hacked Ultegra Di2 long cage rear derailleur

Reader “Comekk” just sent over photos of his Ultegra Di2 rear derailleur hack. Standard units are only available with short cages, as expressed directly from Shimano:

“Existing Shimano Ultegra Di2 and Dura-Ace 7970 Di2 accommodate cassette gear ranges up to 28T (with SS cages) as well as new Dura-Ace 9070 Di2. That is factory spec as recommended by Shimano.

However there are some aftermarket vendors who have created RD cage solutions on their own to widen the gear range and riders have been tinkering with cages from those vendors and even Shimano to widen the gear range. Again, those options are not ‘formally recommended’ by Shimano with existing Di2 systems and the forthcoming Dura-Ace 9070 system.”

So, what’s a guy to do if he wants more gearing?

Comekk simply took the long cage from a standard mechanical Ultegra derailleur and swapped it in, replacing the Di2’s shorter cage. The only adjustment needed was fiddling with the B-screw tension (and, likely, adding a few chain links).

Simple, and far less expensive than the K-Edge units…which could open this up to more mountain bike hacks by simply getting creative with the climber’s button pods.


  1. Flemming Jørgensen on

    Jeg har lavet det samme på den nye Trek Madone 5,9 – 2013, da jeg købte den for 01-10-12, ved at stille på B skruen kan den tage en 12-32 T.

  2. Brandon on

    So the mechanical cage just bolted right on? Awesome news as I have a medium cage Ultegra RD right next to me, a Niner frameset hanging on the wall, and an Ultegra Di2 bike that I never ride…..

  3. Aaron on

    The best part is the GS cage assembly components are available from Shimano (at least for a little while longer…). I have swapped the 6700 cage assembly onto a broken 7900 derailleur for a customer before when he snapped the stock carbon assembly to lower cost of repair.

  4. Shane on

    Obviously he wants to run a triple up front, or a 54/34 chainring combo. As the length of the cage has nothing to do with how big a cogset is.

  5. Brandon on

    Shane: try running 50/34 with a 11-36 cassette and a short cage derailleur and get back to me on how that goes. No triple or 54/34 combo, just a wide-range cassette and a compact.

  6. rick on

    Shane is right. Parallelogram size is what SHOULD change to make a 11 36t work. A longer cage will allow the RD to run more chain to wrap around a 11 36t but it is not ideal. You would have to set the B-screw tension almost at full tension.

    Brandon: I run a short cage MTN RD on a single front ring set up (33t Q ring) with a 11 36t no issues. RD cage size only related to total number of teeth of the drivetrain

  7. Chipollini on

    @ William… Right? Did they do that on purpose? Or is that just the mechanic just throwing on crap pedals for a test ride out of the shop – I bet the guy how built it was not wearing clip-less shoes… “Dude, I just hacked this Di BS with an old mechanical cage.. Totally works, check it it out man!”

  8. Brandon on

    Rick: the longer cage allows for more chain slack. I don’t care if you’re running a single, because your chain wrap on a single with an 11/36 is…wait for it….25 teeth. Guess what the max capacity is on a short cage rear derailleur….wait for it….33 teeth. So like I said before “try running 50/34 with a 11-36 cassette and a short cage derailleur and get back to me on how that goes”.

    Get back to me when you try the scenario I laid out rather than something completely different.

    Chain wrap isn’t dictated only by the cassette (unless you are running a single chainring), but also the chainrings.

    So it’s not so obvious as Shane points out, even a standard or compact crank is going to require some type of mod for it to work with an 11-36 cassette because a short cage rear derailleur can’t handle that much chain wrap.

  9. comekk on

    so this pic is not showing that its on a road bike with a 50-34 and it work, not the best fit but if you are going to run it with a 24-36 , 11-36 or any other mtb it should work
    I did this for a customer and its working on a road bike or you can get a Kedge cage for a little more $ or buy the lang cage from shimano

  10. Gummee! on

    I’ve made several XTegra derailleurs out of XT cages and Ultegra uppers. I’m still running the XTR rear derailleur I made out of 3 ‘trashed’ ones. IME XT and Ultegra rear derailleurs use the similar parts and XTR and D/A use similar parts. Least the older stuff did.

    When you’re broke and have scrounged broken derailleurs, you’ll try anything.

    Good news that DI2 is similar enough to its mechanical counterpart for that to still work!

  11. Shreddie on

    C’mon you dummies. This is a bad idea, period. It’s not about chain wrap capacity – it’s about maximum cog size. Greg is spot on. Brandon, class is in session, listen up: The difference between ‘mountain’ and ‘road’ derailleurs isn’t the cage length (they make short cage mountain ders too) – it’s the length of the knuckle of the derailleur. The distance from the upper pulley to the center of the rear axle is what will determine how large of a cassette cog the derailleur will work on. A shorter knuckle (as typically found on ‘road’ derailleurs) cannot accommodate a 36t cog – no matter how long the cage is.

