Why do wheel spokes/caps break?

For some of the most common questions, give the most simple and direct answers and reasons, but this is not simple. First of all, the simple principles may be complicated to understand. In addition, these common questions may be accompanied by common misunderstandings of answers, so do popularization. Not only the reasons for the correct answers, but also the reasons for the misunderstandings.

Many people have experienced broken spokes or caps of the carbon bike wheel set, and some people have found that after the wheel set is broken and replaced, it breaks again, and the broken one is the new one after the break. What are these reasons? Excluding the carbon one-piece wheel, some people will take it for granted that, Spokes or nipples break due to excessive spoke tension, and many bicycle practitioners think so, but this is wrong. 1. Wheel breakage generally occurs on the spokes with low tension; 2. The spokes with low tension can be said to be "broken" repeatedly for a long time. Let's explain separately.

1. How the Carbon Bicycle Spokes are repeatedly "broken" I try to explain this in a way that is easy to understand and close to life. Suppose you were given a flat spoke of steel, without any tools, how would you break it in the middle? Is this easy? Repeatedly bend and straighten it in one position, it will break after a while. No one is stupid enough to try to break a spoke, right? So in fact, the spokes of the wheel set are broken for the same reason, but it is not as intuitive and obvious as we break it by hand. The principle is called stress concentration. It is determined that the spokes are "broken", then it is necessary to talk about the preconditions. How do the spokes on the bicycle wheel set "bend"? This should be easy to think of, that is, the wheel set after weaving, the spokes are bent. As shown below We all know that as the wheel rolls, the tension of the spokes changes from time to time, so if the spokes on the wheel are bent.

With the repeated changes of the spoke tension, the bending amplitude of the spokes will change repeatedly, and the long-term accumulation will be "broken".

2. Why are the low-tension spokes broken? "High tension spokes are easy to break" is the most common misunderstanding in bicycle circles. There are also so-called professional technicians who write articles to guide readers with this wrong view. Even though the previous point has been said that the spokes are "broken" and not "pulled" at high tension, many people still do not understand why it is happening on low tension spokes.

Many people should be aware of cargo conveyor belts. If you want to transport heavier goods, the surface tension of the conveyor belt will be relatively high. Otherwise, it cannot be transmitted, that is, the conveyor belt is deformed and collapsed by heavy objects. In the same way, we can use the limit method for the change of the spoke tension. For a spoke with a bending point, the spoke tension changes dynamically during the rolling of the wheel set. Then the spoke tension is low to the limit.

becomes zero, then the bending point of the spoke will straighten, and when the tension increases, it will bend again, which makes the spoke produce the maximum "bending and straightening" operation under its conditions, and the stress concentration accumulates faster. If the tensile force (pre-tension) is relatively high when the spokes are being prepared, and the tensile force is always in a relatively high state during rolling, the deformation state of the spokes will change accordingly, and the accumulation of stress concentration will come more slowly, and the probability of breaking the bar will be relatively high. lower.

3. How is the spoke bending on the wheel set produced? The rim and hub rely on the connection between the spokes and the nipple, and the stability of the wheel is maintained by the pulling force of the spokes. Therefore, a good wheel set design should try to keep the spokes in a simpler and more stable state, that is, in addition to the fixed point at the head and the end, the force in the middle of the spokes is less. More directly, this state is that the spokes are as far as possible straight line. Some people may say that the elbow spoke wheel has not broken for a long time. You can ask the roadside car repair master for such a logical problem. Those non-sports bicycles ten or twenty years ago, the ones that broke are Not all at the elbows of the spokes. Of course this is a difference in fixed point spoke types and there is no need to go into too much detail.

Hub Types and Spokes After the (elbow) weaving method is determined, whether the spokes remain in a straight state lies in the spoke holes of the rim. At this point, some people may say that this is simple, the main thing is to calculate the angle of the spoke holes, and just drill the holes according to the calculation? This is probably not possible for carbon fiber rims, because when carbon fiber rims are cured and formed, Sometimes the internal structure will inevitably have uncontrolled changes, we use pictures to demonstrate: The drawn schematic picture will inevitably look a bit exaggerated, but as long as you carefully observe the comparison, you can still find the situation corresponding to the real object. For example, the situation in Figure 2 is the same as the bending of the spokes in the first real picture in this article. Why does the wheel set break, and what happens when it breaks, such a common problem, I personally think it is easy to understand, and I do not recommend anything to explain the principle with video, because the video is too much Intuitive, it will make people lose their thinking, that is to say, the video is only suitable for your mechanical learning operation, rather than in-depth understanding and mastering some knowledge and common sense.

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