I have a Trek Domane with 8100 Ultegra on it. I swapped the front crank to a Rotor ALDHU 50/34 w/ 155mm and want to increase the rear cassette to 11-36 from 11-34. I found Rotor makes an 11-36 although I can’t find it anywhere. What are my options if I’m willing to change out other parts to get something even bigger cassette-wise with Di2 like 11-40? The application is climbing mountains, with artificial joints. Cadence is everything. Thanks for any help.
CJ
Ultegra and DA only officially support a maximum of 34t cassette, although I don’t think I hae heard of anyone having problems with a 36t. 40t may be pushing things a bit, but should be doable, especially with a Wolftooth Roadlink. Will likely need a longer chain too.
The new 105 7100 officially supports 36t, so is probably more 40t friendly, but getting your hands on it at the moment would be difficult.
Also consider perhaps swapping the 34t inner for a 33t inner for a few more rpm (not sure if this is possible with Aldhu?)
Thanks. The Wolflink doesn’t work on 12 speeds they told me. Will try the 11-36 Rotor cassette, if I can find one that is…
Thanks, Chris
From what I hear, Shimano RDs usually accept cassettes that are a bit larger than officially specified. So Rotor’s 12-speed cassette will probably work, but I haven’t tried that. Rotor’s cassettes are from what I hear a thing of beauty. I have an oval chain ring by Rotor on my mountain bike and that is beautifully machined. Rotor even included gorgeously machined bits to screw the chainring into.
They are soooo much easier to mount than those damn “decorative“ XTR counter parts. The latter are a better fit visually, but getting them in straight … and they four separate, non-interchangeable. Who thought this was good design 
Another option is to buy a super compact front chainring. I also have the rotor crankset myself, and you can buy 110bcd four hole chainrings made just for climbing. On the cheap side, there’s these Stone 46/32 chainrings that will work.
Check out this video where the topix gets explored
That looks like an interesting video. I decided to order the Rotor 11-36 cassette (found one in Germany) and a KMC DLC12 chain and see if the Di2 (and knee and ankle) will tolerate these. Thanks for all the great advice.
CJ
I have a 110 BCD Rotor 3D+ crank set, and was able to put a subcompact 46/30 set of chainrings on it for some very steep hilly rides. Everything worked just fine.
Hi Guitar007!
I am an avid climber and I have the same wish as you had 
.
I rode on a modifided Cannondale Synapse with Ultegra 50-34 crankset and a SRAM 11-36 cassette.
I just upgrade to a new bike equipped with Ultegra Di2 12V (50-34 + 11-34). After 2 months I still miss the agilty of my previous configuration and I can’t keep the pace I had before.
I have been surfing the net and I found your post.
May I ask if you had the modification “Rotor 11-36 cassette + KMC DLC12 chain” done? If so, how does it work?
Is it correct that’s enough to change cassette + chain and no other components of the rear derailleur?
Thank you in advance
Any other advice will be appreciated.
Sorry to resurrect this thread. Just want to see if anyone have some successful story on the larger cassette (11-36 or 11-40) and perhaps at the same time to have a smaller chainrings (46-30) with the existing Ultegra 8100 groupset?
Thanks in advance!
Not 12-speed, but I’ve run an 11x36 cassette and 46x30 chain rings with my Ultegra 8050 with no issues. All it took as a B screw adjustment and I think I had to shorten the chain a bit. Shifting wasn’t quite as crisp as normal, but no other issues or problems.
Thanks for your info and it’s great that you can make it work!
What I have been hearing during my research is that the Shimano 12-speeds is less forgiving on changing either the cassette nor chainrings because of the narrower chain width and less space in between the gears sprockets. Also wolf-tooth said their derailleur cage extender does not compatible to the 12 speeds ones. 
Let me share my experience with you.
Currently me and my wife both ride with Shimano DA R9200 (12 speed).
For Cyclo riding in the mountains we use a 11-34 cassette and subcompact cranksets.
I am using a Power2Max with 48x31 chainrings.
My wife is using Shimano GRX800 (11 speed) crankset with 44x28 chainrings (TA Specialites)
For my wife we use a WickWerks Fit-Link adapter to lower the front derailleur. Perhaps that would help my shifting as well, because shifting to big ring for me is not super, for my wife is perfect.
We both finish in the top 25% for a Cyclo, but our cadence is sometimes 1.5x of other riders.
