Yet another broken spoke

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> [email protected] wrote:
>>> Ben C? writes:
>>>
>>>>>>> Loose spokes also wear the holes in the hubs - and wear the
>>>>>>> spokes where they go through the hub.
>>>
>>>>>> I hadn't thought of that. That would explain the phenomenon
>>>>>> (loose spokes breaking) in a way that's consistent with my
>>>>>> understanding of Jobst's earlier suggestion that there is a bit of
>>>>>> clearance down there.
>>>
>>>>>> I thought the reasoning was: the spoke can't easily be bent since
>>>>>> it isn't firmly held in the hub hole but free to wobble a bit.
>>>>>> But if it's wobbling up and down it can wear and that can initiate
>>>>>> fatigue.
>>>
>>>>>>> I've replaced numerous spokes that were worn half way through
>>>>>>> before breaking (and some that had not yet broken)
>>>
>>>>>> Maybe this is something datakoll's practice of putting Teflon wax
>>>>>> in the hub holes before you put the spokes in could help with.
>>>
>>>>> he lives in a hot salty climate, so in his case, it's more likely
>>>>> that it mitigates corrosion or stress corrosion. sticky **** that
>>>>> retains grit sure isn't going to do a thing to mitigate wear.
>>>
>>>> Good point. IIRC he may also have reported this was a way to get
>>>> more life out of generic spokes. I don't know if those are the
>>>> galvanized kind, but also some kinds of Chinese "stainless" steel do
>>>> rust in little spots. I know because I've seen it happen to
>>>> teaspoons.
>>>
>>> You needn't fly your kite on every breeze that comes along. Remember,
>>> it's the jam nut on valve stems that caused stem separations about a
>>> year ago, information that was brought to this newsgroup by the same
>>> folks who say spokes break from low tension.
>>
>> specious **** - rubber usage has /nothing/ to do with metal fatigue.
>
> Does this mean you won't repeat your spew of everything (off topic) you
> disagree with when he posts?

eh?

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>> Peter Cole wrote:
>>>>> jim beam wrote:
>>>>>> [email protected] wrote:
>>>>>
>>>>>>> What is bending the elbow in your perception?
>>>>>>
>>>>>> simple loading!!! the spoke elbow is offset from the spoke axis,
>>>>>> thus is it subject to bending - by definition!!!
>>>>>
>>>>> If
>>>>
>>>> "if"??
>>>>
>>>>> the spoke elbow is fully supported on its inside radius it can't
>>>>> bend. By definition!!!!!!!!
>>>>
>>>> but it's not. so you're bullshitting.
>>>
>>> It is if you've built your wheel right.
>>>
>>>
>>>>>> except that it /is/ being bent back and forth more, simply because
>>>>>> it's interleaved.
>>>>>
>>>>> Do the math. How much force (tension) does it take to fully
>>>>> straighten a clothesline with a 5lb weight in the center?
>>>>>
>>>>
>>>> false example - the usual peter cole deceit.
>>>
>>> No, the interleaving force is in the middle of the spoke, like the
>>> clothesline. Consider the vectors.
>>
>> and we're discussing loosening and /angle/ increase. /you/ are trying
>> to deceive with /tension/ increase and straightening which can never
>> be achieved - as you well know.
>
> I don't know what you are discussing, I was discussing the change in
> angle with the change in tension -- it's immaterial which direction you
> want to reference.

wriggle. squirm. you cite a false example in a deliberate attempt to
mislead. you get called on it. now you start bleating. just address
the freakin' point originally asked and stop playing games.

