It's all about the 1x drivetrain these days, and with the advent of XX1 and XO1 the usable range of gearing range for a 1x system was taken to a level formerly reserved for 2x or even triple-ring setups. For the rest of us, the poor, the weak and the curious, we were left with our 10 speed cassettes and a 36 tooth as the lowest gear. Ok for some, not so for others. Not having a low climbing gear was the biggest problem for the majority of riders, so a few companies have addressed this and one of the first to offer a solution is OneUp Components with their 42 tooth cassette sprocket.
The idea behind the OneUp conversion is simple; remove a smaller cog and slip the 42t in behind your cassette and voila, a wide range 10speed cassette. OneUp recommends removing the 17t, leaving a jump from the 15 to the 19t... I thought I'd get tricky and remove both the 15t and 17t, then replace them with a 16t. All good in theory, but the available cog from Shimano is for a road cassette, and didn't quite mesh properly, making shifts to and from it a bit slow and clunky. I then tried removing the 11t and 13t and replacing them with a 12t, but once again the shifting was compromised by a non-compliant cog. Getting the right cogs would make both conversions possible, and I'll be having another look into this soon.
The cog has a wide contact area with the freehub body, which will help prevent any gouging of alloy freehubs, while the machined spacer tabs towards the outer part of the cog keep it from flexing under load. For Shimano, slip the supplied spacer behind the cog, while SRAM users will place it on the inside of the cog, between it and the next sprocket.
In the end I just did what was recommended, taking out the 17t... the 4 tooth jump isn't too noticeable, and the shifts across the whole cassette are still smooth. Shifting from the 36t to the 42t is trouble free and doesn't seem to be a problem jumping up the 6 teeth.
OneUp advises against using short cage derailleurs, but I was curious to see if my ZEE unit would work with the cog. Wind the B-screw all the way in, and the pulley clears the cog without problem. The short cage appears to be capable of handling any chain growth, and a quick test of letting the air out of the shock and fully compressing the suspension showed no problems. It's unlikely to be bottoming the suspension in the 42t cog anyway, but a few proper rides will reveal more. But if in doubt, go with the recommendation, as a medium or long cage mech should work without issue.
*UPDATE: After a first 'proper' ride, I can report the short cage derailleur worked with no problems...
There's little doubt that the 1x drivetrain is here to stay, and is getting more popular not just with the buzzword set but with a wide range of riders. The simplicity of a single ring and no front derailleur or shifter simplifies the bike and the cockpit. But the decreased range of gearing limited some and the cost of replacing and entire drivetrain put many others off. The appearance of kit like the OneUp cog are helping bring the benefits of 1x to more riders at a cheaper price point.
We'll bring you more as we ride the cog, and try a few combinations of cogs and derailleurs over the next few months.