Tinkering: Attempting the “Impossible”

Derailleur Limits

My good friend Dr. Pain and I (JOM) are known bicycle tinkerers. Our Monster CX creations (Dr. Pain’s rigJOM’s rig) are the result of planning, customization, tinkering; creating bicycles that weren’t commercially available at any bicycle dealer.

JOM has been running a 11-36 11 speed SRAM 1170 cassette in conjunction with a 11 Speed Shimano Ultegra Di2 derailleur for a few months. The Di2 derailleur is rated at a maximum of 32 teeth. With the 36 tooth cog installed and the “B” derailleur screw turned clockwise approximately 2/3rd down, there is plenty of room between the top jockey wheel of the derailleur, and the cog itself.

With a little time to kill over the past weekend, Dr. Pain and JOM wanted answers to what is deemed impossible (or perhaps not recommended) by most manufacturers.

Running a 40 tooth cassette cog with a Shimano mid-cage derailleur, mechanical or otherwise.

Why you ask? While a 34 tooth front chainring with a 40 tooth cassette cog sounds like a low gear on a road bike, on a Monster CX rig with 2.0″ tyres, that low gear is negated because of the increased height of the 29’er tyres.

UltegraDi2-40TCogThe image above tells the story, Ultegra Di2 mid-cage derailleur in use with a 40 tooth cassette cog. Keen eyed observers will note we are testing with a 10 speed cassette – just to prove our theory.

Does it work?

In short, yes, but a small change needs to happen first. The stock Ultegra Di2 “B” screw, turned clockwise until it bottoms out, almost works. The chain will shift perfectly onto the 40 tooth cog, with a minor amount of cog to pulley contact. Obviously this is not acceptable.

The problem is solved by installing a longer “B” screw, which safely pushes the top derailleur pulley clear of the 40 tooth cog for flawless shifting. Win!

Try this modification at your own risk. We are not held responsible if you destroy an expensive derailleur because you didn’t follow the manufacturer’s specifications for maximum rear cog size.

With the legal jargon over, thanks for reading!



  1. K-Dogg K-Dogg

    Why is there spaghetti sauce on your sprocket?
    It’s making me hungry.

  2. Though you might be interested in a write-up on a “hack” I recently did on my Monstercross. It’s not Di2 but similar in nature. Photo album on my Google+.

    Seems like there’s a lot of interest in 10 vs 11 speed as well as Road vs Mountain group compatibility. I just completed a mix/match combination I think your readers would like to hear about. I have a Giant Anyroad Comax with Shimano 105 11-speed group set. The bike has compact 50-34 crank and 11-32 cassette with 32 mm tires from the factory. After taking a group trail ride with mixed cross and Mountain bikes I soon discovered the climbing/traction limitations of the factory tires/gearing. I wanted to be able to swap between wheelsets with 32mm cross tires to as big a tire as I could fit in the frame with even lower gearing. The 11-32 cassette is the lowest geared road 11-speed cassette available and I didn’t want to change to a Mountain crankset. The solution, it seemed, was how to reliably use the XTR 11-40 11-speed cassette. I purchased a used Giant Mountain 10-speed wheelset and mounted Panaracer Firecross 45 mm tire for the front and Bruce Gordon Rock-n-Road 43 mm tire for the rear. Since the XTR 11-speed cassette has the same hub width as a 10-speed cassette it mounted to this new wheelset without modification. With a 50t chain ring and 40t cog the stock 105 medium cag rear derailleur wasn’t up to the task of taking up that much chain and the long cage Mountain derailleurs have a different pull ratio and are therefore incompatible. Also, there isn’t a “B” screw long enough to keep the upper jockey wheel from rubbing that 40t cog. What I needed was a Long Cage 11-speed road derailleur. After examining the exploded views of the Shimano road and Mountain rear derailleurs I found it appeared the long cage from a direct mount Deore LX 10-speed RD-T670-SGS derailleur would interchange with the 105 medium cage. I purchased a used Deore RD-T670-SGS, removed the cage/P-tension spring, removed the 105 cage/P-tension spring and sure enough the mounting/pivot pins are identical. I installed the Deore long cage/P-tension spring on the 105 and now have a Long Cage 105 11-speed rear derailleur. Prior to my rear derailleur “hack” I would switch chains when I would switch between wheelsets. Also, I reversed the “B” screw, had it adjusted all the way out, and had to install a spacer between the screw and derailleur hanger to prevent jockey wheel rub. The Long Cage 105 derailleur “hack” solved 2 problems:
    1) I no longer have to switch chains when switching wheelsets, the long cage easily handles the longer chain.
    2) The upper jockey wheel on the Deore cage is offset slightly away from the cassette compared to the 105 cage. This allows an almost normal “B” screw adjustment even with that big 40t cog.
    The time it takes to swap cages/P-tension springs is about 30 seconds (not including chain) and for me was under $30. I haven’t priced just the cage from Shimano but these Deore derailleurs can be found used for cheap. I can cross chain 50-40 or 34-11 with no issues and it shifts flawlessly.

