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.

Fabike C3: Rear Derailleur Troubleshooting and Final Specifications

While changing the drivetrain on the Fabike C3 from a belt drive to a chain drive, I found some issues with the shifting performance. More specifically, when shifting outwards from the 12T to the 11T sprocket, which is the top gear.

I was using the Ultegra Di2 RD-6870, which was a modified version with a short cage. This rear derailleur has a construction that can support up to a 32T max sprocket, but with a short cage for more ground clearance with small wheels. So, it should actually match well with the 11-32T cassette used on this setup.

However, the distance between the guide pulley and the gear sprocket seems to be rather big at the top few gears (11T and 12T), which could be the reason why the shifting does not occur smoothly. About 1 out of 5 times it will fail to shift properly.

I also noticed a metallic clinking sound when shifting from 11T to 12T. I could not find where it came from, until I looked from the top and saw the chain briefly touch the end of the bolt (for joining the split section of the frame) during shifting. During pedaling in either 12T or 11T, there is no interference. The touching only occurs during shifting, when the chain flexes as shown below.

When shifting from 11T to 12T, the chain will touch the inside of the bolt briefly.

Due to the flex of the chain, it will bend and touch the end of the bolt during shifting.

The touching is not a big issue, just something odd that I discovered. However, I was not happy with the shifting performance, as it would fail to shift about 1 out of 5 times. The other times the shifting was mostly delayed.

Therefore, I swapped in another rear derailleur to try and find the root cause. I took the Ultegra Di2 RD-R8050-SS from the Dahon MuSP, which is rated for up to 30T sprocket only. It is out of specifications, but no harm trying. It is easy to swap in a Di2 rear derailleur as there is no inner cable to fix and adjust.

What I found was that the performance was actually worse, as the chain will sometimes drop off from the top sprocket when shifting outwards from 12T to 11T. This was despite setting up the limit screws properly. The distance between the guide pulley and the 12T / 11T sprockets was quite big as well.

Chain drop from the top gear of the cassette, despite the limit screws being set properly.

At this point, I suspected that the dropout may not be made to the recommended specifications. On the Di2 rear derailleurs, I had to set the electronic position adjustment all the way inwards, in order to align the guide pulley to the gear sprockets correctly for shifting. This was because the resting surface on the dropout for the rear derailleur was too far outwards.

The dropout angle also looked a bit odd, as it seems to tilt back too much, which will also cause the big distance between the guide pulley and the sprocket. Before all the above installation and testing, I had already checked to make sure that the dropout was straight and not bent. However, I was unable to measure the actual dropout dimensions.

Recommended dropout dimensions by Shimano, for optimum shifting performance.

Without measuring the dropout dimensions, I cannot say for sure that the dropout is wrong, but I never had the same issues with the same rear derailleurs on other bikes. 

I still had one Di2 rear derailleur to try, which is the GRX Di2 RD-RX817 rear derailleur. This is quite different from the previous two that I tried (Ultegra 6870 and R8050), as this GRX rear derailleur is front single specific, with a different construction.

As this GRX rear derailleur is meant for a front single drivetrain with a large sprocket size of 40T/42T, it is actually not designed to work with this 11-32T cassette that I have on the Fabike C3. However, my theory was that if the distance between the guide pulley and the smaller sprockets was too big (due to wrong dropout design), this wide range rear derailleur with a big slant angle will allow me to close up this distance.

RD-RX817 has a bigger slant angle, which will theoretically give a closer guide pulley to sprocket distance at the top gears (12T / 11T) if the B-tension is set to be the same at the large 32t sprocket.

Note that this component setup is not recommended (RD-RX817 with 11-32T cassette), if the dropout dimensions are correct. I am only trying this to compensate for the unusual dropout dimensions.

RD-RX817 on 11-32T cassette, at low gear

With the B-tension screw fully loosened, this is the closest I can set the guide pulley to the 32T sprocket. This is because this rear derailleur was designed for a low sprocket size of 40T /42T, not a much smaller 32T sprocket.

At the top gear position, on 11T sprocket

The distance between the guide pulley and the 11T sprocket is much closer than using the other rear derailleurs. Luckily there is enough frame clearance for a large cage with offset guide pulley.

View from the back

With this GRX rear derailleur, shifting performance is now very good! Although this setup is outside the recommended specifications, it works well for this particular bike. The clutch is a bonus, although not necessary.

Latest drivetrain setup, with a front 38T chain ring, rear 11-32T cassette, and GRX Di2 rear derailleur.

With all the accessories removed, except the mudguards. This is one fast commuting bike!

Final Di2 wiring layout. Basically everything is the same, except that I changed the Alfine 11 motor unit to the rear derailleur.

Full bike specifications, with a chain drive setup. Only 8.1 kg without pedals!

