to the # 3 cog (the biggest cog being # 1 and the smallest being # 9).
If you’re on the small ring, it’s cool to run down to # 7, provided you can “trim” the front derailleur to stop the chain from rubbing it, and provided the chain doesn’t tinkle against the big ring.

That’s for normal riding. If you’re racing, all bets are off. In your delirium you can do whatever it takes as long as it doesn’t make you crash.
If you are running triple chainrings, you are either Roberto Heras on the Angliru or you are a geek who’s lost all climbing power. If you are the latter, like me, the idea is to run the chain nearly straight to the cassette.
This means that when on the small inside chainring, use the 3 or 4 largest cogs.
When in the middle ring, run # 8 up to # 2, occasionally # 1 in a pinch. But if that gear isn’t low enough, you’ll have to shift to the small ring and your chain will drop onto the bottom
bracket shell about half the time (unless you have a chain watcher). Prevent this by making your shift to the small ring before you’re up on ol’ # 1.
When on the big chainring, it’s okay to run from # 9 up to # 3 regularly, and # 2 occasionally.
The bottom line is that your 27-speed bike is actually a 19-speed but is even better as an 17-speed.
Never run the small ring/smallest cog combo or the big ring/biggest cog combo, or I will hunt you down and hurt you. You are asking for trouble if you don’t run the chain relatively straight. Those combos put it at the max angle.
The whole purpose of multiple gears is to give you what you need and do it with good chain line. This results in less wear and tear on your equipment, less noise and maximum efficiency.
There isn’t always the “right gear” for the job. Sometimes, it has to be your legs that make the difference. Don’t be afraid to push a little harder or spin a little faster to prevent cross-chaining. It’ll make you a better and stronger rider.