On the virtue of traditional wheels when racing Paris-Roubaix

[Peter Cole]

Ben C wrote:
> On 2008-04-17, [email protected] <[email protected]> wrote:

>> It's exactly because the wheel with the slack spoke is less stiff that
>> an incremental increase in load gets the rim closer to yield strain
>> than a wheel without a slack spoke would be.
>
> I don't think that's right. Never mind strain, just consider yield
> stress.

He is right.


> When the spokes are slack, the structure as a whole is less stiff. But
> by definition the rim yields when the total stress on the rim reaches
> its yield stress. The more stress already on it from the spokes the less
> additional applied stress you need to bring it to yield.


It is less confusing to think of yield strain and consider the
stress/strain response of the wheel as a structure with and without
loaded spokes.

Spoke stress on the rim is primarily compression across the cross
section (rim). Bending stress (wheel radial load) is compression on the
outer surface, tension on the inner.

Since the rim is pre-loaded with compression (spoke tension), that
compression must be added to the skin compression (outer) caused by a
bending load, if you do the math, you find you still come out ahead
(stronger for radial loads) with higher spoke tension.

> But the other side to the story is you also have to consider the wheel
> as a structure, and whether there's any way in which it "collapses"--
> i.e. fails as a structure before any of the components in it actually
> yield, a bit like a tent folding up in a strong wind. In that case
> higher spoke tension may mean it collapses at a higher applied load.

Taco failure is beam buckling. The higher the rim compression (spoke
tension) the less additional lateral load or compressive load is
required to buckle. Taken to the limit, a wheel will spontaneously
buckle just from spoke tension.

In normal use, lateral and compressive wheel loads are small, so the
increase in radial load capacity given by higher spoke tension is more
relevant than the loss in buckling resistance. The other potential
limiting factor is fatigue at the spoke bed, which is accelerated by
increased spoke tension.

The ideal spoke tension is reached by increasing to the maximum that
still leaves sufficient buckling resistance to normal cycling loads --
you may argue what is "normal". Some rims may not reach that limit
because of relatively weak spoke beds, that's a design parameter, and
the manufacturer's spec must be observed.

As Jobst says, the strongest wheel is made by using the highest spoke
tension the rim can bear. That translates into buckling or spoke bed
fatigue, whichever comes first. Buckling can be experimentally
determined by carefully increasing tension until the wheel starts to
deform into a saddle shape. There is no experimental way to know the
tolerable spoke bed fatigue tension, so the only recourse is to use the
manufacturer's maximum. I can't see any reason for using less than that
maximum, doing so will make a wheel with less radial load capacity.

In practical terms, more spoke tension doesn't make the wheel stiffer,
it just increases the load point where the wheel undergoes a stiffness
transition. Beyond that point, the lower stiffness causes relatively
more strain with load until the rim is permanently bent.

 

[jim beam]

[email protected] wrote:
> On Apr 17, 6:04 pm, Ben C <[email protected]> wrote:
>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>
>>
>>
>>> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>>> [...]
>>>>> There are all kinds of messy things going on when you bring a wheel up
>>>>> to tension. A better example is how much you need to unscrew the
>>>>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
>>>>> much. The wheel may continue to deform past spoke slacking, but it's
>>>>> essentially failed.
>>>> Note however that the strength of the wheel is just the same when the
>>>> spokes are slack as when they are tight, it's just less stiff. If the
>>>> spoke tension is excessively high, it actually reduces the strength of
>>>> the wheel (in that case the rim would yield before the spokes went
>>>> slack).
>>>> The reduced stiffness when the spokes go slack means the rim will deform
>>>> more for a given load and that extra deformation _may_ cause it get out
>>>> of shape and buckle. But spokes can go slack and wheels not fail.
>>> You can't really decouple strength and stiffness that way.
>> But strength and stiffness _are_ different things, I'm not decoupling
>> them. They're already decoupled.
>>
>> Strength is yield stress, stiffness is strain per unit stress.
>>
>>> It's exactly because the wheel with the slack spoke is less stiff that
>>> an incremental increase in load gets the rim closer to yield strain
>>> than a wheel without a slack spoke would be.
>> I don't think that's right. Never mind strain, just consider yield
>> stress.
>>
>> When the spokes are slack, the structure as a whole is less stiff. But
>> by definition the rim yields when the total stress on the rim reaches
>> its yield stress. The more stress already on it from the spokes the less
>> additional applied stress you need to bring it to yield.
>>
>> But the other side to the story is you also have to consider the wheel
>> as a structure, and whether there's any way in which it "collapses"--
>> i.e. fails as a structure before any of the components in it actually
>> yield, a bit like a tent folding up in a strong wind. In that case
>> higher spoke tension may mean it collapses at a higher applied load.
>>
>> Peter Cole explains the structure well here:
>>
>> http://groups.google.co.uk/group/rec.bicycles.tech/msg/444e4c7184eef863
>>
>> There are explanations elsewhere in that thread about how increasing
>> spoke tension "borrows" compressive strength from the rim as jim beam
>> puts it.
>>
>> Which failure mode is significant when you get a buckle or a flat spot:
>> the materials yielding, or the wheel collapsing? I don't know and I
>> don't think it's an easy one to call.
>>
>> Having collapsed in the structural sense bits of the assembly may then
>> yield because the geometry has changed and you may get more leverage on
>> parts of the structure that you wouldn't have had before. So post mortem
>> demonstration of yielded parts doesn't prove the failure actually
>> started with the components yielding rather than with the structure
>> collapsing.
>
> You can think about it in terms of stress if you want to. Yield
> strain and yield stress are interchangeable, and are related by
> modulus.

that's confusing and ugly terminology. "interchangeable" is definitely
not the word to use.


> If a rim is able to deflect more under a given applied load
> because of a slack spoke (altered boundary conditions), then the
> stress in the rim must be going up simply by virtue of the fact that
> it's deflecting more. The incremental load thus brings that rim
> closer to failure than a rim that is properly supported by the spokes.

except that at higher spoke tension the rim is closer to compression
buckling. a beam in 3-point loading can support whatever load causes
material loading below [tensile or compressive] yield. but if the beam
is longitudinally compressed /and/ 3-point loaded, it cannot support as
much because the compression loading available on the upper part of the
beam is decreased by the compressive pre-load.


>
> The link you provided does a very good job of explaining how the wheel
> works as a structure. It is meant to operate with all of its
> components intact. If you take a structural support out of play (de-
> tension a spoke) the structure becomes both more compliant and
> weaker. Thus, strength and stiffness are in this case coupled because
> the function of the spokes lends both properties to the function of
> the wheel as a whole.

but the limit is the strength of the rim material, however is it loaded,
either with spoke tension [and thus circumferential compression] or in
bending [road load]. as the circumferential load is cranked up, the
road load that can be supported decreases. an unsupported [unspoked]
open pro rim can support my full 210lb body weight without buckling, so
this image of a rim being a fragile flower that can't bridge the gap of
one or two slack spokes is somewhat over simplistic.

