Sapim Fatigue Test: converting Revs. to Miles

daveornee said:

Can someone explain the meaning of the fatigue charts shown for the
various models of Sapim spokes?
http://www.sapim.be/index.php?st=products&sub=spokes&detail=fatiguetest

When I took the Race model and 1,200,000 Revolutions and converted it
to miles I got under 2,000 miles.
What am I doing wrong?
What is the correct way to interpret the fatigue data?
David Ornee, Western Springs, IL

Dear Dave,

I don't think that you're doing anything wrong--it is
puzzling.

The Race spoke's text details indicate 980,000 wheel
revolutions, at which point something bad presumably
happened to the average spoke:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted

For a 2124 mm circumference 700c tire and 757.7 spins per
mile, 980,000 spins works out to only around 1250 miles.

Few spokes would fatigue enough to fail in only 1250 miles
of normal use, so I suspect that the fatigue test loads and
unloads the spokes considerably more than the weight of a
normal rider in order to speed up the test.

If so, the data is useful only for comparing one Sapim spoke
to the other.

Maybe someone can find more details to explain what actually
happens? The site doesn't seem to offer any real
explanation.

Carl Fogel

jim beam said:

be careful with the term "fatigue limit" that generally implies the
stress at which a given alloy [without an endurance limit] will endure
"n" cycles. it's not the same as an endurance limit, the stress at
which an alloy will endure an infinite number of cycles.

Quoted message said:

In actual use, I would hope that most spokes are cycled at loads
below the fatigue limit. The fatigue limit can vary due to alloy,
treatment and surface condition. You could reduce the fatigue
strength of a polished spoke by roughing it up with fine sandpaper.

that's correct - surface finish has a significant impact on fatigue
life. but if you want to sabotage your opponents spokes, make sure
your scratches are perpendicular to the stress axis, otherwise they
will have little effect.

I notice that sapim spokes have the word "sapim" marked (stamped?) at
the hub end of the spokes. I assume this does not matter in practice,
but wouldn't the lettering be a stress riser?

"jim beam" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Quoted message said:

I think you are right. Ferrous metals typically have a true fatigue limit
or a stress that can theoretically be cycled forever without failure.

most quality spokes are stainless steel which has no fatigue endurance
limit.

be careful with the term "ferrous". technically, that means an alloy
based on iron, which includes all kinds of steels, stainless steels, some
with fatigue endurance limits, some without.

Yes Jim, There are no absolute truths in engineering. In the future I will
add a disclaimer at the end of every sentence so that you can relax. My old
materials engineering textbook says "Some materials, such as ferrous metals
and alloys, show a definite fatigue limit, whereas other materials..." I
suggest you take it up with the author Jastrzebski.

Quoted message said:

Quoted message said:

To be useful a fatigue test has to be at stress levels above the fatigue
limit so that the piece will eventually fail.

"useful"? "useful" testing is that which maps the whole landscape, not
just one small portion of it.

You will note that my "test" is singular and your "testing" implys (to me) a
series of tests. A series of tests would be required to determine the s-n
chart for a single spoke design. I was thinking of a test to compare spokes
of different design or manufacture. If I run a single test on spoke design A
then one on spoke design B, I won't learn much if neither breaks.

Quoted message said:

be careful with the term "fatigue limit" that generally implies the
stress at which a given alloy [without an endurance limit] will endure "n"
cycles. it's not the same as an endurance limit, the stress at which an
alloy will endure an infinite number of cycles.

Geez, you and Jastrzebski have exactly opposite terminology. I will stick
with him until I review your qualifications. Jastrzebski sez: "Fatigue limit
is defined as the maximum stress that can be applied repeatedly an infinite
number of times without fracturing the material."

Quoted message said:

Quoted message said:

In actual use, I would hope that most spokes are cycled at loads below
the fatigue limit. The fatigue limit can vary due to alloy, treatment and
surface condition. You could reduce the fatigue strength of a polished
spoke by roughing it up with fine sandpaper.

that's correct - surface finish has a significant impact on fatigue life.
but if you want to sabotage your opponents spokes, make sure your
scratches are perpendicular to the stress axis, otherwise they will have
little effect.

