Adjust 12-speed Di2 rear derailleur shifting and limits
If you’ve been riding Di2 bikes for a while, you may have read my earlier guide on setting up 10-speed and 11-speed rear derailleurs.
It involves pressing the main junction box button to enter adjustment mode, and then using the shift buttons to adjust the rear derailleur.
The process is very similar for 12-speed Di2 rear derailleurs, but here’s the thing: there is no main junction box.
Well, sort of. All of this functionality (including the LED), has moved to the rear derailleur. This impacts the guide enough to warrant a completely new page, and here we are.
The steps involved
Setting up a 12-speed rear derailleur is done through four steps. They are:
Alternatively, since the rear derailleur has BluetoothLE/ANT functionality built into it, you can now also set up the rear derailleur using the mobile app - E-Tube Cyclist.
Adjust the B-screw
Just like before, the first step is to set up the B-screw. Tightening or loosening the B-screw changes the distance between the cassette sprockets (sometimes referred to as cogs) and the guide pulley.
To get started, first shift the front derailleur to the smallest chainring. Shift the rear derailleur to the largest sprocket, or easiest gear.
Shimano’s dealer manual states that you should be turning the crank arm in reverse as you do this. The training videos don’t mention this at all, so it probably doesn’t matter all that much.
Next, we’ll turn the B-screw to adjust the gap between the guide pulley and the largest sprocket. The recommended gap depends on the cassette used.
This is Shimano’s guideline for Dura-Ace and Ultegra:
- 11-28T - 18mm
- 11-30T - 14mm
- 11-32T - 10mm
- 11-34T - 6mm
And these are the recommended values for 105 Di2:
- 11-34 - 10mm
- 11-36 - 6mm
Alternatively, you can use the GG adjustment gauge. This should be included with the rear derailleur. If it isn’t, and you’d like to buy one, the part number is Y3GK89600.
If you have access to a 3D-printer, you can try printing one. Chris Heerschap published a model of the tool on ThingiVerse.
You set the adjustment gauge on the plate as shown above, and then turn the B-screw to align the tip of the sprocket teeth with the correct line.
If you’re using a 30T cassette, use the top line. If you’re using a 34T cassette, use the lower line. For 28T cassettes you’d use the top of the adjustment gauge.
Micro-adjust the gear shifting
Next you’ll have to fine-tune the gearing, using micro-adjustment.
First, shift the rear derailleur to the 5th gear from the largest sprocket. This will ensure the chain line is as straight as possible.
Press and hold the rear derailleur button for about 2 - 5 seconds to enter adjustment mode. When in this mode, the LED lights up yellow.
Once the system is in adjustment mode, press the button highlighted below, while turning the cranks. This will move the rear derailleur toward the larger sprockets, in tiny steps.
Keep doing this until the chain touches the 4th sprocket, and a slight rubbing noise can be heard.
At this point, you’ll have to back off the rear derailleur a bit to move it to its final position. Press the button shown below five times.
Press and hold the rear derailleur button for about 0.5 seconds, and you’ll exit adjustment mode.
The derailleur should now be set correctly. This is just a starting point though, so shift through all the gears and double-check that they all work well. Make further adjustments if necessary.
Note that at this point you haven’t set the derailleur limits yet, so be careful not to shift the chain off either end of the cassette. Also, if the derailleur doesn’t shift onto the last gears, that is OK at this point. As long as all the other gears are shifting fine, don’t worry.
If you find that your derailleur shifts well in the middle of the cassette, but performance degrades at either end, consider checking your derailleur hanger alignment.
Dave Rome wrote a really good guide on derailleur hanger tools right here: derailleur hanger alignment tools on CyclingTips.com.
Set the low limit
With the derailleur shifting set up, let’s now set the low limit. Shift the rear derailleur to the largest sprocket (easiest gear). Now use a size 2 hex wrench to tighten the low side stopper bolt until it just touches the low side stopper. This is the bolt with an L next to it.
It’ll look similar to this:
If you’re confused and can’t get it right, just send me a message and I’ll do my best to help.
To check whether your limit setting is correct, shift the rear derailleur down three gears and then back to the largest sprocket again. It should shift to the easiest gear just fine.
If it doesn’t, relax the limit screw a bit, and try again.
Set the high limit
Now we’ll set the high stopper bolt.
Shift the rear derailleur to the smallest sprocket or hardest gear. Tighten the top stopper bolt with a size 2 hex wrench until it just touches the top-side stopper. This is the bolt with an H next to it.
Turn the top-side stopper bolt counterclockwise one turn to loosen it and allow for the rear derailleur to overshift.
Note that if you overtighten either stopper bolt the derailleur will not stay in that highest or lowest gear and shift back within five seconds. When this is the case you'll also hear some clicking or ticking, indicating that the derailleur is blocked from fully completing the shift.
If this happens to you, loosen the top side stopper bolt a bit, and check the shifting again.
Shimano says that the following could happen, if your limits are set too tight:
- Chain noise does not stop.
- The battery level drops quickly because extra load is being placed on the motor.
- The motor may be damaged due to overload (irreparable).
Your rear derailleur should now be installed and set up pretty well. If you're having problems, or just cannot get it right - just leave a comment or send me a message and I'll do my best to help you!