My fatbike has a 26t front ring and an 11 x 36t (10spd) rear cassette. The biggest middle ring for the Surly O.D crank is a 26 but that’s not a bad thing for me. Most of the time when fatbiking I’m always looking for a lower granny gear to keep momentum and smooth pedaling to keep traction and a straight line, especially on soft snow. So when I saw the OneUp 42 aftermarket cog for Shimano or SRAM cassettes I had to give it a try. (Wolf Tooth Components also has one.) This is how it works. You remove your 17t cog and put the new 42t behind the 36t, with the supplied small spacer either in front of or in back of the 42 (depending on whether you have a Shimano or SRAM cassette). This is an XT so I put it in back, put on the 42, then put on the rest of the cassette.
Once it’s on your wheel, you have to tighten the B-tension screw until the upper pulley clears the 42, which is quite a few turns. If it still doesn’t shift up there you will have to remove the plastic washer that limits the B-limit screw. I took mine out and added another turn of the screw before deciding that was good enough clearance and good enough shifting. It’s a bit slower shifting across the entire cassette if you leave the clutch on but with the clutch off it’s pretty regular and you hardly notice the 2 new bigger jumps.
While riding, I did notice the jump where the 17t cog was removed. But the jump to the 42 feels natural, probably because when you need that extra low gear the easier the better. I tested it out up the road and i can definitely say that if i hadn’t installed the 42 i would have turned around almost immediately because I wouldn’t have been able to pedal easy enough to stay afloat. As it was, it was slow going in the 4-6″ of wet Sierra cement on top of…cement, but with 3psi in the Lou/Bud combo…i was just able to stay afloat and pedal in the 26×42. I’m pretty sold on these for converting an existing 1×10 setup to a cassette with more range.