Jim Smith said:

jim beam said:

be careful with the term "fatigue limit" that generally implies the
stress at which a given alloy [without an endurance limit] will endure
"n" cycles. it's not the same as an endurance limit, the stress at
which an alloy will endure an infinite number of cycles.

Quoted message said:

In actual use, I would hope that most spokes are cycled at loads
below the fatigue limit. The fatigue limit can vary due to alloy,
treatment and surface condition. You could reduce the fatigue
strength of a polished spoke by roughing it up with fine sandpaper.

that's correct - surface finish has a significant impact on fatigue
life. but if you want to sabotage your opponents spokes, make sure
your scratches are perpendicular to the stress axis, otherwise they
will have little effect.

I notice that sapim spokes have the word "sapim" marked (stamped?) at
the hub end of the spokes. I assume this does not matter in practice,
but wouldn't the lettering be a stress riser?

Dear Jim,

Yes, most spokes have some mark stamped on their flared hub
end.

But in that position, they're being pulled into the hub's
spoke hole, so they're mostly in compression, which doesn't
easily produce a fatigue crack--which is why we don't see
spoke ends cracked like one end of a box of spaghetti
noodles.

That is, the ends of the spokes resemble the end of a wire
being drawn into a die--the stress is squashing things
together, not pulling things apart.

Stamp the letters on the sides of the spoke, and they'd be
effective stress raisers. The threads at the other end of
the spoke are much larger, and that's where about one out of
ten spokes broke in long-ago Wheelsmith testing.

Those threads, of course, are pretty much where the spoke
exerts straight-line stress. Most spokes break at the elbow,
where the stress is going to cause microscopic bending, or
at the flare, where the stess is raised by the even more
abrupt bend.

Carl Fogel

Quoted message said:

Jim Smith said:

jim beam said:

be careful with the term "fatigue limit" that generally implies the
stress at which a given alloy [without an endurance limit] will endure
"n" cycles. it's not the same as an endurance limit, the stress at
which an alloy will endure an infinite number of cycles.

> In actual use, I would hope that most spokes are cycled at loads
> below the fatigue limit. The fatigue limit can vary due to alloy,
> treatment and surface condition. You could reduce the fatigue
> strength of a polished spoke by roughing it up with fine sandpaper.

that's correct - surface finish has a significant impact on fatigue
life. but if you want to sabotage your opponents spokes, make sure
your scratches are perpendicular to the stress axis, otherwise they
will have little effect.

I notice that sapim spokes have the word "sapim" marked (stamped?) at
the hub end of the spokes. I assume this does not matter in practice,
but wouldn't the lettering be a stress riser?

Dear Jim,

Yes, most spokes have some mark stamped on their flared hub
end.

But in that position, they're being pulled into the hub's
spoke hole, so they're mostly in compression, which doesn't
easily produce a fatigue crack--which is why we don't see
spoke ends cracked like one end of a box of spaghetti
noodles.

That is, the ends of the spokes resemble the end of a wire
being drawn into a die--the stress is squashing things
together, not pulling things apart.

Stamp the letters on the sides of the spoke, and they'd be
effective stress raisers. The threads at the other end of
the spoke are much larger, and that's where about one out of
ten spokes broke in long-ago Wheelsmith testing.

Oh my. I wasn't clear. The stamp on the Sapin spokes IS on the side.

Quoted message said:

Those threads, of course, are pretty much where the spoke
exerts straight-line stress. Most spokes break at the elbow,
where the stress is going to cause microscopic bending, or
at the flare, where the stess is raised by the even more
abrupt bend.

Carl Fogel

jim beam said:

Quoted message said:

I think you are right. Ferrous metals typically have a true fatigue limit or
a stress that can theoretically be cycled forever without failure.

most quality spokes are stainless steel which has no fatigue endurance
limit.

