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July 01, 2014 2 Comments
Last post I shared images of exploded spokes. These were ridden off road and when the rim was damaged, the new rim required 2mm shorter spokes. These were shortened and threads re-run, a process done a zillion times a day across the planet.
Only, this time, the spokes blew up, would not accept a nipple. Here are close-ups of the ends. We're in the middle of discovering what happened and images are part of the evidence. By the 3rd post (Chapter 3), we'll be unravelling the cause and effect. By following along, I hope you'll be better acquainted with spoke metallurgy, what to expect, and techniques to get the most from your wheels.
Here are two brand new spoke ends, same brand and gauge. No use whatever.
These two ends are from ridden spokes. Since the use, no work has been done.
These two used spokes were trimmed 2mm, but no threads run.
Finally, three ends from used spokes that were both trimmed and rethreaded. Ghastly deformities, thankfully rare. Nearly no builder we've consulted has seen this so it makes a great puzzle to unravel.
By the next post, we need answers to at least these questions;
(1) Were new spokes too hard or brittle to begin? Did that predispose them to failure? Was the material defective ?
(2) Did riding cause work hardening or other deterioration? Was corrosion a factor?
(3) What are the relationships between hardness, strength, elasticity, and fatigue life in steel wire? What are the trade off's between these? What best suits a spoke? What is contemporary spoke practice?
In order to answer these questions, samples will be metallurgically analyzed. We need to know stuff that doesn't show in these dramatic images. I'll share all the results except the spoke brand. No spoke maker should get kudo's or demerits from such research. These samples are statistically insignificant.
Stay tuned and please contribute if you've been down this, or similar roads, before.
Update: we can confidently explain the explosions of spoke wire! It's the outcome of threading dies set too tight for a given wire diameter. The compressive loads when wire material has nowhere to go are huge. A dramatic "explosion" is the result every time.
If you experience this while threading spokes, back off your die distance. Not only are spokes destroyed but these abnormal pressures make accelerated wear for the dies. Brittle wire material is more vulnerable but stainless work hardens at a high rate compared to many materials. So even well annealed spoke wire can be exploded.
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