Lovacott
Über Member
The protocol (for establishing correct chain length) works for others, so there is something awry (and it isn't Shimano).
Share the cassette range and the chainset tooth numbers, and the chainstay length (BB centre to rear axle centre) and you/we can calculate the chain length needed. Would still like to see an image of the RD from the side with chain in small/small.
Glad that is a candidate 'pearl of wisdom' (upthread) - but I'd just point out that it might seem to work but I suspect you don't know that it works (same with me: as well as that (with new chain and new cassette and all clean) I replaced the worn middle chainring (30,000+km) - chain suck ceased.
"A mitigation I used (and actually have left it there) is a chunky ziptie round the right chainstay up where the chainstay joins the BB shell. The idea was that a chain about to suck would knock against the ziptie and 'un-suck'."
[See the last 'chain suck' thread on here (last month?) https://www.cyclechat.net/threads/p...-when-in-the-small-gears.269738/#post-6228182 ]
I picked up the new rear mech from the Amazon Hub locker about two hours ago and I've fitted it. I checked the chain length with the mech off and it is indeed two links longer than needed to wrap around both large sprockets.
On small to small, there is no sag like I had with the old mech and there is a very noticeable difference in the spring tension. I was also able to adjust B Tension better than I could on the old mech (again, probably a weakened spring).
I've re-indexed and left myself enough barrel adjuster room to either tighten or slacken on the fly on the way to work in the morning.
The chain no longer sucks up when on small to small. I'm going to try and avoid small to small in future but I've left the cable tie in place as a safety measure. Like you, sometimes in the dark, I forget where I am gear wise and when the chain was sucking up yesterday, the cable tie was peeling the chain off and preventing it wrapping.