Bicycle Mechanics - How sharp is too sharp?

We all know not to ride big ring-big sprocket or small ring-small sprocket because the chain is at too sharp an angle. Richard Ballantine in Richard's 21st Century bicycle book says that triple chain rings on 7,8, or 9 speed blocks you should avoid 2 cogs on either side of the extreme, and that the middle ring shouldn't be used with the extreme cogs either. Is there a rational way of figuring out which angles are too extreme? Does the chain width effect the acceptable angles? Does anybody really know these things or is it all trial & error? If so, why don't production bikes come with charts of the "right" gear combos and the no-nos?

A little trial and error I would say. You can really see when you are cross chaining badly. There really is no need for it. You should be able to find a comfortable gear without going to extremes. Personally, I ride 1-4 on the granny gear, 3-6, maybe 7th on the middle ring and 5-8 on the big ring. Just experiment...

Using the inside to outside small to small etc. combinations are a bad ida on 2 chainring bikes as well. On five, six and 7 speed compact clusters do not use one outer or inner cog with the opposite chianring. On 7,8,9 speed clusters two cogs.
My chain rubs on the outer chainring if I try to use either of my outer two cogs when I'm in my small chainring on my 53/39 X 13-23 set-up and others.
Extreme angles cause-
Uneven pressure, thus quicker wear on the bushings in the chainlinks. ( That's where "chain-stretch" comes from).
The chain plates to rub and erode the chain-ring and cogset teeth and against each other.

Ride that chain-line straight
Pat

You should be able to use ALL cogs when you are in the middle ring of a triple ring bike.

I was going to post a thread on why I can't adjust my Ultegra triple to quiet down when at the extremes.

Now, I find I shouldn't be there in the first place.

Once again, unexpectantly, I've learned something new.

Thanks all.

Allowable chain angles are a function of lateral chain flexibility and chainring tooth profile. I agree that one should avoid the large/large and small/small combinations on almost any modern derailleur bicycle, but which (if any) other combinations to avoid depends also on chainstay length, chainring size difference, cogset width, and chainring-to-cog alignment. If you can make the chain run quietly and if it stays on the chainring when you backpedal, you are in a usable gear.

Both of my road bikes, which use 8-tooth chainring drops (50-42 and 52-44) and mid-size 7-speed freewheels (13-26 and 14-28), work well in all combinations except large/large. On my 3/7 mountain bike (48-40-24 / 13-26), I avoid 24/13, 24/15, and 48/26.