Belt drive?
Seems good enough to break the round the world record. I'm sure it's a much better solution than a fully enclosed chain. I'd be interested to know whether Thorn have considered the option.
I looked at the belt drive. Santos, a Dutch bike manufacturer who builds a touring bike in the Raven class, and a variety of other bikes, a longtime Rohloff fan too, has a working proto and are working with Gates, the makers of the belt drive, to develop it.
A few points are worth noting.
1. The belt is manufactured a continuous closed run. There is no facility for breaking it like a chain. You therefore need a special frame with a split stay to fit the drive belt.
2. There is thus very little chance of the belt drive being retrofitted to a conventional bike you already own. Manufacturers who want to keep faith with existing customers (among other things by offering them retrofits whenever possible) will be wary.
3. All the available fittings (special "chainwheel" and special "sprocket) appear to be for internal hub or single speed gears. Gates and Santos both recommend Rohloff. So the belt drive is clearly headed, at least at first, for the most expensive bikes.
4. The belt drive is clean and oil-less, essentially maintenance free, sure enough. But from the Santos and Gates netsites you get the distinct feeling that they aren't aiming at tourers or commuters but at mudpluggers. The Santos proto is a mudracer.
5. Belt drive has had outings before. Bridgestone offered one within living memory, at least in the States (I have no personal experience). It didn't take off.
6. The Gates type is claimed to be the first belt drive to be stretch-free. It is built on carbon fibres as the dimension-stabilizing element. No intended service life is given (that I could see anyway on either the Dutch Santos or Gates sites), but a belt drive that pretty much forces the buyer to use a Rohloff hub had better have a service life at least as long as a Rohloff sprocket (both sides). I don't imagine that Santos would get involved in something flimsy or fragile. From photographs and description the materials and workmanship of the belt drive both seem first class. (You just know from looking at the prototypes that the price will also be first class...) However, a prototype is one thing; the devil is in the productionizing.
7. From the photographs the most striking feature is that there is no tensioning device supplied with the belt drive. Let me say that again: no integrated tensioning device. The frame must have some method of moving the sprocket and crankwheel closer together for fitting the belt drive and then apart again for tensioning, which most purpose-built Rohloff frames already have, and all bikes with long(ish) horizontal slots. Gates are so certain of marketing the belt drive, they are already working on their installation manual, and there is a clear warning against levering the belt on with a screwdriver: it has to be sliders or an ECC.
8. Aesthetically, belt drive could make for a very clean frame, depending on how you handle the split in the stay and the sliding capability. Thorn already has the eccentric bottom bracket for the sliding capability. That might be an expensive way to achieve correct tension if a truly stretch-free belt drive is fitted, because the slider-facility would be used once to install the belt drive, and then not again until it is replaced. But aesthetically, an EBB is the cleanest possible Rohloff installation, and belt drive installation too.
HTH.
Hobbes
