You seem to have covered most of the possible bases. One outside possibility is that the hub is just not up to the job. Are the heads-out or heads-in spokes breaking? How much daylight is there between the bend of the spoke and the flange? The flanges may be a bit thin for the spokes - solvable with one or two 8BA or 2mm washers under the heads - or they may not be correctly canted towards the rim (not a lot of people know this, but flanges should NOT be plane and parallel; they are actually the tip of a very shallow cone). I don't know what design and quality control a couple of quid gets On-One from a Chinese factory.
I've probably built 40 or 50 wheels and never had a spoke break, although I have broken a hub flange. I ride on rough roads, jump kerbs and do all the other stuff that is bad for them.
Roger,
Thanks for that; sadly it is becoming increasingly clear that I have not "covered all the bases". The more I measure, the less I understand the measurements (no smiley).
Son has suffered 2 bikes off the road due to spoke failures. FFS he's been reduced to using the godforsaken car to get to the station. I've replaced spokes on both rear wheels, but have absolute certainty that I haven't solved the problem.
I've looked at the latest broken spokes, with suitable magnification. Both show classic patterns of fatigue failure. There are evident rings originating from inside the spoke elbows: typical crack propagation patterns. The final rupture zone was about 0.4mm wide on the symmetrical wheel, but only 0.3 mm on the spoke from the tourer's dished wheel. I guessed that the ruptured bits were about 20% of the spoke on the symmetrical wheel & only 10% on the dished wheel, which is the opposite of what I would have expected.
I need to do some sums on those observations. Half-term is not an ideal time, though the distractions are not all bad (discovering that a stroppy 5-year-old girl can imagine in 3 dimensions is a deeply unforgettable good bit)
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Meanwhile, son has advised that his weight may be 5% more & baggage probably double OP. Total 135kg in summer...
I'm a long way from a solution in which I would have any confidence.
I'm already planning to build 2 rear wheels with 36 spokes, due to lack of choice. The existing hub is nominally On-one, but looks increasingly like System EX or its many alternative brands. It has other problems that need to be resolved later, so I'll buy a couple of new ones. I think it's flanges are conical. I know the current System EX hubs are, but am uncertain about the old one.
I have 2 choices on spokes: -
1) Thinner spokes, 2.0/1.5/2.0 (from Biggsy: Sapim Laser or DT Revolution).
These would test my wheelbuilding skills in a big way. Halving (-ish) the torsional stiffness is a serious challenge for a wheel which has to have high spoke tension. Son's comments about clicking noises ( presumably unwinding) from repairs that "others" had done suggest there's a lot of scope for error.
Also, I have no technology that will help measure spoke tension on these spokes. I've looked a bit on Google for spoke tensiometers and haven't spotted anything that measures in 21st century units at a sensible price.
The most difficult part is that I've not yet seen anythng that quanitifies the well-known benefit of traditional 2.0/1.8/2.0 spokes vs plain gauge 2.0 cheapies. There is IFAIK nothing that can be extrapolated to 1.5mm spokes. Can anyone help?
2) Thicker elbows, i.e. 2.0/1.8/2.3 spokes, as in DT Swiss Alpine III. I'm doubtful about the benefit, for reasons already explained. But if the broken spokes, as yet to be analysed properly, suggest simple high cycle fatigue failure, then this may be a more a more plausible solution.
No-one has yet reported circumstances in which spokes have broken, whether they were wheels built by "someone else", or personally. The lack of that empirical evidence is beginning to raise doubts about the value of all the advice posted thus far.
