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Posting this as I couldn’t find much online about it, and thought it might be useful for others wondering if it works.
My partners road/cx bike has ultegra di2 R8150. We’ve been living in a hilly area but with peak gradients of about 9%, so a 11-30 cassette on compact chain set has been fine. We’ve recently moved to an area with more gravel, less tarmac, and much steeper hills. A recent ride saw 24% for a sustained period on my garmin!
I was looking for ways to lower her gears. Options included sub compact chainrings, a GRX mech and wider cassette, or moving to a 11-34 and only getting slightly lower gears.
Then I realised - she has to use the synchro function on di2 as she can only change gears using her left hand. If I set the changeover points for the front mech correctly, I could minimise chain growth whilst still getting top and bottom gears, opening up wider cassettes as an option.
I found an old 11-42 cassette in the spares box with some life in it, bought a wolftooth road link (which might not be necessary), a new chain, and got to work.
My calcs suggest that what she’s ended up with is:
- Retained the 4.55:1 top gear
- Now has 0.8:1 lowest gear
- Has 14 effective gears, each with ~11% jump (vs pervious 9% on a road cassette)
- Noise, chain retention and shift speed seem unaltered.
- Chain growth is 30t, well within the 35t limit from shimano.
I know that synchro mode is not for everyone (unexpected front shifts still mess with your cadence) but for her it’s a revelation. It’s been test ridden and she’s really happy with it. And for me, it was almost free!
I have some tables I worked out with changeover points I can post for anyone interested.
Have you disabled the left shifter? The only risk I can see is that an inadvertent manual front mech shift would blow the rear mech to bits
Hmmm, interesting. Which specific cassette did you use? (Brand & groupset level)
I've got a similar setup and was thinking of swapping out the 11-30 cassette. I was curious as to how low I could go, just by changing the cassette though I don't think I'd need anything that low (34 or 36 may do me...)
I already use synchro shift so altering the change over point wouldn't bother me but I'm not sure I'd need to if I just go with a 34...
I have! Just unplugged it. She was inadvertently shifting when braking previously.
To be fair though, because both shifters are in synchro, there’s no risk of over stretching the chain.
Thinking about it, if you can you both hands fully you could configure both buttons on the left for down shifts, both on the right for up shifts. Removes the issue of fiddly buttons with bulky gloves.
Which specific cassette did you use? (Brand & groupset level)
Shimano M8000 11-42, on a hope RS4 with HG freehub.
I think 11-40 would be the sweet spot. 34/42 is a ludicrously low gear!