Yet another broken spoke

[jim beam]

[email protected] wrote:
> Michael Press writes:
>
>>>> The spokes lose huge amounts of pre-tension as they roll under the
>>>> wheel. The individual the spokes all the way around the wheel show
>>>> an increase of only up to 10% in tension, compared to the spoke
>>>> directly under the axle's loss of tension.
>
>>> Right. Under what criteria is a 10% increase in tension
>>> insignificant, as it was described by Brandt? And in your testing,
>>> as well as everyone else's, the greatest loss of tension was in the
>>> spokes perpendicular to the spokes that lost tension.
>
>>> The loss of tension caused by the local flexing of the rim cannot
>>> be balanced by a rise in tension by the rest of the spokes; OTOH,
>>> the flexing of the rim caused by the ovalization of the hoop _must_
>>> be offset by a rise in tension by the rest of the spokes.
>
>> The hoop does not "ovalize" in normal use, the use for which it is
>> intended; to wit: transmitting a compressive load between the
>> contact patch and the axle. The shape of the distortion of a rim
>> under load is lumpy.
>
>> On a thirty six spoke wheel the greatest change in spoke length is
>> at the contact patch where it is -0.153 mm. The next local maximum
>> of absolute spoke length change is four spokes from the contact
>> patch, or one ninth of the circumference where the change is 0.014
>> mm. After that all the spokes are extended by 0.007 mm. The rim
>> remains circular, except for an indentation at the contact patch and
>> a couple lumps adjacent to the contact patch.
>
> To put it a different way, the rim is flattened at the road contact
> area and this flattening increases the radius of the remaining
> circular part of the rim (the previous arc having a shorter linear
> length than when flattened. Of course you can read about this in "the
> Bicycle Wheel" which is what inspired Ian and Henry Gavin to publish
> the same material in their own fora.
>
> http://www.avocet.com/wheelbook/wheelbook.html
>
>>> The latter effect is where the wheel gets its strength; it is
>>> ridiculous to suggest that the rise in tension of the other spokes
>>> is insignificant because without that rise in tension you might as
>>> well be riding a wheel with all the spokes detensioned to the point
>>> that all the wheel strength derives completely from the strength of
>>> the rim alone. To say that the rise in tension of the other spokes
>>> is insignificant is just utterly ridiculous.
>
> If you research the many times this subject has appeared in this
> forum, you'll find that the vertical component of tension increases,
> caused by spreading the wheel circumference, sum to zero, leaving only
> the reduction in downward force of the spokes in the "load affected
> zone" as the sole support of axle loads. The reason this is so, is
> that at either end of the load affected zone, a bulge caused by rim
> stiffness in the transition from the flattened area to the circular
> part does not allow a sudden transition. This may slightly differ
> depending on the bending stiffness of the rim cross section used as a
> model. The ones in the book are MA-2's.
>

and yet interestingly, wheels still manage to support load with no
bottom spokes at all...

http://www.flickr.com/photos/38636024@N00/417157612/

but that picture must be photoshop - there's no way your theory could
/possibly/ be incomplete.

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> >[email protected] wrote:
>
>>> I think we have discussed this at
>>> great length with only a "former metallurgist" claiming that new
>>> materials resolved those causes rather than going back to short elbows
>>> (that DT did) and to properly shape spokes and stress relieve after
>>> building
>>
>> truth is jobst, if you ever properly addressed the points i have had
>> to repeatedly raise with you, or ever bothered to read the cites i've
>> given you, went to the library and did your own homework, or even
>> bought a decent magnifier and bothered to examine fracture surfaces
>> from actual failures, you might, just might, be able to finally start
>> to understand a little about fatigue. instead, you continue to write
>> suppositional ******** based on a shamefully poor grasp of the facts.
>> if you even understood the difference between materials that strain
>> age and those that don't, you might evidence some potential for
>> understanding. but as things stand, you continue to confirm the truth
>> of the saying, "ignorance can be cured, stupid is forever".
>
> Just out of curiosity, I Googled to see where this was originally
> explained to you.
>
> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
> good job. Apparently it didn't stick. I can see why Jobst no longer
> bothers to respond.
>
> http://tinyurl.com/29v4u2

you had to dig back /four years/ to find something that predated luns
tee reminding us all of residual stress profiles? ok, so having done
that, can you now explain how fatigue still nucleates at points with no
residual stress but subject to bending stress? [hint: i included a clue
about the answer - see if you can spot it.]

