Hack! Ultegra Di2 Long Cage Rear Derailleur

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.

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

yup. two different things. max cog and capacity.

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.

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!

shreddle, you are so right 100%

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.

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.

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.

I Want to do the same thing, what derailer did you use?

Thanks