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>> Peter Cole wrote:
>>>>> Ben C wrote:
>>>>>> On 2007-09-04, [email protected] <[email protected]> wrote:
>>>>>
>>>>>>> But the other spoke can only drop from 100 pounds of pre-tension
>>>>>>> down
>>>>>>> to 0. After it loses only 100 pounds of tension, it just rattles.
>>>>>>
>>>>>> I get it! Thanks.
>>>>>>
>>>>>> Of course whether it does rattle harmlessly or flex horribly, rapidly
>>>>>> fatiguing itself to death, is another matter.
>>>>>
>>>>> It could only "flex horribly" (or at all) if the spoke was bowed.
>>>>> Even in that case, you'd have to consider where the flex occurred
>>>>> vs where the spokes broke. The "flexing horribly" speculation also
>>>>> needs to consider the actual amount of rim deflection which bounds
>>>>> the degree of "horribleness".
>>>>>
>>>>> A worst case scenario would be where the spoke elbow angle did not
>>>>> match the angle of the spoke hole to flange. In that case,
>>>>> fluctuations in tension could cause elbow bending when the overall
>>>>> tension wasn't high enough to keep the spoke fully supported. To
>>>>> have that happen the angular mismatch would have had to survived
>>>>> the initial wheel tensioning and stress relief. If a wheel was
>>>>> built with low tension and not stress relieved, and a spoke
>>>>> subsequently became loose enough to lose support at the elbow, it
>>>>> might bend enough to fatigue rapidly, but I would consider this to
>>>>> be the consequence of a bad initial build rather than a loose spoke
>>>>> per se.
>>>>
>>>> wow! how to admit something you've previously denied, while
>>>> phrasing it as further denial!!! quite masterful.
>>>
>>> Only in your world. In the first paragraph, I was referring to the
>>> spoke bending along its whole length, the second only at the elbow --
>>> in case that wasn't clear.
>>>
>>> I think the burden is on you to explain how the spoke elbow is
>>> unsupported (or how it can bend if it isn't).
>>
>> er, the light gap between the hub and the spoke ought to be proof to
>> anyone whose intent is not to ******** and deceive...
>
> Why don't you compute the bending moment for that distance and compare
> its contribution to skin stress to that of the static and dynamic spoke
> tension and get back to us?

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.

 

[[email protected]]

Dear SSTW,

Aaargh! My memory is unreliable and I was wrong!

I apologize.

And I hope that I'll remember this embarrassing lesson not to trust my
memory.

I _did_ do test measurements of a 32-spoke front wheel, checking
tension with a Park gauge with the wheel in the air and then again
with it on the ground and an 8-pound load on the handlebar.

Here's the post:

http://groups.google.com/group/rec.bicycles.tech/msg/369d5902e39c7562

Assuming that my measurements were accurate, I found 5 roughly bottom
spokes losing tension, with spokes 32-01 at about 6'o'clock, the
bottom of the wheel:

bottom
tension
spoke change
kgf
32 -6
01 -18
02 -13
03 -12
04 -13

Six top spokes at about 12 'o'clock also lost tension (or showed 0
change), not expected:

top
tension
spoke change
kgf

17 -15
18 -13
19 -11
20 0
21 0
22 -15

Indeed, the lack of tension change continues around:

23 0
24 0
25 0 this spoke is roughly horizontal

I may try to repeat this test, since the results seem to be quite
different than everyone expects, since I flat forgot it, and since I
made a senile ass of myself insisting that I didn't even do it.

This particular wheel was a nicely true machine-made ma3 that I never
used.

Although true, its spoke tension varied from 52 to 85 kgf, which made
me to wonder if the irregular tension led to misleading results
because the higher tension spokes lost tension when theory and other
tests predicted that they would lose it.

The three spokes at 80, 80, and 85 kgf were the two at the top and one
at about 10 o'clock, and all three unexpectedly lost tension. No other
spokes were over 72 kgf.

Aaaargh!

Carl Fogel

 

[[email protected]]

Carl Fogel writes:

> Although true, its spoke tension varied from 52 to 85 kgf, which
> made me to wonder if the irregular tension led to misleading results
> because the higher tension spokes lost tension when theory and other
> tests predicted that they would lose it.

> The three spokes at 80, 80, and 85 kgf were the two at the top and
> one at about 10 o'clock, and all three unexpectedly lost tension.
> No other spokes were over 72 kgf.

This is a front wheel (I assume, if you are weighting the handlebars).
With that large a tension difference, your results will be misleading.
Try to balance tension before using this wheel as an example of which
spokes are affected by axle loading.

Although the highest tension appears to be above what might cause
misalignment, a loosely tensioned wheel with helter-skelter tension can
be straight because the rim was straight to begin with and tension is
insufficient to distort it. On the other hand, if widely varying
tension is fairly high, tight and loose spokes can be balanced against
one another but the wheel might reveal that in not being round.

Let's start with a good wheel.

