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Wheel building gone wrong - a "twang" and disaster

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Wheel building gone wrong - a "twang" and disaster

Old 11-19-14, 02:11 PM
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Originally Posted by 79pmooney
.... When that chain drops in there, you will regret riding a wheel where the spokes at the top of the hub face forward and have the heads out. Those spokes will actively pull the chain in deeper. Damage to your derailleur, possibly dropout, chain and hub will happen. ....
While I believe you are exaggerating the issue greatly, my real problem is that you have your facts backward unless I don't understand your use of "face forward".

If the spokes on the outside rear the hub, ie. elbow out are leaving the shell in a clockwise direction (RH spiral) then they'll suck the chain down. OTOH if those elbow out spokes are leaving in a counterclockwise direction, they'll tend to lift the chain. One glance at the area near the cassette of any built wheel will make it obvious whether the wheel would tend to suck the chain down or lift it when overrunning.

In any case, chains have been over shifting for ages and the most common worst case scenario is inconvenience because once the chain is pulled below the spoke line, there's enough room for the wheel to turn. The issue is that it's a royal PIA to free the trapped chain without removing the cassette or freewheel.

Also, chain overshift tends to nick or gouge the spokes and that can set up spoke breakage some time down the road. Often it's a long time down the road, but it can also be sooner.
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Old 11-19-14, 09:23 PM
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Originally Posted by Jiggle
Strength is not stiffness.
Once would have sufficed. When writing about how much a thing deforms under force, as opposed to breaking, they are the same.

Originally Posted by Jiggle
And the word you should use is strain, not deflection.
I adopted the word Mr FBinNY introduced; the portion of the post you quote responded to him.

Originally Posted by Jiggle
the stiff center section of the straight gauge transmits all the stress to the elbow, the elbow of the straight gauge spoke will fatigue and break much more quickly.
I don't see this. Why the elbow and not the nipple? (A study conducted at Stanford for Wheelsmith in 1984-5 found
In 68 spokes the failure occurred at the cold-worked elbow; in the remaining 8 spokes the failure occurred at the threads.
Bicycle Wheel Spoke Patterns and Spoke Fatigue, Journal of Engineering Mechanics, vol 122, no. 8, (August 1996 pp. 736-742, Henri P. Gavin, Professor, Department of Civil Engineering, Duke. https://people.duke.edu/~hpgavin/pape...heel-Paper.pdf) I see 2 forces on the spoke: tension, which stretches it, and torque, which bends it. Both have to propagate over the entire spoke. The ends are more fixed than the center so they can't bend as much. If the fatigue of repeated bending breaks spokes they would break in the center. Hold a spoke at both ends, bend it: what happens? Keep on doing that until it breaks: what part breaks?

Originally Posted by Jiggle
Why doesn't the center of the db spoke break instead of the elbow?
Because the elbow moves in the hub, rotating back and forth to transmit torque from the hub (rear wheel) to the tire and inwards and outwards, rubbing off a bit of metal each rotation of the wheel - I can see the wear the spokes have rubbed in the hub and the wear on a spoke's elbow when I take a wheel apart.

Originally Posted by Jiggle
Steel is a wonderful material because it has a fatigue limit.
Wouldn't it be more wonderful it had no fatigue limit? Is there a non-wonderful material that has no fatigue limit?

Originally Posted by Jiggle
stress is greatest at the surfaces of any bending beam
The surfaces? Do you mean the ends? It's at the ends when it's a moment; when it's compression or tension of a uniform material it's the same throughout. A little torque gets lost between the hub and the rim but all the rest has to get transmitted through the spoke. In a uniform material the part that bends the most absorbs the most.

Originally Posted by Jiggle
the elbow is bending while the center of the spoke is only in tension.
The center of the spoke isn't bending? Hold a spoke, push in from both ends: where does it bend? Hold a non-straight spoke, pull: where where does it bend?

I found some threads at :: CyclingForum.com , correspondents include Sheldon Brown and Jobst Brandt, that indicate that butted spokes distribute high-impact stresses more quickly amongst themselves and from the elbow and nipple to the center internally in high-impact stresses. Also, astonishing to me, Mr Brandt wrote
Cyclic stress causes fatigue cracks and a thinner spoke as [sic] a lower N/mm elongation than a fat one
Butted vs. straight-gauge spokes - Page 2 in other words he says a thinner spoke stretches less under tension, the reason why butted spokes don't wear out rims as quickly. This is hard to believe.
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Old 11-19-14, 09:45 PM
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Originally Posted by FBinNY
While I believe you are exaggerating the issue greatly, my real problem is that you have your facts backward unless I don't understand your use of "face forward".