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>> [...]
>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>> bothers to respond.
>>>
>>> http://tinyurl.com/29v4u2
>>
>> Thanks for the link, that's an interesting thread. Before my time.
>>
>> How much more civilized everyone was back then!
>
> Unfortunately, incivility seems to drive the civil away. Perhaps that's
> the motive.
>
>> I might as well just
>> read the archives and not bother with new RBT.
>
> There's a lot to learn in many NG archives, I often get info there that
> I can't find with a web search -- on all kinds of topics.

and yet, you seem to absorb so little!


> The signal to
> noise ratio is tough, ours has declined a lot.

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>
>> why don't /you/? but i forget, you don't contribute. maybe that's a
>> guilt thing since you're playing this game on your employer's time,
>> not your own.
>
> That's none of your business.

it's a damned good indicator of your fundamental dishonesty though.

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>> [...]
>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>> bothers to respond.
>>>
>>> http://tinyurl.com/29v4u2
>>
>> OK I have a question.
>>
>> jim beam> i have ignored residual stress as a factor in these failures
>> jim beam> because the majority of the fractures i've examined initiate
>> jim beam> on the /inside/ of the spoke elbow bend, not the outside
>> jim beam> [although i have examples of each]. residual stress in this
>> jim beam> location is compressive so i'm just looking at the external
>> jim beam> [+cyclic] load.
>> Mike Prime> The inside of the spoke elbow will have TENSILE residual
>> Mike Prime> stress, not compressive, because of the elastic springback
>> Mike Prime> after bending. See below. That 0.5 Sy number is for a beam;
>> Mike Prime> I'm too lazy to derive the number for a circular cross
>> Mike Prime> section right now.
>>
>> Mike Prime> Since that location has tensile residual stress, tensile
>> Mike Prime> applied mean stress from the spoke tension and bending,
>> ^^^^^^^
>>
>> Is the _applied_ stress on the inside of the elbow from spoke tension
>> and bending really tensile?
>>
>> I don't understand that. I thought when you bent a wire you got tensile
>> stress on the outside of the bend and compressive on the inside?
>>
>> I know that the residual stress, after it springs back, is tensile on
>> the inside.
>
> If the spoke has no bending moment (perfectly supported, perfect path),
> the applied stress from spoke tension will be tensile (uniform) across
> the cross section. Whatever bending force that is also present will add
> to that. The bending force can be either way depending on angular
> mismatch. If the spoke elbow is too long, another bending force will act
> to open the spoke angle further, adding to the residual (mfg) stress.

but you're ignoring the actual residual stress profile. just alluding
to residual stress being possible doesn't mean it's actually nucleating
fatigue!

>
> The worst case would be an (initial spoke) angle too acute with elbow
> too long. Both of those factors plus residual stress would all put
> tension on the inside of the elbow.

no, just having a spoke elbow offset from the spoke's longitudinal axis
will create tension on the inside of the elbow.


>
> If the spoke elbows are the right length, and the spoke angle is
> corrected, the only significant stresses should be spoke tension and
> residual.

you're omitting bending. even if the spoke is resting against the hub
flange, it can't protect against tensile stress on the inside of the
elbow, only tensile stress on the /outside/, a place which, while
fatigue does nucleate there from time to time, is not the commonest point.

> By stress relieving, the residual is reduced to non-fatiguing
> levels.

presumptive straw clutching. plastic deformation subsequent to initial
forming operations can /increase/ residual stress.

