Yet another broken spoke



Clare who? writes:

>>> And be SURE the spokes are tensioned adequately. Loose spokes break.
>>> Tight spokes don't (as a general rule of thumb)

>> I keep seeing this admonition yet no one seems to be able to describe
>> the mechanism that causes such spoke failures. Loosely spoked wheels
>> can allow the nipples to unscrew and cause wheel misalignment, but
>> spoke failure is caused by metal fatigue that arises from tension
>> change, caused once with every wheel rotation as spokes pass through
>> the zone of tire contact with the road. This must occur with stress
>> near the yield stress, something that does not readily occur in loose
>> spokes.

>> Please explain.

> A properly tensioned spoke is "pre stretched" and does not flex or
> stretch in operation. A loose spoke bends at least twice every
> revolution, and stretches repeatedly. This is what fatigues a spoke
> and breaks it.

Please explain what bends the spokes. Rim deflection is a few
thousandths of an inch, the spoke holes in the flange have 10 to 20
thousandths clearance and spoke nipples much more. Where is this
fatiguing compression force arising? Spokes do not go into column
buckling when slack.

I have a feeling the concept requires some exaggerated model of a
spoke that is less than properly tight. The bending of which you
speak cannot occur and cause fatiguing stress. To do that the bend
would need to approach yield (permanent deformation). To see what
that is, try how much of a bend it takes to put a bend in a spoke by
manually using it as a walking cane.

Jobst Brandt
 
In article <[email protected]>,
[email protected] writes:
> Clare who? writes:
>
>> And be SURE the spokes are tensioned adequately. Loose spokes break.
>> Tight spokes don't (as a general rule of thumb)
>
> I keep seeing this admonition yet no one seems to be able to describe
> the mechanism that causes such spoke failures. Loosely spoked wheels
> can allow the nipples to unscrew and cause wheel misalignment, but
> spoke failure is caused by metal fatigue that arises from tension
> change, caused once with every wheel rotation as spokes pass through
> the zone of tire contact with the road. This must occur with stress
> near the yield stress, something that does not readily occur in loose
> spokes.
>
> Please explain.

I can't.

But I /do/ know from empirical experience that once one
spoke breaks, its fellows are soon to follow.

I'm dealing with that right now, on my drive-side
rear wheel. Except once that first spoke broke, I
took the wheel out and swapped-in a less desirable
one, just to have some transportation, to keep my
favourite bike going. I've gotta buy 18 short spokes
pretty soon, because I want that wheel back. Hell, I
might as well respoke the whole kit-'n-kaboodle. Actually
I should buy a whole new wheel, as the rim is getting worn.
And I'm talking about a stress-relieved wheel that's put in
a lot of service, and has just ... had it.

Spokes are not immortal. Not even with featherweight riders.
Especially if they've got strong legs and lots of
vertical terrain to ride over.

Spokes get old, and just plain die.


cheers,
Tom

--
Nothing is safe from me.
I'm really at:
tkeats curlicue vcn dot bc dot ca
 
On 2007-09-03, [email protected] <[email protected]> wrote:
> Clare who? writes:
>
>>>> And be SURE the spokes are tensioned adequately. Loose spokes break.
>>>> Tight spokes don't (as a general rule of thumb)
>
>>> I keep seeing this admonition yet no one seems to be able to describe
>>> the mechanism that causes such spoke failures. Loosely spoked wheels
>>> can allow the nipples to unscrew and cause wheel misalignment, but
>>> spoke failure is caused by metal fatigue that arises from tension
>>> change, caused once with every wheel rotation as spokes pass through
>>> the zone of tire contact with the road. This must occur with stress
>>> near the yield stress, something that does not readily occur in loose
>>> spokes.
>
>>> Please explain.

I also thought that, and my reason was the same as Clare's. If a spoke
loses preload it might bend out of line resulting in enough moment to
produce yield stress (or close to it) at the elbow.

I admit I have absolutely no evidence that that's what happens, so I'm
glad you brought this up.

>> A properly tensioned spoke is "pre stretched" and does not flex or
>> stretch in operation. A loose spoke bends at least twice every
>> revolution, and stretches repeatedly. This is what fatigues a spoke
>> and breaks it.
>
> Please explain what bends the spokes. Rim deflection is a few
> thousandths of an inch,

Is that for a basically healthy wheel with one or two loose spokes or
for one where they are all so loose as to be losing their preload?

