Originally Posted by
pacificcyclist
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.
Don't get all nostalgic for the old school cranksets. QC wasn't any better...nor probably any worse...than it is today. New rings and cranks are actually stiffer and shift better than any old school crank you care to name. I would not trade my Shimano XT nor my Race Face Turbine (2000 version) nor my Race Face Deus for a Sugino AT from 1983...and the AT was a good crank and a great touring crank in 1983. But dropped chains and balky shifting are things of the past for the most part because the rings are better made.
As for the problem of ring wobble, do you really think that a little wobble of a chainring is going to have much influence on where a flexible chain is in the rear? We're talking about a chain that will deflect a large percentage of an inch from front to back if you
are cross-chaining and a very small amount of wobble in a chainring (fractions of a millimeter) if there is any. If the chain were a rigid structure chainwheel wobble might be a problem but the chain is far from rigid.