Yellow Saddle
Guru
- Location
- Loch side.
Who is he and what was his assumption?
Why do my spokes keep breaking? - Bike wheel science.
- Thread starter united4ever
- Start date
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Who is he and what was his assumption?
Ming the Merciless
There is no mercy
- Location
- Inside my skull
Well look at my post you replied to, I quoted the persons post it is all there and he assumes the bottom three spokes are supported differently to all the others.
Yellow Saddle
Guru
- Location
- Loch side.
Sorry, at first the diagram in question post didn't show so I assumed you referred to the author of the paper and the last drawing in the thread.
From what I can see, he (@Salar) made no assumptions which were either proven right or wrong. I think the "support means contact or at the very least, within the LAZ. I'll let him comment on his own drawing.
But, I'll stick with the answer that the bottom spokes don't need support to stay upright or straight since they are in tension.
Ming the Merciless
There is no mercy
- Location
- Inside my skull
We are agreed but his support comment about the bottom three spokes seems a strange comment to make. All the spokes are held in tension the same way. We both agree there is no support and it should not be necessary for him to make that incorrect assumption in his modeling.
Ajax Bay
Guru
- Location
- East Devon
"The tests were carried out while the writers were employed at Imperial College (assume London)" ( last paragraph). Stepping on and off the Engineering Department's pater-noster lift was the highlight of my attendance at structures lectures bitd.
Ajax Bay
Guru
- Location
- East Devon
I know you won't mind if I say that I differ. I maintain that saying that the load "stands on the bottom spokes" is misuse of the word 'stands', is therefore not correct terminology. Saying that the load 'hangs' on the upper spokes is false too - ie a misuse of the word 'hangs' and use of either terminology does not help to understand how a wheel supports a load.
Not sure what you mean by "the mechanism is reversed" but I suspect that I'd argue that the 'reversal' absolutely does matter to those trying to understand how a wheel supports a load.
andrew_s
Legendary Member
- Location
- Gloucester
What I meant was that it seems to be readily accepted that, in an actual wheel that you could use on a bike...
a) all the spokes are at a fairly high tension.
b) when the wheel is loaded the tension in the bottom 2 or 3 spokes is reduced.
c) the tension in all the other spokes is more or less unchanged.
It's in translating that information into an explanation of what keeps the hub off the ground that everything falls apart and the hang or stand arguments arise.
Ajax Bay
Guru
- Location
- East Devon
Replying to the posts you made yesterday evening.
"clearly possible from a statics perspective" Perspective which offers no utility, I suggest.
"It's only deformation in the system which defines where the load is taken." In an elastic system, external forces will result in deformation (and movement if unbalanced).
We are considering scenario 3, where the two spokes (one up, one down) are elastic.
Pasting a link to your post for ease of reference: https://www.cyclechat.net/threads/why-do-my-spokes-keep-breaking.244392/post-5498719
Try this. Convert the 'simple' two spoke system to one pre-tensioned 'spoke' spanning the diameter of the rim vertically with a neat attachment point at the centre of this diameter spoke. Then apply a vertical (downwards) load to the centre point (same as a load through the drop outs) and an equal and opposite force upwards where the spoke reaches the rim/ground.
The top half of the spoke has no net increase in tensile force (the resultant of the load and normal force is nil). The bottom half of the spoke has a compressive force applied. Fortunately this force is much less than the magnitude of the pre-tension in the spoke so it doesn't buckle and fail: it stays in tension but less than its starting tension by 'W'.
That's why E (which you defined as the reduction in tension due to elasticity) = W (the load applied (at the centre (hub)).
"if all the change in tension were in the lower spoke, it would reduce in length" absolutely and this is what happens in the real world. The rim in the contact (with the ground) area and adjacent distorts from a perfect circle (by <20 microns for a 622 wheel and 500N load), the lower spokes contract and the tensions in the lower spokes reduce accordingly. When the wheel is loaded the upper spoke(s) do not measurably increase in length (their tension is essentially unchanged from their pre-load tension).
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andrew_s
Legendary Member
- Location
- Gloucester
To get back on topic 
As stated, the OPs spokes keep breaking because they are fatigued, and as many other spokes will have started to crack but not yet actually broken, the only thing to do to avoid having the wheel fixed every few weeks (or even days) is to rebuild the wheel with new spokes.
The root of the problem is that the wheel wasn't stress relieved after being built, as is fairly common in cheaper factory or new bike wheels.
Stress relieving is done by briefly bringing each spoke up to a much higher tension than the static build tension (up to double). This is usually done by squeezing pairs of spokes together hard, forcing the middles of the spokes sideways. If you are squeezing hard enough, you'll probably want gardening gloves to protect the hands.
https://www.sheldonbrown.com/brandt/stress-relieving.html
It is possible that you could give the wheel a good hard stress relief session, replace the spokes that this broke (no loss, they would have broken soon anyway), then stress relieve the wheel again. However, the chances of getting a bike shop to do this are fairly low.
As stated, the OPs spokes keep breaking because they are fatigued, and as many other spokes will have started to crack but not yet actually broken, the only thing to do to avoid having the wheel fixed every few weeks (or even days) is to rebuild the wheel with new spokes.