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2008-04-17, [email protected] <[email protected]> wrote:
>
>>> It's exactly because the wheel with the slack spoke is less stiff that
>>> an incremental increase in load gets the rim closer to yield strain
>>> than a wheel without a slack spoke would be.
>>
>> I don't think that's right. Never mind strain, just consider yield
>> stress.
>
> He is right.
>
>
>> When the spokes are slack, the structure as a whole is less stiff. But
>> by definition the rim yields when the total stress on the rim reaches
>> its yield stress. The more stress already on it from the spokes the less
>> additional applied stress you need to bring it to yield.
>
>
> It is less confusing to think of yield strain and consider the
> stress/strain response of the wheel as a structure with and without
> loaded spokes.
>
> Spoke stress on the rim is primarily compression across the cross
> section (rim). Bending stress (wheel radial load) is compression on the
> outer surface, tension on the inner.
>
> Since the rim is pre-loaded with compression (spoke tension), that
> compression must be added to the skin compression (outer) caused by a
> bending load, if you do the math, you find you still come out ahead
> (stronger for radial loads) with higher spoke tension.
>
>> But the other side to the story is you also have to consider the wheel
>> as a structure, and whether there's any way in which it "collapses"--
>> i.e. fails as a structure before any of the components in it actually
>> yield, a bit like a tent folding up in a strong wind. In that case
>> higher spoke tension may mean it collapses at a higher applied load.
>
> Taco failure is beam buckling. The higher the rim compression (spoke
> tension) the less additional lateral load or compressive load is
> required to buckle. Taken to the limit, a wheel will spontaneously
> buckle just from spoke tension.
>
> In normal use, lateral and compressive wheel loads are small, so the
> increase in radial load capacity given by higher spoke tension is more
> relevant than the loss in buckling resistance. The other potential
> limiting factor is fatigue at the spoke bed, which is accelerated by
> increased spoke tension.
>
> The ideal spoke tension is reached by increasing to the maximum that
> still leaves sufficient buckling resistance to normal cycling loads --
> you may argue what is "normal". Some rims may not reach that limit
> because of relatively weak spoke beds, that's a design parameter, and
> the manufacturer's spec must be observed.
>
> As Jobst says, the strongest wheel is made by using the highest spoke
> tension the rim can bear. That translates into buckling or spoke bed
> fatigue, whichever comes first.

except that jobst doesn't address rim cracking in any way as a problem
with spoke tension, simply some underinformed and misdiagnosed rubbish
about it being the fault of anodizing. post-facto re-definition of
"what jobst really meant to say" needs to include, er, "encouragement"
to get the facts correct both in "the book" and the faq's.


> Buckling can be experimentally
> determined by carefully increasing tension until the wheel starts to
> deform into a saddle shape. There is no experimental way to know the
> tolerable spoke bed fatigue tension, so the only recourse is to use the
> manufacturer's maximum.

indeed. and to state anything to the effect that spoke tension can be
determined by the user, as is supposed by jobst, is a signal failure to
understand the principles involved.


> I can't see any reason for using less than that
> maximum, doing so will make a wheel with less radial load capacity.

except that the dish of a rear wheel is not set in stone - it varies
with different hubs. the more extreme the dish, the more spoke tension
can rise on lateral loading, and thus, the higher spoke tension can come
to the cracking limit, even though static load may appear to be ok.


>
> In practical terms, more spoke tension doesn't make the wheel stiffer,
> it just increases the load point where the wheel undergoes a stiffness
> transition. Beyond that point, the lower stiffness causes relatively
> more strain with load until the rim is permanently bent.

a wheel can be ridden with completely slack spokes. to repeat the
jobstian myth that rims are so weak that they collapse without full
spoke tension is simply ignorance.

[having said that however, there are very good reasons to ride with
tensioned spokes - spoke nipple unscrewing and spoke elbow fatigue being
two practical ones. but again, a simple practical experiment that you
can do at home puts jobstian tension myths to rest.]

 

[Tom Sherman]

Peter Cole wrote:
> [...]
> As Jobst says, the strongest wheel is made by using the highest spoke
> tension the rim can bear. That translates into buckling or spoke bed
> fatigue, whichever comes first. Buckling can be experimentally
> determined by carefully increasing tension until the wheel starts to
> deform into a saddle shape. There is no experimental way to know the
> tolerable spoke bed fatigue tension,[...]

One could get multiple rims of the same model, spoke them to different
tensions, and cyclically load them until fatigue cracking occurs in the
spoke bed. Of course, this is not a very practical solution.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

 

[jim beam]

Tom Sherman wrote:
> Peter Cole wrote:
>> [...]
>> As Jobst says, the strongest wheel is made by using the highest spoke
>> tension the rim can bear. That translates into buckling or spoke bed
>> fatigue, whichever comes first. Buckling can be experimentally
>> determined by carefully increasing tension until the wheel starts to
>> deform into a saddle shape. There is no experimental way to know the
>> tolerable spoke bed fatigue tension,[...]
>
> One could get multiple rims of the same model, spoke them to different
> tensions, and cyclically load them until fatigue cracking occurs in the
> spoke bed. Of course, this is not a very practical solution.
>

but one might be forgiven for thinking it important to undertake if one
wanted to test a theory before publication. it beats misattributing a
failure mechanism to the wrong cause, anodizing, at any rate.

 

[jim beam]

[email protected] wrote:
> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
>> On 2008-04-17, [email protected] <[email protected]> wrote:
>> [...]
>>
>>> There are all kinds of messy things going on when you bring a wheel up
>>> to tension. A better example is how much you need to unscrew the
>>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
>>> much. The wheel may continue to deform past spoke slacking, but it's
>>> essentially failed.
>> Note however that the strength of the wheel is just the same when the
>> spokes are slack as when they are tight, it's just less stiff. If the
>> spoke tension is excessively high, it actually reduces the strength of
>> the wheel (in that case the rim would yield before the spokes went
>> slack).
>>
>> The reduced stiffness when the spokes go slack means the rim will deform
>> more for a given load and that extra deformation _may_ cause it get out
>> of shape and buckle. But spokes can go slack and wheels not fail.
>
> You can't really decouple strength and stiffness that way. It's
> exactly because the wheel with the slack spoke is less stiff that an
> incremental increase in load gets the rim closer to yield strain than
> a wheel without a slack spoke would be.

how so? how does the rim yield?


> So the strength of the wheel
> is not the same. Fatigue failure from overtensioned spokes is a
> different issue all together and has been beaten to death elsewhere.

spokes don't usually fail from over-tension, they fail from bending
fatigue at the elbow if they're too slack.


>
> Yes, spokes can go slack without catastrophic wheel failure. You can
> also knock quite a few holes in an aircraft fuselage without crashing
> it.

if it's un-pressurized, but not if it is.


> Once you're outside of design conditions you're bumping up
> against the edge of, "how much farther until it collapses?", and the
> real world is far too variable to consider that zone to be safe
> operating conditions.


so which is stronger in tension - a rope with a load on it, or the same
rope without?

