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I see outer 130 BCD chainrings with 52T/39T. Why the the 39T in the label? How does the outer care or know what the inner ring is? What would be different about 52T/42T?
Does the 39T care what size the bigger is?
I have a 52T/39T that I want to take down to a like a 48T.
Front derailleurs are optimized for certain gaps between the rings and are limited to that - for example, the FD-6700 series from Shimano has a 16t front chainring difference (relating the outer and inner chainring size). There are also size constraints on the chainrings which determine the maximum and minimum size of chainrings you can use due to the shaping of the cage. If you're below the difference, within the chainring size limits and the derailleur clears the chainrings, you're good. So 52/42 should work. As should a 48 for the large ring on most road FD's.
In the olden days, people had "half step gearing" which the front chainrings were close in size and they would crosschain (partially to get closer gearing and partially because old derailleurs couldn't shift 16t chainring differences). You can still do this now.
52/39's are probably marked that way since a lot of people run that combination (and probably put it in the search bar for either chainring).
