I’m building a project for a gravel bike with 32-inch wheels. Because of the bigger wheels, a boost hub was recommended to me 12x148. The frame is planned with T47 internal shell (edit: 85.5mm) BB.
I plan to build the bike on a new 1x13 XPLR E1. And I’m wondering if the standard XPLR WIDE chainline 47,5mm will provide nice shifting and drivetrain performance?
EDIT: I think it’s worth adding that frame is going to have 490mm chainstays. Maybe this could offset 3mm wider boost rear hub working with standard WIDE CL 47.5mm?
If possible, can you use a 92mm T47 shell and then pair it with an XX1sl crank? the 8 bolt chainring should accommodate whatever ring you want to run and give you a better 52mm chainline. That’s what i would probably shoot for.
edit to say - It sounds like a really exciting project and is still uncharted territory. Can’t wait to see it!
My framebuilder told me that SRAM doesn’t provide T47 bottom brackets for MTB. And I would need to play around with spacers with T47 internal cups to extend the shell for longer spindle. Or to look for other aftermarket options.
I haven’t heard before of 92mm T47 shells. I’m not sure it’s possible with my build.
I don’t know much about MTB cranksets or drivetrains. Seems like SRAM standard MTB with 52mm chainline is supposed to run with -3mm chainring offset. That sound like running the standard xplr wide :o
Hmm honestly I would try to get your chainline to conform to the boost rear hub chainline however that looks for the cranks. SRAM is pretty clear about Road Wide cranks requiring a maximum of 142mm rear hubs -
In my experience with full-mount derailleur set-ups (transmission, XPLR) chainline does matter more than it used to.
I’d be thinking about either building it around a 142mm rear hub, which will always offer a lot more flexibility in hub options (as some hubs don’t offer xdr option for their boost models). OR i would look into a different BB situation - nothing wrong with a classic BSA 73 with a mountain crank.
but with what chainstay length is that for? The longer chainstays may cancel out the wider hubs, meaning the angle the chain goes to could be the same as with shorter chainstay and 142. Five mins with a pencil, ruler and paper could quickly answer that question if you have the chainstay length sram was using for that guidance.
I imagine it’d be fine, and if it wasn’t Alugear offer chainrings with 0, 3 or 6mm offsets to fit SRAM 8 bolt cranks from 26T upwards. A few people have suggested keeping the chainline narrower works as well or better in any case. Bear in mind also that with 32” wheels and XPLR you will need a smaller chainring (than XPLR’s min. 38T) if you want anything resembling a low gear, although Garbaruk have announced a 10-52 13 speed cassetre they say is compatible wth XPLR full mount RDs.
If it was me I’d most likely use a smallish Alugear ring with 3mm less offset than XPLR, and maybe that Garbaruk cassette too.
Yeah that’s an interesting point. I see what you’re saying. SRAM says to use a road wide crank for chainstays over 415mm but seem to leave it there. I suspect it will probably work ok. But, for the money that one spends on a custom bike with the most expensive components how ok is ok? Worth trying for sure if money is no object and can always change it up if it it doesnt work well i suppose. The OP is breaking new ground as far as i can tell so it’ll be interesting to see how it goes.
Thank you once again @Harris_Bucklin and @RichGearing for good advice. I’ve decided to switch to standard 142mm hub and go with road wide chainline. I was thinking similar thing to “how ok is ok” - I don’t want to second guess myself and with the help of gear inches calculator I’ve found out that I’ll need 36T chainring (easier fit) - which should provide enough of clearance for the tire and chainring.
@JC1979 My exact thinking and with help of Taleb (knowledgable greek) and ChatGPT (cause my lazy ass around midnight scratching my head around) it looks like extra 3 mm spacing in the hub would be compensated by extra 50mm in the chainstays (I’ve compared my current gravel bike with the one I’m building). Keeping the same chainline 47.5mm resulted in the same angle of the chain. But as said earlier - can I be 100% certain? 32-inchers are brand new thing - I’ll let other folks discover this staff in the future
Having already spent too much time on the topic of 32“ gravel, I would revert that decision and use boost. Unless you use deep section rims (which to my knowledge do not exist in 32“ yet), you might run into the situation that the chain touches the spokes when on the biggest sprocket. Combining it with a regular XPLR crank should not be an issue, as you can run XPLR with a road crank on 142 hubs, it actually even gives you a better chainline.
I focussed on the chainline question, overlooking the 32 inch wheel stipulation. I’m with Harris … go boost to give the larger wheel the best spoke triangulation. I would suggest that’s more important than the chainline being absolutely spot on.
The chainline recommendations from manufacturers are for perfect performance, and in reality there is often a good bit of leeway.
Also, which gears are you going to be in the most? I deliberately got the highest offset chainring for my gravel bike to move the chainring inward to roughly equivalent of running on inner of a double, because I know that by far most of my time is spent in lower gears. I was happy to sacrifice a little bit of shifting performance in the higher end (and honestly, that drop in performance is nothing compared to what a worn chain, lack of lube, poor cable friction or a load of dirt does to shifting performance).
All you need to make it work is a 3mm or 0mm MTB chainring. Sram’s stock chainring is 6.5mm so the 3mm will get you a 51mm Chainline (Boost) and the 0mm offset will get you a 54mm Chainline (Transmission) on the Road “Wide” Cranks.
I would definitely recommend boost spacing with 32” wheels. The wider chainline will allow you to get shorter chainstays and the wider hub flanges will build much stronger wheels.
I’d also recommend a T47x92 Bottom bracket. It’ll allow the chainstays to be welded wider for shorter chainstays. The “wide” spindle cranks are effectivly the mtb spindle mounted to the road crank arms which allows for compatibility with mtb width bb’s.
