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I have a problem when trying to shift from the smallest sprocket on the rear casette to the bigger one when on the small chainring. It tries but it seems there isn't enough tension on the cable. If I "half click" the lever it goes up. If I tension the cable ever so slightly more then I have a problem with crisp shifts down. My setup is road/MTB so quite unusual since I would like to go with a bigger casette over 34T. My crankset is Shimano Ultegra FC-6600 53/39T, rear casette is Sunrace CSM96 11-34T 9sp, rear derailleur is Shimano XT RD-M772 Shadow, SGS 9sp, brifters are Sora 9sp ST-3500, chain is Sunrace M94 9sp. I've cut my chain using small-small method. I was worried that it is a rear derailleur capacity problem but when I calculated it, it turned out that I still have a lot of room since rear derailleur's total capacity is 45T and my setup is only 37T. Recently I have checked the rear derailleur hanger with a tool and it's straight. Also the casette and chain are almost brand new, cables and housings are brand new Jagwire LEX-SL and the cable is also Jagwire polished version. But I guess it's not a friction problem since the downshifting is good. When I am on the big chainring and smallest sprocket the problem is non existent and I can shift from smallest sprocket to biger one no problem, back and forth. I heard some people mod the rear derailleur and clamp/route the cable on the opposite side of the clamp/pinch bolt (place without the groove). But I think it is applicable for 10sp shifter and 9sp derailleur combo to have a proper pull ratio. I must also mention that I changed the cage spring tension in my rear derailleur by moving the cage spring notch to the increased tension hole to improve the downshifting. Can somebody help to outline where the problem lies?

Combination small chainring-11T sprocket - front: enter image description here Combination small chainring-11T sprocket - rear: enter image description here

Combination small chainring-12T sprocket - front: enter image description here Combination small chainring-12T sprocket - rear: enter image description here Chain sag comparison - 11T enter image description here Chain sag comparison - 12T enter image description here

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    Not sure of your reasoning on the overly-long chain. Normally you'd have enough chain that it doesn't bind up in big-big. Can you please add a clean and clear photo of your rear mech from the right hand side when the chain is in the smallest 11 tooth cog and the shfter is in that position, and a second photo when the chain is on the second-smallest cog and shifter is in the right position for cog2 (12 tooth probably)
    – Criggie
    Jun 21 at 11:32
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    In my experience friction just makes one shifting direction work badly. Which direction it is mostly just depends on the cable tension (barrel adjuster).
    – Michael
    Jun 21 at 11:35
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    More seasoned cyclists may read "low" as the biggest sprocket, since that's the lowest gear. Hence, I edited the title. Feel free to undo the edit if this doesn't convey your intent.
    – Weiwen Ng
    Jun 21 at 12:56
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    FWIW, I've had several bikes with Shimano road shifters and a Shimano MTB rear derailleur. Shifting was never as good as it was on a bike with all road or all MTB components. No matter how I'd adjust things, there was always some combination that didn't quite shift properly. I'd be happy on any of those bikes with only having to coax a shift from the 11t to the 12t with the chain running small-small. Jun 21 at 12:57
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    From the photos you’ve added the chain tension looks quite low. I mean there is still tension, it’s not exactly sagging, but almost. You could temporarily shorten the chain by using a piece of wire like this: by cdn.topeak.com/storage/app/media/product/tools/mini-tools/… (when turning the cranks to check the shifting, just make sure you don’t turn the cranks so far that the wire goes into the derailleur)
    – Michael
    Jun 21 at 14:10

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Most road drivetrain components don’t work with mountain bike components. They have different cable pull ratios and other such discrepancies. But there are exceptions. Here's great site I found that goes into detail on which road shifters go with what MTBs derailleurs:

https://bikepacking.com/gear/guide-to-mullet-drivetrains/

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