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?
09-10-2019, 12:43 PM
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