Broken spoke

[foxyrider]

Hi,


I think this is not the first time that the problem by foxy for that wheel has been posted.
Same advice last time by Brucey IIRC.

Wasn't looking for any advice - it really was an 'out of interest' post as i've never in 40+ years had both parts of a spoke that has broken at the elbow. Each time spokes have been replaced the wheel has been stress relieved - its already done @ 1000km since this latest breakage! As i mentioned earlier, i am looking at retiring the wheel soon anyway, if it starts shedding more spokes that will get brought forward.

Not properly, it hasn't

how do you know? you don't know what i've done at all

[foxyrider]

Hi,
How well does the spoke fit the hub?
Just curios.
Is there any space under the bend?

The replacements are quite tight and by the fact the broken stub stayed in place i'd say that at least some of the originals were equally tight.

[531colin]

Wasn't looking for any advice - it really was an 'out of interest' post .........

Not properly, it hasn't

how do you know? you don't know what i've done at all

What I know is you are still breaking spokes.
What I believe is that spoke breakage at the elbow is due to fatigue, and can be prevented by stress-relief so that one set of spokes will last more than one rim (on a rim braked bike), and thats rears as well as fronts.
But, I'm not looking for an argument any more than you are looking for advice, so if you think spoke failures are somehow inevitable, thats fair enough.

[Vorpal]

how do you know? you don't know what i've done at all

531Colin doesn't have any way to know what you have done. However, he has some very good reasons for saying what he is saying. And I think he is pointing out an opportunity for improvement. I will attempt an objective look at the facts...

In general, steel spokes, properly stress relieved, *should* last longer than yours seem to (based on your posting history about spoke failures). 20k km is a good life for a wheel, however, I would normally expect a wheel to be retired through lack of braking surface, rather than spoke problems.

Part of my job is analysing &/or reverse engineering failures, especially fatigue failures (not on bike wheels, but the principles are the same, whatever the part). When fatigue failures occur in steel under tension that is within design loads/yield strength, the location of the failure is important. In this case, the failure has occurred where the metal had previously been worked. There are three possible causes for a failure in this area: 1) a quality issue, such as a tool mark, 2) stresses from manufacture or installation, 3) over tensioning. If there were a quality issue with the spokes, they probably would have failed much earlier. That leaves us with two possibilities. While over tensioning is possible, the other is more likely. Tension should be relatively easy to check, so it could be the first avenue of investigation. Assuming that the tension is correct, or nearly so, the stress relieving process becomes the most likely culprit. For example, if using the Jobst Brandt method, perhaps using more force, or adopting Colin's more recent approach viewtopic.php?p=1596416#p1596416

I'm not really looking for an argument either, and as I said earlier 20k kms is a pretty good life for a wheel.

[NATURAL ANKLING]

Hi,
On the rim brakes using the bike off road.
I'll be very surprised if I will even get 10 K out of a wheel, and probably a lot less than that.
On winter training rides I can lose over a millimetre off the brake pads in just 30 miles, using quality brand stuff.
I stopped using my touring bike for off-road training in the winter, because I realised I was wearing out expensive to replace parts very quickly.
I only train on my hacks nowadays.

[rogerzilla]

Overtensioning is unlikely to break any half-decent spoke, since the rim will give up first. You might manage it with a steel rim. Tension doesn't increase when the wheel is used, either*. The only significant change in tension when riding is a reduction at the 6 o'clock position, where two or three spokes effectively carry the load.

*only by a barely-measurable amount at the 5 and 7 o'clock positions

[NATURAL ANKLING]

Hi,
A 2mm diameter spoke could support the total of rider bike and luggage.

[foxyrider]

, so if you think spoke failures are somehow inevitable, thats fair enough.

I don't think spoke breakages are inevitable, far from it, i've had far more rims die than spokes break over my 40 plus years of cycle abuse! Yes i'll stand up and admit it, my bikes get abused, once upon a time i'd stop at every raised kerb, carry bikes across bad surfaces etc but i'm past such silliness now, even the CF has seen lots of trail riding alongside its preferred smooth tarmac, sometimes from choice, other times through necessity!

