Min chain width that works with singlespeed sprocket?

Konstantin Shemyak said:

I have a tandem with internal-gear hub, and want to swap the
rear crank for a shorter one, suitable for a child stocker.
I have short crankset with pressed-in chainrings, some old
Altus, believe it was 7 speed. So I want the sync chain to
run over the middle chainring and the main chain to the rear
hub over the large chainring. Obviously this does not work
with wide singlespeed chains. Now I'm starting to wonder
whether it is possible to use so narrow chain that the
sync chain would not rub against the main chain.

I know that 8-speed chains could be used OK with singlespeed
sprockets, but remember reading that 9-speed chains could not.
Is this true?

Generally not. Pretty much any 3/32" chain works with any 3/32"
sprocket.

Another way to do a kidback is to replace the right front crank with
one that has a chainring, and run the chain diagonally from the kidback
to the chainring on the right front.

You can see a couple of variants of this approach here:

http://sheldonbrown.com/images/kidfrontz.jpeg

By the way, it was silly of the designers of your bike to use crossover
drive. Crossover drive only makes sense on tandems that have a front
derailer.

See: http://www.bikexprt.com/bicycle/tancrank.htm

and: http://sheldonbrown.org/picchio

Sheldon "Single Side Drive" Brown
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Harris Cyclery, West Newton, Massachusetts
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Sheldon Brown said:

Quoted message said:

I know that 8-speed chains could be used OK with singlespeed
sprockets, but remember reading that 9-speed chains could not.
Is this true?

Generally not. Pretty much any 3/32" chain works with any 3/32"
sprocket.

Thank you! This will be given a try.

Quoted message said:

Another way to do a kidback is to replace the right front crank with
one that has a chainring, and run the chain diagonally from the kidback
to the chainring on the right front.

In fact the kidback user has already outgrown the device, in my
original post I meant changing from kidback to regular, just shorter,
rear cranks. But about running the kidback chain to the right front
chainring: I did not do it, because I was afraid to break or bend
the frame. The chain tension would have twisted the kidback attachment,
probably no problem with a solid steel frame (as it worked for two of
your tandems!), but mine was aluminium, and I was not eager to work
by trial and error.

Quoted message said:

By the way, it was silly of the designers of your bike to use crossover
drive. Crossover drive only makes sense on tandems that have a front
derailer.

Completely agree. Well, the good side of it is that now, when I
"downgrade" to single-side drive, I got three tandem cranks for
future experiments.

--
Konstantin Shemyak
My tandem for riding with two kids:
http://konstantin.shemyak.com/cycling/Tandem