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I'm planning to upgrade my existing Shimano road 11sp groupset with a GRX 46/30 crankset.

According to Shimano, this means I need to use the FD-RX810 front derailleur. Apparently the 10sp FD-RX400 won't work.

However, I can't see why not. One answer to this question suggests that the pull ratio should be identical, but doesn't go as far as explicitly stating that it'll work. Other than pull ratio, I cannot see any other issue.

The FD-RX810 is currently unavailable where I live (presumably due to Shimano's supply chain issues), so can I just buy an FD-RX400?

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    First I’d try to get it working with your existing front derailleur. – Michael Mar 3 at 12:00
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The 11sp derailleur cage is designed for the externally narrower 11sp chain. therefore the shifting will be less precise using an 11sp chain with a 10sp front derailleur

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  • This was the only thing I could come up with, but apparently the difference is only around 0.5mm. Is that really enough to noticeably diminish shifting performance? – Will Vousden Mar 3 at 8:51
  • Yes. You can use the 10sp one but why would you unless you're using up parts you already have as a stopgap? – JoeK Mar 3 at 10:47
  • See my comment on the other answer. Current lead times where I live are between 4 and 20 weeks on the 11sp version. I'm looking for alternatives. – Will Vousden Mar 3 at 11:14
  • Buy a used 11sp 105 derailleur then, while you have the correct part on order. But don't spoil your new system! – JoeK Mar 3 at 11:54
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Chainline is shifted by 2.5 mm outside on the GRX crankset (compared to a road one), so the road front derailleur may not reach the outer ring on the GRX crankset.

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  • However, both derailleurs in the question are GRX. – Vladimir F Mar 1 at 17:29
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    Oh well, I made the comparison with the existing road one. IMHO, if the FD is changed, why not get the right one corresponding to the chain width ? – Arnaud VILLANOVA Mar 2 at 5:42
  • Yes, why? That's what the question is actually asking. – Vladimir F Mar 2 at 7:40
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    @ArnaudVILLANOVA Because it's out-of-stock everywhere in Europe, thanks to Shimano's supply chain problems. – Will Vousden Mar 3 at 8:08
  • You could probably test for this ahead of time by just letting out the limit screw and seeing how much of a gap you have with the current derailleur. I would imagine that most derailleurs have a pretty good amount of tolerance for something like this to deal with different frame sizes. For instance, band on derailleurs are expected to deal with all the way from 28.6 mm seat tubes all the way up to 34.9 mm seat tubes (6.3mm difference) with a spacer on the band. So I would think that it would most likely work unless you were already at the extreme end of the range. – Kibbee Mar 3 at 15:00

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