 

[jim beam]

Ryan Cousineau wrote:
> In article <[email protected]>,
> Tim McNamara <[email protected]> wrote:
>
>> In article <[email protected]>,
>> Hobbes@spnb&s.com wrote:
>>
>>> On Tue, 15 Apr 2008 22:13:21 -0500, Tim McNamara
>>> <[email protected]> wrote:
>>>
>>>> In article <[email protected]>,
>>>> Hobbes@spnb&s.com wrote:
>>>>
>>>>> On Tue, 15 Apr 2008 14:27:28 -0500, Tim McNamara
>>>>> <[email protected]> wrote:
>>>>>
>>>>>> In article <[email protected]>,
>>>>>> Hobbes@spnb&s.com wrote:
>
> [special traditional-type wheels for P-R]
>
>>> So they waste days of mechanic time and inventory space and many
>>> hundreds of dollars on special wheels?
>> Wheels that they probably already have, because they don't usually use
>> the high-bling carbon wheels for training. They just ride their normal
>> training wheels in this particular race. And the teams that normally
>> ride clinchers ride tubulars for this one for the pinch flat reduction.
>
> That's simply not correct. The stuff that gets broken out for P-R is
> often not only lower-profile stuff than they generally use for training,
> it's often wheel sets using parts that are no longer even available new:
>
> <http://www.cyclingnews.com/road/2008/apr08/wevelgem08/tech.php?id=/photo
> s/2008/tech/features/wevelgem_tech208/Caisse_dEpargne_Campy_rim>
>
> <http://www.cyclingnews.com/road/2008/apr08/rvv08/tech.php?id=/photos/200
> 8/tech/features/flanders_tech108/Saunier_Duval-Scott_rims2>
>
> Here's a team caught by a canny tech writer putting their sponsor's
> stickers on someone else's rim:
>
> <http://www.cyclingnews.com/road/2008/apr08/rvv08/tech.php?id=/photos/200
> 8/tech/features/flanders_tech108/Quick-Step_rims>
>
> Oh, and for those wondering at the logic behind P-R wheel choices, I
> present this photo:
>
> <http://www.cyclingnews.com/road/2008/apr08/roubaix08/tech.php?id=/photos
> /2008/tech/features/paris_roubaix08/Agritubel_Kuota_KOM_Berges_tied>

that's a limited value exercise if the solder doesn't wet the spokes -
as is the case there - their relative movement is not arrested. it does
help with spoke breakage not fouling a wheel, but is that much of a
problem these days? to get wetted spokes, use silver [or deoxidized]
spokes and the correct flux.


>
> Note that several of those photos are from other classics leading up to
> P-R. In some cases, the races are a test-bed for P-R equipment, or may
> have equipment choices completely unrelated to the team's P-R choices.
>

 

[Ben C]

On 2008-04-18, [email protected] <[email protected]> wrote:
> On Apr 17, 6:04 pm, Ben C <[email protected]> wrote:
[...]
>> When the spokes are slack, the structure as a whole is less stiff. But
>> by definition the rim yields when the total stress on the rim reaches
>> its yield stress. The more stress already on it from the spokes the less
>> additional applied stress you need to bring it to yield.
>>
>> But the other side to the story is you also have to consider the wheel
>> as a structure, and whether there's any way in which it "collapses"--
>> i.e. fails as a structure before any of the components in it actually
>> yield, a bit like a tent folding up in a strong wind. In that case
>> higher spoke tension may mean it collapses at a higher applied load.
>>
>> Peter Cole explains the structure well here:
>>
>> http://groups.google.co.uk/group/rec.bicycles.tech/msg/444e4c7184eef863
>>
>> There are explanations elsewhere in that thread about how increasing
>> spoke tension "borrows" compressive strength from the rim as jim beam
>> puts it.
>>
>> Which failure mode is significant when you get a buckle or a flat spot:
>> the materials yielding, or the wheel collapsing? I don't know and I
>> don't think it's an easy one to call.
>>
>> Having collapsed in the structural sense bits of the assembly may then
>> yield because the geometry has changed and you may get more leverage on
>> parts of the structure that you wouldn't have had before. So post mortem
>> demonstration of yielded parts doesn't prove the failure actually
>> started with the components yielding rather than with the structure
>> collapsing.
>
> You can think about it in terms of stress if you want to. Yield
> strain and yield stress are interchangeable, and are related by
> modulus. If a rim is able to deflect more under a given applied load
> because of a slack spoke (altered boundary conditions), then the
> stress in the rim must be going up simply by virtue of the fact that
> it's deflecting more. The incremental load thus brings that rim
> closer to failure than a rim that is properly supported by the spokes.

I'm not saying you are, but it's easy to get confused by the
pre-loading.

If the spokes are already cranked up to a high tension then the rim is
already pre-compressed quite a bit. It might only take a small
additional deflection to yield it.

So if you're measuring what deflection _caused by the load_ (which I'm
calling "applied" load, to distinguish it from the "pre"-load) is
necessary to yield the rim, it's a smaller deflection if the rim is
pre-compressed than if it isn't.

But if there's no pre-compression because the spokes are slack, then you
need a bigger applied load (and therefore a corresponding bigger
deflection due to the applied load) to bring it to yield.

The spoke tension does not increase the load you can apply to the rim
before it yields. On the contrary it reduces it.

But as I said this is only one side of the picture. The other side is
what happens to the structure as a whole and how the configuration
changes during any particular damaging event like hitting a pothole.

With looser spokes you increase the amount of load you can apply to the
rim before it yields, but the tradeoff is it transitions from stiff to
less-stiff at a lower load. The loss of stiffness may be more damaging
than the reduction in strength depending on the conditions. I don't
know.

 

[Ben C]

On 2008-04-18, Peter Cole <[email protected]> wrote:
> Ben C wrote:
>> On 2008-04-17, [email protected] <[email protected]> wrote:
>
>>> It's exactly because the wheel with the slack spoke is less stiff that
>>> an incremental increase in load gets the rim closer to yield strain
>>> than a wheel without a slack spoke would be.
>>
>> I don't think that's right. Never mind strain, just consider yield
>> stress.
>
> He is right.
>
>
>> When the spokes are slack, the structure as a whole is less stiff. But
>> by definition the rim yields when the total stress on the rim reaches
>> its yield stress. The more stress already on it from the spokes the less
>> additional applied stress you need to bring it to yield.
>
>
> It is less confusing to think of yield strain and consider the
> stress/strain response of the wheel as a structure with and without
> loaded spokes.
>
> Spoke stress on the rim is primarily compression across the cross
> section (rim). Bending stress (wheel radial load) is compression on the
> outer surface, tension on the inner.
>
> Since the rim is pre-loaded with compression (spoke tension), that
> compression must be added to the skin compression (outer) caused by a
> bending load, if you do the math, you find you still come out ahead
> (stronger for radial loads) with higher spoke tension.