Jobst Brandt

 

[jim beam]

[email protected] wrote:
> Carl Fogel writes:
>
>> Although true, its spoke tension varied from 52 to 85 kgf, which
>> made me to wonder if the irregular tension led to misleading results
>> because the higher tension spokes lost tension when theory and other
>> tests predicted that they would lose it.
>
>> The three spokes at 80, 80, and 85 kgf were the two at the top and
>> one at about 10 o'clock, and all three unexpectedly lost tension.
>> No other spokes were over 72 kgf.
>
> This is a front wheel (I assume, if you are weighting the handlebars).
> With that large a tension difference, your results will be misleading.
> Try to balance tension before using this wheel as an example of which
> spokes are affected by axle loading.
>
> Although the highest tension appears to be above what might cause
> misalignment, a loosely tensioned wheel with helter-skelter tension can
> be straight because the rim was straight to begin with and tension is
> insufficient to distort it. On the other hand, if widely varying
> tension is fairly high, tight and loose spokes can be balanced against
> one another but the wheel might reveal that in not being round.
>
> Let's start with a good wheel.
>

since tension does not affect elasticity, why would you expect a
different tension delta if the static tensions were different?

 

[[email protected]]

On Thu, 06 Sep 2007 22:05:22 -0600, [email protected] wrote:

>Dear SSTW,
>
>Aaargh! My memory is unreliable and I was wrong!
>
>I apologize.
>
>And I hope that I'll remember this embarrassing lesson not to trust my
>memory.
>
>I _did_ do test measurements of a 32-spoke front wheel, checking
>tension with a Park gauge with the wheel in the air and then again
>with it on the ground and an 8-pound load on the handlebar.
>
>Here's the post:
>
>http://groups.google.com/group/rec.bicycles.tech/msg/369d5902e39c7562
>
>Assuming that my measurements were accurate, I found 5 roughly bottom
>spokes losing tension, with spokes 32-01 at about 6'o'clock, the
>bottom of the wheel:
>
>bottom
> tension
>spoke change
> kgf
>32 -6
>01 -18
>02 -13
>03 -12
>04 -13
>
>Six top spokes at about 12 'o'clock also lost tension (or showed 0
>change), not expected:
>
>top
> tension
>spoke change
> kgf
>
>17 -15
>18 -13
>19 -11
>20 0
>21 0
>22 -15
>
>Indeed, the lack of tension change continues around:
>
>23 0
>24 0
>25 0 this spoke is roughly horizontal
>
>I may try to repeat this test, since the results seem to be quite
>different than everyone expects, since I flat forgot it, and since I
>made a senile ass of myself insisting that I didn't even do it.
>
>This particular wheel was a nicely true machine-made ma3 that I never
>used.
>
>Although true, its spoke tension varied from 52 to 85 kgf, which made
>me to wonder if the irregular tension led to misleading results
>because the higher tension spokes lost tension when theory and other
>tests predicted that they would lose it.
>
>The three spokes at 80, 80, and 85 kgf were the two at the top and one
>at about 10 o'clock, and all three unexpectedly lost tension. No other
>spokes were over 72 kgf.
>
>Aaaargh!
>
>Carl Fogel

Aaargh!

Tension varied from _59_ to 85 kgf, not 52--another careless error on
my part.

CF

 

[[email protected]]

On Sep 6, 9:23 pm, [email protected] wrote:
> On Thu, 06 Sep 2007 18:15:00 -0700, [email protected]
> wrote:
>
> >On Sep 4, 8:24 pm, [email protected] wrote:
>
> >> The spokes lose huge amounts of pre-tension as they xxrollxx ROTATE under the
> >> xxwheelxx HUB. 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.
>
> >And in your testing, as well as
> >everyone else's, the greatest xxlossxx GAIN of tension was in the spokes
> >perpendicular to the spokes that lost tension.
>
> I have never posted any test of tension loss as spokes roll under the
> wheel.

Thanks for proofreading my post.

 

[[email protected]]

On Sep 6, 11:28 pm, [email protected] wrote:
> Carl Fogel writes:
> > Although true, its spoke tension varied from 52 to 85 kgf, which
> > made me to wonder if the irregular tension led to misleading results
> > because the higher tension spokes lost tension when theory and other
> > tests predicted that they would lose it.
> > The three spokes at 80, 80, and 85 kgf were the two at the top and
> > one at about 10 o'clock, and all three unexpectedly lost tension.
> > No other spokes were over 72 kgf.
>
> This is a front wheel (I assume, if you are weighting the handlebars).
> With that large a tension difference, your results will be misleading.
> Try to balance tension before using this wheel as an example of which
> spokes are affected by axle loading.
>
> Although the highest tension appears to be above what might cause
> misalignment, a loosely tensioned wheel with helter-skelter tension can
> be straight because the rim was straight to begin with and tension is
> insufficient to distort it. On the other hand, if widely varying
> tension is fairly high, tight and loose spokes can be balanced against
> one another but the wheel might reveal that in not being round.
>
> Let's start with a good wheel.

