When I was using the smaller bicycle chain sprocket, it wasn't as difficult to do this, however with the new larger 95T #25 sprocket, it has decreased the margin of error to almost nothing, and using a typical derailleur (I am prevented from using some of the better kinds because of the restrictive space in the back caused by the NuVinci linkage) has meant it has never quite behaved as I would like with about a 1/16th of gap between motor and pedal chains.
So last night someone mentioned to me something about a special automatically adjusting chain tensioner Vision sold to the dealers to demo their products allowing for different sized people to ride the same bike with out lengthening/shortening the chain.
I then realized that I could use an old short-cage derailleur to do virtually the same thing, and it solved 99% of the issues I was having!
For the last 1%, I used an old recumbent bike trick, using 1/2 inch irrigation (poly) pipe to act as chain guides to help keep spacing just so where the chains are near each other to keep a healthy gap between them, and surprisingly, the pipe makes virtually no noise!
Here are some pics:
There might be some high-dollar off the shelf tensioner, but this is something you can buy at most bicycle shops, and would be cheap and available often as a used part.
