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Shimano GRX rear derailleur

by bob_c
Tue, Aug 20, 2019 19:06
5

The GRX derailleur is available as 10 speed (RD-RX400) or 11 speed (RD-RX810) versions. The 10 speed is compatible with Tiagra 4700 which has the same cable pull as 11 speed, so is there actually any practical difference between the two other than weight?

I was hoping I could use the RD-RX400 with an ultegra 6800 drivetrain - it's a much cheaper way of getting a clutched derailleur - and it seems to be theoretically possible. Is there anything I'm missing or should this work?

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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5 comments

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fixit | 4 years ago
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finished testing the grx400 with my rs-685 11sp shifters and it works fine. 

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CyDia | 5 years ago
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Hey sorry to dig up this thread, I have a question! 4

So if I get it well, I should be able to work a GRX 11s rear derailler with my 105 5700 10s brifters, am I right?
I mean, cable pull between 11 and 10s shifters is obviously different, but cassette width is the same.
My 2 main concerns would be:
- Will a 10s chain fit into a 11s derailler? I might have to swap the jokey wheels, no big deal IMHO.
- Is the 11s parallelogram similar to the 10s? (Guess it should be, as the difference in travel comes from the shifter…)

I currently ride 105 shifters with a Deore 9s RD (as cable pull lengths between 10s road and 10s MTB are different. 10s road shifters cable pull is the same as 9s MTB by Shimano…)
It works but is suboptimal, plus I'd like a gravel-dedicated derailler that fits my 11-42 cassette. Tricky! crying

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bob_c | 5 years ago
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Great, thanks for that. The chart is very useful too - I hadn't seen it before. I have 11-32 at the back anyway so it's within range.

I suppose the large cog lower limit must be something to do with the geometry of the derailleur and length of B screw not allowing the upper jockey wheel to get close enough to a cassette cog <32T when in the low limit position. In practice, this probably doesn't make much difference and could also be typical Shimano conservatism based on worst-case assumptions on derailleur hanger design.

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kil0ran replied to bob_c | 5 years ago
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bob_c wrote:

Great, thanks for that. The chart is very useful too - I hadn't seen it before. I have 11-32 at the back anyway so it's within range.

I suppose the large cog lower limit must be something to do with the geometry of the derailleur and length of B screw not allowing the upper jockey wheel to get close enough to a cassette cog <32T when in the low limit position. In practice, this probably doesn't make much difference and could also be typical Shimano conservatism based on worst-case assumptions on derailleur hanger design.

Ah, that makes sense. Maybe I've been fortunate but my approach with B-screws has always been don't fiddle with 'em if it's shifting fine. Unless you're running a big cog well out of mech spec it's not going to come into play.

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kil0ran | 5 years ago
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Should work absolutely fine - same pull and will just be a question of tweaking the limit screw to allow it to shift inwards enough. 

Shimano don't list it as compatible, but that's because they see it as a 10-speed RD and, well, don't cross the streams. Can be safely ignored because the pull is the same. 

The only difference I can see other than weight (and the official speed rating) is that the minimum big cog that the RX400 is officially compatible with is a 32T so you might need a new cassette, particularly if you're not already running a 28 or 30. On that subject, the RX-810 is only rated for a 30T minimum big cog.

Does beg the question why are there lower limits for the big cog size?

https://productinfo.shimano.com/#/com?acid=C-454&cid=C-453

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