Why do spokes break?

Spokes break for the same reason any other material does: they are subject to stresses they are unable to withstand.

In the case of a wheel, it can be overloaded by rider weight, cargo, or forceful impacts. Additionally, the presence of loose spokes results in other, properly tensioned spokes bearing more of the wheel's load. Spokes are under tension, so loose spokes are subject to larger swings in stress as they are loaded and unloaded as the wheel rotates. This cycling fatigues the spoke material and can lead to failure.

All of these can be compounded when the spokes are materially compromised from corrosion or other forms of damage.

Spokes typically break due to fatigue failure. Steel has a material property called the fatigue limit, which is the level of stress it can experience repeatedly without weakening. As long as the stress on the material does not exceed the fatigue limit, it is not weakened. If the material exceeds its fatigue limit, tiny cracks form, and repeated cyclic stresses will cause the cracks to propagate and weaken it further until eventually, it fails. Practically speaking, then, spokes fail when something causes them to exceed their fatigue limit. This can happen for a couple of different reasons:

Bad Build - When a wheel is built, the tension in the spokes should be roughly evenly distributed so that none of them come close to the fatigue limit. In a bad build, some spokes are over-tensioned, and they start to exceed their fatigue limit. Eventually, the over-tensioned spokes will fail, and the load they were carrying gets transferred to other spokes, which also start to fatigue, and the problem continues. Replacing the one broken spoke doesn't necessarily address the problem, because other spokes may have been over stressed and started to experience fatigue failure. Especially in the case of using cheap spokes (see below), you can end up chasing broken spokes, Whack-A-Mole style, for a long time and may have to start over with an all-new build. Alternatively, if a spoke is under-tensioned, it may alternate between tension and compression during rotation, with disastrous consequences for fatigue life.

Lack of Maintenance - Wheels do need maintenance, especially if you carry a lot of load, experience impacts, etc. Spoke tension is controlled by a nipple that fits into the wheel, and sometimes these can work loose over time to decrease the tension in a spoke. This causes the transfer of tension to other spokes possibly causing one or more to become over tensioned. This can show up as a wobble in the wheel but it's also possible to have a perfectly true wheel with uneven spoke tensions. It's worth checking spoke tensions from time to time and having your wheel serviced, or adjusting them yourself if you have a tension meter and truing stand. Two quick and easy ways to check spoke tensions are to squeeze crossing pairs of spokes against each other and gauge their stiffness relative to the other pairs of spokes, and to tap each spoke with a screwdriver and listen to the tone. Any spoke that feels too stiff or sounds too high relative to the other spokes indicates the need for maintenance.

Bad Materials - Cheap spokes may have impurities that will lower their fatigue strength (If you're buying the cheapest option from China on Amazon, you're in danger. Good spokes are probably going to cost a dollar or two each). Using cheap spokes to build a wheel or replace a broken spoke is almost guaranteed to cause problems later on, as the spokes experience fatigue and fail in the manner discussed above. I have personal experience with this mode of failure and can testify that although you may not want to spend money on high-quality spokes for building or repairing wheels, eventually, you will have to pay a lot more in time and materials to keep a cheap-spoke wheel working than a wheel built with high quality spokes. In my opinion, your local bike shop is the best source of high-quality spokes.

Anyway I'm wondering what causes spokes to break.

There are multiple reasons for that.

It all starts from improper wheel design. A properly designed wheel uses nearly tangential spokes that are flexible in their center sections and strong in their ends where they are most likely to break. Proper wheels usually have 36 spokes. In proper wheels, the spoke head is matched to the hub flange.

Unfortunately, it is all too common today to see wheels that do not have these design features.

A conventional size wheel with less than 36 spokes puts nearly all of its load on a single spoke, thus making spoke failure very likely. This use of less than 36 spokes also makes nipples unscrew. In an effort to not make nipples unscrew, some designers of improper wheels put thread glue into the threads (or it is possible this thread glue is added by the bike shop as a "quick fix" to apparently make an improper wheel work). Unfortunately, a wheel where spokes go completely slack is operating beyond its design limits and thus likely to fail. This problem is best avoided by insisting on wheels having 36 spokes.

In order to make wheel lacing fast (thus reducing the price of built wheels), hub makers have increased the size of spoke holes to be larger than necessary thus decreasing spoke lacing time. This has created a poor fit for spoke heads, especially if using the superior 1.8mm-1.6mm-1.8mm butted spokes. If building a wheel with these poor modern hubs, one should ideally select a 2.34mm-1.8mm-2.0mm triple butted spoke to better match the spoke head to the hub flange.

The most optimal spokes are butted spokes. They are more expensive, so most ready-built wheels save on costs and use straight gauge spokes. These straight gauge spokes are not as flexible, so the load is concentrated on a smaller number of spokes. This increases chances of spoke breakage.

The spokes should be laced in a nearly tangential manner, but it's unfortunately very common to see radially laced front wheels on front wheels for rim braked bicycles. These wheels lack the spoke crossings that distribute the spoke load to a pair of spokes, and also to prevent hub flange failure, tension on radially laced front wheels cannot be high.

Hub and rim design also play a role. Ideally a rim should be stiff, thus distributing the cyclic loads to a larger number of spokes causing lower peak loads. Disc brake hubs should have large enough flanges, torsionally stiff shaft and lots of spokes, but it's all too common to see hubs having too few spokes with flanges that are clearly too small for the spoke count. Good rims are also manufactured to a high tolerance, which is the enabler for even spoke tension (so the spoke tension doesn't have to fight rim irregularities by unevenness).

Then it is exaggerated by poor wheel building.

Ideally, wheels should:

• Be tensioned to an even and high tension. Unfortunately, many wheels lack both of these -- i.e. the tension is uneven and low on the average.
• Be stress relieved. Unfortunately, many wheels have not been stress relieved.
• Have corrected spoke line especially if using spokes that don't have a particularly thick head section. Unfortunately, cheap wheels may lack this correction of spoke line, thus causing the spoke to hang in thin air. This spoke line correction can be difficult with spokes being 2.34mm thick in their heads, but then again such thick-headed spokes work just fine without spoke line correction as an exception to the rule of needing to correct spoke line.

What finally causes the spoke to break is lots of hard miles. A cyclist who weighs a lot and carries a lot of load on panniers, riding tens if not hundreds of thousands of miles on the same wheels, in an aggressive manner (ride fast over bumps, brake hard, use relatively narrow tires pumped to huge pressures), causes the final breakage event. However, it all started from improper wheel design and was exaggerated by poor wheel building practices. A properly designed wheel, built by a master wheelbuilder, would be durable even if riding lots of hard miles.

And as a final note, historically materials used for making spokes were far worse. This has caused the author of the definite book on bicycle wheels, "The Bicycle Wheel" by Jobst Brandt, to discover what makes a wheel durable. We would not have the book in its present form if spokes had equally good materials 50 years ago.