What can cause wheel spokes to snap in the middle?

Question

While sitting outside with my bike next to me a spoke spontaneously snapped. What could have caused this?

Even worse, after getting the spoke fixed and the wheel retrued at a bike shop, four more spokes have now snapped in the middle while sitting inside my house unridden.

All the spokes are snapping in the middle not at the centre or the rim. The wheel is one I've lightly used without issue for the last four years.

1. Why did this happen?
2. Should the bike shop have noticed anything or done anything different when replacing the spoke a month ago and retrueing?
3. Do I need to worry about this wheel after I replace the four spokes now? Or the other wheel which (so far) has never had any problems?

Answer 1

I've dealt with a number of wheels where there were inexplicable failures at the mid-span of the spokes (far away from the elbow or threads). I haven't heard of instances of this where the failure occurred while the bike was not in motion, but what is the same is that it's affected multiple spokes without any apparent outstanding factors, i.e. no crashes or objects caught in the wheel.

Someone with in-depth knowledge of the metallurgy and production of the spokes might have a better answer, but the practical answer is the spokes are defective in a manner where they're not able to handle the static tensile loads or the cyclic compressive loads they need to. It's likely something that went wrong in either the composition of the material or the heat treatment.

Spokes need to be able to resist fatigue by having the loads on them throughout the part be under the fatigue limit. Accumulated fatigue lowers the tensile strength, which explains why it's possible for it to break while just sitting there, since a wheel is a preloaded structure.

With five down I don't think there's any reason to expect the rest of them to do any better. Either the wheel or all the spokes should be replaced. You're playing the odds either way. With one breakage I don't think that would be a reasonable conclusion to jump to, but at five it is.

With only one such spoke broken in front of them, it wouldn't really be practical or reasonable for a mechanic at a shop to get too cautionary about larger-scale problems. They probably thought that as likely an explanation as anything is that at some point something got caught on that spoke and jammed, weakening it. I don't think that the shop is at fault. That said, a lot of shops would be amenable to an arrangement something along the lines of applying labor costs already spent to the labor part of installing a new wheel, or of rebuilding the one you've got (which may or may not reasonable depending on the parts on it).

I wouldn't worry about the other wheel unless it starts breaking spokes as well. There's every chance that this a problem related to the specific spoke production batch that the bad one used. What you could do is test it by squeezing pairs of the parallel spokes around in the fashion a wheelbuilder does to stress relieve spokes. Any wheel that you trust to ride on should be able to undergo this without issue.

Answer 2

Spokes break at four main points

1. at the J bend becuase they've been ridden too loose, undertensioned
2. In the rear wheel, about 10mm along from the J bend because the chain has jumped the big cog and munched the spokes. Normally the outboard ones take the damage.
3. At the spoke nipple because of excess tension and age/fatigue. The thread can be a stress riser, sometimes exacerbated by a smaller rim where the spoke has to turn shaper
4. Mid-span - this can be any combination of fatigue from old age, along with damage over the life of the bike, or perhaps a manufacturing flaw that finally broke.

For an older bike, especially one with an unknown history, its possible the bike was in an accident or fall and this has put a nick in the spoke.

Rust can also build up over time. This is delayed by having chromed spokes, or anodised on newer bikes, or galvanised on older bikes.

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That five spokes have all gone implies they're not independent events, and something caused damage to them all. Or the whole wheel is made from cheese.

If it were my wheel, I'd give it a clean, then a close inspection. Take it out of the bike, remove tyre/tube and clean it well, right between each spoke.
Tap each spoke in turn around the wheel, both sides. Use something metal like a screwdriver. You should hear "similar" tink-tink-tink sounds, with none of them sounding super high or low. That would imply high or low spoke tension.
I'd also give each crossing pair of spokes a good hard squeeze so the crossing slides. Perhaps wear gloves in case one breaks in your hand.

The result of your inspection should be a decision to replace the broken spokes OR all the spokes, OR to replace the whole wheel.

I've had good luck checking local auction websites with patience, rather than buying brand-new wheels.

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Ultimately, you should learn to replace your own spokes. Each one costs a couple of dollars, but the bike shop will charge 10x that for labour.

The only special tools you need are a spoke nipple wrench that snugly fits your size of nipples, and on the rear wheel you may have to remove the cassette/freewheel to get access to fit in a new spoke. And you'll need the spoke itself (and a nipple if the old one gets lost). Some people use a "spoke prep" lube on the threads that thickens up after assembly, but that's optional.

Answer 3

1. Crappy Chinese spokes. Most likely heat/cold has set off a fatigue failure.
2. No.
3. Probably not. If you are using traditional steel spokes, it is probably an old wheel so it might be time to rebuild it with DT Swiss. If you go for a rebuild, get the shop to do it. Getting the spokes threaded is the easy part. Truing a new wheel with proper dish at the rear could take you days to get right if you've never done it before and don't have the right equipment:
   If the rims are not worth saving, then pick up a pair of Bora One 35s! They don't snap.