But i am a realist, given the age/abuse that this particular wheel has had, an off the peg Trubuild which hadn't seen as much as a spoke key since it left the factory, i would be a fool to think there would never be another breakage. If the remaining 29 original spokes have been fatigued the same way as the 3 that have failed (@ 5000km apart), even after stress relief, there is, i reckon, a better than even chance that another may break. OTOH, the rim probably doesn't have another 5000km in it so that's probably moot - although some of the holes in the roads (i use that word with caution) in north Somerset / south Gloucester could claim the wheel at any point!

In terms of the life expectancy of wheels, i don't think this wheel has done anything extraordinary in terms of mileage, i would hope to get something similar from any reasonable modern wheel build (old school sprints were made of cheese so life could sometimes be counted in days!). For example, I was given a radially spoked sprint race wheel in @ 1979 that was used primarily for racing for over 15 years before being retired to spare/training use - it never went out of true despite the spokes getting a bit soggy, it was rim wear which eventually did for it. Some years it did over 1000 racing kilometres (mostly short TT's and 3/J RR's) plus at least double that riding to/from events, i'd be surprised if it hadn't done at least 30k km before its demise.

[531colin]

how do you know? you don't know what i've done at all

531Colin doesn't have any way to know what you have done. However, he has some very good reasons for saying what he is saying. And I think he is pointing out an opportunity for improvement. I will attempt an objective look at the facts...

In general, steel spokes, properly stress relieved, *should* last longer than yours seem to (based on your posting history about spoke failures). 20k km is a good life for a wheel, however, I would normally expect a wheel to be retired through lack of braking surface, rather than spoke problems.

Part of my job is analysing &/or reverse engineering failures, especially fatigue failures (not on bike wheels, but the principles are the same, whatever the part). When fatigue failures occur in steel under tension that is within design loads/yield strength, the location of the failure is important. In this case, the failure has occurred where the metal had previously been worked. There are three possible causes for a failure in this area: 1) a quality issue, such as a tool mark, 2) stresses from manufacture or installation, 3) over tensioning. If there were a quality issue with the spokes, they probably would have failed much earlier. That leaves us with two possibilities. While over tensioning is possible, the other is more likely. Tension should be relatively easy to check, so it could be the first avenue of investigation. Assuming that the tension is correct, or nearly so, the stress relieving process becomes the most likely culprit. For example, if using the Jobst Brandt method, perhaps using more force, or adopting Colin's more recent approach viewtopic.php?p=1596416#p1596416

I'm not really looking for an argument either, and as I said earlier 20k kms is a pretty good life for a wheel.

Vorpal, there is a thread here viewtopic.php?f=5&t=110419&hilit=screwfix&start=30 which is the best discussion of spoke fatigue and stress-relief that I can find.....we have had a few such discussions on these pages. Can I suggest it belongs in "too good to lose" with the title amended?
Brucey discusses fatigue failure in 3 stages, crack initiation, crack propagation, and sudden failure. He says that initiation is the long phase, and once the crack is initiated, progression to failure is relatively rapid. This appears to chime with your "if there were a quality issue with the spokes, they would probably have failed sooner."

[Cyril Haearn]

Overtensioning is unlikely to break any half-decent spoke, since the rim will give up first. You might manage it with a steel rim
..

If that is true, that a rim of whatever material might break before a spoke breaks, surely spokes are too strong and rims are too weak. Replacing a spoke is easier than replacing a rim

[531colin]

Overtensioning is unlikely to break any half-decent spoke, since the rim will give up first. You might manage it with a steel rim
..