Interesting. Are you saying that the effect of the spoke tension
mitigating the tension on the inner surface outweighs its negative
effect on the outer surface?

Is this because aluminium is stronger in compression than in tension?
(Is it?).

I would be interested to see your math.

>> But the other side to the story is you also have to consider the wheel
>> as a structure, and whether there's any way in which it "collapses"--
>> i.e. fails as a structure before any of the components in it actually
>> yield, a bit like a tent folding up in a strong wind. In that case
>> higher spoke tension may mean it collapses at a higher applied load.
>
> Taco failure is beam buckling. The higher the rim compression (spoke
> tension) the less additional lateral load or compressive load is
> required to buckle. Taken to the limit, a wheel will spontaneously
> buckle just from spoke tension.

Yes although that doesn't guarantee that the propensity to buckle
increases monotonically with spoke tension all the way from zero.

A "collapsed" wheel (lots of spokes going slack) might cause it to get
twisted out of line in some way when hitting a bump resulting in higher
loads and failure.

I had always thought that the argument for high tension was to delay the
onset of collapse. I haven't heard before the reasoning that it actually
increases the radial load capacity of the rim.

> In normal use, lateral and compressive wheel loads are small, so the
> increase in radial load capacity given by higher spoke tension is more
> relevant than the loss in buckling resistance. The other potential
> limiting factor is fatigue at the spoke bed, which is accelerated by
> increased spoke tension.
>
> The ideal spoke tension is reached by increasing to the maximum that
> still leaves sufficient buckling resistance to normal cycling loads --
> you may argue what is "normal". Some rims may not reach that limit
> because of relatively weak spoke beds, that's a design parameter, and
> the manufacturer's spec must be observed.
>
> As Jobst says, the strongest wheel is made by using the highest spoke
> tension the rim can bear. That translates into buckling or spoke bed
> fatigue, whichever comes first. Buckling can be experimentally
> determined by carefully increasing tension until the wheel starts to
> deform into a saddle shape. There is no experimental way to know the
> tolerable spoke bed fatigue tension, so the only recourse is to use the
> manufacturer's maximum. I can't see any reason for using less than that
> maximum, doing so will make a wheel with less radial load capacity.
>
> In practical terms, more spoke tension doesn't make the wheel stiffer,
> it just increases the load point where the wheel undergoes a stiffness
> transition. Beyond that point, the lower stiffness causes relatively
> more strain with load until the rim is permanently bent.

Yes.

 

[Nate Nagel]

Ben C wrote:
> On 2008-04-18, Peter Cole <[email protected]> wrote:
>
>>Ben C wrote:
>>
>>>On 2008-04-17, [email protected] <[email protected]> wrote:
>>
>>>>It's exactly because the wheel with the slack spoke is less stiff that
>>>>an incremental increase in load gets the rim closer to yield strain
>>>>than a wheel without a slack spoke would be.
>>>
>>>I don't think that's right. Never mind strain, just consider yield
>>>stress.
>>
>>He is right.
>>
>>
>>
>>>When the spokes are slack, the structure as a whole is less stiff. But
>>>by definition the rim yields when the total stress on the rim reaches
>>>its yield stress. The more stress already on it from the spokes the less
>>>additional applied stress you need to bring it to yield.
>>
>>
>>It is less confusing to think of yield strain and consider the
>>stress/strain response of the wheel as a structure with and without
>>loaded spokes.
>>
>>Spoke stress on the rim is primarily compression across the cross
>>section (rim). Bending stress (wheel radial load) is compression on the
>>outer surface, tension on the inner.
>>
>>Since the rim is pre-loaded with compression (spoke tension), that
>>compression must be added to the skin compression (outer) caused by a
>>bending load, if you do the math, you find you still come out ahead
>>(stronger for radial loads) with higher spoke tension.
>
>
> Interesting. Are you saying that the effect of the spoke tension
> mitigating the tension on the inner surface outweighs its negative
> effect on the outer surface?
>
> Is this because aluminium is stronger in compression than in tension?
> (Is it?).
>
> I would be interested to see your math.
>

Very few materials (trying to think of one) are stronger in tension than
compression.

nate

--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel

 

[[email protected]]

On Apr 18, 10:52 pm, jim beam <[email protected]> wrote:
> [email protected] wrote:
> > On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
> >> On 2008-04-17, [email protected] <[email protected]> wrote:
> >> [...]
>
> >>> There are all kinds of messy things going on when you bring a wheel up
> >>> to tension. A better example is how much you need to unscrew the
> >>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
> >>> much. The wheel may continue to deform past spoke slacking, but it's
> >>> essentially failed.
> >> Note however that the strength of the wheel is just the same when the
> >> spokes are slack as when they are tight, it's just less stiff. If the
> >> spoke tension is excessively high, it actually reduces the strength of
> >> the wheel (in that case the rim would yield before the spokes went
> >> slack).
>
> >> The reduced stiffness when the spokes go slack means the rim will deform
> >> more for a given load and that extra deformation _may_ cause it get out
> >> of shape and buckle. But spokes can go slack and wheels not fail.
>
> > You can't really decouple strength and stiffness that way. It's
> > exactly because the wheel with the slack spoke is less stiff that an
> > incremental increase in load gets the rim closer to yield strain than
> > a wheel without a slack spoke would be.
>
> how so? how does the rim yield?
>
> > So the strength of the wheel
> > is not the same. Fatigue failure from overtensioned spokes is a
> > different issue all together and has been beaten to death elsewhere.
>
> spokes don't usually fail from over-tension, they fail from bending
> fatigue at the elbow if they're too slack.

We're talking about rims here jim. Try to keep up. We're also
talking about Roubaix, so the service life requirement is only 250
km. These wheels can be built up to tensions that would certainly
result in fatigue failure of the rim if it were to be used for more
than one day.


>
>
>
> > Yes, spokes can go slack without catastrophic wheel failure. You can
> > also knock quite a few holes in an aircraft fuselage without crashing
> > it.
>
> if it's un-pressurized, but not if it is.

Really?
http://www.dailymail.co.uk/pages/li...ews.html?in_article_id=543570&in_page_id=1811

>
> > Once you're outside of design conditions you're bumping up
> > against the edge of, "how much farther until it collapses?", and the
> > real world is far too variable to consider that zone to be safe
> > operating conditions.
>
> so which is stronger in tension - a rope with a load on it, or the same
> rope without?

Well, if the rope has a tensile strength of 300 lbs and one of them is
pre-loaded with 100 lbs, which one is strong enough to hang 210 lbs
with? You can't just focus on the material strength and ignore the
boundary conditions.