Or perhaps if they're all that loose the wheel will collapse anyway
making my question a red herring?
 
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.

has anybody yet bothered to ask the most important question? "what
brand are the spokes"???

this is the most important issue. you need to have good quality spokes
to resist fatigue - they're made from fatigue resistant material. if
your current spokes are some cheap no-name brand, i would re-spoke to
ensure reliability, especially as front wheels don't see as extreme
stress cycles as dished rears.
 
[email protected] wrote:
> Clare who? writes:
>
>>>> And be SURE the spokes are tensioned adequately. Loose spokes break.
>>>> Tight spokes don't (as a general rule of thumb)
>
>>> I keep seeing this admonition yet no one seems to be able to describe
>>> the mechanism that causes such spoke failures. Loosely spoked wheels
>>> can allow the nipples to unscrew and cause wheel misalignment, but
>>> spoke failure is caused by metal fatigue that arises from tension
>>> change, caused once with every wheel rotation as spokes pass through
>>> the zone of tire contact with the road. This must occur with stress
>>> near the yield stress, something that does not readily occur in loose
>>> spokes.
>
>>> Please explain.
>
>> A properly tensioned spoke is "pre stretched" and does not flex or
>> stretch in operation. A loose spoke bends at least twice every
>> revolution, and stretches repeatedly. This is what fatigues a spoke
>> and breaks it.
>
> Please explain what bends the spokes.

interleaving. the spoke that goes slack receives its bending from it's
interleaving neighbor.

and the fact that spokes fatigue at the elbow [as opposed to another
location] means that is where the bending is greatest - by definition.


> Rim deflection is a few
> thousandths of an inch, the spoke holes in the flange have 10 to 20
> thousandths clearance and spoke nipples much more. Where is this
> fatiguing compression force arising? Spokes do not go into column
> buckling when slack.

red herring.


>
> I have a feeling the concept requires some exaggerated model of a
> spoke that is less than properly tight. The bending of which you
> speak cannot occur and cause fatiguing stress.

simply untrue. you don't understand the principles of fatigue.


> To do that the bend
> would need to approach yield (permanent deformation).

er, fatigue happens well below yield - that's why it's called "fatigue"
and not "yield". you really need to study this subject before you can
credibly presume to lecture on it.


> To see what
> that is, try how much of a bend it takes to put a bend in a spoke by
> manually using it as a walking cane.

red herring. the elbow bends in tension, not just compression. the
very fact that the elbow is offset from the spoke axis ensures there is
a bending moment. again, that's by definition.
 
jim beam wrote:
> 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.
>
> has anybody yet bothered to ask the most important question? "what
> brand are the spokes"???
>
> this is the most important issue. you need to have good quality spokes
> to resist fatigue - they're made from fatigue resistant material. if
> your current spokes are some cheap no-name brand, i would re-spoke to
> ensure reliability, especially as front wheels don't see as extreme
> stress cycles as dished rears.
No I don't recall anyone asking the brand of the breaking spokes. I
assume they are no-name cheapos. They are in a set of Alex R500 rims
which seem to be the low end for that brand. They came on a entry level
Fuji Ace.
 
Just A User wrote:
> jim beam wrote:
>> 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.
>>
>> has anybody yet bothered to ask the most important question? "what
>> brand are the spokes"???
>>
>> this is the most important issue. you need to have good quality spokes
>> to resist fatigue - they're made from fatigue resistant material. if
>> your current spokes are some cheap no-name brand, i would re-spoke to
>> ensure reliability, especially as front wheels don't see as extreme
>> stress cycles as dished rears.
> No I don't recall anyone asking the brand of the breaking spokes. I
> assume they are no-name cheapos. They are in a set of Alex R500 rims
> which seem to be the low end for that brand. They came on a entry level
> Fuji Ace.

so there's your problem - and no amount of "stress relief" can fix that.
if you're interested in the idea of wheel building, now is your
opportunity to re-spoke with a quality brand and take care of it.
 
On 2007-09-03, jim beam <[email protected]> wrote:
> 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.
>
> has anybody yet bothered to ask the most important question? "what
> brand are the spokes"???

Actually I did suggest cheap and nasty spokes might be the problem.
 
Ben C wrote:
{jim beam asked}

>> has anybody yet bothered to ask the most important question? "what
>> brand are the spokes"???

> Actually I did suggest cheap and nasty spokes might be the problem.

