I've just finished (well not quite) setting up shifters on my bike. All new cables and outers, chain and cassette with used Sram Force shifters and rear derailleur. Shifting is super smooth and switching from Shimano I really love double tap.
One problem though: I can't shift into the lowest gear / largest sprocket in the back. When I try, it shifts up and immediately back down to the second largest sprocket. The shifter makes a noise which sounds like the usual upshift sound, directly followed by the downshift sound. It feels almost like after the upshift something forces the downshift. I can upshift and hold the lever in the position and it will stay on the largest sprocket until I let loose. The shifter also behaved that way, even when it wasn't hooked up to any cables.
Rear derailleur h+l screws are setup correctly, cable tension is fine, derailleur hanger is not bent.
That leads me to think either something with the indexing in the shifter is broken. Which I have no idea how to troubleshoot and fix. Or something with the derailleur and cassette combination/setup is off.
In case of the second option, it might be related to this: after shifting down to the smallest sprocket, I can make an additional shift down, which leaves the chain on the smallest sprocket and doesn't push it off the cassette. It almost feels like a half-shift. Is this a trimming feature or setup error? If the issue is related to that, could it be that the half-shift would actually shift into the smallest sprocket if setup correctly? Maybe because of a missing spacer or compatibility issue with the cassette or hub? I'm not using any spacers on the hub.
Sram Force DoubleTap Shifters (10x Road)
Sram Force Derailleur (10x Road)
Shimano CS-HG500-10 (10x Mountain)
Shimano XT FH-M756A Rear Hub (10x Mountain)
Thanks for your help!