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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?

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kil0ran [1643 posts] 1 month 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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bob_c [67 posts] 1 month 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 [1643 posts] 1 month 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.