Fnhon Ultegra Di2 12 Speed: Short Cage Conversion using Ultegra RD-R8000

Although I managed to install the Mr TiParts super short derailleur cage onto the Ultegra Di2 RD-R8150 12 speed rear derailleur earlier, it was not a satisfactory modification. There were too many issues with the modifications that can affect the performance.

Therefore, I continued to look for a better solution to have a shorter cage rear derailleur on the Fnhon Ultegra Di2 12 speed folding bike. Then it occurred to me that I should consider original Shimano rear derailleur cages that are shorter, instead of third-party ones.

Drawing from previous experience when I modified the GRX rear derailleur with an extra long cage for the Cervelo Aspero gravel bike, it may be possible to find a compatible shorter cage to replace the stock cage on the Ultegra RD-R8150 rear derailleur.

After removing the Ultegra Di2 RD-R8050 11 speed rear derailleur from the Dahon MuSP, it was available for comparison. All R8000 and R8050 mentioned below are the short cage SS version.

Long outer plate of Ultegra R8150 vs the shorter outer plate on Ultegra R8050 11 speed rear derailleur.

Based on this comparison, the shape and holes are approximately in the same place. A different spring hooking hole will just change the spring tension, but should still work fine.

To study further, I compared the inner plate of R8150 vs R8050.

Cage length of R8050 is 72 mm, which is quite a bit shorter than R8150 which is 100 mm.

To confirm the compatibility, I removed the cage set from Ultegra R8050 (11 speed) and installed it into Ultegra R8150 (12 speed). There is no major issue found, and seems to be nearly 100% compatible. This is how the Ultegra Di2 R8150 12 speed rear derailleur can be modified to have a shorter cage compared to the stock condition.

However, it would be a waste to cannibalize the Ultegra Di2 R8050 11 speed rear derailleur cage, as it is a rather expensive piece of component. I should try to get the inner and outer plates replacement parts instead, and install them onto the R8150 rear derailleur.

Based on this compatibility chart, it states that R8000 inner and outer plate are compatible to R8050 rear derailleur.

My logical deduction is that if R8000 is compatible to R8050 (based on the chart), and R8050 is compatible to R8150 (based on my experimentation above), then R8000 cage set would be compatible to R8150 rear derailleur.

I tried to find R8000 inner and outer plates as replacement parts, but they are rather expensive and not easily available. In the end, I decided to just get a whole Ultegra R8000 rear derailleur instead, and cannibalize the cage set.

Ultegra R8000 mechanical on the left vs R8050 electronic on the right. They look quite similar as they are from the same generation, with the same design language.

Ultegra R8150 12 speed outer plate on the left, vs Ultegra R8000 11 speed outer plate on the right.

Cage length is 100 mm vs 72 mm as measured earlier.

Shaft on the outer plate looks to be 100% identical, which will ensure perfect compatibility.

As shown earlier, this area looks nearly the same, except for the different spring hole locations.

Comparing the inner plates as well. Gloss vs matte surface finishing and different length.

Ultegra R8000 short cage transplanted onto the Ultegra Di2 R8150 12 speed rear derailleur!

Using the 12 speed pulleys with tall teeth for better chain retention.

As mentioned many times earlier, the shorter cage of 72 mm works better on small wheel bikes, although it also reduces the chain capacity to 33T (same as R8000-SS and R8050-SS).

Looks like a mini version of the standard Ultegra R8150 rear derailleur.

Extra short cage version of Ultegra R8150 weighs 249 grams, which is 13 grams less than the stock 262 grams.

There is still interference between the pulley teeth and rear derailleur body, but this can be managed by controlling the chain length to ensure the cage is not fully released even in the top gear.

I'm pleased with this modification, as it is relatively easy to perform, while nearly 100% compatible, short of the pulley interference. It is good that Shimano kept the design of the cage set similar between 11 speed and 12 speed, and between mechanical and electronic rear derailleurs.

You could even use this extra short cage version of Ultegra Di2 R8150 for a 2x12 speed drivetrain, provided you limit the chain capacity to 33T. This is possible with a rear 11-30T cassette (19T difference) and a front 54-40T crankset (14T difference).