Previously I have used Shimano DA R9100 (11-speed) with RoadlinkDM. Shifting rear from 12 to 11 is not good. Used a Sram 11-36 cassette. Shifting larger cogs is super, the smaller you get, the worse is shifts. Worse shifting here means it takes a bit more time. For a cyclo taking me 7 hours that is acceptable, for a crit I would not use it.
Before that I have been using the normal Roadlink with Ultegra 10 speed and SRAM 11 speed.
On SRAM it shifts best. Even forgot to remove the Roadlink once and noticed only a week after.
Used a 33 tooth front ring. No shifting problems, but some cranksets cannot handle it and the chain is lifted slightly and will jump teeth with high torque.
Since Shimano now has GRX-12 speed I would recommend going for that. You can exchange the chainrings for after market (TA Specialites) to go even smaller than the lowest stock option of 46x30.
Yes, you sacrifice your large/small a bit (48x11 instead of 50x11). A bummer in sprints or in high speed (bunch) riding. Very rare and worth it for me to be able to climb for an hour on 10% grade in a pleasant cadence instead of grinding at 50 rpm.
I can only recommend good rpm on climbs.
My way of thinking:
good luck
I haven’t had an issue swapping chainrings.
My bike came with either an ultegra 12 sp crank or i could swap it for an fsa powerbox carbon power meter crank.
I then ordered oversized chainrings for it.
ultegra di2 only officially supports up to 52/36 but i threw a 54/39 made specifically for that fsa crank, and then just used etube to do manual trim adjustment and it shifts perfectly.
Could you describe that process in more detail?
I also run a 54/40 crankset with my 12 speed ultegra Di2 and my shifting is perfect as well.
Sure thing.
If you haven’t used the etube app before, you will download it, then hold the button on your derailleur until it starts flashing.
While it’s flashing it’s able to pair with your etube app ( i have it on ios )
It usually asks if you want to change a passcode, I just skip that.
Once connected and in the app, you will go into the maintenance tab. Select front derailleur adjustment.
This is going to automatically position your gears in a few different positions ( big little, big big etc. ) by making the derailleur change and asking you to pedal so the chain falls into position, and ask you to adjust the physical position of the derailleur by clicking on the slider in the app, and it will make a microadjustment in or outbound depending on the direction you click, until you are 0.5-1.0 mm from the outside of your chainring, then continue, it’ll as you to pedal a few times again as it goes to the next step, reopeat the process, and at the end you will be asked to position the derailleur 0.5 mm from the inside of the small chainring.
This usually will take care of any rub issues on the front chainring, but if you are noticing that it is rubbing in a certain position, then repeat the process and position the derailleur a tiny bit closer or further from the chainring depending on where it’s rubbing.
Eg if the chain is hitting the inside of the derailleur in your biggie small, you know to move the drailleur slightly closer on that step.
It’s fairly intuitive after the first one or two times doing it.
I’ve actually started doing my rear adjustments while riding my bike on the road using the app to totally get rid of any noise, it’s super nice
It sounds like the Ultegra 8100 di2 can be more forgiving and handle bigger chainrings (54/39 or 54/40) easily for more speed, but not many examples to smaller ones (46/32) for climbing.
I have been researching and see if there is any smaller round or oval shape rings (i.e. 46/30) that supports BCD110/4 Shimano’s 12 speeds. I have a spider type power meter as well so my situation is a bit more complicated. I need the 2 pieces 46/30 rings. One piece ones won’t work with my power meter. All of them I could find are for 10/11 speeds only.
Rotor: they supports 12s, but smallest only 50/34
Easton: only for 11s
Absolute Black: 10/11s only
I guess Shimano’s 12 speeds are still a bit new to all 3rd party vendors and or the market is too small to support them.
I am not using a shimano crank on mine, the one I have is an fsa powerbox ( basically an fsa branded ng eco ) which came as a stock option on my bike with the 12 sp groupset on it and it works absolutely seamlessly.
Gives more leeway for different chainring options.
I don’t see why it would not work just as well going smaller.
Here’s the one I have which may well be similar to what you are looking for.
Basically the different between the cranks is being 1 mm different in chainline with the 8100 set has plenty of adjustment in the trim to account for the difference.
I REALLY can’t imagine why some other 11 speed chainrings would not cooperate, despite being listed as 10/11 just like mine are, despite mine coming as an option on my 12 sp bike and operating flawlessly.
Which brand of chainrings did you buy?