Why do bicycle spokes break?

There are many reasons why bicycle spokes break.

We always think that spokes are unbreakable. In fact, spokes are kind of the elements that do their job so well that nobody considers their importance until they finally fail. 

And by fail, I mean they break. To understand why they break, you first need to know how they work.

Why bicycle spokes break

How do bicycle spokes work?

These thin wires that make up the wheels of our bikes, and that we know as spoke or spokes.

They undoubtedly perform one of the hardest and most essential jobs for the movement of our bike.

In general terms: the bicycle spokes are that element in your bicycle that support your weight and transfer power (from your legs) to the wheel (through the gear), besides keeping the shape of the rim integrated.

Spokes are actually one of the most efficient inventions a bicycle has had for a long time. And that is true even for the first-ever bicycles that used spokes of one type or another.

In fact, in the early days, many carts and wagons used spokes in their wheels. This is not the case nowadays, but it is a clear sign of how efficient they are in doing their job, and to date, they are still the preferred way of manufacturing a wheel. A good example of a string is shown below, as the spokes are used on a motorbike wheel.

Spoke string on a motorbike

A spoked wheel is very strong and yet they are only a fraction of the overall weight of a bike. And, I believe, we all agree that minimizing the weight on a bicycle wheel is extremely important to improve your cycling performance.

A determining factor in the strength of a wheel is its assembly and the number of spokes it has. So, do not undermine the role a spoke play. If you miss only one spoke on your wheel, it will become unbalanced. 

I have to say that these thin steel strands are able to withstand enormous loads during locomotion, and they are just one of the most essential part of your MTB.

The spokes of your bike are repeatedly subjected to very difficult forces. They are, no matter what weather conditions and ride you make, compressed with every turn your wheels make. They carry the acceleration forces of pedaling from the pedal/gear to the rim, as well as transmitting the braking forces. 


The spoke string

Spoke string on an MTB

The importance of the spoke lies in the fact that it can transfer forces that are often very large and that are created by the cyclist, the bicycle and the different surfaces of the road on which it is rolled.

When you look at one spoke, it is very fragile, but when you put these on the rim around the wheel, they form what is known as the spoke string. And together the spokes makes a very strong string.

Each of these thin pieces of steel – although there are others made of other materials such as: aluminum or more exotic materials such as carbon fiber or ceramics – is systematically compressed as the wheel turns and the loads are transferred from one spoke to another, and so on. 

Once under tension, the spokes essentially support the edge of the wheel using the gear as the central anchor. Each spoke pulls with the same tension to distribute the load evenly across the wheel while keeping the rim truly circular and rolling in a straight line.

Any spoke string is designed in such a way that it can support the wheel against lateral bending and deformation of the rim. It must also resist the rim being continuously crushed by a vertical load (radial compression) on the wheel. 

This simply means that the string should be able to bend when needed, absorb shocks and come back to its initial position without breaking.

Well, this is the science behind the spoke string. The reality is that these strings break when being subjected to too much shock or tension. More about that below.


Broken spokes – How can they break?

The fracture by crack or fatigue of a spoke is most likely related to the process of degradation of its material.

Through time, there are spokes that degrade due to being overused, over-compressed, or due to the environment they are being stored and used. 

Here is something that I do not understand:

“Each spoke on the wheel of your bicycle is subjected to the compression load that is directly below the gear, and for only a fraction of a second that spoke is compressed before the pressure decreases and it can return to its normal length.”

Well, what this means is simple:

“Every time you use your bike, your spoke will be pressurized, horizontally and vertically. It will, as such, contract and expand all the time while you are cycling.”

It is a relentless cycle that can lead to the cracking and then the failure of any bike’s spoke. After hundreds of thousands of normal wheel rotations, you’ll get some defects.

And any small defect on the surface, even if it is only a few microns deep and completely imperceptible to the human eye, can begin to fatigue the spoke material of your wheel.

At first, it is a slow process, but eventually, it will lead to the breakage of the spokes. And you are done, you need to repair the missing spoke.


Inadequate tightening tension on the spokes

At the time when the wheel is assembled, all the spokes are tightened to a high tension. I mean this is a calculated tension, and the manufacturer knows the appropriate torque to be applied for each type of wheel.

The tension applied to the spoke when fixing these onto the rim is usually higher than that generated by a person’s weight.

What you need to understand is that the applied tension to the spokes (when we tighten the spokes in) preloads the entire rim with compressive stress, enough to prevent the area of the rim underneath the gear from bending.

But, if the tension is not adequate, which can be due to a lack of tension or an excess of it, it causes the spokes to premature failure and/or break.

It sounds contradictory, but what happens is that when applying torque to the wheel’s rim, the spokes pointing to the rear of the bike (as they come out of the gear) are tightened.

So those pointing to the front are loosened, causing that a difference in tension/compression. And when the torque is not adequately applied when manufacturing the wheel and not uniform it makes them highly susceptible to break.

So be warry of you tightening skills. Use appropriate tools to do so and the right tightening tension must be applied.


Excessive load

excessive weight on spokes bike

Every material has a point where it gives way, which makes the object it composes susceptible to excessive pressure and weight loads. And, overloading the resistance of a spoke, will inevitably trigger in the breakage of this. 

