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Technical FAQ: Using Di2 Synchro Shift

Lennard Zinn provides detailed instructions on using Shimano's Synchro Shift for Di2 drivetrains.

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Di2 battery help

Dear Lennard,
I was watching one of your YouTube videos and I have a question. Recently I took my bike in for a tuneup and a new cassette and chain. A week later when I picked up my bike, I noticed the Di2 battery was completely dead. At home I plugged it in and charged it, and when I held down the left shifter, the green light came on, followed by the red light.

I purchased a new Di2 battery, installed it, plugged it into my laptop, and charged it; when I tested it, the same issue happened: the green light came on first, followed by the red light.
— Vic in Chicago

Dear Vic,
It sounds to me like you got a Di2 firmware update when you took your bike in for a tuneup. There was no need to get a new battery; yours appears to have been working perfectly.

What you are looking at on those LEDs on your Junction A is, first, the indicator of battery charge level (the green light alone, meaning your battery was fully charged), followed by the indicator that you were in Manual shift mode (the solid green and red lights together). Since I am assuming from your set of statements that you had not had this occurrence before, you are now the owner of a bicycle set up for Synchro Shift, Shimano’s coordinated front/rear shifting system that was pioneered on mountain bike Di2 systems and is now on road Di2.

I am sure you will find that if you push the mode button twice on the bottom of your Junction A, the red and green lights will flash twice. And if you again push the mode button twice, the red and green lights will flash three times. Finally, the third time that you push the mode button twice, you will be back where you started; the red and green lights will once again come on solid momentarily, and then turn off. And you can then cycle through those three modes again and again.

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The red and green lights coming on solid momentarily and then turning off, as happened to you following the green battery indicator light alone after holding down a shift button, indicates you are in shift mode M, for manual. In this mode, the shifters work as they always have: the front derailleur only shifts when you operate the buttons on the left lever (and nothing you do on the left lever causes the rear derailleur to shift), while the rear derailleur alone is operated by either the buttons on the right lever or by the buttons on the satellite shifter, if you have one.

Alternatively, the red and green lights flashing twice indicates you are in shift mode S1, or Semi-Synchro Shift (unless somebody has reprogrammed the contents of file S1 in your Di2 system). In this mode, every time you shift the front derailleur (with the left lever, yes), you will do a double shift; the rear derailleur will simultaneously shift back two cogs in the other direction so that you make a similar step up or down in gear ratio as a single shift in the rear would cause.

Finally, the red and green lights flashing three times indicates you are in shift mode S2, or fully Synchro Shift (that is, again, unless somebody has reprogrammed the contents of file S2 in your Di2 system). In this mode, you never actually need to shift with the buttons on the left lever to go through your entire gear range; in fact, you could forgo the Di2 levers and just have a climbing shifter plugged into your Junction A. In other words, when you have shifted to preprogrammed points on the rear cassette, the front derailleur will shift, along with the same simultaneous double shift of the rear derailleur two cogs back in the opposite direction to even out the step up or down in gear ratio.

For instance, the standard pre-programmed S2 mode will have you stay in the large chainring when downshifting all the way through the cassette until you try to shift to the largest rear cog. Say you have an 11-28 cassette and 39-53 chainrings. If you start downshifting from the 53×11 combination and only shift with the right lever buttons or the satellite shifter, you will go all the way to the 53×25 combination without a front shift (this is exactly the same so far as what would have happened in M mode). However, when you push a right downshift button once more, instead of going to the 53×28, your front derailleur will instead drop down to the 39-tooth chainring and the rear derailleur will simultaneously drop down two steps — it will pass the 23-tooth cog and stop on the 21-tooth cog. Three more presses of a right downshift button will finally take you to the 39×28 combination, your lowest gear. Similarly, if you now upshift only using right shift buttons, your rear derailleur will take you to the fourth smallest cog, and then the next shift will bring on a double shift of the front derailleur to the big chainring while the rear derailleur shifts two cogs larger.

If you have an internal battery, in order for this firmware update to have accomplished what it did, namely set up your Synchro Shift options, you must have already had the latest rod-shaped battery — the BT-DN110 (or the BT-DN100 if you have the rectangular external battery). These batteries have the extra memory required to run the Synchro Shift, whereas the older versions of these batteries (SM-BTR2 and SM-BTR1) cannot support Synchro Shift. And, clearly, the new battery you bought is also either a BT-DN110 or BT-DN100.

