When to adjust eccentric bottom bracket?
 

The thorn bike manual says adjust when "T1-T2" is between 40 and 60mm. The picture:

http://kelehers.me/pics/t1-t2.png

seems to indicate that T1-T2 is just the difference in diameters of the sprocket and chainring. This makes no sense to me. My bike came w/ a 38-17, difference is around 100mm.

Clearly sag must be important here. But I'm not getting it from this picture.

??

For T1, it looks like they want you to find the midpoint between the sprocket and chainring, pull the chain as far apart as it will go and measure the outside dimension. For T2, use the same midpoint but squeeze the chain in and measure the outside dimension. Their illustration seems to be a bit exaggerated which has lead to some confusion for you. T1 will likely be a good bit smaller than the chainring diameter and T2 will be a good amount larger than the sprocket diameter, unless your chain is really loose.

Aha! That makes lots of sense. The picture just shows an unfortunate coincidence. Thanks.

The drawing is indicating take the measurement ( T1 -T2 ) at the midpoint between cog centers. Further..

divide by the number of pins in the chain and you have an idea how much wear has taken place ..

another technique: on center 24 pins start out as 12'' when they measure 12.125" it's time for the chain to go ..

I found bushingless chains in a single speed drive train, shorter lived than older bushing around the pin, pin sticks out a ways taller side plates, types,
in 3/32, rare these days, more likely to find just 1/8'' in that type.
bushingless type helps side flex for derailing ..

My bike, Koga WTR, came with a Rohloff chain, I like the Whipperman bushing type chain better, that i bought .

Whipperman themselves acknowledge the added longevity of bushings , a larger surface rotates around the pin..

The issue is chain tension. If it's too loose it slaps around a lot and can come derailed easily. If its too tight the bike won't pedal smoothly. The T1-T2 thing is just an objective way to measure how much slack is in the chain.

In real life I just gradually snug up the eccentric until I start to feel some resistance or vibration when I turn the crank by hand - then I back it off until it goes away.

Yep, everyone with single speeds or fixies talks about chain "tension". But the correct tension is just barely no tension at all.

Watch out for the tight spots where the front and rear gearing mounting allowances add up to make the spacing greater than normal. You need to check THOSE points to ensure the chain does not go tight when these occur.