Researching this question I found a thread from 2003 in this forum that included a message by Mr Jobst Brandt. He wrote, 'Cyclic stress causes fatigue cracks and a thinner spoke as a lower N/mm elongation than a fat one. Hence the rim knows as does the nipple, that stress excursions with thinner spokes are lower,'
This sounded counter to what I think, that he wrote that a thinner spoke stretches less for the same force/distance applied , both the 'lower elongation' and 'lower excursion'. Is it too late to ask? If so, I apologize.
spokes acts as springs. Pull at them and they stretch. Stop pulling and they return to normal. Unless you pull harder than the spring can handle. Then it'll be permanently extended when you remove the tension. Or pull really long and the spoke will snap. A thinner spoke -assuming the same material - will act as a weaker/lighter spring, and stretch further than a thicker spoke for the same amount of force or load. And also the reverse, for a given amount of stretch, the force in the thin spoke will be lower than the force in the thick spoke. So if you have a rim being pushed sideways a bit, some of the spokes will stretch. And a thinner spoke will carry less force into the hub/rim. For a bicycle wheel, most of what it sees is rim buckling inwards a little at the contact point with the road, causing spokes to lose tension. For a thinner spoke - that is stretched more - the proportional loss of tension is lesser than for a thicker spoke. Let's say a thick spoke wheel has the spokes stretched 1mm and a thin spoke wheel has then stretched 2mm. And that the rim buckles inward with 0.5 mm at the contact point. That's a 50% drop for the thick spoke and a 25% drop for the thin spoke. A lot better deal for fatigue avoidance.
