Sunday, November 10, 2019

Canyon Endurace: GRX Rear Derailleur and Cassette

The most important change needed to convert a standard road bike to a gravel bike would be a new set of gravel tires. There is a huge difference when you ride a gravel trail on a set of slick tires, versus a set of gravel specific knobbly tires. I used the Panaracer GravelKing SK tires on a separate DT Swiss G 1800 wheelset, and it has worked well for me.

In order to have a separate gravel wheelset that can be swapped quickly with the standard road wheelset, it is necessary to have an additional cassette and disc brake rotors on the gravel wheelset as well. This way, the wheelset can be swapped in without adjusting or changing any other component.

In this case, a new cassette is needed on the gravel wheelset. If I were building a gravel bike from scratch, with a new frame, most likely I will go for a front single drivetrain, with a wide ratio cassette at the back. For example, a 40T chain ring with a 11-42T cassette at the back.

However, since I am modifying the Canyon Endurace to be a part-time gravel bike, I need to maintain the front double drivetrain that is needed for road riding. Just to recap, the Canyon Endurace is equipped with a full Dura-Ace R9170 groupset, with the 50/34T crankset and a 11-30T 11 speed cassette.

A front double crankset is actually pretty good for a gravel setup as well, as the small 34T inner chain ring can provide a low gear ratio for off-road riding. Usually, a gravel setup will have a lowest gear ratio of 1:1, to enable grinding up steeper slopes. In this case, since the small chain ring is already 34T, a 11-34T cassette will achieve a lowest gear ratio of 1:1, or roughly 28 gear inches with 700C wheels.

A larger 11 speed cassette would be 11-36T or even 11-40T, but may not be necessary with the low 34T chain ring available. Besides, the gear sprocket difference between the gravel wheelset (11-34T) and road wheelset (11-30T) should be minimized for better shifting performance, as will be explained later on.

Ultegra grade CS-HG800-11 cassette, 11-34T 11 speed.

The compatibility graphic on the packaging is a bit confusing, because this 11 speed HG800 cassette can actually be used on 8/9/10 speed freehub bodies, just by omitting the 1.85 mm spacer that comes with the cassette. A 11 speed MTB freehub is the same as a 8/9/10 speed freehub.

Ultegra grade finishing on the sprockets, but it is not classified under Ultegra R8000. Probably because the cassette structure is a bit different, as it can be used on 8/9/10 speed freehub bodies as well, not exclusively on 11 speed road freehub bodies.

Weighs 337 grams, including the 1.85 mm spacer. 131 grams heavier than the 11-30T Dura-Ace cassette.

Gear sprockets are 11-13-15-17-19-21-23-25-27-30-34. 

I don't like the large 11T to 13T jump at the top end of the cassette, as explained before here. That is the case if all the gears are used, such as on a 1x11 speed drivetrain where the full gear range is utilized.

However, I realized that on this 2x11 speed Di2 drivetrain, the cross-chaining is electronically limited. Basically, if you are in the small 34T chainring, the rear derailleur does not allow you to shift to the top two gears on the cassette (11T and 13T in this case). The reason is to prevent chain touching on the large chain ring due to the cross chain angle, and also to ensure that the chain capacity of the rear derailleur is not exceeded.

In other words, when I am using the small 34T chainring, I am limited to 9 gears at the back (15T - 34T). The top 2 gears on the cassette are only accessible when in the large chain ring, where it is hardly used as the gear ratio is usually too high for me. In conclusion, the large jump between 11T and 13T does not come into play at all, as it is not selectable (when in small chainring) or the gear ratio is too high (when in large chainring).

The 11 speed 11-34T cassette disassembled.

Rear view of the cassette with the large aluminium spider. Note that the largest sprocket is cantilevered out over the spider, beyond the freehub body. This is what enables it to be mounted on older 8/9/10 speed freehubs.

The largest 3 sprockets (27T, 30T, 34T) are mounted on an aluminium spider, and is specific to this cassette.