Good date material, however.

Bill "slow morning" S.
 
On 2007-09-03, jim beam <[email protected]> wrote:
> [email protected] wrote:
[...]
>> I have a feeling the concept requires some exaggerated model of a
>> spoke that is less than properly tight. The bending of which you
>> speak cannot occur and cause fatiguing stress.
>
> simply untrue. you don't understand the principles of fatigue.
>
>
>> To do that the bend
>> would need to approach yield (permanent deformation).
>
> er, fatigue happens well below yield - that's why it's called "fatigue"
> and not "yield".

Yes, and as I think you've pointed out before, the steel typically used
for spokes doesn't have a fatigue limit, so there is always fatigue in
use.

The question is time to failure-- a few hundred miles or tens of
thousands.

Fatigue happens below yield, but premature fatigue happens close to
yield, I think is the point.

So the interesting question is, what does the stress cycle for a
too-loose spoke look like?

[...]
>> To see what
>> that is, try how much of a bend it takes to put a bend in a spoke by
>> manually using it as a walking cane.
>
> red herring. the elbow bends in tension, not just compression. the
> very fact that the elbow is offset from the spoke axis ensures there is
> a bending moment. again, that's by definition.

So you're saying that if a spoke goes slack enough to exceed its
preload, it will pretty much inevitably get at or close to yield stress
around the bend?

Jobst was suggesting I think that there's a bit of slop around the hub
hole and the rim hole to take up first, that's typically bigger than the
amount of rim deformation. Seems a reasonable argument. It all depends
on the amounts these things move by which we have some idea of from FEAs
and things.
 
Ben C wrote:
> On 2007-09-03, jim beam <[email protected]> wrote:
>> [email protected] wrote:
> [...]
>>> I have a feeling the concept requires some exaggerated model of a
>>> spoke that is less than properly tight. The bending of which you
>>> speak cannot occur and cause fatiguing stress.
>> simply untrue. you don't understand the principles of fatigue.
>>
>>
>>> To do that the bend
>>> would need to approach yield (permanent deformation).
>> er, fatigue happens well below yield - that's why it's called "fatigue"
>> and not "yield".
>
> Yes, and as I think you've pointed out before, the steel typically used
> for spokes doesn't have a fatigue limit, so there is always fatigue in
> use.
>
> The question is time to failure-- a few hundred miles or tens of
> thousands.
>
> Fatigue happens below yield, but premature fatigue happens close to
> yield, I think is the point.
>
> So the interesting question is, what does the stress cycle for a
> too-loose spoke look like?

that is indeed the question.

http://www.flickr.com/photos/38636024@N00/1313347532/

what's the elastic deviation here? a couple of degrees? what's the
stress delta between that and straight for the elbow?

>
> [...]
>>> To see what
>>> that is, try how much of a bend it takes to put a bend in a spoke by
>>> manually using it as a walking cane.
>> red herring. the elbow bends in tension, not just compression. the
>> very fact that the elbow is offset from the spoke axis ensures there is
>> a bending moment. again, that's by definition.
>
> So you're saying that if a spoke goes slack enough to exceed its
> preload, it will pretty much inevitably get at or close to yield stress
> around the bend?

not yield, but there will be a stress cycle, and it's cycling that
causes fatigue.

>
> Jobst was suggesting I think that there's a bit of slop around the hub
> hole and the rim hole to take up first, that's typically bigger than the
> amount of rim deformation.

rim deformation is a red herring - yes, the rim deforms, but we're not
worried about rim fatigue, we're worried about spoke fatigue, so let's
figure out what's going on there.


> Seems a reasonable argument. It all depends
> on the amounts these things move by which we have some idea of from FEAs
> and things.

right, but you don't need fea to calculate the stress rise for a bending
elbow. it's not a simple calculation, but the whole fea thing is
water-muddying.
 
On 2007-09-03, jim beam <[email protected]> wrote:
> Ben C wrote:
[...]
>> So the interesting question is, what does the stress cycle for a
>> too-loose spoke look like?
>
> that is indeed the question.
>
> http://www.flickr.com/photos/38636024@N00/1313347532/
>
> what's the elastic deviation here? a couple of degrees? what's the
> stress delta between that and straight for the elbow?

Good question, I don't know.