If the spokes on the outside rear the hub, ie. elbow out are leaving the shell in a clockwise direction (RH spiral) then they'll suck the chain down. OTOH if those elbow out spokes are leaving in a counterclockwise direction, they'll tend to lift the chain. One glance at the area near the cassette of any built wheel will make it obvious whether the wheel would tend to suck the chain down or lift it when overrunning.
You understood me right. I get your logic, but that isn't what happens. Examine wheels that have suffered chain overshift and you will see a clear pattern that backs what I said. (And I cannot explain it. My best reasoning always comes up with the same result you did, but I have enough experience to know that my $$s are better spent defying that logic.)

Now, you have to see a few wheels to get the pattern. A fully "stomped" inside pulling will have a lot more damage than an outside pulling where the rider instantly backed off and braked hard. (A place where locking up the rear brake can save you big $$s! Forget that tire!)

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Old 11-19-14, 09:49 PM
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Originally Posted by 79pmooney
You understood me right. I get your logic, but that isn't what happens. Examine wheels that have suffered chain overshift and you will see a clear pattern that backs what I said. (And I cannot explain it. My best reasoning always comes up with the same result you did, but I have enough experience to know that my $$s are better spent defying that logic.)

Now, you have to see a few wheels to get the pattern. A fully "stomped" inside pulling will have a lot more damage than an outside pulling where the rider instantly backed off and braked hard. (A place where locking up the rear brake can save you big $$s! Forget that tire!)

Ben
Not trying to change your opinion. Simply posting another so readers can see both sides and decide for themselves.

BTW- you're not consistent. You advocate for inside pulling (head is out, spoke is in) then say inside pulling suffers more damage.
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Old 11-19-14, 10:01 PM
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Originally Posted by RandomTroll
Sounds like a job for Spoke protector!
Yup! But I wont.

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Old 11-19-14, 10:14 PM
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Originally Posted by FBinNY
BTW- you're not consistent. You advocate for inside pulling (head is out, spoke is in) then say inside pulling suffers more damage.
I'm just saying if you stomp on the pedals after you dump your chain into a correctly laced wheel, you may do more damage than the guy who backs off instantly and locks up the rear brake with other lacing; that when you sample the wheels/bikes after this has happened, you will see overlap in the extent of damage. But view enough and a clear pattern emerges. It is possible I got my words mixed up. What I intended all along is that "inside pulling" rules. (The pulling spokes being the ones that come off the top of the hub and rotate to the rear.)

Originally Posted by FBinNY
Not trying to change your opinion. Simply posting another so readers can see both sides and decide for themselves.

Opinion? This has been my experience, as a rider, working in bike shops and examining and noting spoking patterns in every chain dump I have seen over 40 years. Have you seen otherwise? If so, tell us about them.

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Old 11-19-14, 10:21 PM
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Originally Posted by 79pmooney
Opinion? This has been my experience, as a rider, working in bike shops and examining and noting spoking patterns in every chain dump I have seen over 40 years. Have you seen otherwise? If so, tell us about them.

Ben
So I have a few more years of experience, (since 1968), and observed the opposite. But a few caveats, Unlike you I don't consider chain dumps inevitable, and most riders don't experience wheel damaging dumps even in a lifetime of riding.

One fact that might differentiate the outcome, isn't the spoke pattern, but the cassette/spoke clearance. If there's enough clearance a wheel laced as you do will quickly suck the chain down below the spoke elbows where the wheel may spin with less damage. OTOH- if the clearance is tight, spokes sucked down can immediately shear the elbows or at the least shave the outside of the spoke.

So, let's simply agree to disagree, and readers can weigh both opinions (because that's what they are, no matter how they were derived) and decide for themselves.
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Old 11-20-14, 07:29 AM
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Originally Posted by RandomTroll
I found some threads at :: CyclingForum.com , correspondents include Sheldon Brown and Jobst Brandt, that indicate that butted spokes distribute high-impact stresses more quickly amongst themselves and from the elbow and nipple to the center internally in high-impact stresses. Also, astonishing to me, Mr Brandt wrote Butted vs. straight-gauge spokes - Page 2 in other words he says a thinner spoke stretches less under tension, the reason why butted spokes don't wear out rims as quickly. This is hard to believe.
My friend, you are so totally confused that I do not know where to begin. Brandt said "a thinner spoke has a lower N/mm elongation than a fat one". So a lower force will elongate it more. Sounds right. That means the opposite of what you posted it means.

For the rest, I recommend the following book. It has all the mechanics formulas that we are talking about as well as the proper definitions for words like stress, strain, torsion, and tension so when you use them it won't be confusing to a reader.

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Old 11-20-14, 09:56 AM
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Originally Posted by Jiggle
Brandt said "a thinner spoke has a lower N/mm elongation than a fat one". So a lower force will elongate it more. Sounds right. That means the opposite of what you posted it means.
You interpret his comment to mean 'the same elongation for a lower N/mm'; I interpret it to mean 'a lower elongation for the same N/mm'. Syntactically both are possible; I think mine is more likely; comments in forums can be written carelessly (witness Mr Brandt's typo, which you silently corrected when you repeated it). But the conclusion he drew was that the rim would crack more with the thicker spoke. Doesn't flexing make the rim crack? If the thinner spoke elongates more doesn't the rim flex more?