Next 3 sprockets (21T, 23T, 25T) are also mounted on an aluminium spider. This is different from other Ultegra cassettes which use carbon fibre for the second spider.

This second spider is also specific to this cassette only.

It has an aluminium lock ring, but then uses resin spacers instead of aluminium spacers of other Ultegra cassettes.

An additional 1.85 mm spacer is provided, which should be used ONLY if you are mounting this spacer on 11 speed freehub bodies.

If done correctly, the top 11T sprocket should protrude from the freehub body thread by a couple of millimetres.

If you miss out the 1.85 mm spacer when mounting on an 11 speed freehub body, the whole cassette will sit too far inwards, and the lockring cannot secure the casette properly. This picture shows the wrong setup.

In my opinion, branding the cassette as a non-series HG800 is correct, as it is a bit short of the Ultegra grade quality expected. Besides using resin spacers instead of aluminium spacers, the second spider does not utilize carbon fibre material, which is another difference.

CS-HG800 11-34T 11 speed cassette mounted on the DT Swiss rear wheel.

With the cassette settled, let's move on to the rear derailleur. The Canyon Endurace is currently using the Dura-Ace R9150 Di2 rear derailleur, which is rated for up to 30T large sprocket and a maximum chain capacity of 33T. If I use the 11-34T gravel cassette, the total chain capacity required would be (50-34) + (34-11) = 39T.

If we discount the top 2 sprockets (11T and 13T) as it is limited by the Di2 system, the chain capacity required would be (50-34) + (34-15) = 35T.

In this case, the R9150 rear derailleur cannot achieve both the required max sprocket size and also the chain capacity. Therefore, a new rear derailleur is needed to match with the larger 11-34T sprocket.

Remember, my objective is to enable the road bike to be transformed into a gravel bike, just by a change of wheelset. The rear derailleur and chain must be compatible to both the road and gravel setup, without any adjustment required during the swap. My plan is to set up the rear derailleur and chain length to match the larger 11-34T gravel cassette, and this same rear derailleur and chain setup also needs to work for the 11-30T road cassette.

The Ultegra RD-R8050-GS (mid cage) Di2 rear derailleur is compatible with a 34T large sprocket, but if I'm going to make a gravel setup, I might as well use components from the new GRX gravel groupset. There are a few new GRX rear derailleurs, to suit different gravel setups (1x or 2x, mechanical or Di2 shifting). What I need in this case is the GRX Di2 rear derailleur for a 2x11 speed drivetrain.

Nice packaging graphic to signify the GRX gravel component.

4 types of GRX rear derailleurs available. I would be using the RD-RX815 which is the Di2 rear derailleur for a 2x11 speed drivetrain.

GRX RD-RX815 Di2 rear derailleur. Shadow construction, similar to RD-R8050, but with an aggressive outer link design. The design language is in-between road and MTB, which works well for gravel.

The aluminium cage is quite long, to achieve a max chain capacity of 38T. This is helped by the large 13T tension pulley at the bottom. Otherwise, the structure is similar to the RD-R8050 used on the Dahon MuSP.

Most importantly, this GRX rear derailleur is equipped with a clutch! This increases chain tension to minimize chain slapping and eliminate dropped chains. Already in use on MTB rear derailleurs for many years. This will be super useful for gravel riding.

Weighs 287 grams, which is 90 grams heavier than the Dura-Ace rear derailleur. This is expected as the GRX rear derailleur has an additional clutch, a longer cage, and does not use carbon fibre outer and inner plates.

Comparing the GRX rear derailleur with the Dura-Ace rear derailleur which is still on the Canyon Endurace.

Comparing them side by side. Similar Shadow construction, but the GRX rear derailleur seems to be the more capable one, with a larger chain capacity and a clutch function.

In order for this road/gravel setup to work, this new GRX rear derailleur needs to match both the 11-34T cassette on the gravel wheelset, and also the 11-30T cassette on the road wheelset. I will basically have to set up the rear derailleur B tension screw using the 34T sprocket, and hope that it will still shift well on the 30T sprocket on the road wheelset.