    Hack away guys,


    • JOM JOM

      Nice solution Tom! I forgot that reversing the “B” screw can also solve the jockey wheel rub issue. Speaking for myself, there won’t be a 40T cog going on my bike, rather this was to help my friend Dr. Pain perform a proof of concept. With an MTB crank on my Monster CX rig (which started life as a 29’er), I’m limited by chainring size. A 42 / 28 setup has proven pretty good in mountainous terrain, and I am loving the 28 x 36 11 speed low gear 🙂

  3. Hello.
    Do you think it would be possible to run 6870 long cage and a long b-screw with 42T? I’m currently running XTR 9050 (x1) with a XX1 10-42 cassette on my race bike and it works well. However, I need more gear range on my 2nd and 3rd bike which today got 6770 (x1). I really like the quietness and smoothness of the Ultegra. The XTR is much more noisy and harsh, not to mention more than twice the price of 6870. That’s why I’m looking into 6870 + XX1 10-42 or XT 11-42. I’d be more than happy if you could answer this. Thanks!

  4. Avatar John

    I’m confused. SRAM’s website indicates that the 11-36 11 speed SRAM 1070 cassette you refer to is not 11 speed, but 10 speed. Can you clarify? I’ve got an Ultegra 11-speed long cage rear derailleur and would love an 11-36 11 speed cassette but can’t find one anywhere. Ideas?

    • JOM JOM

      Oops… typo! That should say PG1170 cassette. 1070 is 10-speed and 1170 is 11-speed. Sorry about that.

  5. Avatar Matt

    Where does one get a longer b screw?

    • JOM JOM

      We found a suitable screw at a local hardware store. Sometimes you get lucky

  6. Avatar Fe

    I want to switch from 11-32 to 11-40, too. Maybe you can help with the following question.

    Current setup:
    – Aivee Edition One road rear hub
    – SRAM Rival 22 group (11-speed) with medium size derailleur
    – SRAM Rival PG-1130 cassette (11-fach) 11-32

    My plan:
    11-40 tooth or 11-42 cassette
    Wolf Tooth Roadlink

    My question:
    Which of the following (MTB) cassettes can I install on the hub and which will work with the road SRAM Rival 22?
    – SRAM MTB cassettes with 11-40 teeth
    – Sunrace CSRX8 RX 11-Speed Wide Ratio Cassette 11-40
    – Shimano Deore XT 11-40t Kassette
    – Shimano XT CS-M8000 11-40t Kassette

    Someone told me that I can install any of these but that I need a 1.85mm spacer for the MTB cassette even if this is 11-speed. Does that mean the cassette is smaller? Will that cause performance issues?

    Hope you can help.

    • Avatar jwilli

      First I don’t have your answer, however I do have a similar issue. I have Di2 and want to run an XT or XTR cassette.
      If any of the expert trendsetters watching have knowledge that will help either of us, please chime in!

  7. Avatar Red

    I am currently using an ultrgra crank with 52-36 i switched my cogs with a 11-42 xt my local bike shop just changed the b screw with longer one i cant go big big my question is it better use a road link?

  8. I would not tinker around. Just get an 11speed XT or SLX durailleur and use it with your 105/ULTEGRA/Duraace.

    You need to get a Jtech shiftmate and it will work just fine !
    You can now use your road shifter with an MTB derailleur.

    There are only in brand-shiftmates. At this moment you cannot pair up your Sram Red with XTR or on the contrary.
    Only Sram to Sram and Shim to Shim.

    • JOM JOM

      But we like tinkering, so that isn’t going to stop. Very familiar with Shiftmate (formerly Jtek Engineering), use them on a couple of my older Campy mechanical road bikes.

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