Re: Spoke Failure--surface quality or stress relief?

[[email protected]]

Rault00 who? writes:

> And in my experience, the few failures I've seen at the nipple end
> were clearly due to the spoke being stressed beyond the yield point
> of the threaded section; the failure was obviously caused by tension
> in excess of the spoke's load capacity. (I will note that I have
> yet to see a spoke break at that point without a spokejam being
> involved, also.)

I think the picture at:

http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm

shows a common failure that occurs at the threads, not caused by the
mechanisms you mention.

[email protected]

 

[Zog The Undeniable]

[email protected] wrote:

> I think the picture at:
>
> http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>
> shows a common failure that occurs at the threads, not caused by the
> mechanisms you mention.

That spoke is screwed much too far into the nipple. Possibly the "new"
thread cut by the nipple created a weak point, as it isn't rolled like a
proper spoke thread.

 

[[email protected]]

On Sat, 09 Apr 2005 18:51:42 +0100, Zog The Undeniable
<[email protected]> wrote:

>[email protected] wrote:
>
>> I think the picture at:
>>
>> http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>>
>> shows a common failure that occurs at the threads, not caused by the
>> mechanisms you mention.
>
>That spoke is screwed much too far into the nipple. Possibly the "new"
>thread cut by the nipple created a weak point, as it isn't rolled like a
>proper spoke thread.

Dear Zog,

Good eye! Without seeing the other end, we can't tell if the
spoke was screwed into the nipple past its threads, but it's
a possibility.

Come to think of it, how did they fix the spoke for the
rotation and bending? With no tension, the spoke would
either have unscrewed when rotated one way, or else screwed
itself in up to the hilt when rotated the other way.

Maybe you could fix the spoke in place by adding a second
nipple as a jam nut to the protruding spoke end?

Now I'm squinting at that picture again more carefully.

Do the uppermost two threads of the stump of the spoke look
as if they've deformed?

Or is it a trick of the light and whatever they did to cut
the nipple and show the cross-section?

Carl Fogel

 

[jtaylor]

"Zog The Undeniable" <[email protected]> wrote in message
news:425814eb.0@entanet...
> [email protected] wrote:
>
> > I think the picture at:
> >
> > http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
> >
> > shows a common failure that occurs at the threads, not caused by the
> > mechanisms you mention.
>
> That spoke is screwed much too far into the nipple. Possibly the "new"
> thread cut by the nipple created a weak point, as it isn't rolled like a
> proper spoke thread.

Nipples are almost always brass; spokes are almost always steel; care to
share your experience with brass tools cutting steel?

 

[[email protected]]

On Sat, 9 Apr 2005 21:31:49 -0300, "jtaylor"
<[email protected]> wrote:

>
>"Zog The Undeniable" <[email protected]> wrote in message
>news:425814eb.0@entanet...
>> [email protected] wrote:
>>
>> > I think the picture at:
>> >
>> > http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>> >
>> > shows a common failure that occurs at the threads, not caused by the
>> > mechanisms you mention.
>>
>> That spoke is screwed much too far into the nipple. Possibly the "new"
>> thread cut by the nipple created a weak point, as it isn't rolled like a
>> proper spoke thread.
>
>Nipples are almost always brass; spokes are almost always steel; care to
>share your experience with brass tools cutting steel?

Dear J.T.,

A good comment on sharp observation!

Sounds as if my question about how fix a rotating and
bending spoke with its end free is answered--just let it
turn itself until it jams itself into a brass nipple.

I like the way that something so apparently simple leads to
posts like yours and Zog's.

Thanks,

Carl Fogel

 

[[email protected]]

On Sat, 09 Apr 2005 18:51:42 +0100, Zog The Undeniable
<[email protected]> wrote:

>[email protected] wrote:
>
>> I think the picture at:
>>
>> http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>>
>> shows a common failure that occurs at the threads, not caused by the
>> mechanisms you mention.
>
>That spoke is screwed much too far into the nipple. Possibly the "new"
>thread cut by the nipple created a weak point, as it isn't rolled like a
>proper spoke thread.

Dear Zog,

Browsing through things again, I realized that the spoke
broken off in the nipple seems to be just a real-life
example and not from the actual testing--though it's still
screwed awfully far into the nipple.