The chain length obviously has to be long enough to suit the 34T cassette, so a few links need to be added onto the existing chain. The extra chain length (+6 links) will not be an issue as the longer cage can take care of that.

Areas to take note is the indexing of the rear derailleur on the cassette, as each cassette's position might differ slightly. If they are close enough, shifting performance should be good enough across the two different cassettes and wheelsets.

With the 11-34T HG800 cassette and GRX rear derailleur installed on the Canyon Endurace. 

Using the GRX rear derailleur means that the Dura-Ace groupset is incomplete, as the Dura-Ace rear derailleur is no longer used. For me, this is OK as I am no longer obsessed with the weight of the Canyon Endurace any more, unlike previously when it was new.  Besides, using the gravel wheelset and tires already add much more weight than the little difference a change of rear derailleur makes.

Testing the gravel setup!


  1. Hi,
    Can share where you purchase your
    Bicycle parts?
    You purchase online via taobao
    Or via other websites?
    Can share the websites?



    1. You can buy the parts from the local bike shop.

  2. May I know where can you get the gravel rear derailleur and 11-34T cassette? What’s the price like?


    1. You have to check with your local bike store for the prices. There are 4 specifications of GRX RD as shown above. The cassette should be about $100 SGD.

  3. Hi
    Great build.. I am eyeing the same modifications .. Wondering how your build turned out.. could not find the completed project blog - if you can please share the final blog.

    Also, did you consider XT di2 instead of GRX? if yes, what made you decide for the GRX?

    1. The Canyon Endurace modifications can be found here.
      I'm using GRX 2x11 as I am also using this bike as a road bike. If it was a dedicated gravel bike, a 1x setup will also be feasible.
      XT Di2 RD would not be able to be used with the road Di2 FD.
      If running 1x, either the XT or GRX RD will work.

  4. Thanks for the detailed post! I'm looking to do something similar to my cross bike (2 wheelsets) with a compact crank. I've got an 11-32 on my road set, and am considering either an 11-36 or 11-40 for the gravel set (some gravel where I am is very steep), but I wonder if jumping from 32 to 40 when changing wheelsets might be too much? Also do you have any thoughts about the GRX rear derailleur compared to the Ultegra RX800 in this use case? Thanks!

    1. A compact crankset has a 50/34T chain ring. You don't need a large 11-40T cassette.
      The RD is not able to cater for the large cassette different, neither does it have the chain capacity to run a front double with a 11-40T cassette.
      A 11-40T cassette is for front single use, not for front double.
      The RX800 was a temporary RD until GRX was ready. So just get the GRX RD, unless you have a good discount on the RX800.

  5. Thanks for your detailed post! I'm thinking about doing something similar with my cross bike with compact cranks and have 2 wheelsets. I have 11-32 on the road set, and am considering either 11-36 or 11-40 on the gravel. Your post makes me worried about the difference going from 11-32 to 11-40 while switching wheels. Also curious if you have thoughts about this GRX rear derailleur compared to the Ultegra RX800. Thanks!

  6. Hello,
    I also have an Endurace that I would like to ride as a gravel bike occasionally. I currently have a 52/36 crankset so I would try to mount a 11-36 cassette to achieve a 1:1 ratio.
    I can see on the picture of the box of the GRX RD that max T capacity is 34 for the 2x11s models but then you said that "The aluminium cage is quite long, to achieve a max chain capacity of 38T.".
    I wonder if a Shimano 105 + this type of 36T adapter would works.
    Thank a lot for this great blog :)

  7. Chain capacity means front difference + rear difference.
    Rear difference is the max sprocket size minus the min sprocket size. For 11-36T cassette, the rear difference is 25 teeth.
    Front difference is thus 52-36=16 teeth.
    For your setup, chain capacity needed is thus 41 teeth.
    Since it is slightly beyond the RD rated capacity, you will get some chain slack when in the front low and rear top combination. For Di2, I think this combination is blocked anyway.
    Check out this post, it is exactly what you need. The Sunrace RX1 is a good 11-36T cassette.