Originally Posted by
Bad Lag
Are these tensile test results?
I would say yes.
I have to say is this - I dislike when people provide data for comparison in graphical format but plot that data on different scales. It makes it impossible to compare without mental gymnastics (recalculating and replotting in your head to understand how they actually compare).
Not my graphs. I stole them from Pillar as the examples I can find of actual spoke strength testing. And, to make the matter worst, both axes vary from graph to graph. The units on the Y axis are 14 and 16 units depending on the graph. Yea, it’s hard to compare. It’s be nice if they stuck with the same color scheme as well.
If I did it correctly (in my head), the butted spokes are effectively stiffer (steeper slope) and have higher breaking strength. Is that correct?
The double butted elongate less before breaking. The straight gauge (P14) break at a bit over 5mm of elongation while the double butted (1415) breaks at bit above 4.5mm. The triple butted (2018) seems to break at around 5.6mm.
Is this from hardening associated with the butting process or do they get annealed? Harder (stiffer) is almost always stronger.
From what I’ve seen, the butting process is essentially drawing of a straight wire through a die. They don’t get annealed following the drawing that I’ve ever seen.
Is breaking strength the key parameter for a spoke, or is it yield or fatigue strength?
Fatigue strength is more important in a spoke but it would be more difficult to measure. Tensile strength isn’t directly relatable to fatigue strength but for comparison purposes of wire spokes, it’s probably not a bad estimate. If you compare spokes to spokes and on spokes to bolts, it’s probably a fair comparison.
Failure mode is also an important characteristic in these tests? Did they all fail in the same way, at the same location?
It’s hard to say. For spokes, failure is almost always at the head. Occasionally, they fail at the spoke nipple but seldom do they fail mid-spoke. I would suspect that the tensile strength measurement may break them mid-spoke but can’t say for sure.
Absent answers to those, I do not want to draw any conclusions.
It’s not perfect but I think the data is valid. It fits real world experience and suggestions made by others that butted spokes and butted spokes with heavier heads result in less spoke breakage than straight gauge spokes.
I say that because I have broken many, many good quality spokes in my riding - butted spokes, usually not at the head or threads. A typical failure would look like a tensile failure in the thinner section near or right after the transition from the thick to thin section. Was it simple tensile failure (loading) or was it a fatigue failure? New York City roads were notoriously bad.
I’ve never broken a spoke mid-shaft nor have I seen spokes in wheels at my co-op broken mid-shaft. I’ve seen a very few broken at the spoke nipple but the vast majority have been broken at the head.
I have some good quality wheels with good rims and double butted spokes that have lasted me 40 years. They are showing their age but have been a blessing. I've never broken even a single spoke but roads out here are relatively smooth.
I’ve broken lots of spokes. The triple butted spokes were suggested to me by a local shop owner back in the early 2000 and I’ve never looked back. They are more robust and durable than anything I’ve used previously. That first wheel lasted me 10 years of mountain biking without issue which isn’t something I could have said previously. They probably would have lasted longer but I loaned the bike to an intern who “adjusted” the shifting and shifted into the spokes. I had reached many of the same conclusion with regard to the durability of triple butted spokes as that of the Hjertberg article I linked to before I read his article. His article explained
why but I was already aware of the benefits.
By the way, I don’t look at the failure of something as bad. I’ve learned a lot over my years of bicycling and my science career through failure. I’ve always felt I learn a lot more from failure than from constant success. Success is good but failure tells you where the limits are.