Broken spoke

[Vorpal]

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.

It is possible to over tension a spoke to the point that it yields but does not break, without breaking the rim. But steel (or stainless) have to stretch quite a bit to break in tension, and wheel building doesn't really have the space for that kind of elongation. I suppose I could calculate how much a spoke has to stretch before it breaks if I were bothered about it. But I know someone who stretched some spokes 3 - 4 mm before he realised that he had the units wrong on his tension meter.

A spoke that has yielded due to over tensioning may very well break due to fatigue at low mileage, but it will fail at the point of elongation, which I think is unlikely to be the bend, as this has been work hardened in the manufacturing process.

[531colin]

I once came across a batch of (stainless) spokes which were too weak to build (and stress-relieve) a wheel.
They elongated at the elbow bend, and were obviously thinner there than elsewhere. If you stress-relieved sufficiently hard to do any real good, these spokes would part at the elongated section.

[Vorpal]

I once came across a batch of (stainless) spokes which were too weak to build (and stress-relieve) a wheel.
They elongated at the elbow bend, and were obviously thinner there than elsewhere. If you stress-relieved sufficiently hard to do any real good, these spokes would part at the elongated section.

I think I would call that a quality issue

[531colin]

I once came across a batch of (stainless) spokes which were too weak to build (and stress-relieve) a wheel.
They elongated at the elbow bend, and were obviously thinner there than elsewhere. If you stress-relieved sufficiently hard to do any real good, these spokes would part at the elongated section.

I think I would call that a quality issue

At least it meant we didn't send out any wheels with those spokes!

[foxyrider]

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.

Indeed that's what I'm saying - I have similar experience in wheels that I have built myself.

The wheel in question, as I've pointed out before, is a machine build that was checked for true and fitted and never touched again until the first spoke breakage whereupon it was treated to stress relief when replacement was made. It had done a lot of miles up to that point so the remaining original spokes, whilst now stress relieved, may well have hidden damage which may cause further failures. Its not inevitable but it is possible.

[jb]

Took me a while to realize stress relieving didn't mean placing the wheels in an oven at 300°C
It's more accurate to say they are cold set to the required shape to prevent stress. But I suppose that's a bit pedantic.

[andrew_s]

It's more accurate to say they are cold set to the required shape to prevent stress. But I suppose that's a bit pedantic.

Cold setting doesn't mean there's no resulting stress.

When a spoke is bent to form the J-bend, the metal on the inside and the outside of the bend sees enough stress to permanently deform, but in the middle of the spoke, between the compressive stress on the inside and the tensioning stress on the outside, these are regions where the metal is both unstressed, and stressed not quite enough to deform.
If you cold set something, you'll notice it springs back some when you let go. That's the un-deformed metal in the middle attempting to return to its original position.

The closer a bit of metal is to the deforming stress, the fewer stress cycles it will take before it starts to crack.
Stress relief is adding enough extra stress, temporarily, that the metal that was previously just about to deform does so. This means that the remaining undeformed metal isn't so close the the deforming stress under normal use.

[jb]

Well yes, but cold setting (in my book anyway) means you set the component to the the right form so that there are no forces trying to spring it back, this being achieved in this case by taking it slightly beyond its elastic limit to permanently keep the desired form.
The more common way I mentioned (rather tongue in cheek) is heating the component to allow the crystalline structure to move around and relax any built up stresses. Obviously not an option for cycle wheels.

[Vorpal]

Took me a while to realize stress relieving didn't mean placing the wheels in an oven at 300°C
It's more accurate to say they are cold set to the required shape to prevent stress. But I suppose that's a bit pedantic.

Actually, whether or not it should be called stress relieving has already been argued endlessly. It is not a technically accurate term with regards to materials engineering, but it is a process that relieves inbuilt stresses, so it makes sense to use it. And more importantly, it is one that people who build wheels understand.

[NATURAL ANKLING]

Hi,

Took me a while to realize stress relieving didn't mean placing the wheels in an oven at 300°C
It's more accurate to say they are cold set to the required shape to prevent stress. But I suppose that's a bit pedantic.

Or sticking your head in the freezer

In the past I've tried in vain to come up with a technical term for that "stress relieving" spoked wheels,
just means something else of course normally in metalwork.

I guess it's just a short name abbreviation for what you are actually doing with the spoke.

It's a little bit like MA -

https://www.definitions.net/definition/ ... 0Advantage
"Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms. An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal mechanism does not include a power source, and is frictionless and constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account friction, deformation and wear."

[Cyril Haearn]

So, what could 'stress relieving' be called instead? Stress equalization?

[Vorpal]

So, what could 'stress relieving' be called instead? Stress equalization?

I think stress relieving is okay. People talking about wheels built with steel or stainless spokes understand it.

Sress equalisation doesn't really make sense. Stress isn't being equalised. The only problem with using stress relieving, realyl is that it is *also* used to mean other things. But that applies to lots of words & phrases in English, and the meaning can easily be discerned by context.

[Mike Sales]

How about just calling it "stressing"?
I see it as giving the assembly a dose of what it will undergo in use, and correcting any settling in that this might cause, before use.

[Jdsk]

So, what could 'stress relieving' be called instead? Stress equalization?

I think stress relieving is okay. People talking about wheels built with steel or stainless spokes understand it.

Sress equalisation doesn't really make sense. Stress isn't being equalised. The only problem with using stress relieving, realyl is that it is *also* used to mean other things. But that applies to lots of words & phrases in English, and the meaning can easily be discerned by context.

Agreed x2.

Jonathan

PS: And of course the most common use of "stress relieving" in English is in the context of mental wellbeing, and especially in the promotion of proprietary remedies of dubious efficacy. But cycling often works. : - )

[531colin]

.......................Cold setting doesn't mean there's no resulting stress.

When a spoke is bent to form the J-bend, the metal on the inside and the outside of the bend sees enough stress to permanently deform, but in the middle of the spoke, between the compressive stress on the inside and the tensioning stress on the outside, these are regions where the metal is both unstressed, and stressed not quite enough to deform.
If you cold set something, you'll notice it springs back some when you let go. That's the un-deformed metal in the middle attempting to return to its original position.

The closer a bit of metal is to the deforming stress, the fewer stress cycles it will take before it starts to crack.
Stress relief is adding enough extra stress, temporarily, that the metal that was previously just about to deform does so. This means that the remaining undeformed metal isn't so close the the deforming stress under normal use.

Andrew; thanks for this. Its something I don't find easy to understand in depth. (I have a background in biology, not engineering.)
Can I just check with you that I have understood properly your sentence which I have turned red?

"When metal is stressed into shape (ie. not permanently deformed) cyclic stresses imposed in addition to the static stress will initiate a crack. Crack initiation is quicker if the static stress is close to that stress which would cause permanent deformation, and with greater cyclic stresses."