CJ wrote:CJ wrote:You just don't have to mind that the chain dangles loosely in the little ring and the two or three smallest sprocket combinations you never need anyway. That dangling never, ever does any harm in my very long experience (there was even a bunch of 1930s Frenchies used to ride like that all the time, search 'flottantistes'), just makes a bit of noise...
PPS: Not that long experience. I don't really remember the 1930s!
uh-oh... without knowing it I may have slowly been turning into a latter-day 'flottaniste' for some years...
I used to use a short arm RD on my MTB, even though it didn't tension the chain properly in various small-small gears, figuring that I didn't use those except by accident and as long as it didn't break anything should I accidentally select those gears, that was 'good enough'. Actually it was better than 'good enough' most of the time.
The basic observation is that gravity always gives a certain amount of 'chain tension', so (unless you are on very bumpy surfaces or are hell-bent on getting airborne) you probably don't need a sprung-loaded tensioner per se, within limits.
Practical experimentation has demonstrated to me that with suitable chain and sprockets, quite large amounts of chain slack can be tolerated in an IGH setup without the chain derailing by itself on a regular basis. How much slack? Well, about 3" of vertical movement on the lower run seems OK in many cases. This is usually limited by wear, in that by the time the chain gets this slack, the sprocket is usually hooked and this causes 'skipping' (much as you might find with a new chain and worn sprockets in a derailleur system) once there is enough slack for the chain to climb over the teeth.
I keep meaning to try with a new chain and see how much slack can be tolerated with with an unworn sprocket.
Anyway the conclusion is that you can certainly run with enough slack to allow a chain to be deliberately derailed if needs be, which opens up some interesting (and tensioner-free) gearing setup possibilities.
cheers