 

[[email protected]]

On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
> Ben C wrote:
> > On 2008-04-18, Peter Cole <[email protected]> wrote:
>
> >>Ben C wrote:
>
> >>>On 2008-04-17, [email protected] <[email protected]> wrote:
>
> >>>>It's exactly because the wheel with the slack spoke is less stiff that
> >>>>an incremental increase in load gets the rim closer to yield strain
> >>>>than a wheel without a slack spoke would be.
>
> >>>I don't think that's right. Never mind strain, just consider yield
> >>>stress.
>
> >>He is right.
>
> >>>When the spokes are slack, the structure as a whole is less stiff. But
> >>>by definition the rim yields when the total stress on the rim reaches
> >>>its yield stress. The more stress already on it from the spokes the less
> >>>additional applied stress you need to bring it to yield.
>
> >>It is less confusing to think of yield strain and consider the
> >>stress/strain response of the wheel as a structure with and without
> >>loaded spokes.
>
> >>Spoke stress on the rim is primarily compression across the cross
> >>section (rim). Bending stress (wheel radial load) is compression on the
> >>outer surface, tension on the inner.
>
> >>Since the rim is pre-loaded with compression (spoke tension), that
> >>compression must be added to the skin compression (outer) caused by a
> >>bending load, if you do the math, you find you still come out ahead
> >>(stronger for radial loads) with higher spoke tension.
>
> > Interesting. Are you saying that the effect of the spoke tension
> > mitigating the tension on the inner surface outweighs its negative
> > effect on the outer surface?
>
> > Is this because aluminium is stronger in compression than in tension?
> > (Is it?).
>
> > I would be interested to see your math.
>
> Very few materials (trying to think of one) are stronger in tension than
> compression.
>
> nate
>
> --
> replace "roosters" with "cox" to reply.http://members.cox.net/njnagel

A fiber composite can be stronger in tension than compression, but in
continuum materials the ratio of tensile to compressive strength is
bounded by one for ductile materials (like steel) and approaches zero
for brittle materials (like concrete). Not that it is at all helpful
to think of this problem in terms of pure tension or compression. A
rim is not a rope, and if yielding occurs from impact with a
cobblestone it will be the result of localized shear strains.

 

[jim beam]

[email protected] wrote:
> On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
>> Ben C wrote:
>>> On 2008-04-18, Peter Cole <[email protected]> wrote:
>>>> Ben C wrote:
>>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>>>>> It's exactly because the wheel with the slack spoke is less stiff that
>>>>>> an incremental increase in load gets the rim closer to yield strain
>>>>>> than a wheel without a slack spoke would be.
>>>>> I don't think that's right. Never mind strain, just consider yield
>>>>> stress.
>>>> He is right.
>>>>> When the spokes are slack, the structure as a whole is less stiff. But
>>>>> by definition the rim yields when the total stress on the rim reaches
>>>>> its yield stress. The more stress already on it from the spokes the less
>>>>> additional applied stress you need to bring it to yield.
>>>> It is less confusing to think of yield strain and consider the
>>>> stress/strain response of the wheel as a structure with and without
>>>> loaded spokes.
>>>> Spoke stress on the rim is primarily compression across the cross
>>>> section (rim). Bending stress (wheel radial load) is compression on the
>>>> outer surface, tension on the inner.
>>>> Since the rim is pre-loaded with compression (spoke tension), that
>>>> compression must be added to the skin compression (outer) caused by a
>>>> bending load, if you do the math, you find you still come out ahead
>>>> (stronger for radial loads) with higher spoke tension.
>>> Interesting. Are you saying that the effect of the spoke tension
>>> mitigating the tension on the inner surface outweighs its negative
>>> effect on the outer surface?
>>> Is this because aluminium is stronger in compression than in tension?
>>> (Is it?).
>>> I would be interested to see your math.
>> Very few materials (trying to think of one) are stronger in tension than
>> compression.
>>
>> nate
>>
>> --
>> replace "roosters" with "cox" to reply.http://members.cox.net/njnagel
>
> A fiber composite can be stronger in tension than compression,

that's the form, not the material itself. like rope.


> but in
> continuum materials

"continuum materials"???


> the ratio of tensile to compressive strength is
> bounded by one for ductile materials (like steel) and approaches zero
> for brittle materials (like concrete).

it's not "bounded by", the ratio /is/ close to one for an unflawed
isotropic ductile material, and /is/ close to zero for a flawed brittle
material.


> Not that it is at all helpful
> to think of this problem in terms of pure tension or compression. A
> rim is not a rope, and if yielding occurs from impact with a
> cobblestone it will be the result of localized shear strains.

localized? yes. shear? most unlikely. there may be a shear component
[hydrostatic], but mainly it's just bending tension and compression.

 

[jim beam]

[email protected] wrote:
> On Apr 18, 10:52 pm, jim beam <[email protected]> wrote:
>> [email protected] wrote:
>>> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>>> [...]
>>>>> There are all kinds of messy things going on when you bring a wheel up
>>>>> to tension. A better example is how much you need to unscrew the
>>>>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
>>>>> much. The wheel may continue to deform past spoke slacking, but it's
>>>>> essentially failed.
>>>> Note however that the strength of the wheel is just the same when the
>>>> spokes are slack as when they are tight, it's just less stiff. If the
>>>> spoke tension is excessively high, it actually reduces the strength of
>>>> the wheel (in that case the rim would yield before the spokes went
>>>> slack).
>>>> The reduced stiffness when the spokes go slack means the rim will deform
>>>> more for a given load and that extra deformation _may_ cause it get out
>>>> of shape and buckle. But spokes can go slack and wheels not fail.
>>> You can't really decouple strength and stiffness that way. It's
>>> exactly because the wheel with the slack spoke is less stiff that an
>>> incremental increase in load gets the rim closer to yield strain than
>>> a wheel without a slack spoke would be.
>> how so? how does the rim yield?
>>
>>> So the strength of the wheel
>>> is not the same. Fatigue failure from overtensioned spokes is a
>>> different issue all together and has been beaten to death elsewhere.
>> spokes don't usually fail from over-tension, they fail from bending
>> fatigue at the elbow if they're too slack.
>
> We're talking about rims here jim. Try to keep up.

actually, we're talking materials. try to keep up.



> We're also
> talking about Roubaix, so the service life requirement is only 250
> km. These wheels can be built up to tensions that would certainly
> result in fatigue failure of the rim if it were to be used for more
> than one day.

what is the point? are you falling for the jobstian misconception of
thinking that increasing tension somehow, mystically, increases strength?


>
>
>>
>>
>>> Yes, spokes can go slack without catastrophic wheel failure. You can
>>> also knock quite a few holes in an aircraft fuselage without crashing
>>> it.
>> if it's un-pressurized, but not if it is.
>
> Really?
> http://www.dailymail.co.uk/pages/li...ews.html?in_article_id=543570&in_page_id=1811

yes, really. pressure vessel rupture. there is a whole science evolved
for its testing and mitigation.


>
>>> Once you're outside of design conditions you're bumping up
>>> against the edge of, "how much farther until it collapses?", and the
>>> real world is far too variable to consider that zone to be safe
>>> operating conditions.
>> so which is stronger in tension - a rope with a load on it, or the same
>> rope without?
>
> Well, if the rope has a tensile strength of 300 lbs and one of them is
> pre-loaded with 100 lbs, which one is strong enough to hang 210 lbs
> with? You can't just focus on the material strength and ignore the
> boundary conditions.
>

do you even know what a "boundary condition" is? you use that phrase
liberally, but your context is always incorrect. and you're fudging the
question.