>> [...]
>>>> To see what
>>>> that is, try how much of a bend it takes to put a bend in a spoke by
>>>> manually using it as a walking cane.
>>> red herring. the elbow bends in tension, not just compression. the
>>> very fact that the elbow is offset from the spoke axis ensures there is
>>> a bending moment. again, that's by definition.
>>
>> So you're saying that if a spoke goes slack enough to exceed its
>> preload, it will pretty much inevitably get at or close to yield stress
>> around the bend?
>
> not yield, but there will be a stress cycle, and it's cycling that
> causes fatigue.

Yes but, at the risk of stating the obvious, the spoke has a stress
cycle anyway whenever you ride the bike. But the mean stress in that
cycle is low enough, or so you hope, that the spokes last a good long
time.

>> Jobst was suggesting I think that there's a bit of slop around the hub
>> hole and the rim hole to take up first, that's typically bigger than the
>> amount of rim deformation.
>
> rim deformation is a red herring - yes, the rim deforms, but we're not
> worried about rim fatigue, we're worried about spoke fatigue, so let's
> figure out what's going on there.

I was thinking that the rim deformation is what brings the threaded end
of the spoke closer to the hub and therefore allows it to bend.

>> Seems a reasonable argument. It all depends
>> on the amounts these things move by which we have some idea of from FEAs
>> and things.
>
> right, but you don't need fea to calculate the stress rise for a bending
> elbow. it's not a simple calculation, but the whole fea thing is
> water-muddying.

Unless one actually provides the FEA, yes. Otherwise it just is a way of
doing that calculation.
 
Ben C wrote:
> On 2007-09-03, jim beam <[email protected]> wrote:
>> Ben C wrote:
> [...]
>>> So the interesting question is, what does the stress cycle for a
>>> too-loose spoke look like?
>> that is indeed the question.
>>
>> http://www.flickr.com/photos/38636024@N00/1313347532/
>>
>> what's the elastic deviation here? a couple of degrees? what's the
>> stress delta between that and straight for the elbow?
>
> Good question, I don't know.
>
>>> [...]
>>>>> To see what
>>>>> that is, try how much of a bend it takes to put a bend in a spoke by
>>>>> manually using it as a walking cane.
>>>> red herring. the elbow bends in tension, not just compression. the
>>>> very fact that the elbow is offset from the spoke axis ensures there is
>>>> a bending moment. again, that's by definition.
>>> So you're saying that if a spoke goes slack enough to exceed its
>>> preload, it will pretty much inevitably get at or close to yield stress
>>> around the bend?
>> not yield, but there will be a stress cycle, and it's cycling that
>> causes fatigue.
>
> Yes but, at the risk of stating the obvious, the spoke has a stress
> cycle anyway whenever you ride the bike. But the mean stress in that
> cycle is low enough, or so you hope, that the spokes last a good long
> time.

the axial spoke tension is relatively low as a faction of yield, but
spoke /bending/ is unaddressed in consideration by the ancients. pretty
fundamental omission since the spoke is not axially loaded due to its
offset and fatigue evidence _proves_ bending to be the stress cycle of
interest.


>
>>> Jobst was suggesting I think that there's a bit of slop around the hub
>>> hole and the rim hole to take up first, that's typically bigger than the
>>> amount of rim deformation.
>> rim deformation is a red herring - yes, the rim deforms, but we're not
>> worried about rim fatigue, we're worried about spoke fatigue, so let's
>> figure out what's going on there.
>
> I was thinking that the rim deformation is what brings the threaded end
> of the spoke closer to the hub and therefore allows it to bend.

don't swallow the herring - you don't need to know that to get to the
point. what you /do/ need to know is that the spoke is

a. loaded, and
b. subject to bending.

then you can try to figure out by how much.


>
>>> Seems a reasonable argument. It all depends
>>> on the amounts these things move by which we have some idea of from FEAs
>>> and things.
>> right, but you don't need fea to calculate the stress rise for a bending
>> elbow. it's not a simple calculation, but the whole fea thing is
>> water-muddying.
>
> Unless one actually provides the FEA, yes. Otherwise it just is a way of
> doing that calculation.
 
In article
<[email protected]>,
daveornee
<[email protected]>
wrote:

> 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.
> Quality spokes and a quality build will help lengthen the life of the
> spokes.
> The time it takes for an experienced builder may be a good investment.
> It depends on you inclination, mechanical aptitude, budget, and how you
> would like to spend your time.
> Barnetts (the bicycle mechanic school) has dedicated a chapter to the
> subject, and Jobst Brandt has a whole book "The Bicycle Wheel".
> I like to build wheels, but many people think it is a waste of time and
> effort when you can just buy the finished product. You can often buy a
> completely built wheel for less than you can buy the same parts that
> make up the wheel.

