I've found Shimano has problems with their specs on their products such as derailleurs. No doubt others do as well. For instance stating an Ultegra RD has a 27T cut-off. Or a FD-M961 is at it's limit with a 48T chainring. Both of these are utterly false. Hence I suggest trying things to see what works best, rather than relying on the companies' spec-sheet.
I agree that some shimano tech info isn't great. However it should be pretty easy to find out if the rings are made to match. Either they're ramped and pinned to work as a set, or they are generic non-ramped and non-pinned. I looked into this about a year ago for my cross bike setup and found good info on rings from shimano and fsa.
pbayne, the 50/39 doesn't seem to be so common. Honestly I'm a little sceptical about the whole matched thing. That being said I'm taking it as IMPORTANT when I get the next set, why take a chance, especially when it has to do with the ride.
I may be making mountains out of molehills. There are people on these boards with much more tech knowledge than I have.
I do have setups with non-matched chaingrings. They work just fine. I think of it as a trade off. If I want the super smooth, fast, quiet shifting experience I make sure everything is matched up (and clean, adjusted etc, etc.)
For my non-standard setup I really wanted the specific chainring combo for the gearing it gave me. Shifting works fine, its just a little slower and I can hear it more. It is a noticeable difference but it works.
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