We think that probably all of you know Schlumpf Innovations and their Mountain Drive or Speed or High Speed drive two speed gears inside cranks. It would be wasting time to describe here all the advantages and properties so who is interested more in such system just visit www.schlumpf.ch
We would like to show on couple of pictures, taht it is pretty easy to assemble it and the only special thing you need is reamer which can be rented from some dealer or possibly also directly from Schlumpf. But Schlumpf also offer a version which doesn´t require this tool.
If you have any good or bad experience with Schlumpf systems, we would like to read about it here in comments.
Just to inform you. This one was assembled on Dahon Cadenza 8 folding bike and it works great.