Here's the example spoke, not from testing (though it's
natural to assume otherwise):

http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm

But the next page in the series mentions that "The spoke
material is held in a collett and rotated about its long
axis." And the diagram shows a collett, not a nipple:

http://www.princeton.edu/~humcomp/bikes/design/desi_19.htm

And two pages later is a diagram (on the right) showing a
distance of 0.75 inches from the "grips" (not a spoke
nipple) to where the spoke is loaded:

http://www.princeton.edu/~humcomp/bikes/design/desi_21.htm

So the test used some sort of special holder, not a spoke
nipple.

Carl Fogel

 

[Zog The Undeniable]

jtaylor wrote:

> Nipples are almost always brass; spokes are almost always steel; care to
> share your experience with brass tools cutting steel?

I don't know the relative hardnesses of the particular grades of s/s and
brass used here, but although brass is generally softer, it's not *that*
much softer. We're not talking about the sort of hardened steel used in
chains, anyway.

Now if those spokes I cut out the other day are still in the bin, I'll
go and screw some of the nipples on really tight and see whether it
makes new threads.

 

[Zog The Undeniable]

OK, jtaylor is probably right. Forcing a spoke into the nipple leaves a
faint helical mark on the unthreaded section, but it's not deep enough
to be called a thread, and probably doesn't weaken it any more than the
rubbing action of two spokes crossing, which never seems to cause breaks.

 

[[email protected]]

The Indecipherable writes:

>> Nipples are almost always brass; spokes are almost always steel;
>> care to share your experience with brass tools cutting steel?

> I don't know the relative hardnesses of the particular grades of s/s
> and brass used here, but although brass is generally softer, it's
> not *that* much softer. We're not talking about the sort of
> hardened steel used in chains, anyway.

> Now if those spokes I cut out the other day are still in the bin,
> I'll go and screw some of the nipples on really tight and see
> whether it makes new threads.

An easy test is to screw a spoke nipple on past the end of the spoke's
threads and see what it does to the spoke. You'll find that the spoke
mashed the threads flat in the spoke nipple and leaves no marks on the
spoke. In the days of tubulars, people regularly used too long a
spoke and caused this to happen, the spoke nipples being in fairly
deep sockets beneath the tire base tape.

[email protected]

 

[Phil, Squid-in-Training]

[email protected] wrote:
> On Sat, 09 Apr 2005 18:51:42 +0100, Zog The Undeniable
> <[email protected]> wrote:
>
>> [email protected] wrote:
>>
>>> I think the picture at:
>>>
>>> http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>>>
>>> shows a common failure that occurs at the threads, not caused by the
>>> mechanisms you mention.
>>
>> That spoke is screwed much too far into the nipple. Possibly the
>> "new" thread cut by the nipple created a weak point, as it isn't
>> rolled like a proper spoke thread.
>
> Dear Zog,
>
> Good eye! Without seeing the other end, we can't tell if the
> spoke was screwed into the nipple past its threads, but it's
> a possibility.
>
> Come to think of it, how did they fix the spoke for the
> rotation and bending? With no tension, the spoke would
> either have unscrewed when rotated one way, or else screwed
> itself in up to the hilt when rotated the other way.
>
> Maybe you could fix the spoke in place by adding a second
> nipple as a jam nut to the protruding spoke end?
>
> Now I'm squinting at that picture again more carefully.
>
> Do the uppermost two threads of the stump of the spoke look
> as if they've deformed?
>
> Or is it a trick of the light and whatever they did to cut
> the nipple and show the cross-section?
>
> Carl Fogel

Carl, they would have had to have taken out the spoke (broken threaded part
or not) in order to make the cross section of the nipple visible. Thus, how
do we know they threaded it back into the nipple for the picture exactly as
far as it was when it was taken out? Thus, we have no idea if the spoke was
cut by the nipple by overthreading, and that argument is useless.
--
Phil, Squid-in-Training

 

[[email protected]]

On Tue, 12 Apr 2005 16:46:34 GMT, "Phil, Squid-in-Training"
<[email protected]> wrote:

>[email protected] wrote:
>> On Sat, 09 Apr 2005 18:51:42 +0100, Zog The Undeniable
>> <[email protected]> wrote:
>>
>>> [email protected] wrote:
>>>
>>>> I think the picture at:
>>>>
>>>> http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
>>>>
>>>> shows a common failure that occurs at the threads, not caused by the
>>>> mechanisms you mention.
>>>
>>> That spoke is screwed much too far into the nipple. Possibly the
>>> "new" thread cut by the nipple created a weak point, as it isn't
>>> rolled like a proper spoke thread.
>>
>> Dear Zog,
>>
>> Good eye! Without seeing the other end, we can't tell if the
>> spoke was screwed into the nipple past its threads, but it's
>> a possibility.
>>
>> Come to think of it, how did they fix the spoke for the
>> rotation and bending? With no tension, the spoke would
>> either have unscrewed when rotated one way, or else screwed
>> itself in up to the hilt when rotated the other way.
>>
>> Maybe you could fix the spoke in place by adding a second
>> nipple as a jam nut to the protruding spoke end?
>>
>> Now I'm squinting at that picture again more carefully.
>>
>> Do the uppermost two threads of the stump of the spoke look
>> as if they've deformed?
>>
>> Or is it a trick of the light and whatever they did to cut
>> the nipple and show the cross-section?
>>
>> Carl Fogel
>
>Carl, they would have had to have taken out the spoke (broken threaded part
>or not) in order to make the cross section of the nipple visible. Thus, how
>do we know they threaded it back into the nipple for the picture exactly as
>far as it was when it was taken out? Thus, we have no idea if the spoke was
>cut by the nipple by overthreading, and that argument is useless.

Dear Phil,

You may be right, but I don't think so.

The spoke looks as if it broke in two and left most of the
threaded section in place inside the nipple.

Then the nipple (still holding the stump of the spoke) was
ground or cut down to show a cross-section--no removal and
re-insertion, just a cross-section with the broken spoke
section in place.

The stump of the spoke in the example is broken off right
where I'd expect a spoke to crack at the nipple--just a
thread into the nipple.

In any case, this spoke was not part of the actual testing,
which involved fixing the test spokes in a collett, not a
spoke--I needed to read further and more carefully in the
series.

You can duplicate the basic test quite easily and quickly by
shoving the threaded end of a spoke into a drill press
chuck, raising the platform so that the spoke goes through
the drill hole, and then swivelling the platform to one side
so that the spoke is bent. Turn the drill on, and the spoke
will spin, constantly bending, and soon break--how soon
depends on how much of a bend you put in it.

The test used a wheel pressing with up to 10 pounds of force
against the spoke quite close to the chuck (0.75" is shown
in a diagram). Bending the spoke to one side with the hole
in the drill press platform is much cruder, even if you oil
the spot where the spoke revolves against the platform.

If you take a spoke, bend a small complete loop in its
middle (a super elbow), and run it through this test, the
spoke usually breaks in the middle of the loop if you use a
mild overall bend and wait about five minutes.

My library has ordered the book for me, so in a month I
might have the rest of the text.

Carl Fogel

 

[jtaylor]

<[email protected]> wrote in message
news:[email protected]...
>
> You may be right, but I don't think so.
>
> The spoke looks as if it broke in two and left most of the
> threaded section in place inside the nipple.
>
> Then the nipple (still holding the stump of the spoke) was
> ground or cut down to show a cross-section--no removal and
> re-insertion, just a cross-section with the broken spoke
> section in place.

Tricky work, that; cutting the nipple in half, right down to the root of the
threads, and not touching the spoke...

 

[[email protected]]

On Tue, 12 Apr 2005 21:13:07 -0300, "jtaylor"
<[email protected]> wrote:

>
><[email protected]> wrote in message
>news:[email protected]...
>>
>> You may be right, but I don't think so.
>>
>> The spoke looks as if it broke in two and left most of the
>> threaded section in place inside the nipple.
>>
>> Then the nipple (still holding the stump of the spoke) was
>> ground or cut down to show a cross-section--no removal and
>> re-insertion, just a cross-section with the broken spoke
>> section in place.
>
>Tricky work, that; cutting the nipple in half, right down to the root of the
>threads, and not touching the spoke...
>

Dear J.T.,

D'oh!

Good eye--I missed that. Now I think that Phil and you are
right about the stump being removed, the nipple being cut or
ground, and the stump being reinserted.

Being wrong about that, I'd better hurry and re-think my
theory that the spoke would break about one thread down into
the nipple . . .

Hmmm . . . I don't think that they'd break off deeper into
the nipple, well past the beginning of the threads, since
they'd presumably tear out the nipple's threads.

But maybe I'm dead wrong about them breaking off a thread
into the nipples?

Maybe the spokes tend to break well out of the threaded part
of the nipple, where the spoke threads end and the smooth
shaft of the spoke begins? But the maximum stress would seem
to be right at the spoke-nipple interface, not up above the
nipple.

Any thoughts on exactly where the spoke would break on the
nipple end?

Carl Fogel