Which Shimano RD for 2x Allroad? GRX vs Ultegra RX

[billspreston]

I currently have a R9150 rear derailleur on my allroad/gravel bike and get a lot of chain slap. Building a new frame and would like to lessen that with one of the newer clutched rear derailleurs. Having trouble deciding between the GRX and Ultegra RX rear derailleurs to go along with the below config:


Shifters & Brakes: R9170
Front Derailleur: GRX RX815 (2.5mm offset chainline)
Crank: Rotor Aldhu 2x 48/32 (2.5mm offset "disc" spindle)
Cassette #1: 11-32 on 38c Knobby Wheelset
Cassette #2: 11-28 on 32c Slick Wheelset

This bike sees a fair mix of pavement and gravel and I'll change the wheelset/cassette/chain depending on the conditions. I initially thought I would go with a GRX 2x rear derailleur to match the front derailleur, but in reading through the specs I noticed the following:


GRX RD-RX815 - Low sprocket: Max. 34T/Min. 30T, Total capacity: 38T
Ultegra RD-RX805 - Low sprocket: Max. 34T/Min. 28T, Total capacity: 39T

Does anybody know why the GRX rear derailleur has a min low sprocket size of 30t while the Ultegra RX has a min of 28t? I run a 28t cassette on one wheelset because I like having the closer ratios on pavement heavy routes. Knowing that Shimano is pretty conservative with their specs, I'm wondering if I can run a 28t cassette on the GRX derailleur with great shifting or if I would be better off getting the Ultegra RX?

https://shimano-h.assetsadobe3.com/is/image/content/dam/global/cg1SHICCycling/final/products/cg2SHICComponent/cg3SHICRearDerailleur/cg4SHICRearDerailleurDi2spec/P-RD-RX805-GS_BC19Nd0003.jpg?id=QhMaB3&fmt=jpg&fit=constrain,1&wid=400&hei=400https://shimano-h.assetsadobe3.com/is/image/content/dam/global/cg1SHICCycling/final/products/cg2SHICComponent/cg3SHICRearDerailleur/cg4SHICRearDerailleurDi2spec/RD-RX815_C219_1.jpg?id=KcJaR1&fmt=jpg&fit=constrain,1&wid=400&hei=400

[icepick_trotsky]

Not sure why the difference in spec, but I would wager that your set up will work with either derailleur. If anything, maybe the Ultegra has more B screw adjustability.

[John H.]

For a 32 tooth- either one will work.
RX is more like an Ultegra derailleur. Basically a road derailleur with a clutch option.
GRX is a more substantial/robust derailleur. More like an XTR.

I am using GRX 817- But I use an 11-40 cassette. The GRX handles the 11-40 really well. No need to lighten the clutch adjustment. And it feels very strong and machine-like. More like the XTR DI2 derailleur.

[Gummee]

I have a couple of Ultegra RX derailleurs on my CX bikes. They work well for my 11*28 and 11-34 cassettes.

Haven't tried it with a 25 to say whether it'll work despite what Shimano says

M

[tony_mm]

For a 32 tooth- either one will work.

RX is more like an Ultegra derailleur. Basically a road derailleur with a clutch option.

GRX is a more substantial/robust derailleur. More like an XTR.



I am using GRX 817- But I use an 11-40 cassette. The GRX handles the 11-40 really well. No need to lighten the clutch adjustment. And it feels very strong and machine-like. More like the XTR DI2 derailleur.



The GRX is like Ultegra or XT.

XTR is a different class. Also in terms of pricing....[emoji2957]

[K3RRY]

Get whichever is cheaper. Pretty much the same. If you ride pavement more it’ll leans towards the RX

[Lovetoclimb]

I highly doubt the system will care if you’re using a 28t cog on a GRX. But I agree get whichever is cheaper.