Simple Physics Question re SS Gearing

[dddd]

Originally Posted by bikinchris

This. The chain itself will have to articulate less and needs less energy to operate. Also, the chain pulling the rear cog will be much straighter when leaving the rear cog. Larger cogs and chainrings WILL allow the chain drive to be more efficient.
The ten tooth cogs on the modern systems is completely wrong in my opinion.

True, there is a proportional (to chainring size increase) reduction in the articulation of the links.

But, there is also a proportional increase in the number of links articulating per second.

The increase in swing angle of the links moving around smaller cogs is accommodated entirely under the condition of dynamic friction, while any increase in the number of links flexing incurs more in the way of static-friction threshold events (static friction being much higher than dynamic friction).

As I said earlier, the theoretical considerations are complex.

And I do think that the OP's suggested increase from 17t to 21t would perhaps have much less effect on efficiency than the aforementioned increase from 14t to 19t.
So, well less than 1 watt difference even at the relatively high output of 250W. How might that equate to the added weight and air resistance of the bigger sprockets and longer chain I wonder?

Still, I would expect the larger sprockets to increase the power transfer stiffness/resistance noticeably, seemingly giving this rider more efficient use of my muscles at higher force levels and cadences.