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I've built about half a dozen wheels for my commuter bikes and never had any problems, but the last rear wheel broke some spokes.

Everything on the wheel was brand new, 32 spokes, had only done about 15 miles. I was pedaling hard, probably accelerating from 15mph up to 25 mph, and I broke a few spokes. I checked the wheel and it was okay to pedal slowly to my office as it was only a few minutes away, and I leaned on the bars to keep as much weight off the rear wheel as I could. Ended up breaking six (!) spokes by the time I got there.

All but one broke at the rim, and all but one were non-drive side leading spokes (i.e. the last set of spokes to be installed as per https://www.sheldonbrown.com/wheelbuild.html#leading).

There was obviously something wrong, maybe too much/too little tension, or a tension imbalance somewhere, but what could have caused just one set of spokes to fail so greatly? I would have expected drive side and trailing spokes to break first under stress.

Just so I don't make the same mistake again. In retrospect I should have checked the tension on the remaining spokes after the first two broke, that's what I did in the evening to get me home six spokes down and it was still fairly solid.

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  • Sounds like it might have been a bad run of spokes ? To break at the rim would be exactly where they bend to enter the nipple. Perhaps they were rolled a bit deep in the thread, leaving a weakness ready to shear ? Have you tried poking a surviving spoke about ? – Criggie Feb 20 '20 at 0:45
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    I think I probably rushed the first build, not properly checking tension before going onto fishing and truing. Fitted the replacement spokes and got them up to tension and it all seems to be behaving itself now. Would be interested if you have a link to something that says how the differently laced spokes function in a wheel, when they take up tension and when they slacken etc. – Wilskt Feb 20 '20 at 12:28
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    Another thought - is your rear rim symmetrical? If it was asymmetric and installed backward, then the nipples would have problems seating at a good angle. – Criggie Jan 4 at 19:24
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Spokes generally break when they are not tensioned enough, because they can then cycle from being taut and being loose as the wheel rolls, and because of this they fatigue and fail. Specifically, as the wheel rolls on the ground, when a spoke is pointing up, away from the ground, it does not carry any of the bike and rider weight and is tensioned, but when it is pointing down, it carries weight and this counteracts its tension, so an insufficiently tensioned spoke will lose all its tension and become loose. This taught-loose fatigue cycle repeats every wheel turn, which quickly adds up to a lot of cycles and breaks the spoke. As the non-drive side spokes of the rear wheel are less tensioned to begin with, it makes sense for them to be more prone to this issue, and the point at which a spoke breaks is usually at the rim, because that is the location of the spoke threads, which cause stress concentraction (they can be thought of as little cuts in the otherwise smooth spoke surface).

I have seen same wheels, one properly tensioned and one not (easily checked by squeezing the spokes together by hand), behave completely differently becasue of this. The tensioned wheel was virtually undestructible, whereas the loose wheel had its spokes break all the time. The owner of the loose wheel thought his wheel was junk, when in fact all he needed to do is tension its spokes properly. The moral is, when you buy wheels, check that they have been tensioned properly out of the factory, it makes a world of difference.

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  • Good points - but I've noticed that loose spokes tend to break at the J bend, not at the nipple While tension may be part of the problem. I suspect there's something more going on. It may have been too high-tension rather than low tension. – Criggie Jan 4 at 19:23
  • Usually pretty tough to get too high tension on NDS spokes though. – Noah Sutherland Jan 4 at 19:37
  • @Criggie Interesting, although I have read this is common, I have never seen a spoke breaking at the J bend. Granted, the particular wheels I was referring to have straightpull spokes, but I have seen spokes snap because of low tension on other wheels too, and they also snapped at the nipple/rim. I think I read somewhere that a brake at a J bend is usually associated with spokes being too narrow for the holes in the hub, so there is freeplay. A loose spoke would wiggle in the hole because of that so it could very well get damaged and break. – Mick Jan 5 at 9:37
  • @ Noah Sutherland If the drive side spokes are not tensioned enough in the first place, then yes, no way the non drive side spokes can be taut enough. But if you tension the drive side enough and get the dishing right (i.e. if the rim is centered, and not pulled closer to the drive side), the tension in the non drive side should be enough to avoid spokes loosening. – Mick Jan 5 at 9:47
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How much tension did you put on them? Since they broke at the rim, was it the nipple which broke or really the spoke? In any case you probably had insufficient tension on the non-drive side spokes¹. You should go as high as possible. This means tensioning the drive side spokes up to the maximum allowed tension (usually 1200N) and then truing and dishing the wheel. You should end up with ~700N on the non-drive side this way, which is barely enough to ensure that the spokes are always under tension in normal use.

There are asymmetric rims which improve the nipple and spoke angles and have better (higher) non-drive side spoke tension. Of course, if you install them incorrectly they have the exact opposite effect.

¹: But I’ve had aluminium nipples break due to saltwater corrosion. I use brass now and had no issues so far (on a 28 spoke rear wheel with 15kg luggage used for touring).

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  • +1 for brass - aluminium just isn't worth the tiny weight saving. – Criggie Jan 4 at 19:20

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