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Swaged spokes come in 5 varieties:
* Single-butted spokes are thicker than normal at the hub end, then taper to a thinner section all the way to the threads. Single-butted spokes are not common, but are occasionally seen in heavy-duty applications where a thicker than normal spoke is intended to be used with a rim that has normal-sized holes.
* Double-buttedspokes are thicker at the ends than in the middle. The most popular diameters are 2.0/1.8/2.0 mm (also known as 14/15 gauge) and 1.8/1.6/1.8 (15/16 gauge).
Double-butted spokes do more than save weight. The thick ends make them as strong in the highly-stressed areas as straight-gauge spokes of the same thickness, but the thinner middle sections make the spokes effectively more elastic. This allows them to stretch (temporarily) more than thicker spokes.
As a result, when the wheel is subjected to sharp localized stresses, the most heavily stressed spokes can elongate enough to shift some of the stress to adjoining spokes. This is particularly desirable when the limiting factor is how much stress the rim can withstand without cracking around the spoke hole.
* Triple-butted spokes, such as the DT Alpine III, are the best choice when durability and reliability is the primary aim, as with tandems and bicycles for loaded touring. They share the advantages of single-butted and double-butted spokes. The DT Alpine III, for instance, is 2.34 mm (13 gauge) at the head, 1.8 mm (15 gauge) in the middle, and 2.0 mm (14 gauge) at the threaded end.
Single- and triple-butted spokes solve one of the great problems of wheel design: Since spokes use rolled, not cut threads, the outside diameter of the threads is larger than the base diameter of the spoke wire. Since the holes in the hub flanges must be large enough to fit the threads through, the holes, in turn are larger than the wire requires. This is undesirable, because a tight match between the spoke diameter at the elbow and the diameter of the flange hole is crucial to resisting fatigue-related breakage.
Since single- and triple-butted spokes are thicker at the head end than at the thread end, they may be used with hubs that have holes just large enough to pass the thick wire at the head end.
