Originally Posted by
cyccommute
Let’s start with tensile strength. The tensile strength being measured isn’t the wheel but the resistance of the
spoke to breakage. Tensile strength measurements of the spoke gives you insight into how much it takes to fatigue the spoke to the point where it will break. Higher tensile strength (or resistance to breakage) will result in a strong, more durable wheel.
Pillar spokes is about the only place where I’ve seen actual measurements of tensile strength of various types of spokes. You have to look at multiple graphs to see what effect butting has on strength but it’s pretty clear when you compare them. For example a 2.0mm (14ga) spoke breaks at 270 kgf (kilograms force which is a really dumb unit). A 2.0/1.8/2.0mm breaks at 290 kgf (about) and a 2.2/1.8/2.0mm spoke breaks at about 330 kgf. They have a 2.3/1.8/2.0mm triple butted spoke that breaks at about 420 kgf. For comparison, their 2.3mm straight spoke breaks at about 360 kgf.
Their charts show pretty conclusively that butting the spokes increases their strength.
I'm pretty sure Jobst Brandt put data on spoke tensile strength into his book, The Bicycle Wheel. When I read it I was in engineering school and it seemed like the results of conventional technical measurements I was learning about. But since I graduated as an EE rather than an ME, you should probably just take this as a suggestion rather than a statement.
Brandt's overall conclusion was that spokes break because of excessive cycling into inelastic deformation. A thick spoke has a stiffer shaft which elongates less with an increase in tension, compared to a more noodly thin spoke shaft. If strain is common between the thick and thin spokes, then more strain occurs at the nipple and the elbow of a thick spoke, than with a thin spoke. The accumulation of fatigue damage should be faster for a thick-spoke wheel than for a thin-spoke wheel. At least this how I understand Jobst. But again, this is just a suggestion.