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Old 11-20-14, 10:58 AM
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Originally Posted by RandomTroll
You interpret his comment to mean 'the same elongation for a lower N/mm'; I interpret it to mean 'a lower elongation for the same N/mm'. Syntactically both are possible;..... Doesn't flexing make the rim crack? If the thinner spoke elongates more doesn't the rim flex more?
Two points, not quoting JB, but I hope you're aware that both same for lower, and lower for equal, though both possible, are the opposite of each other.

In any case, I have to wonder if you're intentionally trying to live up to your name, and your agenda is to gull some of us into wasting time refuting fatuous arguments.

OTOH- on the off chance that you're genuinely confused, I'll offer one last short hint to (maybe) point you in the right direction. You have your horse and cart reversed and are confusing causes and effects. Think about how suspension forks reduce frame stress, or similar applications where the deflections that happen under stress are directed to one place rather than another.
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Old 11-21-14, 10:48 AM
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Originally Posted by FBinNY
Think about how suspension forks reduce frame stress, or similar applications where the deflections that happen under stress are directed to one place rather than another.
Is that their purpose? I thought it was for the comfort of the rider. I don't have one. Do competitors use them? They add weight and divert energy from propulsion.

We're talking about different things: a structural member has a stress limit; to protect it from breaking at its stress limit one can suspend it flexibly or incorporate flexibility, which stretches out the duration of a stress thereby decreasing its maximum: all a stress's energy still goes through the member, just over a longer time so it's not as high at any one moment. If a breaking-stress happens at one point and one can distribute it through the member more quickly one can keep a single point from bearing as much stress. I'm talking about a spoke breaking from gradual wear, from millions of events all of them far below its stress limit. In this case the less a part flexes the longer it lasts. The less it flexes the less the atom rearrange. I quoted Mr Brandt because he talked about high-impact events, not gradual wear.

A rim deforms, spoke tension resists its deformation; the magnitude of the rim's deformation is a function of the strength of the spokes: the stronger, the less the spokes stretch, the less the rim deforms. This deformation can prevent breaking from a single event severe-enough to break a rim were the spokes perfectly stiff, but it also work-fatigues a rim, leading to earlier failure from fatigue.

Crossing spokes makes wheels less stiff; according to Bicycle Wheel Spoke Patterns and Spoke Fatigue it doesn't make them stronger under radial loads (but does under lateral loads):
The fatigue life of the spokes of a rear wheel supporting radial loads is therefore not significantly influenced by the spoke pattern.
Some people solder spokes together where they cross to make wheels stiffer: does this make them more-likely to break?
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Old 11-21-14, 10:58 AM
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Originally Posted by RandomTroll
You interpret his comment to mean 'the same elongation for a lower N/mm'; I interpret it to mean 'a lower elongation for the same N/mm'. Syntactically both are possible; I think mine is more likely;
You think it is more likely that an experienced mechanical engineer would get his stress/strain relationship backwards, an equation that every MechE student learns before the first test in their first class on the mechanics of solids in their undergrad year.

El-Oh-El. You are an entertaining troll.
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Old 11-21-14, 11:51 AM
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Originally Posted by RandomTroll
Is that their purpose? I thought it was for the comfort of the rider. Yes, one purpose is rider comfort, and another is handling and control. Feel free to research the principles of vehicle suspension on your own. However, the existence of suspension reduces maximum stress on the rest of the frame members and engineers factor that...

-------------------

Some people solder spokes together where they cross to make wheels stiffer: does this make them more-likely to break?
I'm not going to waste time on this (rhetorical?) question. You ask questions than want to debate the answers. If you know, don't ask, if you don't know then give due consideration to the answers. As I said earlier, I suspect you are exactly what your name implies, more interested in wasting people's time than the answers to the questions you ask.
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Old 11-21-14, 09:23 PM
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I'm far from a MechE, but even I can see how bogus RandomTroll's claims are. They are full of crap.
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Old 11-22-14, 11:02 PM
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Originally Posted by Jiggle
You think it is more likely that an experienced mechanical engineer would get his stress/strain relationship backwards, an equation that every MechE student learns before the first test in their first class on the mechanics of solids in their undergrad year.
That quote from Mr Brandt continues
Cyclic stress causes fatigue cracks and a thinner spoke as a lower N/mm elongation than a fat one. Hence the rim knows as does the nipple, that stress excursions with thinner spokes are lower,
Originally Posted by Jiggle
You are an entertaining troll.
Must be why you want to be my friend. I can't wait for the copy of Gere & Goodno's Mechanics of Materials on my next birthday.

Originally Posted by FBinNY
However, the existence of suspension reduces maximum stress
By increasing the excursion of the member by much more than that of a rigid member.
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Old 12-04-14, 07:04 AM
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page 88-91 https://prawda.org/_/The%20Profession...elbuilding.pdf
I got vers. D
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