For comparison, the previous internal hub + belt drive setup was about 9.1 kg without pedals, so this is a big weight difference. From the very first Avanti Inc 3, there has been drastic weight savings for each iteration. All weights listed below are without pedals and mudguards.

Avanti Inc 3 Alfine 11 Di2 drop bar: 10.9 kg

Fabike C3 Alfine 11 Di2 drop bar: 9.1 kg

Fabike C3 Ultegra Di2 drop bar: 8.1 kg

Pedals will add about 400 grams, while mudguards will add about 500 grams.

There has been a big weight difference compared to the Avanti Inc 3, and it is a difference that can be felt when riding, and most importantly, when carrying the bike. Now it rides almost like a pure road bike, which is what I wanted: A road bike that I can ride in all-weather for commuting.

Early Rider Seeker 14: Conversion to Pedal Bike

After training on the push bike conversion for a while, my kid is now ready to ride on a pedal bike! Basically he has learnt how to use the brake levers on the Early Rider Seeker 14 kids bike, plus how to balance properly on this new bike. So, on goes the drivetrain, as I had removed it previously.

Reinstalling the bottom bracket, crankset, belt and pedals.

Full kids bike with pedals!

Belt drive avoids the need for a chain case to protect against sharp chain ring teeth or chain lube getting on the legs.

This is a good looking kids bike that has many premium components, plus it is pretty lightweight at 5.8 kg including pedals.

Fabike C3: Fulcrum Racing 5 Disc Brake Rear Wheel

When I modified the Fabike C3 commuting bike from an internal hub + belt drive setup to a conventional derailleur + cassette + chain, I needed a rear wheel for that purpose. Previously the Fabike C3 was using a custom-built rear wheel with an Alfine 11 Di2 internal hub, and a DT Swiss ER 1600 Spline DB 32 mm front wheel.

The rear dropout on the Fabike C3 was converted from a nutted axle type to a 12 mm thru axle type. Therefore, I needed an E-thru rear wheel that was disc brake compatible. When I changed the road wheelset on the Cervelo Aspero to the Ascent Bikes Zenith Elite, the previous Reynolds Assault wheelset was available.

However, I had just sold the Reynolds wheelset a few weeks ago, before I had the idea to convert this Fabike C3. That Reynolds wheelset would have been perfect for this Fabike C3, as it was also 12 mm E-thru front and rear with disc brakes. I greatly regretted selling that wheelset so hastily, and fretted over it for quite a while. However, I am unable to undo it, and up till today I still regret it. Moral of the story, don't sell off good components, unless you are absolutely sure that you will not use it on another bike.

In the meantime, I borrowed a spare rear wheel from a friend, so that I can test out the derailleur setup on the Fabike C3. I also needed a narrow rim so that I can install my existing Continental Grand Prix 4 Season tires without having frame-tire clearance issues. These rims are narrow at just 17 mm wide, which is what I needed. Check it out below!

Fulcrum Racing 5 Disc Brake Rear Wheel. Aluminium rims with a modest 26 mm tall rim profile.

Ceramic coated freehub body for durability, and a large flange for a better spoke angle.

Straight pull spokes, and Centerlock disc brake mounting.

Test fitting it into the Fabike C3 frameset. With the non-turn washers removed from the frame, the OLD changes from 135 mm to 142 mm to fit road 12 mm E-thru wheelsets.

The Centerlock rotor fitting is unconventional, as it does not use a normal lock ring. This is due to the different axle and bearing system used by Fulcrum.

Weight of the special steel lock ring and shim weighs 14 grams.

Rear wheel weighs 964 grams, which is rather heavy. Another regret from selling the more lightweight Reynolds Assault wheelset.

Dura-Ace RT900 rotors installed, and fixed using the special lock ring. A Hollowtech BB tool is needed to tighten this external lock ring, instead of the usual tool for internal lock rings.

Using the DT Swiss thru axle that I had bought earlier to fix this wheelset onto the bike.

New drivetrain! Also 1x11 speeds.

Chain drive conversion completed. Looks even more like a road bike with this new drivetrain.

View of the bike from the other side. This is a fast all-weather commuting bike!

Testing it on the slopes of NTU, on a cool night after rain.

The gearing seems perfect, with a 38T front chain ring and a rear 11-32T cassette.

In order to test out the gearing, I went to the NTU slopes to try it out. I had to use the lowest gear (32T) on the steepest parts of the slope, but it was sufficient. As for the highest gear (11T), it was enough to go fast, but it will spin out once you go past 45 km/h. This is also sufficient for me, because I would not want to go too fast anyway.

The best takeaway from the test ride was how useful it is to have a road bike that you can ride in light rain or on wet roads, without having to do maintenance on the bike afterwards. Normally I would not ride my road bike on wet roads, as it means having to clean up the bike afterwards which will be time consuming.

With the Fabike C3, I now have a bike that rides just like a proper road bike, but is weather proof with the new drivetrain and mudguards.