The root of the problem is that the wheel wasn't stress relieved after being built, as is fairly common in cheaper factory or new bike wheels.
Stress relieving is done by briefly bringing each spoke up to a much higher tension than the static build tension (up to double). This is usually done by squeezing pairs of spokes together hard, forcing the middles of the spokes sideways. If you are squeezing hard enough, you'll probably want gardening gloves to protect the hands.
https://www.sheldonbrown.com/brandt/stress-relieving.html
It is possible that you could give the wheel a good hard stress relief session, replace the spokes that this broke (no loss, they would have broken soon anyway), then stress relieve the wheel again. However, the chances of getting a bike shop to do this are fairly low.
andrew_s
Legendary Member
- Location
- Gloucester
The two-spoke system may be simple, but it's not a wheel (you couldn't put it on a bike and ride), so there's no point thinking about it
Ajax Bay
Guru
- Location
- East Devon
The rim is defined as rigid so perhaps you could. Fair bit of tension in the two spokes when they went through horizontal, mind. If the spokes could take compression then perhaps a three or four spoke wheel would roll OK.
roubaixtuesday
self serving virtue signaller
You've missed the point entirely.
I offered the two spoke model not as a practical wheel, but as a way to help think conceptually about the problem.
Far more illustrious thinkers have used similar approaches.
https://en.wikipedia.org/wiki/Einstein's_thought_experiments
Ajax Bay
Guru
- Location
- East Devon
Are you suggesting that the spokes of properly (?and regularly) stress relieved wheels do not fatigue and finally part?
Have you ever actually managed to break a spoke during "a good hard stress relief session" (
)?I see Jobst Brandt said he'd done this:
"My first experience with this [squeezing spokes to achieve much higher tension levels temporarily] was years ago when I had frequent spoke failures and wanted to be done with it by attempting to break any spoke that was about to break. That is where I stumbled onto stress relieving. I broke two more spokes by forcefully stress relieving the
wheel and then there were none for a long time."
Yellow Saddle
Guru
- Location
- Loch side.
There is no point in regularly stress-relieving a spoke. Stress relief is done once, after finishing building the wheel, to normalise internal stress in the spoke which derived from the bending, stamping and threading steps in manufacture. Stress relieving later on in the wheel's life is useless.
Have you ever actually managed to break a spoke during "a good hard stress relief session" (
I see Jobst Brandt said he'd done this:
"My first experience with this [squeezing spokes to achieve much higher tension levels temporarily] was years ago when I had frequent spoke failures and wanted to be done with it by attempting to break any spoke that was about to break. That is where I stumbled onto stress relieving. I broke two more spokes by forcefully stress relieving the
wheel and then there were none for a long time."
That incident doesn't indicate regular stress relieving. He just called it stress relieving because he used the same squeezing method to see if he could find spokes which were just about to break. He did. I've done that plenty of times on wheels that came in with spokes broken. I prefer to break one or two more in the workshop than have the customer return a week later with more spokes.
A stress relieved wheel with double-butted spokes (Revos or Competition, it doesn't matter) has a near infinite spoke life, provided the right wheel was designed for the job. By design, I mean that you didn't attempt to fob a 24-spoke wheel off to a 120kg guy, or build tandem wheels with only 36 spokes. My own wheels are built with Revos, 28 spokes on Mavic Open Pro. The wheels have done in excess of 200 000 kms without a single broken spoke, and one one, "tune" to true a slight wobble.
Our little factory built about 30k wheels in 7 years. Each wheel went out with a lifetime (of the wheel) spoke gaurantee. In 14 years (I've beenout of it for 7 years now but someone else is looking after come-backs), we've had less than 6 broken spokes. Four were from sticks, two from fatigue. We had a contract with a reseller of Zipp wheels who brought the wheels into the country as components for tax reasons. We assembled several hundred of those and never had a single spoke break, even though they were ridiculously under-specc'd. However, there were plenty of comebacks on the nipples, which of course being Zipp, were aluminium. I've never had a single brass nipple fail on us. Each time an aluminium nipple breaks on a Zipp wheel, it is high drama. It usually costs more to repair than the original build fee.
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rogerzilla
Legendary Member
I doubt Brandt broke spokes merely by squeezing pairs of spokes together with his hands, although spokes were not as good back in the day. However, some people also like to torture the spokes by wedging an old crank into the crossings and using it as a lever, and I suppose that could break a spoke if you have no feel for this kind of thing.
For what it's worth, I've only ever had one spoke break, and that was on a very cheap 27" machine-built wheel with "rustless" spokes, that had done quite a lot of winter miles. Decent modern spokes don't break anything like as often as the rubbish ones of the past, even in an indifferently-built wheel; the purpose of good wheelbuilding is more about ensuring the wheel remains true and the nipples never need touching again, not until the rim has worn out.
For what it's worth, I've only ever had one spoke break, and that was on a very cheap 27" machine-built wheel with "rustless" spokes, that had done quite a lot of winter miles. Decent modern spokes don't break anything like as often as the rubbish ones of the past, even in an indifferently-built wheel; the purpose of good wheelbuilding is more about ensuring the wheel remains true and the nipples never need touching again, not until the rim has worn out.