So the quoted fatigue endurance limits for such common stainless alloy
steels as 301,302,303,304,307,309,310,316,321,347, and so on are
manufacturer's goofs?

Or are the steel companies in league with Jobst?

Quoted message said:

jim beam said:

Quoted message said:

I think you are right. Ferrous metals typically have a true fatigue limit or
a stress that can theoretically be cycled forever without failure.

most quality spokes are stainless steel which has no fatigue endurance
limit.

So the quoted fatigue endurance limits for such common stainless alloy
steels as 301,302,303,304,307,309,310,316,321,347, and so on are
manufacturer's goofs?

Or are the steel companies in league with Jobst?

I think Mr. Beams error was one of English usage, not metallurgy. He
seems to have used a "which" where he wanted a "that." Try it, you
will see your complaint disappears.

Quoted message said:

The only Sapim spokes that I have handy are some straight
Leader spokes, not the CX aero spokes. The plain Leader
spokes have no side stamp in real life, or in the picture on
the Sapim site.

Actually, the ones I am looking at are their regular round double
butted (yeah yeah, I know, swaged) spokes. I don't go in much for the
fancy bladed stuff.

Jim Smith said:

Quoted message said:

Jim Smith said:

jim beam <[email hidden]> writes:

> be careful with the term "fatigue limit" that generally implies the
> stress at which a given alloy [without an endurance limit] will endure
> "n" cycles. it's not the same as an endurance limit, the stress at
> which an alloy will endure an infinite number of cycles.
>
>> In actual use, I would hope that most spokes are cycled at loads
>> below the fatigue limit. The fatigue limit can vary due to alloy,
>> treatment and surface condition. You could reduce the fatigue
>> strength of a polished spoke by roughing it up with fine sandpaper.
>
> that's correct - surface finish has a significant impact on fatigue
> life. but if you want to sabotage your opponents spokes, make sure
> your scratches are perpendicular to the stress axis, otherwise they
> will have little effect.

I notice that sapim spokes have the word "sapim" marked (stamped?) at
the hub end of the spokes. I assume this does not matter in practice,
but wouldn't the lettering be a stress riser?

Dear Jim,

Yes, most spokes have some mark stamped on their flared hub
end.

But in that position, they're being pulled into the hub's
spoke hole, so they're mostly in compression, which doesn't
easily produce a fatigue crack--which is why we don't see
spoke ends cracked like one end of a box of spaghetti
noodles.

That is, the ends of the spokes resemble the end of a wire
being drawn into a die--the stress is squashing things
together, not pulling things apart.

Stamp the letters on the sides of the spoke, and they'd be
effective stress raisers. The threads at the other end of
the spoke are much larger, and that's where about one out of
ten spokes broke in long-ago Wheelsmith testing.

Oh my. I wasn't clear. The stamp on the Sapin spokes IS on the side.

Quoted message said:

Those threads, of course, are pretty much where the spoke
exerts straight-line stress. Most spokes break at the elbow,
where the stress is going to cause microscopic bending, or
at the flare, where the stess is raised by the even more
abrupt bend.

Carl Fogel

Dear Jim,

Aaargh! You would have been clear enough to anyone who
actually looked at the right picture! Sorry.

The only Sapim spokes that I have handy are some straight
Leader spokes, not the CX aero spokes. The plain Leader
spokes have no side stamp in real life, or in the picture on
the Sapim site.

Assuming that the bladed aero spokes have the "SAP" stamped
on the round part of the elbow above the blade, as shown in
the site pictures, you've raised an interesting question.

Do the bladed spokes break where normal spokes do, in the
elbow or at the flare of the hub-button? Or do they fail
where the round shaft flattens and widens out into the
blade?

I don't think that I've ever seen anything about whether
bladed spokes fail at a different spot than normal spokes.

On the one hand, the transition from round to bladed looks
like a stress raiser, but on the other hand, butted spokes
are kinda-sorta like that where they narrow, but they don't
fail at that point.