> But, if the spoke has (tensile) stress levels near yield in
> parts of the cross section, those will be reduced as they are forced to
> yield by the momentary overload

and the regions of highest residual stress are on the /inside/ of the
wire, not the inside or outside of the bend where fatigue nucleates.

> -- whatever the source. It's a "can't
> lose" proposition.

eh? you need to read this:
http://en.wikipedia.org/wiki/Fatigue_(material)#Miner.27s_rule

particularly this:
"In some circumstances, cycles of high stress followed by low stress
cause more damage than would be predicted by the rule."

sounds like our much-advocated "stress relief" scenario to me.

 

[jim beam]

[email protected] wrote:
> Ben C? writes:
>
>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>> bothers to respond.
>
> http://tinyurl.com/29v4u2
>
>> OK I have a question.
>
>> jb i have ignored residual stress as a factor in these failures
>> jb because the majority of the fractures i've examined initiate on
>> jb the /inside/ of the spoke elbow bend, not the outside [although i
>> jb have examples of each]. residual stress in this location is
>> jb compressive so i'm just looking at the external [+cyclic] load.
>
>> MP The inside of the spoke elbow will have TENSILE residual stress,
>> MP not compressive, because of the elastic springback after
>> MP bending. See below. That 0.5 Sy number is for a beam; I'm too
>> MP lazy to derive the number for a circular cross section right now.
>
>> MP Since that location has tensile residual stress, tensile applied
>> MP mean stress from the spoke tension and bending,
>
>> Is the _applied_ stress on the inside of the elbow from spoke
>> tension and bending really tensile?
>
> As I mentioned, don't fly your kite on every breeze that comes along,
> especially those sent aloft by jb.

as opposed to red herrings by JB?


> The elbow of a spoke is the last
> operation in spoke manufacture and it is accomplished by extending the
> head end of the spoke an appropriate length from a collet as a blunt
> piston goes by to bend it just enough to make an obtuse angle.

fantastic. spokes get bent!


>
>> I don't understand that. I thought when you bent a wire you got tensile
>> stress on the outside of the bend and compressive on the inside?
>
> These loads tend to open the elbow angle so that causes tensile
> stress.

we make progress!!!


> As Mike mentioned above, springback makes the stress reverse
> from that during forming. I spent a few exchanges on that issue at
> the time. You could also find them. When you bend a spoke it takes a
> set only after you exceed yield stress in the extreme "fibers" of the
> skin of the spoke. The farther you bend, the deeper the yield stress
> reaches.

and yet, we don't observe fatigue to nucleate at the regions where high
residual stress are... how inconvenient.


>
> For instance, a thin wire will not easily take a bend because it must
> be bent severely to go beyond yield. That's why we use braided cables
> made of fine strands that do not reach yield in the bends encountered
> in curves to reach the derailleur or brake to which it is attached.

red herring.


> In addition, there is no length change in these being helically wound
> cables, all of whose strands pass through the inside and outside of
> each curve (equal path length).

still red herring.


>
> Thus, the core "fibers" of a spoke never go to yield (being thin
> wires) and want to spring back while the outer "fibers" yields and
> wants to stay bent. This causes springback when bending a wire.

absolutely not - residual stress does NOT cause or evidence springback.
springback is simply elasticity and relaxation of load. /relaxation/
causes residual stress - it's not present when the material starts to
spring back. - it's all /applied/ stress.


> The
> reason it doesn't spring all the way back to straight is that the
> outer "fibers" resist,

eh? it doesn't return to straight because it's yielded!!!!


> having taken a new shape.

yes, "yielding"!!!


> This resistance is
> residual stress which appears as tension on the inside of the bend and
> compression on the outside.

no. see above.

>
> You'll find that jb learned about this on this forum just as he
> learned about fretting damage to bearings while fighting it all the
> way, denouncing every explanation with ridicule.

eh? would you be somehow "misremembering" our little episode on headset
bearings where you said they don't brinell? and "forgetting" that i,
fogel, and subsequently "chas" all proved the opposite?

