you really need to get the GS (longer cage)
your math is incorrect chain wrap (capacity) is Diffference of smallest largest rear + difference high low front
in this case capacity needed = (28-12) + (48-26) or 16 + 22 or 38
I think your spec on the short cage are wrong also, what I see for the GS (medium cage) is 40 warp capacity and 32 max rear cog and 22 difference up front the short cage seems to be 34 wrap
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