I have no idea what my front mech is up to.

It is shifting from middle to granny without being asked. It does this only in the lowest 4 gears or so at the back and only under load.

It’s not the mech – I’ve adjusted it.

It’s not the cable – I’ve fitted a new one.

It’s not the shifter – when it down changes it feels as though there is no resistance trying to re-change it back to middle (cos the shifter is in middle and the cable tension isn’t there) – if that makes sense.

It’s only just started doing this and it’s pee-ing me off, royally.

Thoughts?

Front mechs can be pigs to setup; adjust it again. You could just throw it over a hedge with the granny ring, big ring, shifter and cable and fit a cheap chain device.

Bizarre as it sounds, the front mech that I fitted to a bike I recently built for my partner was shifting from the granny ring to the big ring in just one click and nothing I did seemed to work. Eventually I changed the plastic ferrule at the shifter end and it’s worked perfectly since; I’ve no idea why. Just keep trying and hopefully you’ll get there in the end.

I’m clueless to what yours is up to but I know that when I clean my chain by running it through a rag, moving the chain out of line can cause it to drop from the middle ring to the granny ring (de-railing it I guess) I wonder whether a bent or mis-aligned rear mech. might cause something similar (just guessing really).

Is there any cahin rub at the front mech in the gear just before the ‘auto shift’ ?
If yes, I would guess the mech cage may not be running parallel to the front rings (ie the mech has moved slightly clockwise when viewed down the line of the seat tube).
When the chainline alters (ie low gears on cassette) there will be contact between front mech/chain sufficient to nudge it onto granny.

Front mechs can sustain damage and wear and you might not notice it. I had tons of trouble with the front shifting on one bike. Mech was not particularly old and hadn’t been damaged that I could think of, but I replaced it and it was back to perfect shifting like new.