fooferdoggie 

with long wheel recumbents and long chains why not use this method instead of having to retake the bottom bracket? it would be far easier to deal with and easier to fix or take off a chain.

jccaclimber

As someone who had built a few recumbents and ridden a lot more, many of them have absolutely appalling chain management.

Other than old or really cheap frames, tandems tend not to have them for the same reason fixed gear bikes don’t. The lower chain portion can end up in tension, which tends to be hard on the idlers. Most also want the chain length to be fixed, IE no rear derailer type idlers. Without the fixed length chain the sync between captain and stoker floats a bit and the top can lose tension.
Finally, a half decently made eccentric is more secure and less easily damaged than an idler.

Don’t get me started on plastic tubes being used as chain guides.

Leisesturm

The main issue is that there needs to be a way to fine tune the tension in the system to allow for 'chain stretch. The previous answer is correct in so far as the need to have tension on the return run of chain, but an idler could allow for that.The horrible tension adjusters fitted on department store TSO's are, in fact, idlers in a sliding carrier. The *usually) extremely small diameter of the pulley wheel, and the overall lack of refinement of such designs makes it easy to overlook that this is actually an idler system at work.

jccaclimber

It isn’t just that it needs some tension to not derail. It’s also that the captain backpedaling makes the lower chain portion a high tension segment rather than just a slack tension segment.

Carbonfiberboy 

IMO an absolutely tight chain between captain and stoker is a must. Each rider needs to be able to feel the pedaling of the other rider. I think that conventional timing chains have too much slack. When we changed from a chain to a tight Gates belt, it made a big improvement to our pleasure-in-togetherness we get on the tandem. An idler pulley would make it much worse.

headwind15

I was a bicycle store owner for over 12 years and used to sell tandems, some of the less expensive ones had idler wheels (Crestline tandems come to mind), The truth is one thing is for sure: Idler wheels are really, really really noisy. Trust me, I you want a half way decent tandem, you don't want one with idler wheels)

Chilepines

Interesting observation about the timing chain being a way for the captain and stoker to communicate. We have a timing chain (as most do) and have no problem at all telling what the other is doing. Maybe that comes from riding the tandem together for many years. And if I don't get the message through the cranks there are other less subtle ways to communicate!

MNBikeCommuter

I came upon this realization several years after beginning riding with my main two stokers: my kids. We got going when they were 9 and 11 so they adopted my style right from the start. I hadn't realized how automatic things were until there'd be an occasional break from the routine pedaling--I might have a false start with pedaling and have to back off right away and they weren't expecting it, etc. My dad joined me on many rides too and our style was so similar it was the same with him--no words needed.

The one time I remember having to explicitly yell something was riding up a steep hill in the South Dakota Badlands with my son. I was trying to drop into the granny gear but he wasn't catching my backing off just a little and I couldn't shift!