Spoke failing when releasing tyre pressure

[FifeingEejit]

Just got a bit of a surprise after starting to change a tyre and having a spoke snap at the base of the thread and throw itself out of its seating when I was releasing the tyre pressure.

Um is that "normal", my previous spoke failures have been due to either catching the spoke on a bit of root sticking out the ground (took two out that day...) or from battering it off a rock (On Rúm with no spares or ability to do anything about it) or J-Bend failures on older wheels; but this wheel is about 1550km of batterings over Scottish roads old... oh!

Also it could easily have had something leant against it on the trains to/from the Blackpool-Glasgow 600 so It's more the way that the broken spoke presented itself to me than the fact that it broke that concerns me.

[Brucey]

spoke failures are basically never 'caused' by a final (minor) event; all this does is administer the coup de grace to a spoke that is already cracked most of the way through.  The crack is invariably a fatigue crack, and this will have been merrily propagating for (at least) several hundred miles previously. Exceptions to this are catastrophic events like stuffing a pedal into the spokes.

Letting the tyres down increases the spoke tension in all the spokes by up to about 10% or so (depending on the rim design and the tyre loading on it) and this is quite enough to cause a spoke that was on the verge of failure to go 'ping'.  If it didn't happen then, it would have happened pretty soon after anyway.

If you look at a fatigue fracture it will usually show a large flat area and a final part (at one edge) that is not flat. This second part is the final part to break, and it will have broken when there was simply insufficient cross-sectional area in the remaining part of the spoke to sustain the load. Probably it will be between 1/3 and 1/4 of the fracture area, something like that.


this looks a bit like a spoke, but is actually a failed bolt that is ~ x15 the size; the final overload fracture is to the right

It is possible that the one spoke that broke was different in some way from the others, e.g. defective or  saw an abnormal load of some kind a few hundred miles ago (eg a thump near the nipple from a lock in the spokes) but the most likely thing is that the wheel wasn't stress-relieved properly when it was built and/or the spoke ends were not 'set' before stress relief to allow for any kinks in the spokes near the nipples.   If so, you should get those things done to the wheel asap.

You may well break one or two further spokes yet, even if you stress relieve the wheel; these will already be cracked, and there is basically nothing to you can do about it.

Wheels that are not stress-relieved properly can be evenly tensioned, perfectly straight, and....

.... complete rubbish.    Breaking spokes after the first ~1000 miles is exactly the sort of thing that happens.

cheers

[FifeingEejit]

spoke failures are basically never 'caused' by a final (minor) event; all this does is administer the coup de grace to a spoke that is already cracked most of the way through.  The crack is invariably a fatigue crack, and this will have been merrily propagating for (at least) several hundred miles previously. Exceptions to this are catastrophic events like stuffing a pedal into the spokes.

Letting the tyres down increases the spoke tension in all the spokes by up to about 10% or so (depending on the rim design and the tyre loading on it) and this is quite enough to cause a spoke that was on the verge of failure to go 'ping'.  If it didn't happen then, it would have happened pretty soon after anyway.

If you look at a fatigue fracture it will usually show a large flat area and a final part (at one edge) that is not flat. This second part is the final part to break, and it will have broken when there was simply insufficient cross-sectional area in the remaining part of the spoke to sustain the load. Probably it will be between 1/3 and 1/4 of the fracture area, something like that.


this looks a bit like a spoke, but is actually a failed bolt that is ~ x15 the size; the final overload fracture is to the right

It is possible that the one spoke that broke was different in some way from the others, e.g. defective or  saw an abnormal load of some kind a few hundred miles ago (eg a thump near the nipple from a lock in the spokes) but the most likely thing is that the wheel wasn't stress-relieved properly when it was built and/or the spoke ends were not 'set' before stress relief to allow for any kinks in the spokes near the nipples.   If so, you should get those things done to the wheel asap.

You may well break one or two further spokes yet, even if you stress relieve the wheel; these will already be cracked, and there is basically nothing to you can do about it.

Wheels that are not stress-relieved properly can be evenly tensioned, perfectly straight, and....

.... complete rubbish.    Breaking spokes after the first ~1000 miles is exactly the sort of thing that happens.

cheers

Thanks Brucey that was along the lines I was thinking in terms of failure but didn't know/think that releasing tyre pressure would cause that much of a change to spoke tension.

Although now I think of it, it is fairly obvious that an inflated tyre will exert a force on the wheel therefore compressing the rim and so the spokes and releasing it will increase spoke tension.

I was terrible at structural engineering at school, I managed to design bridges that were both comically over engineered and so flimsy that Thomas bouch would be delighted...

Sent from my BKL-L09 using Tapatalk

[rogerzilla]

I know tubs are slightly different but if you've ever fitted one "dry" to stretch it, it goes on fairly easily but, inflated to 180psi, it squeezes down onto the rim pretty hard.  It's all due to the angle of the threads in the carcass.

[Brucey]

the forces are somewhat different in a wire-on rim/tyre.  IME the rims that lose spoke tension most when tyres are fitted are often those which have a distinct  'V' shape (but with curved sides); as well as the tyre pressure squeezing the rim inwards as normal, the 'V' opens up under tyre pressure loads and this forces the spoke bed inwards slightly.

This effect is so pronounced that the first time I built a rim of that sort, I had to go back and increase the tension in all the spokes; they went from being 'within tolerance' on the bare wheel to 'too slack' once the tyre was inflated.

cheers

[rogerzilla]

Tyre pressure is also the reason (apart from the tyre getting in the way a bit) you shouldn't true a wheel with a tyre fitted.  An inflated tyre tends to make the wheel run more true, and masks what's really going on.

[FifeingEejit]

And after 100km today another bang, this time no pressure loss, just bang, think it had been pinging for a bit before the bang but couldn't tell as my chain was also pinging.
Got it another 110km though with the borked spoke zip tied to it's neebur.

One again failed at the nipple.
Hm...

[Brucey]

check to see that the nipples are not slightly angled w.r.t. the length of the spoke; if they are then the spokes are more likely to fatigue at the base of the spoke thread, inside the nipple.

Note also that if the spoke tension is slack, spokes tend to flex more when the wheel is under load and can also fail at the nipple. NDS spokes in rear wheels can go this way.

If you want to retension and stress-relieve the wheel now you may be able to improve matters. However it is quite likely that at least one more spoke is now already cracked and is going to fail regardless (probably within a few hundred miles). At best the stress-relief ought to prevent further spokes beyond those from starting to crack.

cheers