Originally Posted by
del690
Thanks for info guys. I think if I ever do this I would have to diligently remember that big-big is off limits, even accidentally. I figured to get the chain sized comfortably on what would be a 48-36 front/rear combination (big-second big) and just make sure I was in the smaller ring when I went to the 42. Current setup on the bike is 48/34 up front with a 11x34 cassette, 10 speed. Rear dérailleur is a shimano R350, which I believe is med cage.
“The best laid schemes o' mice an' men / Gang aft a-gley.”- Robert Burns
You would need a new derailleur to be able to handle that 42 tooth cog anyway, and such a derailleur would have enough take up capacity to handle the extra chain length, so there would be little point in sizing the chain that way. But no matter what you do there will be that time when you will forget that you are already on the 48-34 combination and go for the easier gear. The consequences could be unpleasant