Originally Posted by
HTupolev
Straight-gauge spokes have the same thickness everywhere.
Double-butted spokes use the same thickness at the two ends, but are thinner in the middle.
Triple-butted spokes use three different thicknesses, typically being thickest at the head, thinner at the threaded end, and thinnest in the middle.
Because spokes usually fail on the ends, butting does allow weight savings with minimal compromises to durability.
However, the main advantage of butting is only visible when considering the wheel as a whole.
What's especially critical in wheelbuilding isn't so much that each spoke is strong, but that they distribute load well. To distribute load well, the spokes need to be sufficiently sprung. The more elastic the spoke, the less raw tension you need to get it sufficiently sprung. Thinner spokes are more elastic; by butting the spoke, you can make it more elastic without sacrificing strength on the ends.
Suppose you have a really lightweight rim that can't handle high spoke tension.
If you try to build it strong by using ultra-thick straight-guage spokes, you have a problem. Because they're not very elastic, sufficiently tensioning them to distribute load could hurt the rim, and you'd have a weaker wheel. On the other hand, undertensioning the thick spokes to avoid ripping the rim apart would result in lots of fatigue, since loads wouldn't be well-distributed; you'd ironically end up with a wheel that's really prone to stuff like spoke failure.
If you try to build the wheel using ultra-narrow spokes, things sort of look better. The spokes can be brought to appropriate tension without overstressing the rim. However, the spokes simply aren't very strong, so the wheel still isn't quite as awesome as it could be.
Butted spokes allow you to get the best of both worlds. A butted spoke that's thick on the ends would be more resilient than the thin spokes, and would also be able to be brought to proper tension on the lightweight rim.