 

[[email protected]]

On Apr 19, 9:54 am, jim beam <[email protected]> wrote:
> [email protected] wrote:
> > On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
> >> Ben C wrote:
> >>> On 2008-04-18, Peter Cole <[email protected]> wrote:
> >>>> Ben C wrote:
> >>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
> >>>>>> It's exactly because the wheel with the slack spoke is less stiff that
> >>>>>> an incremental increase in load gets the rim closer to yield strain
> >>>>>> than a wheel without a slack spoke would be.
> >>>>> I don't think that's right. Never mind strain, just consider yield
> >>>>> stress.
> >>>> He is right.
> >>>>> When the spokes are slack, the structure as a whole is less stiff. But
> >>>>> by definition the rim yields when the total stress on the rim reaches
> >>>>> its yield stress. The more stress already on it from the spokes the less
> >>>>> additional applied stress you need to bring it to yield.
> >>>> It is less confusing to think of yield strain and consider the
> >>>> stress/strain response of the wheel as a structure with and without
> >>>> loaded spokes.
> >>>> Spoke stress on the rim is primarily compression across the cross
> >>>> section (rim). Bending stress (wheel radial load) is compression on the
> >>>> outer surface, tension on the inner.
> >>>> Since the rim is pre-loaded with compression (spoke tension), that
> >>>> compression must be added to the skin compression (outer) caused by a
> >>>> bending load, if you do the math, you find you still come out ahead
> >>>> (stronger for radial loads) with higher spoke tension.
> >>> Interesting. Are you saying that the effect of the spoke tension
> >>> mitigating the tension on the inner surface outweighs its negative
> >>> effect on the outer surface?
> >>> Is this because aluminium is stronger in compression than in tension?
> >>> (Is it?).
> >>> I would be interested to see your math.
> >> Very few materials (trying to think of one) are stronger in tension than
> >> compression.
>
> >> nate
>
> >> --
> >> replace "roosters" with "cox" to reply.http://members.cox.net/njnagel
>
> > A fiber composite can be stronger in tension than compression,
>
> that's the form, not the material itself. like rope.
>
> > but in
> > continuum materials
>
> "continuum materials"???

Yes jim, it's a material which may be treated as effectively
continuous rather than a structure of discrete components. You didn't
learn about this in metallurgy school?

>
> > the ratio of tensile to compressive strength is
> > bounded by one for ductile materials (like steel) and approaches zero
> > for brittle materials (like concrete).
>
> it's not "bounded by", the ratio /is/ close to one for an unflawed
> isotropic ductile material, and /is/ close to zero for a flawed brittle
> material.

"Bounded" means that materials may lie somewhere between unflawed
isotropic ductile and flawed brittle, and that the physics dictate
that the ratio cannot be greater than one for a continuum material.
You didn't learn about this in metallurgy school?

>
> > Not that it is at all helpful
> > to think of this problem in terms of pure tension or compression. A
> > rim is not a rope, and if yielding occurs from impact with a
> > cobblestone it will be the result of localized shear strains.
>
> localized? yes. shear? most unlikely. there may be a shear component
> [hydrostatic], but mainly it's just bending tension and compression.

Metals do not fail in bending, tension, or compression. Metallic
bonding between atoms is too strong to simply pull them apart in
tension. The metal will fail on a slip plane that is loaded in
shear. You didn't learn about this in metallurgy school?

 

[jim beam]

[email protected] wrote:
> On Apr 19, 9:54 am, jim beam <[email protected]> wrote:
>> [email protected] wrote:
>>> On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
>>>> Ben C wrote:
>>>>> On 2008-04-18, Peter Cole <[email protected]> wrote:
>>>>>> Ben C wrote:
>>>>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>>>>>>> It's exactly because the wheel with the slack spoke is less stiff that
>>>>>>>> an incremental increase in load gets the rim closer to yield strain
>>>>>>>> than a wheel without a slack spoke would be.
>>>>>>> I don't think that's right. Never mind strain, just consider yield
>>>>>>> stress.
>>>>>> He is right.
>>>>>>> When the spokes are slack, the structure as a whole is less stiff. But
>>>>>>> by definition the rim yields when the total stress on the rim reaches
>>>>>>> its yield stress. The more stress already on it from the spokes the less
>>>>>>> additional applied stress you need to bring it to yield.
>>>>>> It is less confusing to think of yield strain and consider the
>>>>>> stress/strain response of the wheel as a structure with and without
>>>>>> loaded spokes.
>>>>>> Spoke stress on the rim is primarily compression across the cross
>>>>>> section (rim). Bending stress (wheel radial load) is compression on the
>>>>>> outer surface, tension on the inner.
>>>>>> Since the rim is pre-loaded with compression (spoke tension), that
>>>>>> compression must be added to the skin compression (outer) caused by a
>>>>>> bending load, if you do the math, you find you still come out ahead
>>>>>> (stronger for radial loads) with higher spoke tension.
>>>>> Interesting. Are you saying that the effect of the spoke tension
>>>>> mitigating the tension on the inner surface outweighs its negative
>>>>> effect on the outer surface?
>>>>> Is this because aluminium is stronger in compression than in tension?
>>>>> (Is it?).
>>>>> I would be interested to see your math.
>>>> Very few materials (trying to think of one) are stronger in tension than
>>>> compression.
>>>> nate
>>>> --
>>>> replace "roosters" with "cox" to reply.http://members.cox.net/njnagel
>>> A fiber composite can be stronger in tension than compression,
>> that's the form, not the material itself. like rope.
>>
>>> but in
>>> continuum materials
>> "continuum materials"???
>
> Yes jim, it's a material which may be treated as effectively
> continuous rather than a structure of discrete components. You didn't
> learn about this in metallurgy school?
>
>>> the ratio of tensile to compressive strength is
>>> bounded by one for ductile materials (like steel) and approaches zero
>>> for brittle materials (like concrete).
>> it's not "bounded by", the ratio /is/ close to one for an unflawed
>> isotropic ductile material, and /is/ close to zero for a flawed brittle
>> material.
>
> "Bounded" means that materials may lie somewhere between unflawed
> isotropic ductile and flawed brittle, and that the physics dictate
> that the ratio cannot be greater than one for a continuum material.
> You didn't learn about this in metallurgy school?
>
>>> Not that it is at all helpful
>>> to think of this problem in terms of pure tension or compression. A
>>> rim is not a rope, and if yielding occurs from impact with a
>>> cobblestone it will be the result of localized shear strains.
>> localized? yes. shear? most unlikely. there may be a shear component
>> [hydrostatic], but mainly it's just bending tension and compression.
>
> Metals do not fail in bending, tension, or compression.

really??? you'd better let instron know that then!


> Metallic
> bonding between atoms is too strong to simply pull them apart in
> tension.

really????


