I do 49/17 but live in the most hilly(is that a word?) area of SF. I have a 15 tooth cog but have never used it. I want to try 49/16 when I get some more money. I only skid when I am really bored. 49 is a good chain ring because almost all cogs will have max skid patch. Though I guess 47 would be most idea for skidding.