Well, I couldn't find the book, so I did an Internet search.

I couldn't find one site that had the complete answer, but I'll just combine some of them.

I still haven't found the proof I want.

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For any given gear ratio, say 2:1, many different gear diameters and numbers of teeth will work, as long as they have a ratio of 2:1.

However, as the diameter of the gear pairs increase, the chain remains more straight as it reverses direction, and thus, each chain segment bends less.

As each chain segment bends less, the entire drive train system experiences less "chain segment bending loss."

One can further reduce chain segment bending loss, which involves not only the interaction between the pins and bushings, but the interaction between the pins and bushings and the faces of the gear teeth as well.

The straighter the chain stays, or the larger the "corner" it turns, the less chain segment bending loss.

Additionally, one can further reduce chain segment bending loss by replacing the bushings with bearings, so that the meeting surfaces between the gear teeth and the chain experience no friction.

The so-called bushings of high-end 1/8" track chains correspond to bearings rather than bushings.

At some point, the combined weight of the larger diameter gears and the more complex chain undoes the advantages of bearing surfaces between chain and gear teeth, and a straighter, less-bent chain.

Where do the two curves cross?

I don't know.

However, somewhere between a small chain ring and cog combo and a large chain ring and cog combo; and somewhere between the simplest lightest 3/32" bushingless chain, and the heaviest, most complex chain with bushings/bearings, lies the optimum in terms of minimal friction, bending loss, and weight.