    Sometimes the max cog size has a little bit of wiggle room to it – the length and offset of the rear derailleur hanger can play a role in this. Bottom line though, it’s a poor life decision to risk ruining your rear derailleur, frame, AND rear wheel, in order to use a 36t cassette with a derailleur that isn’t designed (and a longer cage does NOTHING to address the design issue [knuckle length]) to work with it.

  12. uzurpator on


    You are incorrect. Knuckle lengths on road/mountain mechs, while a tad different, are very similar. What is different is the parralelogram slant is different.

    Road mechs are meant to track narrow range cassettes, thus have shallower slant angle then mountain mechs. This limits somewhat the maximum cog you can use on a road derailleur. However I have successfuly used road mech on a 11-34 cassette ( heck, even 11-38 custom job worked fine 🙂 ). It was just rougher.

    As for this mod. You can use some sort of touring tiriple ( say, 52-38-26 ) with a 11-28 cassette and you are looking at a long cage requirement.

  13. Shreddie on

    I’m standing by my original post uzurpator. Slant angle has nothing to do with max cog size. Take a look at SRAM’s XX1 rear derailleur for a study on this. It has a slant angle of 0 (or 90, depending on your interpretation) degrees (shallower than the road mechs you’re referencing), yet it is still able to accommodate a 42t cog. When shifting to a larger cog the lower pulley moves as there is less slack in the chain for it to take up. When the lower pulley moves forward, the upper pulley moves rearward and downward (the pivot is not concentric about the pulley like on most derailleurs, but rather is offset), effectively increasing the distance between the center of the rear axle and the upper pulley – the knuckle length is dynamic, rather than static. Lesson being that knuckle length is the critical piece to the puzzle (ability to accommodate a large cassette cog), not the slant angle.

  14. Brandon on

    Shreddle: I’m sorry, but without enough chain wrap capacity you’d rip the derailleur right off or have the chain bouncing off of the outside of the cage with no tension (or both), which is exactly what a longer cage prevents. I only started pointing out to Shane that this derailleur mod isn’t only needed for some goofy front chainring setup, but that a standard Ultegra Di2 rear derailleur cage can’t make up for the chainwrap of a standard or compact road double and a wide-range MTB cassette.

    With all of that said, if it still boils down to the knuckle size, why does this mod, or the Ki2 mod, work? I’ll tell you: cage length. If this is all about knuckle length, adding a long-cage shouldn’t allow the derailleur to do what it’s clearly doing in the video.

  15. Shreddie on

    Brandon (and all others interested) — See http://www.bbinstitute.com/dl/dx_demo_chapter_32.pdf, pages 32-19 through 32-21. Barnett’s is about as legitimate a source as can be had. Sutherland’s confirms as well, but their manuals are not available without either a paid subscription or ownership of the hard copy.

    You’ll notice that pp. 19-21 refer only to the distance from the top pulley to the rear cog – not the cage length. The measurement of the knuckle from the center of the top pulley to the center of the der hanger, along with the distance from the center of the der hanger to the center of the rear axle, will determine how large a cog can be used with a given derailleur. Changing the cage length (regardless of how long or short it is made) will have ZERO effect on how big a cog can be accommodated.

  16. Brandon on

    Shreddie: Then why, pray tell, do conversions like this one and the Ki2 allow a derailleur previously only labeled capable of a 28t largest cog allow them to work with a 36t cog?

    Anyway, you stopped one page too soon, read on. Check out page 22. That covers the fact that maximum cog isn’t the only consideration. Know what the other one is? Check page 22 to find out.

    Anyway, here’s my point: clearly the knuckle size on the road derailleurs is fine, or the derailleur in the video and the Ki2 conversion wouldn’t work. What’s needed to make them work? A longer cage. I’m not saying it’s ideal, but it works.

    I understand what you are saying, I’ve put a D/A derailleur mated to a 30t cassette and if you’re not careful the top pulley will jam up into the largest cog. BUT, with a little B-limit fiddling it works so long as you don’t exceed the wrap capacity. But, add a medium length cage and all of a sudden you can use a larger cassette. MAGIC!

    Let’s go back to when you said “A shorter knuckle (as typically found on ‘road’ derailleurs) cannot accommodate a 36t cog”. That’s just flat out wrong. How do I know? Because I’ve seen it work.

  17. Shreddie on

    Brandon, I’m going to address each of your paragraphs (1-5) in separate paragraphs following. Then, I’m done; you know what they say about arguing on the internet…

    1 – As stated in the first 3 sentences on page 32-19 of the previously referenced Barnett’s manual, derailleur hangers are not all the same length or offset as one another. This changes where the derailleur will mount, in relation to the center of the rear wheel. A hanger that is longer or has greater offset will allow for a larger max cog size than a hanger that is shorter or has less offset, which in some cases can result in a derailleur not even being capable of accommodating it’s advertised max cog size. This is again restated on page 32-21, under the subheading Component Compatibility : Maximum Cog Size in the full first paragraph.

