R9100 rear derailleur capacity

[Peter Salt]

I'm planning a new bike build and have an opportunity to get a used R9100 groupset on the cheap. I've some questions around the rear derailleur capacity for this model. Manufacturer specification says:

Max. front difference	16T
Total capacity	35T
Low sprocket|Max.	30T

Which is identical to the R8000 'short cage' version.

The R8000 'medium cage', which I normally use, has a wider range:

Max. front difference	16T
Total capacity	39T
Low sprocket|Max.	34T

There seems to be no 'medium cage' variant for the R9000 and the only difference between the 'short' and 'medium' for the R8000 is the size of the cage (i.e., the derailleur body itself appears to be identical).

Question: Are the cages on the R8000 and R9100 cross-compatible? I.e. can I take the longer cage from a R8000 and fit it on the R9100? If yes, would that effectively bring it to the spec of the larger R8000?

 

[T4tomo]

I don't know, and yes, as its the cage length that dictates capacity.

Or you could just used the ultegra Rd with the rest of the DA group set.

 

[Cycleops]

You'll find the recommended specs are usually on the conservative side so you exceed them by a small margin.
You don't say what tooth the largest sprocket on the cassette you want to use.

 

[Peter Salt]

I don't know, and yes, as its the cage length that dictates capacity.

Or you could just used the ultegra Rd with the rest of the DA group set.

OK, cool. Yes, there's always that, but then what's the point of buying the whole groupset if I don't end up using the RD? I suppose there's also the wolf tooth solution? But that would be an additional thing to buy - and £20 (before postage) for a small piece of metal seems a bit ridiculous. Whereas I already have a spare long cage.

You'll find the recommended specs are usually on the conservative side so you exceed them by a small margin.
You don't say what tooth the largest sprocket on the cassette you want to use.

Good point. I do use a 11-34 cassette. If my experience with the Ultegra short cage is anything to go by - it works pretty well with 11-32 but at 11-34 it gets a bit janky and you do need the longer cage - wouldn't expect it to be any different with the Dura Ace.

 

[Svendo]

Here ya go:
https://si.shimano.com/en/pdfs/ev/RD-R8000-SS-4284/EV-RD-R8000-SS-4284A.pdf

If you look at the parts list, 10 & 12 are ‘B’ compatible with RD9100. They’ll work perfectly but might look janky.

For the cage I think it’d actually look fine, just different to the DA cage. When I put an ultegra chain ring on a Dura ace crank it worked perfectly (and weighed only ~10g more than the worn DA one it replaced!) but the sculpting of the arms didn’t line up. But the £60+ saving helped get over that!

 

[Peter Salt]

Here ya go:
https://si.shimano.com/en/pdfs/ev/RD-R8000-SS-4284/EV-RD-R8000-SS-4284A.pdf

If you look at the parts list, 10 & 12 are ‘B’ compatible with RD9100. They’ll work perfectly but might look janky.

For the cage I think it’d actually look fine, just different to the DA cage. When I put an ultegra chain ring on a Dura ace crank it worked perfectly (and weighed only ~10g more than the worn DA one it replaced!) but the sculpting of the arms didn’t line up. But the £60+ saving helped get over that!

Ah! Very useful - gives me the answer I was looking for. Thank you.