I was just wondering if anyone has build a Di2 tandem, 11-36 cassette using JIS square tapered bottom brackets. Rear spacing is 145mm & I'll be using the Gates centertrack timing belt. I'll also be using a double crank set, not a triple converted to a double. BB shell is 68mm. If you have done this & happen to remember the lengths of the bottom brackets that have worked well for you, it would be very helpful.
Best regards

Can't comment on the BB lengths yet (the bike goes in this weekend to evaluate BB lengths and chainlines for the purchase of a new crankset), but I would recommend getting an extended clamp (unless you braze-on, which may make things more difficult) like the one from K-Edge (assuming that's where you are having the RD modified). I think this moves the FD about 2.5mm outward. With the belt drive, I'm pretty sure the front and rear BB lengths must match (a chain drive gives you a little wiggle room on this).

Enjoy that DI2... No more frequent cable adjustments, and once you get the system dialed in for your bike shifts are spot-on every time (and as a stoker on a good day I can hear the motors just before the shifts so we don't need to communicate them).

More on BBs when I get the info.

Belt alignment looks like a great reason to use Phil Wood BBs in the appropriate length. A small fine tuning adjustment to "belt line" can be made without using washers or BB spacers.

true, I've pulled a Phil BB axle thru the bearings myself ,, the spline-ring installation tools shoulder against the bearing
and the crank-arm fixing bolt, thru a big washer over the end pulls it through the sleeve created ..

I meant that the design allows for adjusting alignment by loosing one cup in the BB and then tightening the other cup. The cup presses against the BB shell and will move the entire cartridge without moving the bearings on the axle. The result is an asymmetric BB but good for a few mm of adjustment.

Yea , that too ..

In theory you'd want to move your crankset 7.5mm outwards (assuming that your FD can reach that far) so if you're dealing with a symmetrical BB you'd want a spindle 15mm longer than recommended for a half bike.

If your timing crank allows you to mount the chainring (or is that beltring?) on either side of the spider you can get away with a narrower captain's BB.

What crankset are you using?

I'll be using Da Vinci design billet cranks
They recommend: 103 -107 spindle length for captain & 107 - 113 spindle length for stokers crank

As I read it, the o.p. is using a conventional system for the main drive. Only the timing between capt. and stoker is belt drive. IMO nothing fancy need be done to the FD beyond what would work for any other road setup. I think also that the o.p. is in the best situation to determine what chainline is going to work. Or their LBS. I'd start with the spindle length recommended by the crank manufacturer and work shorter from there if need be.

H