Help me to understand how that would not require a different fixed length chain. Since the diameters are different, surely the larger cog will take up more length of chain than the smaller, especially the 17/21 combination they offer?

Maybe I am missing something. I recently changed my SA cog from the 19 to 24 and it required a much longer chain.

Double cog, double chain ring as a 2spd could theoretically be a fixed chain length (with some slack to shift) as low gear is small chain ring/large cog, and high gear is large chain ring/small cog.

However as a 4 speed, you would need a chain tensioner since the 2 middle gears would then be small chain ring/small cog and large chain ring/large cog.

That would take good math, or a bunch of trial and error to see which combination would result in no change of chain length!LOL
Kinda negates the advantage of the double crank and double cog as the two middle gears would be useless.

And so back to the discussion of derailleur drag, would a chain tensioner add the same about of drag as a RD?

Sleepcactus, what are the size of the chainrings you are running? And do you need a chain tensioner to take up the slack? And do you have to stop to manually switch back to the larger chainring?[/QUOTE]

There is a blogpost about the bike and its details here
I use a derailleur as a chain tensioner, set under the one cog

One other observation: I never get saddle-sore on the 2 speed, because I spend so much time standing while cranking up hills.