Then if you buy the wheels with good parts, you can
lubricate the threads, retension, stress relieve, and
true. Also repack the bearings.

--
Michael Press
 
I wrote:

>>>> And be SURE the spokes are tensioned adequately. Loose spokes
>>>> break. Tight spokes don't (as a general rule of thumb)

>>> I keep seeing this admonition yet no one seems to be able to
>>> describe the mechanism that causes such spoke failures. Loosely
>>> spoked wheels can allow the nipples to unscrew and cause wheel
>>> misalignment, but spoke failure is caused by metal fatigue that
>>> arises from tension change, caused once with every wheel rotation
>>> as spokes pass through the zone of tire contact with the road.
>>> This must occur with stress near the yield stress, something that
>>> does not readily occur in loose spokes.

>>> Please explain.

>> A properly tensioned spoke is "pre stretched" and does not flex or
>> stretch in operation. A loose spoke bends at least twice every
>> revolution, and stretches repeatedly. This is what fatigues a
>> spoke and breaks it.

> Please explain what bends the spokes. Rim deflection is a few
> thousandths of an inch, the spoke holes in the flange have 10 to 20
> thousandths clearance and spoke nipples much more. Where is this
> fatiguing compression force arising? Spokes do not go into column
> buckling when slack.

> I have a feeling the concept requires some exaggerated model of a
> spoke that is less than properly tight. The bending of which you
> speak cannot occur and cause fatiguing stress. To do that the bend
> would need to approach yield (permanent deformation). To see what
> that is, try how much of a bend it takes to put a bend in a spoke by
> manually using it as a walking cane.

I should add that the subject of this thread is short term spoke
failure, not infinite life or long term fatigue. For short term spoke
failure, as this one seems to be from the description, low spoke
tension cannot be the cause and similarly for others that have raised
the specter of low tension. If the spokes are not reused or poor
quality spokes, stress concentrations that have not been relieved are
the cause of failure as has been discussed at length in this forum.

Waving the long term fatigue flag does not answer the question of how
low spoke tension causes spoke failure; a claim that appears in this
newsgroup often. I believe the example of rim deflection under riding
loads (which is the amount by which spoke preload is reduced) is the
appropriate parameter for stress change and it shows that compression
buckling is not possible in that respect as I pointed out.

Jobst Brandt
 
[email protected] wrote:
> I wrote:
>
>>>>> And be SURE the spokes are tensioned adequately. Loose spokes
>>>>> break. Tight spokes don't (as a general rule of thumb)
>
>>>> I keep seeing this admonition yet no one seems to be able to
>>>> describe the mechanism that causes such spoke failures. Loosely
>>>> spoked wheels can allow the nipples to unscrew and cause wheel
>>>> misalignment, but spoke failure is caused by metal fatigue that
>>>> arises from tension change, caused once with every wheel rotation
>>>> as spokes pass through the zone of tire contact with the road.
>>>> This must occur with stress near the yield stress, something that
>>>> does not readily occur in loose spokes.
>
>>>> Please explain.
>
>>> A properly tensioned spoke is "pre stretched" and does not flex or
>>> stretch in operation. A loose spoke bends at least twice every
>>> revolution, and stretches repeatedly. This is what fatigues a
>>> spoke and breaks it.
>
>> Please explain what bends the spokes. Rim deflection is a few
>> thousandths of an inch, the spoke holes in the flange have 10 to 20
>> thousandths clearance and spoke nipples much more. Where is this
>> fatiguing compression force arising? Spokes do not go into column
>> buckling when slack.
>
>> I have a feeling the concept requires some exaggerated model of a
>> spoke that is less than properly tight. The bending of which you
>> speak cannot occur and cause fatiguing stress. To do that the bend
>> would need to approach yield (permanent deformation). To see what
>> that is, try how much of a bend it takes to put a bend in a spoke by
>> manually using it as a walking cane.
>
> I should add that the subject of this thread is short term spoke
> failure, not infinite life or long term fatigue. For short term spoke
> failure, as this one seems to be from the description, low spoke
> tension cannot be the cause and similarly for others that have raised
> the specter of low tension. If the spokes are not reused or poor
> quality spokes, stress concentrations that have not been relieved

be careful with that usage - "stress concentrations" are what occur at
stress risers, e.g. surface defects. a stress concentration is not
"residual stress" which is what you're talking about.