>
>> I know that the residual stress, after it springs back, is tensile
>> on the inside.
>
> That's the one that counts and adds to the tensile and working load
> stress of a spoke. That is why spokes need stress relieving after the
> wheel is tensioned.

unproven suppositional rubbish. the process of "stress relief" simply
beds spokes in so they don't go slack in use. and that in turn can help
with fatigue. you neither invented it nor have proved any efficacy
outside of the above. and you certainly haven't managed to
differentiate from the effects of using materials /formulated and
proven/ to be more fatigue resistant.

 

[clare at snyder.on.ca]

All this talk about "stress relieving" spokes - I thought "stress
relieving" was a heat treatment. Dig out the Bernzo-Matic and heat 'em
up cherry red. Quench. Heat to (straw or whatever temper temperature)
and let air cool. NOW the spokes are stress relieved.
But better de-bur the holes (on both sides) and polish all the tooling
marks out of the spokes to elininate "stress rizers" too.
All standard aircraft building technique.

Na- easier to just make sure the spokes are tensined propery and the
wheel is overdesigned and underengineered like a German car.

(ducking and running for shelter)

--
Posted via a free Usenet account from http://www.teranews.com

 

[jim beam]

clare at snyder.on.ca wrote:
> All this talk about "stress relieving" spokes - I thought "stress
> relieving" was a heat treatment. Dig out the Bernzo-Matic and heat 'em
> up cherry red. Quench. Heat to (straw or whatever temper temperature)
> and let air cool. NOW the spokes are stress relieved.

they are, but they're also now softer and weaker - not strong enough for
their application. that's why manufacturers don't do this.


> But better de-bur the holes (on both sides) and polish all the tooling
> marks out of the spokes to elininate "stress rizers" too.

you could, but we're talking bikes - economics prevail. better to just
take reasonable steps on initial formation rather than rely on refinishing.


> All standard aircraft building technique.

indeed.


>
> Na- easier to just make sure the spokes are tensined propery and the
> wheel is overdesigned and underengineered like a German car.
>
> (ducking and running for shelter)

no need - the stupidity contestants will bleat themselves into a stupor
eventually. they're harmless.

 

[Ben C]

On 2007-09-07, [email protected] <[email protected]> wrote:
> Ben C? writes:
[...]
>> MP Since that location has tensile residual stress, tensile applied
>> MP mean stress from the spoke tension and bending,
>
>> Is the _applied_ stress on the inside of the elbow from spoke
>> tension and bending really tensile?
[...]
>> I don't understand that. I thought when you bent a wire you got tensile
>> stress on the outside of the bend and compressive on the inside?
>
> These loads tend to open the elbow angle so that causes tensile
> stress.

Just to recap, because I thought this was (roughly) the picture:

1. I put an outbound spoke in. Its natural elbow angle is a bit too wide.
2. I tighten it up, the elbow bends a bit, making the elbow angle
smaller. It wants to spring back, but it can't, because it's
installed in the wheel and held in place.
3. This leaves applied stress that's tensile on the outside of the elbow
and compressive on the inside.
4. Momentary overload and relaxation leaves a spoke with reduced
stresses.

Do I have this (fundamentally) wrong?

Perhaps the point is it's the other way around for an inbound spoke,
whose elbow gets opened a bit by being installed in the wheel.

> As Mike mentioned above, springback makes the stress reverse
> from that during forming.

Yes, I think I understand that part. That's residual stress from spoke
forming, not retained stress from wheel-building, as I understand it.
During wheelbuilding the spoke is not able to spring back, so an
outbound spoke remains in tensile stress on the outside and compressive
on the inside until you stress-relieve.

After stress-relieving, the stresses may be the other way round again,
but more importantly, reduced in magnitude.

