Putting 11-40 cassette on Ultegra di2?

[Terry Kroenung]

Is it possible to put an 11-40 cassette on Ultegra di2 (8150, medium cage)? Bike is a Fezzari Empre SL road endurance bike with Zipp 300s wheels (Shimano micro spline hubs). Chainrings are 50/34. Alternatively, can I put a 32T chainring in front? I'm 65 with a heart transplant and ride in REALLY high/steep Colorado mountains. Thanks.

[ReapThaWhirlwind]

Low sprocket max is 34T.

Even though the total capacity is 39T, you would need to swap out the cage for an SGS long cage to hope to have a crack at 40T.

In summary, you can put any chainring on the front that they make for the BCD of your crank. However, the max difference for your derailleur is 16T. Meaning, if you ran a 32T on the second ring, you would need to run no larger than a 48T on the main ring. For what you do, I would recommend a 46/36 combination. Maybe swap out the 36 for a 34. I can't say that a 32 is going to help tremendously more than what you can achieve with a 34T and the cassette range you're asking about.

They are rare, but they do have the 32T in BCD 110 road.

https://mikesbikes.com/products/fsa-gossamer-abs-chainring?variant=40459184832581

[ReapThaWhirlwind]

Another option that could be available is to swap out your Ultegra crank for one of these.

Oval rings really help on climbs, and this gives you options to run 48/32 (as mentioned) or even 46/30 (if necessary).

https://www.tradeinn.com/bikeinn/en/rotor-q-rings-dm-oval-chainring/136714069/p

[Sybian]

I am out of my range of experience here since my touring bike is 52/38 up front, and a 7 speed 13 to 34 (13, 15, 18, 22, 26, 30, 34) cluster. Is it possible using a long cage (previous comment) on the rear derailleur to have a 42T large cog, and retain the same front ring setup? I've never had a problem shifting a front derailleur well beyond its rated maximum tooth differential (mine is rated for 10T max., but easily shifts 14T). You generally should be shifting (front or rear) without extreme loading on the chain regardless of whether or not you are within a derailleur's tooth differential range for smooth shifting. Manufacturer's specs are given as a general guidance for optimal performance, but not as limitation of the functional range (my old Campy short cage rear mech does 2 more teeth than it specs). I would experiment only by changing one item first and see how it works. Obviously your indexing must still match your rear cluster setup (10 speed?). If you haven't cycled with the gearing you are going to try how do you know it will be adequate? I would try the simplest and least expensive option first by staying with your original rear cluster and opting to use the 32T front ring and see how it feels and shifts for your needs. If front shifts okay, but you are still struggling with the low gear in the rear then increase the rear cog size and see if you can still shift okay, as well as being comfortable in that gear. If it doesn't shift well but you like the gearing then change to a long cage on the rear derailleur. Hopefully, you will be able to shift smoorhly front and rear, and be able to cycle using the gearing.
As previously mentioned, some professional help and advice might be a good option if you are not comfortable doing the work yourself. I would get a bike shop's opinion regardless and see what they have to say about it.

[ReapThaWhirlwind]

The differential for the rear derailleur is important because it correlates to the chain length. And if you exceed the difference, when you shift to the smaller ring, you're going to have chain tension issues.

[Sybian]

(04-14-2023, 02:53 PM)ReapThaWhirlwind Wrote:  The differential for the rear derailleur is important because it correlates to the chain length. And if you exceed the difference, when you shift to the smaller ring, you're going to have chain tension issues.

@Terry Kroenung
You won't know until you try. My derailleur has cage tensioning adjustments, and I can also adjust the hub position in the dropout to further be able to tweak tension. I am not saying it will work; just saying it will be an easy and inexpensive part to test the drivetrain. It might be that the derailleur still has the tensioning ability, but without testing on the bike it is hard to say definitively whether it will or will not function.
On a dual chainring configuration (whether using the 34T or 32T) you should never be riding on the smallest front and rear cogs at the same time, nor on the biggest front and rear cogs at the same time. Aside from cross-chaining, you are essentially chewing up your front rings when you have the chain at its greatest angle causing a steel chain to cut into aluminum rings to engage the teeth at those angles. It also causes the tensioning (lower) jockey pulley/cog on the derailleur to be worn quicker; along with making for a noisier drivetrain when pedalling. Gear ratios overlap between using the large and small rings so there is no reason to be riding both small cogs or both large cogs at the same time. My bike's gear ratio overlaps when when riding 52/30 or 38/18. I never need to be riding my 2 largest rear cogs on my 52T ring, I have already shifted to my small ring when still on the rear 18T cog. I have easy shifting, proper chain tension, quiet operation, and very little ring wear over a long period of time. The problem is that many riders are not aware of their gear ratios and shift from ring to ring far later than they should. This is very apparent when looking at the large ring which will have obvious wear on the outer leading edge of the ring's teeth. Of course a poor chainline setup can affect this wear even more.
Terry, if I were you I would calculate (and chart) your gear ratios for both rings so that you have an idea where your general overlap is. This will allow for better shifting, less wear, and quieter operation. If you are building a rear cluster from scratch and selecting each cog then it will allow you to custom make the ratio overlap range (my setup is very tight with a 7 speed, essentially only 1 gear ratio overlap and another being very close) between large and small chainring usage.