> are
> the cause of failure as has been discussed at length in this forum.

no, they have been asserted at length by those who don't understand the
subject sufficiently and who have never bothered to examine a fracture
surface and correlate that with residual stress profiles.


>
> Waving the long term fatigue flag does not answer the question of how
> low spoke tension causes spoke failure; a claim that appears in this
> newsgroup often. I believe the example of rim deflection under riding
> loads (which is the amount by which spoke preload is reduced) is the
> appropriate parameter for stress change and it shows that compression
> buckling is not possible in that respect as I pointed out.
>
> Jobst Brandt
 
In article <[email protected]>,
[email protected] writes:
>
> Waving the long term fatigue flag does not answer the question of how
> low spoke tension causes spoke failure; a claim that appears in this
> newsgroup often. I believe the example of rim deflection under riding
> loads (which is the amount by which spoke preload is reduced) is the
> appropriate parameter for stress change and it shows that compression
> buckling is not possible in that respect as I pointed out.

So, would a newly-installed, brand-new spoke that's too loose,
immediately break due to rim deflection, or break before its
adjacent brethren (that have hitherto held-up so well) do?

What exactly /is/ rim deflection, and how does it happen?
What makes it happen? Are we talking about the momentary
bottom of the rim tending to squash flat on the surface it
runs on and thereby compressing spokes, (I guess not, since
you above rule-out compression buckling,) or are we talking
about putting lateral 'S' curves in the rim, and thereby
bending spokes? Or maybe it's about torque in a hard-driven
wheel, between the rim and hub, where the hub tends to rotate
faster than the rim can keep up with, and the spokes in-between
bear the brunt, and the rim is deflected from the POV of
the hub (and the connecting spokes?)

I don't know about these things, but I am curious,
and I'd truly like to understand.


cheers,
Tom


--
Nothing is safe from me.
I'm really at:
tkeats curlicue vcn dot bc dot ca
 
On 03 Sep 2007 05:36:32 GMT, [email protected] wrote:

>Clare who? writes:
>
>>>> And be SURE the spokes are tensioned adequately. Loose spokes break.
>>>> Tight spokes don't (as a general rule of thumb)
>
>>> I keep seeing this admonition yet no one seems to be able to describe
>>> the mechanism that causes such spoke failures. Loosely spoked wheels
>>> can allow the nipples to unscrew and cause wheel misalignment, but
>>> spoke failure is caused by metal fatigue that arises from tension
>>> change, caused once with every wheel rotation as spokes pass through
>>> the zone of tire contact with the road. This must occur with stress
>>> near the yield stress, something that does not readily occur in loose
>>> spokes.
>
>>> Please explain.
>
>> A properly tensioned spoke is "pre stretched" and does not flex or
>> stretch in operation. A loose spoke bends at least twice every
>> revolution, and stretches repeatedly. This is what fatigues a spoke
>> and breaks it.
>
>Please explain what bends the spokes. Rim deflection is a few
>thousandths of an inch, the spoke holes in the flange have 10 to 20
>thousandths clearance and spoke nipples much more. Where is this
>fatiguing compression force arising? Spokes do not go into column
>buckling when slack.
>

I used the wrong terminology - they do not so much bend as flex or
"stretch" but that stretch DOES allow the bend where the spoke goes
through the hub to actually fles, or bend - and THAT is where the
spoke usually breaks, isn't it????
>I have a feeling the concept requires some exaggerated model of a
>spoke that is less than properly tight. The bending of which you
>speak cannot occur and cause fatiguing stress. To do that the bend
>would need to approach yield (permanent deformation). To see what
>that is, try how much of a bend it takes to put a bend in a spoke by
>manually using it as a walking cane.
>
>Jobst Brandt
Does happen.

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

>> Waving the long term fatigue flag does not answer the question of
>> how low spoke tension causes spoke failure; a claim that appears in
>> this newsgroup often. I believe the example of rim deflection
>> under riding loads (which is the amount by which spoke preload is
>> reduced) is the appropriate parameter for stress change and it
>> shows that compression buckling is not possible in that respect as
>> I pointed out.

> So, would a newly-installed, brand-new spoke that's too loose,
> immediately break due to rim deflection, or break before its
> adjacent brethren (that have hitherto held-up so well) do?