> The metal will fail on a slip plane that is loaded in
> shear.

ever heard of polycrystalline materials? ever heard of dislocation
theory?


> You didn't learn about this in metallurgy school?

are you jtaylor?

 

[Nate Nagel]

[email protected] wrote:
> On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
>
>>Ben C wrote:
>>
>>>On 2008-04-18, Peter Cole <[email protected]> wrote:
>>
>>>>Ben C wrote:
>>
>>>>>On 2008-04-17, [email protected] <[email protected]> wrote:
>>
>>>>>>It's exactly because the wheel with the slack spoke is less stiff that
>>>>>>an incremental increase in load gets the rim closer to yield strain
>>>>>>than a wheel without a slack spoke would be.
>>
>>>>>I don't think that's right. Never mind strain, just consider yield
>>>>>stress.
>>
>>>>He is right.
>>
>>>>>When the spokes are slack, the structure as a whole is less stiff. But
>>>>>by definition the rim yields when the total stress on the rim reaches
>>>>>its yield stress. The more stress already on it from the spokes the less
>>>>>additional applied stress you need to bring it to yield.
>>
>>>>It is less confusing to think of yield strain and consider the
>>>>stress/strain response of the wheel as a structure with and without
>>>>loaded spokes.
>>
>>>>Spoke stress on the rim is primarily compression across the cross
>>>>section (rim). Bending stress (wheel radial load) is compression on the
>>>>outer surface, tension on the inner.
>>
>>>>Since the rim is pre-loaded with compression (spoke tension), that
>>>>compression must be added to the skin compression (outer) caused by a
>>>>bending load, if you do the math, you find you still come out ahead
>>>>(stronger for radial loads) with higher spoke tension.
>>
>>>Interesting. Are you saying that the effect of the spoke tension
>>>mitigating the tension on the inner surface outweighs its negative
>>>effect on the outer surface?
>>
>>>Is this because aluminium is stronger in compression than in tension?
>>>(Is it?).
>>
>>>I would be interested to see your math.
>>
>>Very few materials (trying to think of one) are stronger in tension than
>>compression.
>>
>>nate
>>
>>--
>>replace "roosters" with "cox" to reply.http://members.cox.net/njnagel
>
>
> A fiber composite can be stronger in tension than compression, but in
> continuum materials the ratio of tensile to compressive strength is
> bounded by one for ductile materials (like steel) and approaches zero
> for brittle materials (like concrete). Not that it is at all helpful
> to think of this problem in terms of pure tension or compression. A
> rim is not a rope, and if yielding occurs from impact with a
> cobblestone it will be the result of localized shear strains.

I guess I should have qualified, I was considering a homogeneous
material, not either a fiber or a fiber-matrix composite.

nate

--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel

 

[[email protected]]

On Apr 19, 9:56 am, jim beam <[email protected]> wrote:
> [email protected] wrote:
> > On Apr 18, 10:52 pm, jim beam <[email protected]> wrote:
> >> [email protected] wrote:
> >>> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
> >>>> On 2008-04-17, [email protected] <[email protected]> wrote:
> >>>> [...]
> >>>>> There are all kinds of messy things going on when you bring a wheel up
> >>>>> to tension. A better example is how much you need to unscrew the
> >>>>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
> >>>>> much. The wheel may continue to deform past spoke slacking, but it's
> >>>>> essentially failed.
> >>>> Note however that the strength of the wheel is just the same when the
> >>>> spokes are slack as when they are tight, it's just less stiff. If the
> >>>> spoke tension is excessively high, it actually reduces the strength of
> >>>> the wheel (in that case the rim would yield before the spokes went
> >>>> slack).
> >>>> The reduced stiffness when the spokes go slack means the rim will deform
> >>>> more for a given load and that extra deformation _may_ cause it get out
> >>>> of shape and buckle. But spokes can go slack and wheels not fail.
> >>> You can't really decouple strength and stiffness that way. It's
> >>> exactly because the wheel with the slack spoke is less stiff that an
> >>> incremental increase in load gets the rim closer to yield strain than
> >>> a wheel without a slack spoke would be.
> >> how so? how does the rim yield?
>
> >>> So the strength of the wheel
> >>> is not the same. Fatigue failure from overtensioned spokes is a
> >>> different issue all together and has been beaten to death elsewhere.
> >> spokes don't usually fail from over-tension, they fail from bending
> >> fatigue at the elbow if they're too slack.
>
> > We're talking about rims here jim. Try to keep up.
>
> actually, we're talking materials. try to keep up.
>
> > We're also
> > talking about Roubaix, so the service life requirement is only 250
> > km. These wheels can be built up to tensions that would certainly
> > result in fatigue failure of the rim if it were to be used for more
> > than one day.
>
> what is the point? are you falling for the jobstian misconception of
> thinking that increasing tension somehow, mystically, increases strength?
>
>
>
> >>> Yes, spokes can go slack without catastrophic wheel failure. You can
> >>> also knock quite a few holes in an aircraft fuselage without crashing
> >>> it.
> >> if it's un-pressurized, but not if it is.
>
> > Really?
> >http://www.dailymail.co.uk/pages/live/articles/news/worldnews.html?in...
>
> yes, really. pressure vessel rupture. there is a whole science evolved
> for its testing and mitigation.
>
>
>
> >>> Once you're outside of design conditions you're bumping up
> >>> against the edge of, "how much farther until it collapses?", and the
> >>> real world is far too variable to consider that zone to be safe
> >>> operating conditions.
> >> so which is stronger in tension - a rope with a load on it, or the same
> >> rope without?
>
> > Well, if the rope has a tensile strength of 300 lbs and one of them is
> > pre-loaded with 100 lbs, which one is strong enough to hang 210 lbs
> > with? You can't just focus on the material strength and ignore the
> > boundary conditions.
>
> do you even know what a "boundary condition" is? you use that phrase
> liberally, but your context is always incorrect. and you're fudging the
> question.

Same rope under different conditions. Both rigidly fixed on one end.
One has a hundred pound weight on the free end. One of them is strong
enough for you to hang yourself with. The other one isn't.

