Originally Posted by
Carbonfiberboy
So few people understand this. My understanding is that:
while higher tension would seem to make a stiffer wheel, the difference in tension increase necessary to stretch a 100 kgf spoke vs. a 140 kgf spoke for 1mm is very small, if any. The purpose of adequate tension is to stretch the spoke enough to prevent detensioning and thus fatigue. Which is the reason that some builders' heavier straight gauge builds fail sooner than a double butted build, tension as you say being determined by the rim and heavier spokes stretch less at the same tension.
However, that last being the case, I don't understand why you don't think a heavier spoke will make a stiffer wheel, all other things being equal, since it will require more force to get a 1mm stretch than a thinner spoke. Is the difference negligible for common spoke gauges?
Good point and that may in fact account for the change in feeling of stiffness between say a DT comp vs a Sapim CX Ray build. In general though the main difference is in the 'primary gauge' - or the amount of material there to begin with. The end shape has an effect on the aerodynamics and the area moment of inertia of the spoke in deflection (something we are not looking to experience), but the rough cross sectional area and the resulting strain from the tension load applied are roughly the same throughout a family of spokes.
In your example - straight gauge spokes - larger area, resulting in a lower strain, lower stretch, and a higher probability of unloading - especially with a heavy rider in a low spoke count. Double butted - less material, lower area, higher strain, higher stretch/elongation, lower probability of unloading. Change that double butted into an "aero" shape. Same area, same strain, same elongation, etc.
The CX Ray is actually made out of a laser spoke though. So less material to begin with - lower cross sectional area, higher strain, higher elongation by a large factor. It was work hardened through the process they use to make it so it ends up with a very high strength and high fatigue life, but trades it off with a bit of brittleness.