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Anyone successfully install Shimano 7970 DI2 on a tandem?

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Anyone successfully install Shimano 7970 DI2 on a tandem?

Old 10-05-23, 02:20 PM
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Anyone successfully install Shimano 7970 DI2 on a tandem?

Hi Folks,

I'm thinking of re-purposing my original DI2 7970 series group on a tandem. Yes, I still have the original DA DI2...still works like a champ, so I thought I'd bling up the tandem.

The issue I'm having is that the junction wire (5 pin) must be extra long to get from the head tube area to the stoker bottom bracket.

Here are the options I've found for the rear harness:
  • Option 1: EW-7972-B: battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell with external front and rear derailleur routing in conjunction with a short battery mount for neat fitment.
  • Option 2: EW-7973-1: (720mm) battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell then use internal routing in the chainstay for the rear derailleur and up the seatpost tube for the front derailleur in conjunction with a short battery mount for neat fitment.
  • Option 3: EW-7973-2: (1100mm) battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell then use internal routing in the chainstay for the rear derailleur and up the seatpost tube for the front derailleur in conjunction with a short battery mount for neat fitment.
I'm thinking my best bet would be Option 1 since I need a longer junction wire. My understanding is that this harness has an external "collection" of the wires that split to the front and rear derailleurs, and the battery pack. The "internal" wire is for the junction wire that goes inside the downtube. I would simply leave that cable of mine external as I don't have holes in my frame. I'm thinking this extension wire should work....

https://www.modernbike.com/shimano-d...on-wire-1140mm

There's also a 750mm extension wire, which I'm wondering if that would also connect into the above wire to make it even longer if necessary.

I currently have the all external BB harness, but the junction wire is already preset with the plug and can't reach the front junction box on a tandem, hence my question.

The other option is to splice the junction wire. I've spliced and reconnected E-tubes before, but I'm hesitant on the 7970 wires because they're different from e-tube and the availability of replacement 7970 wires is practically nil or super expensive for an entire harness.

Has anyone successfully spliced and extended the 7970 wires? Is the process similar?

Thanks so much!
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Old 10-10-23, 12:07 PM
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I have the 7970 on our Santana. When I installed the WolfTooth Roadlink, the rear derailleur wire was too short so I spliced in a section. I used heat shrink to seal it. If I recall correctly it was only two wires.
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Old 10-10-23, 12:56 PM
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Thanks DKMcK. I realized last night that my new problem would be needing a longer RD wire because I also plan on using a Roadlink as well to get up to a 34 cog...From my lazily hand mounted estimation from the harness at the bottom bracket, it's looking awfully close to the limits of being able to connect as is....

Was it a simple splice? The eTube wires were super fine, but easily manageable, and if I messed it up, I could simply buy a longer wire.. Now that you've said its two wires inside, I'm assuming then the 7970 cables would be similar?

I'm only adverse to the risk of splicing because all the wires are pre-connected to the external bottom bracket mount....i.e. I can't mess it up because I don't want to have to buy another harness!


To follow up on my previous question for those interested, I took apart my internal 7970 wiring on another bike and was able to plug in the internal 5-pin junction cable from that bike into my external junction box. The internal head fit into the pre-fabricated junction end of my external junction, so I'm fairly confident if I get the "internal" junction wire, it allow it to extend to connect the two harness sections together on the tandem...

Now I'm just waiting on those cables!

And looking for an Octalink double crank 170mm with a 110 BCD for compact rings....if anyone has one laying around....
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Old 10-10-23, 03:23 PM
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I did solder the wires. It was fairly easy with a small soldering iron so not to melt the insulation. Good luck!

On another note, I was able to replace the RD cage with a long one off a manual Ultegra RD of the same vintage (my local bike shop ordered the parts for me). I can run a 36 cassette without chain slop.
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Old 10-11-23, 10:51 AM
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Yea, my Giant road bike came with 7970 and the RD had the Berner oversized carbon cage installed. I've been able to do 11/32 with 50-34 front without any modifications...hoping for the same on the tandem, but am ready to go with a hanger extender if needed.

How did you extend the 7970 wire to reach from your bar/stem to the rear?
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Old 10-17-23, 09:45 AM
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The bike came that way. It looks like an extension cable added.
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Old 04-08-24, 04:00 PM
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Originally Posted by Duuuuuude99
Hi Folks,

I'm thinking of re-purposing my original DI2 7970 series group on a tandem. Yes, I still have the original DA DI2...still works like a champ, so I thought I'd bling up the tandem.

The issue I'm having is that the junction wire (5 pin) must be extra long to get from the head tube area to the stoker bottom bracket.

Here are the options I've found for the rear harness:
  • Option 1: EW-7972-B: battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell with external front and rear derailleur routing in conjunction with a short battery mount for neat fitment.
  • Option 2: EW-7973-1: (720mm) battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell then use internal routing in the chainstay for the rear derailleur and up the seatpost tube for the front derailleur in conjunction with a short battery mount for neat fitment.
  • Option 3: EW-7973-2: (1100mm) battery to derailleur cable set where the cables use internal down tube routing which then exits around the bottom bracket shell then use internal routing in the chainstay for the rear derailleur and up the seatpost tube for the front derailleur in conjunction with a short battery mount for neat fitment.
I'm thinking my best bet would be Option 1 since I need a longer junction wire. My understanding is that this harness has an external "collection" of the wires that split to the front and rear derailleurs, and the battery pack. The "internal" wire is for the junction wire that goes inside the downtube. I would simply leave that cable of mine external as I don't have holes in my frame. I'm thinking this extension wire should work....

https://www.modernbike.com/shimano-d...on-wire-1140mm

There's also a 750mm extension wire, which I'm wondering if that would also connect into the above wire to make it even longer if necessary.

I currently have the all external BB harness, but the junction wire is already preset with the plug and can't reach the front junction box on a tandem, hence my question.

The other option is to splice the junction wire. I've spliced and reconnected E-tubes before, but I'm hesitant on the 7970 wires because they're different from e-tube and the availability of replacement 7970 wires is practically nil or super expensive for an entire harness.

Has anyone successfully spliced and extended the 7970 wires? Is the process similar?

Thanks so much!
How did this work out?
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Old 04-08-24, 07:03 PM
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Got it all installed but rear derailleur cable wasn’t quite long enough to get the rear derailleur where it needed to be with the hanger…

ended up keeping the original xtr mtb rear derailleur and everything shifts just fine and dandy…put the 7970 in my cross bike instead.
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