If the spoke was initially not loose, as spokes are on a new wheel, It
would only be deemed too loose if load deflection of the rim at the
load affected zone (where the tire meets the road) is greater than
elastic elongation of spokes from tensioning. That is the problem
with many wheels these days where rims crack if tightened to a
reliable tension that will mot slacken under load. That is
exacerbated by using fewer spokes so that the preload of one or two
spokes alone support the load.

> What exactly /is/ rim deflection, and how does it happen? What
> makes it happen? Are we talking about the momentary bottom of the
> rim tending to squash flat on the surface it runs on and thereby
> compressing spokes, (I guess not, since you above rule-out
> compression buckling,) or are we talking about putting lateral 'S'
> curves in the rim, and thereby bending spokes? Or maybe it's about
> torque in a hard-driven wheel, between the rim and hub, where the
> hub tends to rotate faster than the rim can keep up with, and the
> spokes in-between bear the brunt, and the rim is deflected from the
> POV of the hub (and the connecting spokes?)

When a wheel bears a load applied to its axle, that force is
transmitted to the road n=by compressing the spokes in roughly the
tire contact patch so that they lose preload equivalent to the axle
load. All other spokes remain essentially unchanged in tension
(especially the top ones that have been believed to get tighter).

You can check this by plucking spokes with and without loading the
wheel and not the tone. A lower tone indicates lower tension, a
higher tone indicated higher tension then initially when the wheel was
not loaded.

> I don't know about these things, but I am curious, and I'd truly
> like to understand.

You can get a better picture of this in "the Bicycle Wheel" in which
this is described in detail with computed graphs of wheel deformation.
This subject comes up often enough that this book on the shelves of
most bicycle shops. It is also available from Amazon and ABE, among
others.

http://sheldonbrown.com/harris/books.html#brandt
http://www.amazon.com/exec/obidos/ISBN=0960723668/1361-7743389-379578
http://tinyurl.com/3d7a49

Jobst Brandt
 
Clare who? writes:

>>>>> And be SURE the spokes are tensioned adequately. Loose spokes
>>>>> break. Tight spokes don't (as a general rule of thumb)

>>>> I keep seeing this admonition yet no one seems to be able to
>>>> describe the mechanism that causes such spoke failures. Loosely
>>>> spoked wheels can allow the nipples to unscrew and cause wheel
>>>> misalignment, but spoke failure is caused by metal fatigue that
>>>> arises from tension change, caused once with every wheel rotation
>>>> as spokes pass through the zone of tire contact with the road.
>>>> This must occur with stress near the yield stress, something that
>>>> does not readily occur in loose spokes.

>>>> Please explain.

>>> A properly tensioned spoke is "pre stretched" and does not flex or
>>> stretch in operation. A loose spoke bends at least twice every
>>> revolution, and stretches repeatedly. This is what fatigues a
>>> spoke and breaks it.

>> Please explain what bends the spokes. Rim deflection is a few
>> thousandths of an inch, the spoke holes in the flange have 10 to 20
>> thousandths clearance and spoke nipples much more. Where is this
>> fatiguing compression force arising? Spokes do not go into column
>> buckling when slack.

> I used the wrong terminology - they do not so much bend as flex or
> "stretch" but that stretch DOES allow the bend where the spoke goes
> through the hub to actually flex, or bend - and THAT is where the
> spoke usually breaks, isn't it????

What you haven't explained is where force makes loose spokes break
more readily than properly tensioned ones. That the elbow is a place
of residual stress from manufacture and installation has been
discussed here often. The stress in the elbow of a looser spoke is
lower than in one that has more tension and this stress can only go to
zero if the spoke becomes slack. With a tighter spoke the stress
variation can have a larger excursion than that of a looser spoke. It
is cyclic stress changes that causes failure.

>> I have a feeling the concept requires some exaggerated model of a
>> spoke that is less than properly tight. The bending of which you
>> speak cannot occur and cause fatiguing stress. To do that the bend
>> would need to approach yield (permanent deformation). To see what
>> that is, try how much of a bend it takes to put a bend in a spoke
>> by manually using it as a walking cane.

Spoke failure in fewer than 10,000 miles, for instance is short term
fatigue and is caused by residual stress in stress concentration
locations such as threads and spoke elbows. These tresses must be
relieved after the wheel is tensioned.

Jobst Brandt
 
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