I have an FSA OS-140 stem with carbon fiber face. It looks like it may be more common for a road set up, but is there any reason not to use it on a mountain set up I am building (Santa Cruz Superlite)?
No, it's solid carbon. But they're actually stronger than the alloy version so you're good to go. Both faceplates are really strong, though. A big improvement over the old style machined faceplates from 2004.
I use these stems on mountain bikes all the time, the OS-140 is a great general purpose stem. I have run the OS-115 with no problems also. I once saw an OS-115 stem that was broken by 710mm DH riser bars on a singlespeed once, so maybe don't run that massive-torque combo.
I have an FSA OS-140 with AL face on my Norco Fluid (5.2 inch) AM bike and it works fine. I believe the carbon on the faceplate is only an overlay covering an AL piece, so it should be fine.
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