I have the latest Di2 on one of my road bikes. Recently I had to really get to grips with all the settings, the rear hanger need straightening, so the rear mech also needed tweaking too. What I discovered...
The rear mech being electronic is setup in the opposite way to mechanical. This means you setup the shifting first, then adjust the limit screws. The shifting or trim as its called, can be adjusted using the latest Shimano eTube app. I have this on my iPhone. The advantage of using the app instead of doing it manually is you have a number on-screen to tell you precisely where the trim is set to. Manually you don’t really have an idea apart from guessing.
It’s easy to use, the app guides you, the chain is automatically put onto the 5th sprocket on the cassette and the inner ring on the front. You then have a range of + or - 16 steps either side of ‘0’ in the centre. You are advised to move the derailleur inwards in small steps until the chain is making a noise against the 4th sprocket, then back off towards the centre by 4 steps and it should be fine. All done remotely with buttons on the phone screen.
What I found is when I disconnected the app and went back to normal mode to check the shifting, it would get stuck going towards the smallest sprocket, never the other way, towards the largest. When on the inner ring at the front the chain would find it hard to go from 4 to 5. When on the largest chain ring at the front the same would happen going from 6 to 7. I only really noticed this when riding, it didn’t show up too well in the workshop.
I had to re-trim a few times, moving the trim position a few clicks towards the smallest sprocket. It ended up being at position +7 and now shifts really well.
Limit screws conundrum...
Lower Limit: This is a bit baffling as the Shimano manual tells you to ‘tighten the bolt until it just touches the left link’. No online (YouTube mainly) help here with what is this ‘left link’. I can only guess it’s some kind of cryptic Japanese translation...? What I ended up doing is undoing the screw completely and then tightening until I could see the end touching the relevant part of the rear-mech.
Amusingly, if it’s too tight, the mech wont stay in 1st gear, automatically shifting to 2nd to protect the motor. If it didn't do this it would always be trying to force itself into 1st and drain the battery and possibly wreck the motor.
Similarly with the High limit. I put the chain on the 11th sprocket with the screw completely undone, and then tried to tighten. But this time you can’t see anything directly. When the screw touches ‘the left link’ you back off one complete turn of the hex key so the shifter can over shift then return under the 11th gear.
Thoughts and queries!!
The shifting works very well with both limit screws completely undone and ineffective. The electronics know where they are all the time and can’t over-shft into the spokes or off the end of the cassette.
Adjusting the limit screws doesn’t seem to be done to actually move the rear mech in any way, ie position it in the right place, this is done electronically. They really are just hard limits to travel, so to speak.
So, can the limit screws just be forgotten about, are they there just to reassure traditionally minded people that the electronics haven’t taken over completely, or do they do anything useful?
Could my mechanical bike’s rear mech be setup in the same way? Get the trim / shifting working properly using the cable adjustment, then use the limit screws afterwards just to stop any excess movement, not to fine tune any positioning?
Any thoughts much appreciated!