As we are moving to a Di2 installation, there is an example of a Single-Side-Drive (SSD) and Gates CenterTrack belt that is interesting me.
As you probably know, Di2 currently (and for the foreseeable future) can handle only 2 chainrings. This restriction may leave open the opportunity to use the outer chainring location on the drive side for a Gates Belt, preferably the CDX CenterTrack setup as shown in the following photo:
The implementation above utilizes a standard FSA SL-K Light triple crankset, not a tandem specific model. The front cranks are standard FSA SL-K Light doubles. The setup includes some chainring spacers and longer bolts to provide enough separation between the big chainring and the CDX ring so that the Gates belt does not rub the chain at the two points (upper and lower) where the 12mm wide belt passes forward from the CDX ring.
The photo also shows the use of a standard FSA 53t outer ring simply moved to the inside of the 130BCD spider. Our implementation would use a Specialites TA 48t instead. The clearance requirements between our big ring and the CDX should be no different than in the photo, as the largest clearance required is at the points where the belt passes the chain. Ring pins are inconsequential to the clearance needed as the chain width is the biggest factor.
Another point likely needing a solution, is the use of an outer ring on a middle position. Middle rings alway have a bolt recess built in, which helps minimize the bolt head sticking out and interferring with the chain when it is on the granny ring. When moving an outer ring to the middle, there is no such recess in the inside. I have no idea about how difficult it may be to machine new recesses in an outer ring. Filing down bolt or nut heads may not be a great idea, as in this case these are holding both the big ring and the Gates ring... probably should use strong steel bolts & nuts (no alloy).
Apparently Specialites TA does make middle rings in a 48T, but so far no luck sourcing one. Wiggle.com seems to have the best selection of TA rings found anywhere, but no big middle rings on hand. I currently have them on the hunt with their suppliers for a 48t middle.
Anyone see a technical reason why the implementation above would be a problem?
Here's another example (below) using the Gates CDC belt setup. The CDC ring has a flange on one side, plus a built-in belt offset that helps eliminate (or substantially reduce) the need of chainring spacers. The narrower 10mm belt helps too.