for Sram XX1- XØ1 11 speed 10/42 cassettesoriginal 42teeth cog can be replaced with ARI 39teeth cog without any additional modification on your bikethis new project is created to meet the needs of cyclists who practice cyclocross and road in monoring configuration42teeth cog in these two fields forces with a too agile pedaling, often not usable with efficacystudied for roadies and 'crossers 39teeth permits a more regular and easily usable scale replacement is easy and fast material used is hard anodized ERGAL 7075T6 all range of colours is available in the colours table weight of 39teeth cog is 77 grams