Carl Fogel

Jim Smith said:

[email hidden] writes:

Quoted message said:

Quoted message said:

The only Sapim spokes that I have handy are some straight
Leader spokes, not the CX aero spokes. The plain Leader
spokes have no side stamp in real life, or in the picture on
the Sapim site.

Actually, the ones I am looking at are their regular round double
butted (yeah yeah, I know, swaged) spokes. I don't go in much for the
fancy bladed stuff.

Dear Jim,

Where is the "SAP" (or whatever) stamped on the regular
round double butted spokes? On the inside of the elbow, the
outside curve, or on one side?

The "SAP" appears on the outside curve of the elbow in the
site pictures for the aero spokes:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3959&id=3388&detail=aero

This would be the back of a submarine's periscope, for lack
of a more articulate description.

The round double butted spokes show only a side view, so any
stamp on the outside curve isn't apparent on the site:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3386&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3387&detail=butted

[Nervously re-checks straight Leader spokes, just to be
sure. Whew! No stamps.]

I'm wondering now if the outside curve of the elbow goes
into compression and therefore doesn't suffer much when
stamped or scratched, while the failures originate in the
inside curve of the elbow, where the curved inner surface
might go into tension.

Carl Fogel

Quoted message said:

Jim Smith said:

[email hidden] writes:

Quoted message said:

Quoted message said:

The only Sapim spokes that I have handy are some straight
Leader spokes, not the CX aero spokes. The plain Leader
spokes have no side stamp in real life, or in the picture on
the Sapim site.

Actually, the ones I am looking at are their regular round double
butted (yeah yeah, I know, swaged) spokes. I don't go in much for the
fancy bladed stuff.

Dear Jim,

Where is the "SAP" (or whatever) stamped on the regular
round double butted spokes? On the inside of the elbow, the
outside curve, or on one side?

The "SAP" appears on the outside curve of the elbow in the
site pictures for the aero spokes:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3959&id=3388&detail=aero

This would be the back of a submarine's periscope, for lack
of a more articulate description.

The round double butted spokes show only a side view, so any
stamp on the outside curve isn't apparent on the site:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3386&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3387&detail=butted

[Nervously re-checks straight Leader spokes, just to be
sure. Whew! No stamps.]

I'm wondering now if the outside curve of the elbow goes
into compression and therefore doesn't suffer much when
stamped or scratched, while the failures originate in the
inside curve of the elbow, where the curved inner surface
might go into tension.

The ones I have got are the "Race" model 2.0 1.8 double butted. The
"SAP" is printed on the outside of the elbow.

Quoted message said:

Where is the "SAP" (or whatever) stamped on the regular
round double butted spokes? On the inside of the elbow, the
outside curve, or on one side?

The "SAP" appears on the outside curve of the elbow in the
site pictures for the aero spokes:

The straight 2.0mm spokes I order from Dan's Competition (when I need a
special length cut) are stamped in exactly the same manner as the aero
spokes in the photo. In the past, I have bought straight 2.0mm spokes
form thorusa.com and they were the same.

I wondered whether it was a bad design idea, but so far I've had no
failures. That's not saying much, since spokes are one of the few bike
parts I seldom break.

Chalo Colina

say with the sapim experts in here
would one tell
what the SAPIM NIPPLE LOCK DEVICE is and if it works?

Quoted message said:

say with the sapim experts in here
would one tell
what the SAPIM NIPPLE LOCK DEVICE is and if it works?

Dear Gene,

It's basically a Nylock nut in the shape of a spoke nipple,
just some semi-soft plastic to lock the threaded end of the
spoke.

The idea is to prevent the unscrewing that some people
complain about (not everyone likes the taste of linseed oil
or Loc-Tite) and to give the rest of us something to debate.