It seems that residual stress from forming would be mitigated and/or
dwarfed in magnitude by retained applied stress from the build? So
perhaps residual stress from forming _is_ a red herring?

 

[Ben C]

On 2007-09-08, jim beam <[email protected]> wrote:
> [email protected] wrote:
[...]
>> As I mentioned, don't fly your kite on every breeze that comes along,
>> especially those sent aloft by jb.
>
> as opposed to red herrings by JB?

Please both of you calm down. I'm not about to fly any kites OR swallow
any herrings. You are both much better at explaining things when you
aren't blinded by paranoia or rage respectively.

 

[Ben C]

On 2007-09-08, jim beam <[email protected]> wrote:
> [email protected] wrote:
[...]
>> As I mentioned, don't fly your kite on every breeze that comes along,
>> especially those sent aloft by jb.
>
> as opposed to red herrings by JB?

Please both of you calm down. I'm not about to fly any kites OR swallow
any herrings. You are both much better at explaining things when you
aren't blinded by paranoia or rage respectively.

 

[Ben C]

On 2007-09-07, Peter Cole <[email protected]> wrote:
[...]
> If the spoke has no bending moment (perfectly supported, perfect path),
> the applied stress from spoke tension will be tensile (uniform) across
> the cross section.

I think I basically agree with this, though not absolutely. If the spoke
is perfectly supported, but being pulled around a corner, there will
surely still be a bending moment on parts of the spoke, but the distance
component of that moment will never be greater than half the diameter of
the spoke.

Moments that small (assuming the force is in the range of normal spoke
tensions) are low enough not to worry about-- they're not bringing
anything anywhere near dangerously high stress levels for fatigue.

> Whatever bending force that is also present will add
> to that. The bending force can be either way depending on angular
> mismatch. If the spoke elbow is too long, another bending force will act
> to open the spoke angle further, adding to the residual (mfg) stress.
>
> The worst case would be an (initial spoke) angle too acute with elbow
> too long. Both of those factors plus residual stress would all put
> tension on the inside of the elbow.

Yes, in that case residual stress from forming would be the same way
around as applied bending from spoke tension-- i.e. compressive outside
the bend, tensile inside.

> If the spoke elbows are the right length, and the spoke angle is
> corrected, the only significant stresses should be spoke tension and
> residual.

But if you correct the spoke angle, you're changing things right? It was
bent in the factory by a little piston or whatever, and wants to spring
back, leaving it with residual stresses. Then you go and bend it again
to make the elbow a bit bigger or smaller. What happens to the residual
stresses from the factory?

I imagine they reduce greatly in magnitude and move around a bit. I
imagine you'd have to hunt around quite a bit with the X-ray diffractor
to find them and distinguish them from the new applied stresses you've
just put in.

> By stress relieving, the residual is reduced to non-fatiguing levels.
> But, if the spoke has (tensile) stress levels near yield in parts of
> the cross section, those will be reduced as they are forced to yield
> by the momentary overload -- whatever the source. It's a "can't lose"
> proposition.

I would have thought so, yes, unless you really overdo it.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> Ben C wrote:
>>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>>> [...]
>>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>>> bothers to respond.
>>>>
>>>> http://tinyurl.com/29v4u2
>>>
>>> Thanks for the link, that's an interesting thread. Before my time.
>>>
>>> How much more civilized everyone was back then!
>>
>> Unfortunately, incivility seems to drive the civil away. Perhaps
>> that's the motive.
>>
>>> I might as well just
>>> read the archives and not bother with new RBT.
>>
>> There's a lot to learn in many NG archives, I often get info there
>> that I can't find with a web search -- on all kinds of topics.
>
> and yet, you seem to absorb so little!

I think the evidence (for who is not absorbing) is there for anyone who
wishes to take the trouble to find it. It's all in the archives.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> jim beam wrote:
>>
>>> why don't /you/? but i forget, you don't contribute. maybe that's a
>>> guilt thing since you're playing this game on your employer's time,
>>> not your own.
>>
>> That's none of your business.
>
> it's a damned good indicator of your fundamental dishonesty though.

