|
Well technically, the Zipp wasn't broken spokes, it was a broken flange.
Contrary to common perception, spokes don't often break from a single, extreme overload, but instead they fatigue over time. (Sadly, fatigue is a poorly named phenomenon, which implies that the metal somehow gets weaker, until it is so weak that it breaks. In actuality, fatigue is caused by crack initiation and propagation. When final failure occurs, it is not because the metal is weaker, it is because the crack has grown so large that the un-cracked cross section of the metal is too small to bear the final load). The reason that sometimes spokes break when 'just riding along' is because they have fatigued (cracked) to a point where one final load cycle is enough to cause the final failure - and the spokes go through a load cycle on every revolution of the wheel.
So the question is, why did the spokes fatigue? Fatigue cracks can initiate due to manufacturing defects, stress concentrations, or locked in stresses (stressed locked into the cross-section when spokes are hardened, either through cold-working or heat treating), or a combination of all 3. Spoke manufacturer has gotten much better these days, so spoke breakage due to manufacturing defects is much lower than it used to be. Stress concentrations and locked-in stresses can occur due to wheel design and manufacture processes. Sadly, many wheel builders either don't know enough, or don't care enough about how to mitigate these factors.
Out of curiosity, where did the spokes break? The most common points are at the elbow, at the first thread, or at the head, so I'm guessing it was one of these. Would it be possible to get a sharp close-up shot of the failure? This can help determine what the cause of the failure was - fatigue cracks have certain tell tale features left behind on the fracture surfaces, so the point of crack initiation might be able to found. There are methods to mitigate the factors that contribute to spoke fatigue, so knowing the point of crack initiation might help determine what methods might be useful to prevent them from happening again.
|