Originally Posted by akexpress

My guess is you have "typical tandem" rear hub spacing of 145mm. If that is the case you are probably going to have some chain issues in the small chain ring and smaller cogs. Since the local shop thought that the cassette can be further outboard with a disc hub you need to find a better shop that actually has a clue ( sorry to be so blunt). Di2 on a tandem is exactly the same as a single except for longer wires and the possibility of a wider rear hub. I had similar issues on the small cogs on my previous Calfee with 145 rear hub. Since changing to 135 on our new frame it is only a problem on the smallest (11). There is really no need to ride the smallest chairing and 2-3 smallest cogs combinations. About the only time I do that is when I have a brain fart and forget I am in the small chain ring and try to get moving fast with a group or something. If you can give us what you are using for cranks someone on here will have info on whether you can improve the chainlink. BTW we had 1000's of happy Di2 miles on a 145 mm rear hub and frame so you will have a great bike. Also I would rather be able to totally cross chain in the big ring if there is a compromise to be made.