That the best you got?

 

[jim beam]

Ben C wrote:
> On 2007-09-07, [email protected] <[email protected]> wrote:
>> Ben C? writes:
> [...]
>>> MP Since that location has tensile residual stress, tensile applied
>>> MP mean stress from the spoke tension and bending,
>>> Is the _applied_ stress on the inside of the elbow from spoke
>>> tension and bending really tensile?
> [...]
>>> I don't understand that. I thought when you bent a wire you got tensile
>>> stress on the outside of the bend and compressive on the inside?
>> These loads tend to open the elbow angle so that causes tensile
>> stress.
>
> Just to recap, because I thought this was (roughly) the picture:
>
> 1. I put an outbound spoke in. Its natural elbow angle is a bit too wide.
> 2. I tighten it up, the elbow bends a bit, making the elbow angle
> smaller. It wants to spring back, but it can't, because it's
> installed in the wheel and held in place.
> 3. This leaves applied stress that's tensile on the outside of the elbow
> and compressive on the inside.
> 4. Momentary overload and relaxation leaves a spoke with reduced
> stresses.
>
> Do I have this (fundamentally) wrong?
>
> Perhaps the point is it's the other way around for an inbound spoke,
> whose elbow gets opened a bit by being installed in the wheel.
>
>> As Mike mentioned above, springback makes the stress reverse
>> from that during forming.
>
> Yes, I think I understand that part. That's residual stress from spoke
> forming, not retained stress from wheel-building, as I understand it.
> During wheelbuilding the spoke is not able to spring back, so an
> outbound spoke remains in tensile stress on the outside and compressive
> on the inside until you stress-relieve.

only parts of it. read this from luns tee:
http://groups.google.com/group/rec.bicycles.tech/msg/af080b93a59cca03

most notably:
"For a more severely bent wire, the yielded layers extend deeper,
and the residual stress pattern becomes more like:

cccTCttt "

so here's the problem - that [simplified but useful] depiction shows
where the residual stress profiles would be. if residual stress were
causing fatigue, we would observe fatigue initiating at a "T" point.
instead, we observe it initiating at /both/ "c"'s and "t"'s.

"engineers" can argue all they want about what they think should be
happening, but if observed facts tell a different story, it's just so
much hot air.

>
> After stress-relieving, the stresses may be the other way round again,
> but more importantly, reduced in magnitude.
>
> It seems that residual stress from forming would be mitigated and/or
> dwarfed in magnitude by retained applied stress from the build? So
> perhaps residual stress from forming _is_ a red herring?

truth is, outside of the lab and in carefully controlled environments,
fatigue is *always* observed to initiate at surface defects. these can
be from processing, corrosion, or even inclusions within the material.
addressing each of these is observed to directly affect fatigue life.
among these, electron microscopy shows inclusion content to be a
significant fatigue initiator. removing inclusions is _proven_ to
extend fatigue life considerably.

that's why spoke manufacturers spend lots of money on expensive vacuum
degassed materials. if cheap materials could offer superior fatigue
life by way of simple stress relief, you'd better believe they'd be used.

 

[jim beam]

Ben C wrote:
> On 2007-09-07, Peter Cole <[email protected]> wrote:
> [...]
>> If the spoke has no bending moment (perfectly supported, perfect path),
>> the applied stress from spoke tension will be tensile (uniform) across
>> the cross section.
>
> I think I basically agree with this, though not absolutely. If the spoke
> is perfectly supported, but being pulled around a corner, there will
> surely still be a bending moment on parts of the spoke, but the distance
> component of that moment will never be greater than half the diameter of
> the spoke.
>
> Moments that small (assuming the force is in the range of normal spoke
> tensions) are low enough not to worry about-- they're not bringing
> anything anywhere near dangerously high stress levels for fatigue.

don't forget, virtually every fatigue failure there's ever been is
because of "unanticipated" factors. just because stress levels aren't
/thought/ to be high, *observed failures* tell us that there /is/
bending sufficient to cause fatigue!!!


