Pretty new to SS and have been riding my SS (BD Clockwork) mostly on the flats. Now riding in S.F. and along our N. California coastline and my knees are sore with 46x16 and in a head wind. Suggestions as to a good "compromise" rear cog number..18 rear? Any S.F. riders have a favorite number? Mostly riding in upper Haight/Divisadero area and across the bridge into Sausalito.