Originally Posted by
starving
The derailleur is Sora RD-3400. It says 27 maximum cog on the box. I guess I will change the chainring instead.
Yeah, that's "total capacity"- the difference in sizes of the chainrings plus the difference in sizes of the cogs. With my setup that I just proposed (52/42/24 front, 13-26 rear), you'd need a total capacity of 41 teeth to make it all work perfectly in all gears. Like I said, the small/small gears need to be avoided, but it's no big deal.