Tempted by a 1x10 drivetrain but haven’t got the legs for the hills? Used with a 30t chainring, OneUp’s 42t ‘crawler cog’ gives the same gear ratio as a 24t granny ring and 34t sprocket, helping you to make short work of the climbs.
It’ll fit most SRAM and Shimano 11-36t cassettes in seconds – simply ditch the 17t sprocket to make space and swap the 15t cog for a 16t (included) to avoid a jump between gears. The 36t to 42t gap looks huge, but upshifts are surprisingly seamless. Double shifts back out of the big cog can lead to grinding though, as the rear derailleur struggles to cope with the big downwards and sideways shift. This can be avoided by downshifting one cog at a time, but shifting is definitely less tolerant of poor setup than usual.
The three-sprocket jump either side of the 16t cog is noticeable because it’s in the most used area of the cassette. But it’s an acceptable compromise for the extra climbing ability. Our sprocket is showing little sign of wear even after several months’ use with an old chain. We’d definitely recommend using it with a new chain though. OneUp offer a 40t version (US$85/£52) if you want less of a jump, plus a RAD mech cage for smoother shifting with Shimano set-ups (US$35/£2).