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I made the plunge and have purchased a new 2016 Stumpjumper FSR carbon frame. The post on eBay--from the Specialized Outlet Store, no less--was incorrect when it stated the frame had 142mm rear dropouts. It actually is one of the first Boost 148 frames which was confirmed in email correspondence. I'm ok with this as the bike is a 29/27.5+ aka: 6Fattie.

The problem is I bought rims for the build which are 110 boost front but 142 rear. No problems, I says, and spent an additional $25 on a 142 to 148 boost hub conversion. This is simply (2) 3mm spacers for the ends of the axle area and spacer to move the brake rotor out to meet the wider sitting caliper.

I've been acquiring the components for this build over the last few months and wanted to go 2x11 with an XTR crankset (M9020) 36-26. Two-by is not possible on this frame, so...

The questions I have is do I really need a Boost compatible crankset since I'm really running a 142 rear hub and the cassette will NOT be 3mm more outboard since the conversion happens outside the cassette area? Note: Boost 148 hubs are wider than 142's, +3mm from centerline on each side for a total of +6mm. With these hubs the cassette is kicked +3mm outside, requiring +3mm from the chainring to maintain chain line. This is achieved at the spider and that is a Boost crankset.

Because I'm still running a 142 hub with the cassette in the 142 standard position, can I simply convert the 2x crankset to 1x and go with that? If not, why? Is my thinking correct that a boost crankset in this set-up would create a messed up chain line since the cassette isn't +3mm out?

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I think a slightly better solution is to sell the 142 wheel while you still can and replace it with a 148 wheel. Knowing your skills, you could probably rebuild it with just a 148 hub too. The crankset can be similarly replaced.

I don’t see the need to faff around with adaptors and chainring clearance when the proper configuration is readily accessible. You’re losing out on the stiffness + strength benefits of 148 while also watching your existing wheel/hub’s resale value drop by the day.

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  • Good points (dang it!). You're absolutely correct I should be doing it right instead of stop-gap measures. I'm a bit deluded by the opportunity to sport my first XTR crankset (gently used, great price), which I'm normally unable to justify purchasing when the equally capable but more economical XT is among the choices. Perhaps on a future hardtail build.
    – Jeff
    Commented Jun 30, 2021 at 0:03
  • @Jeff A boost XTR might pop up soon near you. There's actually one on sale on PinkBike for $200CAD, with a 32t ring. XTR cranks are a decent 100g weight savings over XT, so not a bad choice: pinkbike.com/buysell/3086155
    – MaplePanda
    Commented Jun 30, 2021 at 0:46
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You are correct. The cassette hasn't moved, so the rear chainline hasn't changed, so you don't need to compensate for it.

Note however that a non-boost crank in a boost frame might have chainstay clearance issues, depending on the chainring size. You'll need to check your particular combo for this.

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