The spokes of the wheel work in such a way that a part of them pulls the gear upwards to prevent those underneath the gear from succumbing to the weight and pressure. This is why it is very important that the wheels are properly grooved, as the effective distribution of weight and workload depends on it. 

If our wheel is stretched unevenly, it will cause a wobble in the wheel and allow a much greater amount of movement of each spoke during each turn of the wheel, and this leads to the spokes going beyond their designed strength, causing them to break.

Notwithstanding the above, although difficult because they always work in groups, it is not impossible that a lightning bolt silks up when faced with a weight that goes beyond its point of fatigue.

But this can be solved by changing to reinforced rays that are precisely manufactured to support greater weight and tension loads. 

So, in this case, you can break your spokes by applying too much weight (say transporting goods that are way over the weight limit of the bike spokes). You can also break them by jumping etc. Like it is shown in the picture above.


Chain and/or derailleur, etc

Chain causing spoke breaking

There are other factors that can lead to breaking a spoke or two on a wheel. A chain or derailleur misalignment is another reason why a spoke can break on a bicycle.

When it happens that your transmission chain derails, the chain tends to go over the wheel spokes.

Although it will be difficult for it to break the spokes, it will surely bend those that have received the blow.

This will cause their material to begin to fatigue and bend much more than they should.

This will lead to premature failure of the spoke. 


What to do if you have a broken spoke

high wheel bicycle missing two spokes

As you can see, the high and low wheels have a couple of missing spokes. And would not recommend to cycle with that bike under these conditions.

Generally, spokes break more often where the spoke head hooks into the gear – but it can happen anywhere in the structure – in the innermost part of the wheel.

This is due to the curved spoke head is the weakest part and yet it has the ability to carry much of the weight and strength of the power transfer.

If this happens to you, stop and inspect your wheels. Make sure the broken spoke does not fall where it could become entangled with your frame or chain as the wheel turns. To keep it safe, you can tape it to a neighbor or unscrew it from the nipple and remove it completely.

However, as already mentioned, in certain cases it is possible to pedal a short distance even with a broken spoke. You can, for instance, finish your ride home.

However, it is not advisable to ride your mountain bike with a broken spoke for a long period of time because it puts additional stress on the rest of the spokes. This can cause them to break prematurely somewhere along the way too, throwing your wheel off balance.

So, taking into account that the strength of a wheel lies in the number of spokes with which it is built, so the greater the number of spokes that make up a wheel, the more strength it will have.

The rationale behind this is that the tension and the weight load is distributed among a greater number of spokes that make up its circumference and therefore making it stronger.


Broken spokes – Some key points

The lesser the number of spokes, the greater the tension applied to each and every spoke.

As such, the load distributed among them is higher and any shock must be absorbed by a few of them. For this reason, if a spoke breaks, the wheel generally becomes pretty unstable and needs to be repaired almost immediately. But there are certain exceptions that will allow you to continue with your ride before changing the broken spoke.

This is possible when your wheel has more than 30 spokes. With this number of spokes it is possible that the load of the broken spoke is redistributed without major damage.

Obviously, this is good until you manage to change the broken spoke (do this at the earliest convenience). In contrast, when you have wheels with less than 30 spokes, it is necessary to know that each broken spoke creates a problem if it is not changed as soon as possible. So do not ride the bike in this case.

If you feel bold, you can try to replace it yourself. It’s an easy repair if it’s on the front wheel or the non-driven side (away from the chain and sprockets) on the rear. Here’s what you should do.


Replacement of a spoke

Get the right size spoke for your wheel.  Pass the spoke through the rim, matching the pattern exactly to fit the existing spoke sequence.

Pass the spoke through the other spokes, again matching the pattern (over some neighboring spokes, under others) on the way to the nipple. It is ok to bend the radius a considerable amount while working it in place, nothing will happen to it.

Line up the threaded end of the spokes with the nipple, and with a spoke wrench, screw it into place. Tighten it so that its tension is approximately equal to that of its neighbors. Remember, it shouldn’t be too loose, but it shouldn’t be too tight either.

This is the easiest way to replace a broken spoke, but it is always advisable that a new spoke is made when the wheel has already been driven many kilometers, for which an expert will help you to do the best job or you can do it yourself by following the instructions below to center the wheel.

How To Replace A Broken Spoke | Fix Your Road Bike With GCN

Final words

I decided to write this article because a friend of mine who practices a lot cycling asks me the same question as you may have done today, that is: why do my bicycle spokes keep breaking?

I explained the different reasons and it turned out that he was over tightening them. Now he understands what is like to tighten the spokes to the manufacturer’s guidelines. No more breaking, well not because of this issue anyway.

Finally, and without a doubt, spokes have been a smart way to optimize the weight, cost, and performance of a wheel. And I think that because of their efficiency, it’s no wonder that the spoked wheel in cycling has remained largely unchanged in design for just over 200 years.

I hope the above was clear and that we can continue to exchange on cycling and biking. Speak soon.

Source: http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/J%20Krizek/index%20page%201.html

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