>
>> Whatever bending force that is also present will add
>> to that. The bending force can be either way depending on angular
>> mismatch. If the spoke elbow is too long, another bending force will act
>> to open the spoke angle further, adding to the residual (mfg) stress.
>>
>> The worst case would be an (initial spoke) angle too acute with elbow
>> too long. Both of those factors plus residual stress would all put
>> tension on the inside of the elbow.
>
> Yes, in that case residual stress from forming would be the same way
> around as applied bending from spoke tension-- i.e. compressive outside
> the bend, tensile inside.

see other post about profiles.

>
>> If the spoke elbows are the right length, and the spoke angle is
>> corrected, the only significant stresses should be spoke tension and
>> residual.
>
> But if you correct the spoke angle, you're changing things right? It was
> bent in the factory by a little piston or whatever, and wants to spring
> back, leaving it with residual stresses. Then you go and bend it again
> to make the elbow a bit bigger or smaller. What happens to the residual
> stresses from the factory?
>
> I imagine they reduce greatly in magnitude and move around a bit. I
> imagine you'd have to hunt around quite a bit with the X-ray diffractor
> to find them and distinguish them from the new applied stresses you've
> just put in.

see other post on profiles. since residual stress profiles are observed
/not/ to be a factor in fatigue, further discussion is pointless -
unless applied stresses are discussed also.


>
>> By stress relieving, the residual is reduced to non-fatiguing levels.
>> But, if the spoke has (tensile) stress levels near yield in parts of
>> the cross section, those will be reduced as they are forced to yield
>> by the momentary overload -- whatever the source. It's a "can't lose"
>> proposition.
>
> I would have thought so, yes, unless you really overdo it.

 

[jim beam]

Ben C wrote:
> On 2007-09-08, jim beam <[email protected]> wrote:
>> [email protected] wrote:
> [...]
>>> As I mentioned, don't fly your kite on every breeze that comes along,
>>> especially those sent aloft by jb.
>> as opposed to red herrings by JB?
>
> Please both of you calm down. I'm not about to fly any kites OR swallow
> any herrings. You are both much better at explaining things when you
> aren't blinded by paranoia or rage respectively.

so what's the question?

 

[TomP]

I don't know that in your case "knowing" the reason why the spokes are
breaking is important.

They are breaking for what ever.

My philosophy is:
When the second spoke breaks in a wheel; it's time for new spokes. In other
words rebuild the wheel.

Just A User wrote:

> It happened again! I broke ANOTHER spoke on my road bike. This makes the
> second break in a month maybe a month and a half. Now I know I don't
> have the lightest riding style compared to some riders. And I am not the
> lightest of all riders. But then again I am riding on 32 triple cross
> wheels. What I don't understand is why am I breaking them on the front
> wheel only? I thought the back wheel carried more weight. So I have a
> few extra spokes I bought when I had the wheel at the lbs for the last
> spoke replacement. But now I am thinking that a new / better machine
> built wheel, or cough, a handbuilt wheels might be a more reliable way
> to go. When I say handbuilt, I mean with my hands, that have no
> experience building wheels. All opinions welcome.
>
> J.A.U.

--
Tp,

-------- __o
----- -\<. -------- __o
--- ( )/ ( ) ---- -\<.
-------------------- ( )/ ( )
-----------------------------------------

No Lawsuit Ever Fixed A Moron...

