I have built some tandem frames and can tell you a few inherently difficult issues you will be faced with: 1) the position of an essentric is a really exact calculation. You should have it positioned to where the bottom bracket is exactly aligned with the seat tube when finished. You have to allow for what size timing chainrings are to be used, it can be pretty tricky to get it right. 2) The essentric cannot be crocked or the chain will keep falling off. The problem with idler wheels is that they tend to be really noisy.