seing 9 and 10th tooth cogs mentioned, heres a question for all of you. I find that on both my bikes that have 11 cogs, I dont use the 11s that often and a big part of that is because whenever Im in them, it feels "crunchy" slightly. Ive always assumed its because of the tightness of the chain over so few cogs. I mean I use it when I have to, moreso on the mtn bike with the 42/32/22 on downhills as I get into the 42/11 fairly quickly, but I generally avoid it in going up to the next chainring when I can (never go into it in the granny)
Do others feel the same? I dont know if its my rd setup that does this, or if in fact it is the chain being on such a small diameter cog with so few teeth. Just as cross chaining can produce a sound from a chain that seems to say that it is being worked harder, it seems to me that on a 11t it does also, and I assume a 10 or 9t even more? I guess being in it for short periods is never going to be an issue, but I do know that some riders may go onto it often, and I wonder if it shortens the life of a chain?
The life of the chain when cross chaining is determined by the length of the chainstays and which chain ring the cross originates. Less with a 42T because more teeth are supporting the chain (stress evenly distributed through more teeth) as opposed to a 24T where less tooth are supporting to a 11T. Touring bikes have the longest chainstays which makes the chain angle less acute, so it lessens the wear compared to say a road bike or a triathlon bike. A triathlon bike has the shortest chainstays, because they have to make the rear wheel be as close as the seat tube for aerodynamic purposes and to allow riding geometry to allow being closer to the aerobars. Chain will wear sooner on a triathlon bike than a touring bike if the rider cross chain often. This is a common complaint from people coming from a touring or rando bike and into a tri bike as many of them probably started triathlon this way. Their cross chaining habits migrated into their tri bikes with only 2 chain rings.
Having said that, as long as you have a good chain line, the cause is not your rear derailleur. The cause is with your front chain rings or front crankset.
Most chain rings these days will have a slight wobble. 100% quality control is pretty much non-existant today in many industries including bikes as it is expensive to keep good quality because you need to keep paying QC people in house. So they make you, yes the consumer, as QC people. It's cheaper to replace a defective chain ring or crankset free of charge rather than go through every single chain ring or cranks. It's the norm now. Besides, how would you, as a consumer, know the products are sub-standard quality. See, they can get away with many of these products out in the market! In the old school days, you can see mounting marks on the chain ring bolt area and the BB slot of the cranks to show that they actually mount it to a dial indicator gauge to see chain ring "RUN OUT". Today, you are doing this. It sounds to me that you have what is called "chain ring run out". To solve chain ring run out with the not so flat chain ring, they use 4 and 5 pattern bolts to hopefully pull all the sides in to make the ring flat. But that's cheating and rarely provide a good result as your experience seemed to indicate.
Chain ring wobble is a pretty big issue with cross chaining because it really does not leave a lot of room between the front derailleur cage and the 11 cog on the 42T chain ring if you adjust the FD limit screw. If you adjust this, then shifting to the middle chain ring may not be smooth, but if you don't that the chain rubbing noise in a form of crunching noises will annoy you. Keep in mind that your frame flexes a bit too and that just compounds the problem. Most people avoid this combo, except me going downhill. Another problem with chain ring wobble is the transition between small chain ring to a bigger chain ring that has more than a 10 tooth difference. If the chain ring wobble and if you have a larger tooth differential like a 18T such as mine, the bigger the difference the harder it is to shift because the larger area of wobble prevents it from catching the ramp as opposed to a small tooth differential like what Shimano and others recommend which is 10 tooth. So now you know what the industry recommends only up to a 10 tooth difference between chain rings.
I use my old school crankset and old school chain rings I have on my touring double, there is no chain rubbing or hesitation in shifting even if I have a 18 tooth difference between 42T and 24T.
To solve your problem, either switch to an old school crank of the 70s and 80s and chain rings in good shape of that era as their tolerances are much tighter or go for better cranks from T.A and chain rings from T.A or a Rene Herse crankset from Jan Heine of Compass Cycles. As the old saying says, you will always get what you paid for. To be sure if the rings are flat, you should have a flat metal slab and a dial indicator gauge handy. The bike co-op I frequent in has those! Since chain rings wear much longer than a cassette, it would be a good idea to get these rings you can put on the cranks.
Same principle with a tire wheel. Unbalanced wheel compared to a well balance wheel. Which is more stable and create less noise?