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>
>>>> why don't /you/? but i forget, you don't contribute. maybe that's
>>>> a guilt thing since you're playing this game on your employer's
>>>> time, not your own.
>>>
>>> That's none of your business.
>>
>> it's a damned good indicator of your fundamental dishonesty though.
>
> That the best you got?

there's more. but let's just focus on the facts.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> Ben C wrote:
>>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>>> [...]
>>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>>> bothers to respond.
>>>>
>>>> http://tinyurl.com/29v4u2
>>>
>>> OK I have a question.
>>>
>>> jim beam> i have ignored residual stress as a factor in these failures
>>> jim beam> because the majority of the fractures i've examined initiate
>>> jim beam> on the /inside/ of the spoke elbow bend, not the outside
>>> jim beam> [although i have examples of each]. residual stress in this
>>> jim beam> location is compressive so i'm just looking at the external
>>> jim beam> [+cyclic] load. Mike Prime> The inside of the spoke elbow
>>> will have TENSILE residual
>>> Mike Prime> stress, not compressive, because of the elastic springback
>>> Mike Prime> after bending. See below. That 0.5 Sy number is for a beam;
>>> Mike Prime> I'm too lazy to derive the number for a circular cross
>>> Mike Prime> section right now.
>>>
>>> Mike Prime> Since that location has tensile residual stress, tensile
>>> Mike Prime> applied mean stress from the spoke tension and bending,
>>> ^^^^^^^
>>>
>>> Is the _applied_ stress on the inside of the elbow from spoke tension
>>> and bending really tensile?
>>>
>>> I don't understand that. I thought when you bent a wire you got tensile
>>> stress on the outside of the bend and compressive on the inside?
>>>
>>> I know that the residual stress, after it springs back, is tensile on
>>> the inside.
>>
>> If the spoke has no bending moment (perfectly supported, perfect
>> path), the applied stress from spoke tension will be tensile (uniform)
>> across the cross section. Whatever bending force that is also present
>> will add to that. The bending force can be either way depending on
>> angular mismatch. If the spoke elbow is too long, another bending
>> force will act to open the spoke angle further, adding to the residual
>> (mfg) stress.
>
> but you're ignoring the actual residual stress profile. just alluding
> to residual stress being possible doesn't mean it's actually nucleating
> fatigue!

No, I'm not. You cited Luns in the context of stress profile, as far as
I could see he only confirmed Jobst's explanation and rejected yours.
I'm the only one on this NG (as far as I know) who actually tried to
measure residual stress, and found my results agreed with predictions:

http://tinyurl.com/356ru7


>
>>
>> The worst case would be an (initial spoke) angle too acute with elbow
>> too long. Both of those factors plus residual stress would all put
>> tension on the inside of the elbow.

>> If the spoke elbows are the right length, and the spoke angle is
>> corrected, the only significant stresses should be spoke tension and
>> residual.
>
> you're omitting bending. even if the spoke is resting against the hub
> flange, it can't protect against tensile stress on the inside of the
> elbow, only tensile stress on the /outside/, a place which, while
> fatigue does nucleate there from time to time, is not the commonest point.

I am not. You snipped it.

>
>> By stress relieving, the residual is reduced to non-fatiguing levels.
>
> presumptive straw clutching. plastic deformation subsequent to initial
> forming operations can /increase/ residual stress.

Of course, but stress relief doesn't cause bulk deformation. If wheel
building did, stress relief mitigates those residuals same as those from
manufacture.


>
>> But, if the spoke has (tensile) stress levels near yield in parts of
>> the cross section, those will be reduced as they are forced to yield
>> by the momentary overload
>
> and the regions of highest residual stress are on the /inside/ of the
> wire, not the inside or outside of the bend where fatigue nucleates.

See Luns Tee (again).


>> -- whatever the source. It's a "can't lose" proposition.
>
> eh? you need to read this:
> http://en.wikipedia.org/wiki/Fatigue_(material)#Miner.27s_rule
>
> particularly this:
> "In some circumstances, cycles of high stress followed by low stress
> cause more damage than would be predicted by the rule."
>
> sounds like our much-advocated "stress relief" scenario to me.

Sounds like you have no real understanding of Miner's rule.

Take the nominal -20% cyclical change in tension, add *1 cycle* of +50%
for stress relief and plug it into the formula.