Fnhon Ultegra Di2 12 Speed: Super Short Rear Derailleur Cage by Mr TiParts

While planning for the Ultegra Di2 12 speed setup on the Fnhon folding bike, there are a few issues which needed to be solved. One of them was to create a custom Di2 battery holder, so that I can hold the Di2 battery on the Fnhon frame.

The other issue comes from the smaller wheel size of the Fnhon folding bike. I will continue to use the custom built 451 wheelset from Ascent Bikes, as it is a disc brake wheelset and is still very useful. The problem is that the new Ultegra Di2 RD-R8150 12 speed rear derailleur has a pretty long cage, even more so than the previous Ultegra Di2 RD-R8050 11 speed rear derailleur.

Based on the Fnhon DB12 setup, it seems that the rear derailleur cage does not have much clearance with the rim or tire. Therefore, the new R8150 rear derailleur cage might be too long. It may still work in its stock condition, but there would not be much clearance.

Anyway, I decided to try changing the derailleur cage to a shorter version, to get more clearance. In any case, I don't need such a long derailleur cage as I will be running the drivetrain as a 1x12 speed drivetrain.

Mr TiParts makes third-party rear derailleur cages that are really short. I decided to give it a try and see whether it works or not.

The R7000 version was what I had previously for the United Trifold project, but didn't use it. It doesn't seem to be compatible to R8150 which is why I got a new set on the left, for R8000/R9100.

Mr TiParts super short cage weighs 61 grams.

The pulley to pulley pitch is only 57 mm! Still not as short as the Saint RD-M820 rear derailleur.

Comparatively, the cage length on the Ultegra RD-R8150 is much longer at 100 mm.

Removing the cage is quite simple. Unscrew the cage stopper from the Ultegra cage using a screwdriver.

Then use a Torx tool to remove the retaining bolt located on the pulley member as shown.

The cage set can be removed, along with the spring and the rubber seal.

Side by side comparison, showing the big difference in length.

Some differences in the shaft design, but looks like it would fit.

Found that the shaft length is different, with the Mr TiParts shaft being longer.

Test fitting the Mr TiParts short cage onto the Ultegra R8150 rear derailleur, without the spring or retaining bolt installed.

Shorter cage will contact the derailleur body when the cage is fully released. This is not a big issue as the chain length can be adjusted to prevent the cage from being fully released.

Now to actually install the cage. The inner plate of the cage and the pulleys have to be removed first, as the cage stopper has to be accessed from the rear, which is annoying.

Using the original Ultegra spring and rubber seal.

Comparing the stock Ultegra resin pulley (left) vs the aluminium pulley (right). The Ultegra pulley has much taller teeth heights to improve chain retention.

Managed to install the Mr TiParts cage onto the Ultegra rear derailleur! It was very difficult as the cage stopper was barely accessible, while having to work against the spring tension.

I used the original Ultegra pulleys with taller teeth, to ensure better chain retention. As shown earlier, the cage will touch the body of the rear derailleur when fully released.

Unfortunately the modification did not go well. After reassembling the pulleys into the cage set, I found that the top guide pulley did not rotate properly at all. Upon close examination, I found that the bushings have been deformed.

Deformed bushing on left, normal flat bushing on right.

The deformed bushing is no longer flat, and this causes the flanges to interfere with the pulley, preventing smooth rotation.

I found that this is due to the sunken in thread on the Mr TiParts outer plate, which deforms the bushing when the pulley bolt is tightened.

The original aluminium pulleys that come with the Mr TiParts cage set did not have the same issue, as the bearing covers have a curvature that fit into the sunken in area on the cage. My solution is to add a small washer, so that the Ultegra bushing doesn't get deformed when installed.

Adding a washer to prevent the bushing from being deformed when it is clamped together by the inner and outer plates of the cage set.

This pushes the guide pulley out slightly, but should not be an issue. The guide pulley is now able to spin freely. There is a rather big gap at the rubber seal area.

Bottom tension pulley does not face the same issue. Cage touches the derailleur body as shown, when there is no chain tension.

Modified Ultegra RD-R8150 weighs 252 grams, which is a small 10 gram reduction from the stock configuration.

Although the Mr TiParts short cage seems to work, it doesn't really fit very well. These are the problems which I encountered:

1) Cage touches derailleur body, when the cage is fully released with no chain tension.

2) Guide pulley requires additional washer, when used with Ultegra pulleys.

3) Lateral looseness between cage set and plate member, due to the slightly longer shaft. This will lead to poor shifting performance later on.

4) Lateral looseness means no proper sealing for the plate spring, which will lead to dirt and water ingress over time.

5) Cage stopper is really hard to install due to difficult tool access.

As such, I continued to look for satisfactory short cage solutions, which I found and will show in the next post.