Jim’s Tech Talk
By Jim Langley
This week I’m sharing a small adjustment that makes a world of difference in the shifting of Shimano Di2 electric rear derailleurs. Recently I ran into and fixed this issue for a couple of roadie friends. Which makes me think that not everyone understands this critical adjustment.
My theory is that the confusion comes from the natural assumption that rear derailleurs are all adjusted the same. And also from the instructions provided with the Di2 components, which I sometimes find awkwardly written, incomplete and confusing.
Plus, for every new version/model year of Di2 and different Shimano groups that use the electronic technology, there are changes in the components and the instructions. That creates confusion and misunderstanding.
To help, I’m going to explain the basic problem, the cause, and the solution.
The Basic Problem
If your Di2 rear derailleur has been adjusted incorrectly, you’ll be experiencing inconsistent shifting. Sometimes the chain will make the shift you want. Sometimes it won’t. And, sometimes your bike will “ghost” shift, meaning shift when you didn’t actually shift it – and didn’t want it to.
From my experience fixing this problem, I believe it occurs mainly because whoever did the initial install of the rear derailleur adjusted it the way standard (not Di2 electric) derailleurs are adjusted. I can appreciate how this could happen, because visually, it does seem like the electric derailleur is designed to be setup exactly like manual rear derailleurs.
However, when it comes to setting the limit screws, there’s a small yet critical difference. On both electric and manual rear derailleurs there are limit screws that set the sweep or in/out travel of the derailleur. They are adjusted to ensure that the derailleur can move the chain far enough each way to reach all the cassette cogs, smallest to largest.
With manual derailleurs, these limit screws are adjusted so that they hit stops on the derailleur. When they hit the stops, it prevents the derailleur moving any further. It’s an important adjustment to get right. Even only a half turn off with the limit screw can mean the difference between the chain staying on the cog or coming off. And, if it overshifts and comes off it can cause a crash or, in the case of shifting into the spokes, serious wheel and frame damage.
It turns out that Di2 derailleur limit screw adjustment is NOT always done the same way as it is on mechanical rear derailleurs. And, also that if you do adjust the screws as if you’re working on a standard derailleur, you might actually damage or ruin your Di2 rear derailleur.
The reason is that Di2 components are operated by electrical shifters, a brain and motors. And one of their most interesting and sophisticated functions is the ability to self trim to fine-tune shifts.
On a properly setup Di2 rear derailleur you can see this in action if you watch closely. It’s easiest to see when you are shifting from the second-smallest cog to the smallest. Continue watching for a bit after the shift is complete and you’ll see the derailleur move inward closer to the second-smallest cog a tiny amount (assuming the limit screw is adjusted correctly – otherwise you won’t see it). If it’s quiet where you’re doing this test, you might hear the auto trim, too.
Don’t Block the Auto-trim Di2 Feature
The reason my friends’ Di2 rear derailleurs were shifting strangely and ghost shifting, was because the limit screws were turned in too far. This results in them preventing the derailleur from overshifting slightly and then auto-trimming. Worse it can drain the battery because the derailleur continues trying to move further than it can.
To fix this is as easy as backing out the limit screw(s) until there’s clearance between it and the stop. Once you have a small gap, the Di2 derailleur will work correctly again and your electronic shifting perfection will return.
Before you work on yours, though, you should search for its manual online and read it. Because not all Di2 rear derailleurs are the same. On some, only the high gear limit screw is adjusted this way. On others both the low and high limit screws are.
Finding the Correct Instructions
One way to find the right manual is to use Shimano’s new Di2 for Dummies site, here: https://di2center.com/2019/08/24/di2-for-dummies-beginners-guide-to-di2/. On this resource there are links to Shimano manuals for some of their recent Di2 groups. If your Di2 is not listed, you should search on “Shimano Di2 Manual” adding the model number of your derailleur, which will bring up the results for your exact derailleur.
The screenshots below show the directions for a high and low gear limit screw adjustment to give you an idea what directions you need to find for your rear derailleur.
PLEASE NOTE, that you need to find and follow the correct instructions for your specific Di2 rear derailleur. The screenshots shown are just an example of what to look for, and also the warnings of what can go wrong if the adjustment is not done right.
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