 

[jim beam]

[email protected] wrote:
> On Apr 19, 9:56 am, jim beam <[email protected]> wrote:
>> [email protected] wrote:
>>> On Apr 18, 10:52 pm, jim beam <[email protected]> wrote:
>>>> [email protected] wrote:
>>>>> On Apr 17, 2:41 pm, Ben C <[email protected]> wrote:
>>>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
>>>>>> [...]
>>>>>>> There are all kinds of messy things going on when you bring a wheel up
>>>>>>> to tension. A better example is how much you need to unscrew the
>>>>>>> nipple to get a spoke on a tensioned wheel to go slack. It's not very
>>>>>>> much. The wheel may continue to deform past spoke slacking, but it's
>>>>>>> essentially failed.
>>>>>> Note however that the strength of the wheel is just the same when the
>>>>>> spokes are slack as when they are tight, it's just less stiff. If the
>>>>>> spoke tension is excessively high, it actually reduces the strength of
>>>>>> the wheel (in that case the rim would yield before the spokes went
>>>>>> slack).
>>>>>> The reduced stiffness when the spokes go slack means the rim will deform
>>>>>> more for a given load and that extra deformation _may_ cause it get out
>>>>>> of shape and buckle. But spokes can go slack and wheels not fail.
>>>>> You can't really decouple strength and stiffness that way. It's
>>>>> exactly because the wheel with the slack spoke is less stiff that an
>>>>> incremental increase in load gets the rim closer to yield strain than
>>>>> a wheel without a slack spoke would be.
>>>> how so? how does the rim yield?
>>>>> So the strength of the wheel
>>>>> is not the same. Fatigue failure from overtensioned spokes is a
>>>>> different issue all together and has been beaten to death elsewhere.
>>>> spokes don't usually fail from over-tension, they fail from bending
>>>> fatigue at the elbow if they're too slack.
>>> We're talking about rims here jim. Try to keep up.
>> actually, we're talking materials. try to keep up.
>>
>>> We're also
>>> talking about Roubaix, so the service life requirement is only 250
>>> km. These wheels can be built up to tensions that would certainly
>>> result in fatigue failure of the rim if it were to be used for more
>>> than one day.
>> what is the point? are you falling for the jobstian misconception of
>> thinking that increasing tension somehow, mystically, increases strength?
>>
>>
>>
>>>>> Yes, spokes can go slack without catastrophic wheel failure. You can
>>>>> also knock quite a few holes in an aircraft fuselage without crashing
>>>>> it.
>>>> if it's un-pressurized, but not if it is.
>>> Really?
>>> http://www.dailymail.co.uk/pages/live/articles/news/worldnews.html?in...
>> yes, really. pressure vessel rupture. there is a whole science evolved
>> for its testing and mitigation.
>>
>>
>>
>>>>> Once you're outside of design conditions you're bumping up
>>>>> against the edge of, "how much farther until it collapses?", and the
>>>>> real world is far too variable to consider that zone to be safe
>>>>> operating conditions.
>>>> so which is stronger in tension - a rope with a load on it, or the same
>>>> rope without?
>>> Well, if the rope has a tensile strength of 300 lbs and one of them is
>>> pre-loaded with 100 lbs, which one is strong enough to hang 210 lbs
>>> with? You can't just focus on the material strength and ignore the
>>> boundary conditions.
>> do you even know what a "boundary condition" is? you use that phrase
>> liberally, but your context is always incorrect. and you're fudging the
>> question.
>
> Same rope under different conditions. Both rigidly fixed on one end.
> One has a hundred pound weight on the free end. One of them is strong
> enough for you to hang yourself with. The other one isn't.

so what is the "boundary condition" of using incorrect terminology and
avoiding the question then? something to do with confusing load with
strength?

 

[[email protected]]

On Apr 19, 10:42 am, jim beam <[email protected]> wrote:
> [email protected] wrote:
> > On Apr 19, 9:54 am, jim beam <[email protected]> wrote:
> >> [email protected] wrote:
> >>> On Apr 19, 7:28 am, Nate Nagel <[email protected]> wrote:
> >>>> Ben C wrote:
> >>>>> On 2008-04-18, Peter Cole <[email protected]> wrote:
> >>>>>> Ben C wrote:
> >>>>>>> On 2008-04-17, [email protected] <[email protected]> wrote:
> >>>>>>>> It's exactly because the wheel with the slack spoke is less stiff that
> >>>>>>>> an incremental increase in load gets the rim closer to yield strain
> >>>>>>>> than a wheel without a slack spoke would be.
> >>>>>>> I don't think that's right. Never mind strain, just consider yield
> >>>>>>> stress.
> >>>>>> He is right.
> >>>>>>> When the spokes are slack, the structure as a whole is less stiff. But
> >>>>>>> by definition the rim yields when the total stress on the rim reaches
> >>>>>>> its yield stress. The more stress already on it from the spokes the less
> >>>>>>> additional applied stress you need to bring it to yield.
> >>>>>> It is less confusing to think of yield strain and consider the
> >>>>>> stress/strain response of the wheel as a structure with and without
> >>>>>> loaded spokes.
> >>>>>> Spoke stress on the rim is primarily compression across the cross
> >>>>>> section (rim). Bending stress (wheel radial load) is compression on the
> >>>>>> outer surface, tension on the inner.
> >>>>>> Since the rim is pre-loaded with compression (spoke tension), that
> >>>>>> compression must be added to the skin compression (outer) caused by a
> >>>>>> bending load, if you do the math, you find you still come out ahead
> >>>>>> (stronger for radial loads) with higher spoke tension.
> >>>>> Interesting. Are you saying that the effect of the spoke tension
> >>>>> mitigating the tension on the inner surface outweighs its negative
> >>>>> effect on the outer surface?
> >>>>> Is this because aluminium is stronger in compression than in tension?
> >>>>> (Is it?).
> >>>>> I would be interested to see your math.
> >>>> Very few materials (trying to think of one) are stronger in tension than
> >>>> compression.
> >>>> nate
> >>>> --
> >>>> replace "roosters" with "cox" to reply.http://members.cox.net/njnagel
> >>> A fiber composite can be stronger in tension than compression,
> >> that's the form, not the material itself. like rope.
>
> >>> but in
> >>> continuum materials
> >> "continuum materials"???
>
> > Yes jim, it's a material which may be treated as effectively
> > continuous rather than a structure of discrete components. You didn't
> > learn about this in metallurgy school?
>
> >>> the ratio of tensile to compressive strength is
> >>> bounded by one for ductile materials (like steel) and approaches zero
> >>> for brittle materials (like concrete).
> >> it's not "bounded by", the ratio /is/ close to one for an unflawed
> >> isotropic ductile material, and /is/ close to zero for a flawed brittle
> >> material.
>
> > "Bounded" means that materials may lie somewhere between unflawed
> > isotropic ductile and flawed brittle, and that the physics dictate
> > that the ratio cannot be greater than one for a continuum material.
> > You didn't learn about this in metallurgy school?
>
> >>> Not that it is at all helpful
> >>> to think of this problem in terms of pure tension or compression. A
> >>> rim is not a rope, and if yielding occurs from impact with a
> >>> cobblestone it will be the result of localized shear strains.
> >> localized? yes. shear? most unlikely. there may be a shear component
> >> [hydrostatic], but mainly it's just bending tension and compression.
>
> > Metals do not fail in bending, tension, or compression.
>
> really??? you'd better let instron know that then!

I really should, because my specimens keep breaking on shear planes
that aren't orthogonal to the load I'm applying. Maybe I'm doing
something wrong?


>
> > Metallic
> > bonding between atoms is too strong to simply pull them apart in
> > tension.
>
> really????
>
> > The metal will fail on a slip plane that is loaded in
> > shear.
>
> ever heard of polycrystalline materials? ever heard of dislocation
> theory?
>
> > You didn't learn about this in metallurgy school?
>
> are you jtaylor?