TL;DR: You can use your existing RaceFace Cinch 136mm spindle with your new Superboost Pivot Switchblade frame. You will only need a zero-offset Cinch chainring to get the correct 56mm chainline.
My new Pivot Switchblade uses SuperBoost Plus 157mm rear hub and frame spacing, which is a new hub spacing “standard” (at least to me). There aren’t many manufacturers using this new standard yet, but Pivot is all Gung-Ho on it. Benefits are stronger and stiffer wheels, yada-yada…But the most noticeable difference is that you will be in a world of hurt trying to figure out how to get the ideal ~56mm chainline with your RaceFace Cinch crankset.
I’m running the first generation RaceFace Next SL crankset (which have been bombproof for me), and I’ve been moving them from bike to bike over the years. The standard spindle length that comes installed for non-Boost/Boost is 136mm. All of the bikes I’ve used with this crankset had BB92 Pressfit bottom brackets, so I’ve also been moving that BB to every bike as well.
The Pivot Switchblade also uses a BB92 bottom bracket, but the stock XT/XTR build comes with an OEM-only RaceFace Aeffect R crankset, that has a longer 143mm spindle. This confused me, and I thought I needed to replace my 136mm spindle with a longer 143mm one. So I bought the 143mm spindle and swapped it for the 136mm one. Following the RaceFace bottom bracket installation guide, it says that the 143mm spindle requires 4.5mm of spacers on each side of the spindle when used with a BB92 bottom bracket.
For the chainring, I had previously bought a OneUp Switch Cinch (Boost) carrier and Shimano 12s oval chainring. Once installed back on the bike, I did my best to measure the chainline and got about 57.5mm from the center of the BB shell. 57.5mm is a little wider than the ideal 56mm, but decided to leave it and called it good.
Having ridden the bike for a couple months now, I was noticing some chain noise during hard climbing (i.e. in the lower gears) – the kind of noise that sounds like the chain is not perfectly matching up and is rubbing another gear.
Yesterday, I finally took some time to get a closer look at the chainline and try to find out if it could be improved. I had also ordered a OneUp Switch Cinch Superboost carrier to try out.
My first attempt was to take out the Boost carrier and replace it with the SuperBoost one. That actually resulted in a worse (wider) chainline, as the SuperBoost carrier has a zero offset, while Boost is 3mm(?).
I then tried replacing the chainrings with some older ones I had lying around over the years. I tried a Boost round ring and also a non-Boost oval ring. non-Boost actually has a 6mm (inward offset), so fitting that one improved the chainline in the lower gears. This chainring may have solved my rubbing issues, but it isn’t technically Shimano 12s MTB compatible.
After some Googling for “Superboost Chainline”, I came across this 4y old Reddit thread:
Super Boost was designed to maintain compatibility with current RF and Sram cranks, so you should be all good as long as your crankarms clear the chainstays by at least a couple mm.
After letting that sentence digest in my brain, I realized that I may have not needed the longer 143mm Cinch spindle after all. After a quick eyeball for the crank arm clearance, I dug the original 136mm spindle out of my parts bin and swapped it onto the crankset and installed it the same as any previous BB92 setup; no add’l spacers required. Crank arm clearance was plentiful (~1cm) with no tire or frame clearance issues either.
I also realized that the OneUp Switch carriers simply provide the required offset when used with standard spindle length cranksets. So, using the OneUp SuperBoost carrier and Switch ring, it provides a zero-offset ring and the ideal 56mm chainline when used in a Superboost frame.
So, now I have the correct 56mm chainline, but have yet to try it on a trail/climb to see if resolves the rubbing issues. When I look at the chainline in the lowest or highest gears, it still looks pretty extreme to me, so I am not too optimistic that it will resolve all of the noise just yet. But if it doesn’t, I may actually swap back in the Boost carrier to move the chainline inwards by 3mm; this would effectively provide better chainline for lower gears (which I use much more often than the higher ones), which should get rid of the noise for sure. It’s interesting to note that “ideal” chainline is really more for frame and tire clearance than for optimal shifting, so it’s “ok” to adjust your chainline to optimize your shifting as long as there aren’t any issues with your frame/tire clearance. And since there is more chain/slack in the higher gears, shifting in those should still work ok (but may have more noise).
Another benefit of going back to the standard spindle is that the Q-factor (distance between crank arms) is less wide, which is better for ergonomics and pedaling efficiency. (Sidenote: Since getting this bike, I have actually noticed some knee pain, but not sure if it’s from the wider Q-factor or not; I couldn’t find much information online to support this theory.)
But why did Pivot spec the wide 143mm spindle Aeffect crankset in the first place? Maybe it was an availability issue, but as I don’t have that crankset anymore to double-check, my assumption is that Pivot may have used a non-Boost chainring to push the chainline inwards by 6mm, in order to compensate for the wider spindle length. That should effectively provide the proper chainline when used with Superboost spacing. And thinking about that makes me wonder how the Pivot Mach 5.5 I sold (Boost spacing) is handling that crankset and ring offset?! Oops…