    2 – I did not stop one page too soon. Page 32-18 begins a section regarding Component Compatibility. There are multiple considerations that must be made in order to determine whether a particular rear derailleur will work with a given frame (hanger length) and drivetrain (cassette max cog, cassette range, and chainring range), among others. Maximum Total Capacity is the sum of the rear cassette differential (in this case, 11-36 = 25) and front chainring differential (in this case, 50-34 = 16). In order for the rear derailleur to take up the chain slack in the 34-11 combination, and still keep the chain from ripping the derailleur off the frame in the 50-36 combination, it will require a cage with a Maximum Total Capacity (as called by Barnett’s) or Chain Wrap Capacity (more commonly referred to by the manufacturers) of 25+16= 41 teeth. A long cage is required for these reasons, but having a long cage does not enable the derailleur to shift into a 36t rear cog. A longer-than-normal rear hanger, one with greater than average offset, a hanger with a stop tab angle of greater than 35 degrees (see page 32-19, again), an extra-long B-screw, or a rear derailleur with a longer knuckle than is common on road derailleurs will be required in order to move into the 36T cog.

    3 – You have this backwards; you’ve missed the point. The knuckle length on most road rear derailleurs is NOT fine for use on a 36t cassette. The knuckle length is fine if the derailleur has a max cog size equal or greater than the size of the largest cog in the cassette, OR if a myriad of other factors (again, a longer-than-normal rear hanger, one with greater than average offset, a hanger with a stop tab angle of greater than 35 degrees, an extra-long B-screw, or a rear derailleur with a longer knuckle than is common on road derailleurs) all work out to allow for a greater capacity than the rear derailleur is designed for.

    4 – You clearly don’t understand what I’m saying, or we wouldn’t have gotten to this point. In this paragraph you’ve admitted that YOU have put derailleurs and cassettes together that aren’t designed to be compatible with one-another, and that “the top pulley will jam up into the largest cog.” THAT IS THE PROBLEM! Putting a longer cage on the same derailleur DOES NOT CHANGE THE UPPER PULLEY’S POSITION. It ONLY changes how much slack (please refer to my section #2 for this as I shouldn’t need to re-type it here) the rear derailleur can potentially take up. FURTHER, I say “potentially,” not “can,” as road derailleurs often have lighter springs in them than their mountain counterparts – the weight of the additional chain when in the 34-11 combination can be too much for some derailleurs to overcome, leading to a potential derailment as a result of the chain coming off the under-side of the front chainring. That’s another totally different story though, so I’ll leave it for now. Suffice it to say that you shouldn’t need to modify a part to make it ‘work’ – instead the correct part should be used the first time.

    Not going to bother addressing your MAGIC comment.

    5 – You contradict yourself here. In your previous paragraph you admit to having tried pairing a D/A derailleur to a 30t cassette THAT DIDN’T WORK. Don’t bother telling me that ‘it works;’ if the upper pulley is running on the teeth of the cassette, IT DOESN’T WORK. Further, re-read page 32-22 (that you asked me to refer to in your second paragraph); the manual test method Barnett’s says to use will confirm that this is a non-working combination.

  18. KKelly271 on

    While I am very impressed with the technical knowledge of several commenters, I’d just like to know if there’s an option for installing a 32-34T rear cassette on my 2014 Giant Defy. Bike has a compact crank (50-34) and Ultegra Di2 shifters. I understand that Shimano has released / is releasing a mid-cage electronic rear derailleur. Understanding that the hanger length is critical to moving the upper pulley away from the largest cog of the cassette, what measurements do I take to determine if the longer derailleur is even an option? Currently, I have a second bike (Cannondale Synapse) which has mechanical 105 components. I have installed an 11-32t rear cassette and a mid-cage derailleur. Shifting is fine but when on the 32t cassette, there’s noise which indicates minimal clearance and it’s probably not the best case situation. Assistance without criticism is greatly appreciated.

  19. Bill on

    KKelly271 and Chuck, from my reading of Shreddie’s epic lesson on mech/cogset/hanger compatibility a 34t cogset can’t be effectively operated using a mid-cage RD-6870 Di2 mech. That model has a max tooth count of 32t and attempts at extending the capacity to 34t or 36t may result in damage to the components.

    • Warwick Langebrink on

      I have an old Shimano-Ultegra-Di2-RD-6770-SS-10-Speed-Rear-Derailleur short cage Di2 10 speed RD on my super light Paketa Tandem. Desperately need more gears, 11-32 or 34 cassette (instead of 25). I would prefer a 10 speed setup (don’t want to replace several sets of good tandem wheels, hubs). Can I replace my old 10 speed DI2 RD with the new mid cage RD-6870 DI2 11-speed RD and use this on 11-32 or 11-34 10 speed cassettes? Will New 11sp RD-6870 RD work with old 6770 10 speed shifters? Or should I try modify the old derailleur with G’s medium cage and longer B screw. What’s my best solution?


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