Do you mean what combo rubs the FD? I run a 50/34 w/ 11-23 cassette and none of the crosschaining combos rub 'cause I just trim the FD. Same with my 50/36 w/ 12-26 on my cross bike and the same when I ran a 53/39 w/ 12-25. Just trim the FD if the chain starts to rub.