Originally Posted by
xroadcharlie
The bike is mostly stock and 2018. I had the Shimano Tourney derailleur replaced with an Altus at the bike shop after clumsy me stepped on it. Im pretty sure it has the correct parts.
I didn't know the Barrel adjuster at the shifter was for cable tension. Very good to know, thank you. I assumed it was for alignment since the rear derailleur is moving a small amount when I turn it a lot. It makes sense now. Perhaps the cable moves a little differently with changes to the tensioner allowing for minor alignment adjustments.
It doesn't really adjust "tension", it adjusts cable effective length, like chain "stretch" is really pin & roller "wear". That's why you see the derailleur move. The upper pulley on a Shimano index derailleur has side to side float to allow for minor misadjustment to not affect shifting. The pulley moves slightly to match the cog. Proper hanger alignment is also important. "Stepped on" could cause this, although I'd like to think the shop checked/adjusted when installing a new RD.