Cervelo Aspero: Special GRX Rear Derailleur with Long Cage

For gravel riding, having lower gearing is always good, as it allows you to climb steep off-road slopes without having to get off and walk. If you stand up to pedal, you will most likely cause the rear wheel to spin out due to lack of traction.

My current gravel setup is with a 11-34T cassette, which means a low 1:1 gear ratio when used with the 50/34T Dura-Ace double crankset. Having a 1:1 low gear ratio is usually enough, but I want to try and see if I can get it lower.

If I want to use a larger cassette for the gravel wheelset, I will need a rear derailleur with a bigger chain capacity. The current GRX RD-RX815 rear derailleur is already near its limit, so in order to change to a larger cassette that is bigger than the current 11-34T cassette, I need a rear derailleur with a longer cage.

The next largest cassette from Shimano is the 11-40T MTB cassette, which I have used on the Bike Friday Haul-A-Day cargo bike. This is a big jump from the current 11-34T cassette, so I will need to try a different rear derailleur.

The other GRX rear derailleur that is rated for larger cassettes is the RD-RX817. It is a front single specific rear derailleur, so it can reach the largest sprockets, but it has lesser chain capacity than the RX815 which is for front double systems. This post will show the differences between these two GRX rear derailleurs.

My idea is to increase the chain capacity of the RX817 rear derailleur, which has a relatively short cage. If this can be done, the chain capacity may be able to accommodate a front double drivetrain.

The way to increase chain capacity is to use a longer cage. After much research, I found that the Deore XT RD-M8000 has a long cage SGS version, and the cage design looks to be similar to the GRX RD-RX817. Perhaps the M8000 SGS cage can be transplanted to the GRX RD?

GRX RD-RX817 on the left, Deore XT RD-M8000 SGS on the right.

The M8000 cage is much longer than the RX817 cage.

The amount of pulley offset, the general shape, and the pulley sizes look similar.

The stopper pin location also looks similar.

At this point, it seems that there is a good chance that the M8000 SGS cage can be transplanted onto the RX817 rear derailleur. Note that all these are experimental, so change at your own risk! From the dealer manuals, I can find the instructions to change the cage, as it is a service part. As long as the modifications are reversible, I think there is no harm trying.

Close up look at the M8000 rear derailleur

The spring hooking location and pulley offset look similar to the RX817 rear derailleur.

This is a SGS cage (long cage) version, which was designed for Deore XT 3x11 speed drivetrains!

Check out the stopper pin position. Also looks similar to the RX817 rear derailleur.

Let's start the modification! In order to remove the cage, the clutch mechanism first needs to be removed, in order to loosen the axle. Follow the steps carefully and take pictures along the way, so that it can be reassembled correctly.

Clutch cover with 3 screws removed. Many years ago, I studied this clutch mechanism on another rear derailleur.

Clutch mechanism removed

How it looks with the clutch mechanism and stopper pin removed.

This axle needs to be loosened, in order to remove the cage. It can be quite tight as there is loctite on the threads.

Cage removed! The spring also comes out.

Repeat the same steps for the other rear derailleur, as the disassembly method is the same.

Both cage sets disassembled and placed side by side for comparison.

Both axles look the same

Both springs look the same as well

The rubber seals are different though, if you look closely.

Cage comparison! This also shows that the cage design is similar, and can be transplanted with a high chance of success.

The spacer on the axle looks different.

RX817 has a step on the spacer, while the M8000 spacer is just a plain flat spacer.

This different spacer design is due to the different design on the mating part.

As the step on the axle is a different depth, different spacers are needed to maintain same cage position relative to the axle.

As this point, I have determined that the rubber seal and the spacers are different. Regardless of which cage is used, the rubber seal and spacer needs to be matched to the linkset.

Reassembly is quite straightforward, just need to follow the steps as shown in the instructions, found in the dealer manual.

Special edition GRX RD-RX817 with SGS long cage! Looks pretty good too.

View of this special rear derailleur from the rear. The chain capacity has been increased from 31T to 47T, according to the specifications.

Weight of this special rear derailleur is 325 grams, only 4 grams more than the stock version.

The modification is successful, but how will it be used? With the increased capacity, it can theoretically be used for a wide ratio cassette with a front double drivetrain. However, there is likely to be other issues which I am still not aware of. That will be tried in the next post where I will pair this special rear derailleur to a wide ratio cassette.