I encourage you to experiment with the other Synchro Shift modes, rather than just the M mode your bike is currently in. I leave mine in S2 all the time. In this mode, I do most of my shifting with just my right-lever buttons or with my climbing-shifter buttons, and I can still anticipate the terrain ahead and also shift my front derailleur when I want.

For instance, in S2, as I enter a hill and as it gets steeper and steeper, I keep repeatedly requesting a lower gear with my right hand as I need it, eventually ending up on the inner chainring and one of the largest three cogs if the hill is long enough and/or steep enough. But when I come over the top of the hill, I know I’m going to have a rapid increase in speed and don’t need to take similarly small steps up in gear ratio. So I just shift up to the big chainring with my left hand when my speed picks up, just as I would have in M mode or with a cable derailleur — and I can do that cool shift up to the big chainring while sprinting out of the saddle over the crest of a hill that only an electronic front derailleur can perform.

I continue shifting up with my right hand as I get going faster and faster down the hill (and I can still hold down a right shift button to activate multi-shift through a bunch of cogs if I want, just as in M mode). If you follow my gist, you see that I lose nothing by being in S2 versus M mode, and I gain the freedom from performing the double shifts myself; when it’s appropriate to have a double shift, I just let the system do it for me. I find that I never use S1 (Semi-Synchro), as it takes away my ability to just do a front shift without a double shift in the rear.

If you’re not expecting a double shift, it can be a bit alarming when the chain starts clunking around in both directions when you didn’t expect it! But if you have a Garmin 1000, 820, or 520, you can pair it with your Di2 system via an ANT+ connection, and it will not only show you what gear you are in but will also tell you if you’re in Synchro mode and when your next shift will be a double shift. The mountain bike Di2 systems offer a handlebar-mounted digital display, which actually gives you a double beep when you’re about to get a double shift; like the Garmin, it also displays M, S1, or S2, along with the gear you’re in.

With the Garmin, when you’re about to get a double shift, you instead get a visual warning. For instance, if you’re on the big chainring and the second-biggest cog, the gear-indicator display field on the Garmin will show the second largest rear cog in orange, the large chainring in orange, and the small chainring will flash orange, warning you that the next downshift with a right shift button will drop the chain to the inner chainring (and drop down two smaller cogs in the rear).

On my Garmin, I turned on one of my training pages that was off and made it a Di2 page with four fields: one field displays battery percentage, another displays the shift mode (M, S1, or S2) I’m currently in, a third field graphically shows the gear I’m in, and a fourth one displays my heart rate. It’s nice to know the exact percentage of battery charge, rather than just the color codes on Junction A that correspond to very large changes in percentage. I also like seeing which shift mode I’m in without needing to hold down shift buttons and look at Junction A. The gear field displays front chainrings and rear cogs as gray bars with the chainring and cog the chain is on as orange bars; I’ve grown to like seeing what gear I’m in without looking down and back. And in order to monitor whether I’m going into, or in danger of going into, my atrial tachycardia heart arrhythmia, I have a heart rate field open on every active training page I can on my Garmin.

As you are Synchro-shift enabled, you can reprogram S1 and S2. If you, for instance, want the double shift in one direction to go only one cog back in the rear upon a front shift rather than two, or perhaps step back three cogs instead of two, you can set it up that way. You can do this the old-school way (probably using the same interface as got you the firmware update) by using the Shimano E-Tube software on a Windows PC plugged into your Junction A charging port with your battery-charge wire or with Shimano’s SM-PCE1 interface device and a USB cable. If you have a “D-Fly” unit, which is a small inline Bluetooth transceiver usually located on your seatstay just above your rear dropout and plugged into your dear-derailleur e-tube wire, you can instead use the new Shimano E-Tube app for tablet computers and iPhones and Android smartphones. Either way, you can set the shift steps you want on a Synchro-Shift program of your design, and you can drag it into either the S1 or S2 slot. Say you put it in the S2 slot, when you double-click the mode button on Junction A until the red and green LEDs flash three times (and if you have a paired Garmin, it displays “S2”), you’ll be using your custom shift mode.

By the way, rather than holding down a shift button as you did to check battery level (and, now, which Synchro Shift mode you’re in), you can instead hold down the pair of shift buttons on either lever or the pair of buttons on the “climbing” satellite shifter. The advantage to this is that it does not cause a shift; it only gives you battery and mode information, whereas holding down a single shift button causes a shift. This way, you can check battery level and shift mode while riding as well, without shifting.
― Lennard