If that is true, that a rim of whatever material might break before a spoke breaks, surely spokes are too strong and rims are too weak. Replacing a spoke is easier than replacing a rim

A slight over-simplification.
A new (un-fatigued) spoke will pull through a new (un-fatigued) rim.....thats true, but has very little relevance to wheel failures in use.
Spokes which fail in a crash, or hitting a pothole were previously fatigued.
Rims which fail by cracking at the spoke hole were previously fatigued, eg stress-corrosion-cracking in alloys.
As well as supporting spoke tension locally, a rim needs to be stiff enough to work, and (for rim brakes) to have enough material for a braking surface.
Spokes need to be stiff enough to work but elastic enough to share applied load between multiple spokes ....this is why butted spokes are often said to be "better" than plain gauge.
The solution to spoke failure is stress-relief, which is usually done badly, and not at all in machine built wheels.
The solution to stress corrosion cracking is to use enough material, of an appropriate alloy, with an appropriate finish, and to wash off road salt frequently.

[Vorpal]

Vorpal, there is a thread here viewtopic.php?f=5&t=110419&hilit=screwfix&start=30 which is the best discussion of spoke fatigue and stress-relief that I can find.....we have had a few such discussions on these pages. Can I suggest it belongs in "too good to lose" with the title amended?

I added it to the 'too good to lose', but I muddled up the title. Apparently I need to change the title before I moved it. I will see if I can fix it, but at least it's there, now

[531colin]

, so if you think spoke failures are somehow inevitable, thats fair enough.

I don't think spoke breakages are inevitable, far from it, i've had far more rims die than spokes break over my 40 plus years of cycle abuse! Yes i'll stand up and admit it, my bikes get abused, once upon a time i'd stop at every raised kerb, carry bikes across bad surfaces etc but i'm past such silliness now, even the CF has seen lots of trail riding alongside its preferred smooth tarmac, sometimes from choice, other times through necessity!

But i am a realist, given the age/abuse that this particular wheel has had, an off the peg Trubuild which hadn't seen as much as a spoke key since it left the factory, i would be a fool to think there would never be another breakage. If the remaining 29 original spokes have been fatigued the same way as the 3 that have failed (@ 5000km apart), even after stress relief, there is, i reckon, a better than even chance that another may break. OTOH, the rim probably doesn't have another 5000km in it so that's probably moot - although some of the holes in the roads (i use that word with caution) in north Somerset / south Gloucester could claim the wheel at any point!

In terms of the life expectancy of wheels, i don't think this wheel has done anything extraordinary in terms of mileage, i would hope to get something similar from any reasonable modern wheel build (old school sprints were made of cheese so life could sometimes be counted in days!). For example, I was given a radially spoked sprint race wheel in @ 1979 that was used primarily for racing for over 15 years before being retired to spare/training use - it never went out of true despite the spokes getting a bit soggy, it was rim wear which eventually did for it. Some years it did over 1000 racing kilometres (mostly short TT's and 3/J RR's) plus at least double that riding to/from events, i'd be surprised if it hadn't done at least 30k km before its demise.

Are you saying you don't think spoke failure is inevitable in general, but there is something wrong with this particular wheel which makes spoke failure likely?

For context, in all the wheels I have built for myself (and stress relieved, obviously) I have never broken a spoke. Never, ever, not one.
Rears, fronts, wheels where the rim has worn out and I have re-rimmed using the existing spokes.

[Jdsk]

Overtensioning is unlikely to break any half-decent spoke, since the rim will give up first. You might manage it with a steel rim

If that is true, that a rim of whatever material might break before a spoke breaks, surely spokes are too strong and rims are too weak. Replacing a spoke is easier than replacing a rim

That doesn't follow. Modern spoked wheels are a staggeringly efficient solution to a very complex problem. Overtightening is an abuse condition. Making rims stronger in order to tolerate that would cause an adverse trade-off in the optimisation for their true purpose in life.

Shirley

[rogerzilla]

I remember from my Chem Eng degree (now mostly forgotten, I never used it) that stress corrosion cracking is a problem with some stainless steels, e.g. 304, in the presence of chlorides. Any idea what grade of SS spokes are made from? 316, I think, was relatively immune to SCC.

Road salt can certainly accelerate cracking of poorly-specified hub flanges. I lost a chunk of a Goldtec track hub in only its third winter. The drive side, being protected by oil spatter, was fine but the left side just crumbled.