Just do the test you originally suggested: push down on a wheel while
you are plucking the horizontal spokes: the tone will rise
significantly. I suspect that the effect will become smaller on a more
highly tensioned wheel.

This is why Beam was able to show a wheel with the more vertical
spokes removed supporting a lot of weight- the more horizontal spokes
support the load on the wheel.

 

[jim beam]

[email protected] wrote:
> On Sep 6, 11:28 pm, [email protected] wrote:
>> Carl Fogel writes:
>>> Although true, its spoke tension varied from 52 to 85 kgf, which
>>> made me to wonder if the irregular tension led to misleading results
>>> because the higher tension spokes lost tension when theory and other
>>> tests predicted that they would lose it.
>>> The three spokes at 80, 80, and 85 kgf were the two at the top and
>>> one at about 10 o'clock, and all three unexpectedly lost tension.
>>> No other spokes were over 72 kgf.
>> This is a front wheel (I assume, if you are weighting the handlebars).
>> With that large a tension difference, your results will be misleading.
>> Try to balance tension before using this wheel as an example of which
>> spokes are affected by axle loading.
>>
>> Although the highest tension appears to be above what might cause
>> misalignment, a loosely tensioned wheel with helter-skelter tension can
>> be straight because the rim was straight to begin with and tension is
>> insufficient to distort it. On the other hand, if widely varying
>> tension is fairly high, tight and loose spokes can be balanced against
>> one another but the wheel might reveal that in not being round.
>>
>> Let's start with a good wheel.
>
> Just do the test you originally suggested: push down on a wheel while
> you are plucking the horizontal spokes: the tone will rise
> significantly. I suspect that the effect will become smaller on a more
> highly tensioned wheel.

it can't - materials do not become stiffer as tension is increased. the
tone will be different as a function of tension, but that's an harmonic
thing, not a strength of structures thing.

>
> This is why Beam was able to show a wheel with the more vertical
> spokes removed supporting a lot of weight- the more horizontal spokes
> support the load on the wheel.
>

 

[Michael Press]

In article
<[email protected]>
,
[email protected] wrote:

> On Sep 4, 8:24 pm, [email protected] wrote:
>
> > 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.



> 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.

--
Michael Press

 

[Peter Cole]

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

 

[Peter Cole]

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.

 

[[email protected]]

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.

Jobst Brandt

 

[Ben C]

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! I might as well just
read the archives and not bother with new RBT.

 

[Ben C]

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.

 

[[email protected]]

On Fri, 07 Sep 2007 15:30:08 -0500, Ben C <[email protected]> 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.

Dear Ben,

Possibly things change from bending to pulling?

That is, if you clamp a spoke in a vise and push it sideways with your
hand, you get the tensile stress on the outside of the curve and the
compressive on the inside of the curve when the metal yields.

If you then pull hard enough on the end of the bent wire, you start to
change the inside curve from compressive, to neutral, to tensile.

Just to complicate things, the bend itself narrows--check a spoke
elbow with a micrometer.

For such reliable and apparently simple things, wire-spoke wheels are
awfully tricky.

Right now, I'm getting ready for some high-tech spoke-tension testing
of helpless front wheels lashed into my Mark II testing rig:

http://i9.tinypic.com/4p97ur7.jpg

SSTW has reminded me that the spokes in the Mark I test failed to
behave as expected and predicted. Putting up a picture of the new and
improved test rig will goad me to go through the tedious business of
measuring and re-measuring spoke tension on umpteen spokes on several
wheels.

I suspect that the stupid spokes on real wheels with inflated tires
will stubbornly refuse to lose tension on only the bottom spokes,
defying the predictions of theory and single-spoke rolling tests.

If so, I'll wonder if the expected less-than-even tension or the
cross-3 lacing is the cause.

Cheers,

Carl Fogel

 

[[email protected]]

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. 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.

> 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. 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.

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.
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).

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. The
reason it doesn't spring all the way back to straight is that the
outer "fibers" resist, having taken a new shape. This resistance is
residual stress which appears as tension on the inside of the bend and
compression on the outside.

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.

> 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.

Jobst Brandt

 

[Peter Cole]

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

 

[Peter Cole]

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.

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. 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.