Carl Fogel

Jim Smith said:

Quoted message said:

Jim Smith said:

[email hidden] writes:

Quoted message said:

>The only Sapim spokes that I have handy are some straight
>Leader spokes, not the CX aero spokes. The plain Leader
>spokes have no side stamp in real life, or in the picture on
>the Sapim site.

Actually, the ones I am looking at are their regular round double
butted (yeah yeah, I know, swaged) spokes. I don't go in much for the
fancy bladed stuff.

Dear Jim,

Where is the "SAP" (or whatever) stamped on the regular
round double butted spokes? On the inside of the elbow, the
outside curve, or on one side?

The "SAP" appears on the outside curve of the elbow in the
site pictures for the aero spokes:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3959&id=3388&detail=aero

This would be the back of a submarine's periscope, for lack
of a more articulate description.

The round double butted spokes show only a side view, so any
stamp on the outside curve isn't apparent on the site:

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3386&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3385&detail=butted
http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3387&detail=butted

[Nervously re-checks straight Leader spokes, just to be
sure. Whew! No stamps.]

I'm wondering now if the outside curve of the elbow goes
into compression and therefore doesn't suffer much when
stamped or scratched, while the failures originate in the
inside curve of the elbow, where the curved inner surface
might go into tension.

The ones I have got are the "Race" model 2.0 1.8 double butted. The
"SAP" is printed on the outside of the elbow.

they are stress risers, but as carl points out, they are less extreme
than the threads. the stress riser effect is a function of depth and
tip radius. the letters stamped into the spoke are pretty much flat
bottomed, much like the channel stamped into the top of a soda can to
create the opening tear, so the radius is minimized. a smaller but
sharper [smaller radiused] scratch or chain gouge can be much more serious.

Dan said:

"jim beam" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Quoted message said:

I think you are right. Ferrous metals typically have a true fatigue limit
or a stress that can theoretically be cycled forever without failure.

most quality spokes are stainless steel which has no fatigue endurance
limit.

be careful with the term "ferrous". technically, that means an alloy
based on iron, which includes all kinds of steels, stainless steels, some
with fatigue endurance limits, some without.

Yes Jim, There are no absolute truths in engineering. In the future I will
add a disclaimer at the end of every sentence so that you can relax. My old
materials engineering textbook says "Some materials, such as ferrous metals
and alloys, show a definite fatigue limit, whereas other materials..." I
suggest you take it up with the author Jastrzebski.

sorry dan, that's a mis-description. the *endurance limit* is the flat
line of the graph, like that for mild steel, showing an infinite fatigue
life below a threshold stress.

a *fatigue limit* is the stress at which a component will last an
arbitrary number of stress cycles, usually something like 10^7, but that
in no way implies an infinite fatigue life. quite the contrary in fact
because the lack of infinite life /forces/ you to "pick a number and
call it something". unfortunately, because both pertain to fatigue
durability, both phrases are often mistakenly interchanged. even i make
that mistake in conversation sometimes - it's /so/ pervasive.

unfortunately, it's therefore also common for people to assume that when
discussing "steel", anything made of any form of steel has an endurance
limit like low alloy carbon steel. that's not the case. it's a
function of crystal chemistry and carbon content. ferritic steels with
a moderate carbon content exhibit fatigue endurance because carbon can
diffuse in the comparatively open packed lattice and lock into
dislocations halting their progress, and fatigue with it. austenitic
steels, for example stainless steels, don't have the same crystal
chemistry and don't have the same carbon content, so there is no
mechanism by which dislocations can be arrested and fatigue can prosper
regardless.

Quoted message said:

Quoted message said:

Quoted message said:

To be useful a fatigue test has to be at stress levels above the fatigue
limit so that the piece will eventually fail.

"useful"? "useful" testing is that which maps the whole landscape, not
just one small portion of it.

You will note that my "test" is singular and your "testing" implys (to me) a
series of tests. A series of tests would be required to determine the s-n
chart for a single spoke design. I was thinking of a test to compare spokes
of different design or manufacture. If I run a single test on spoke design A
then one on spoke design B, I won't learn much if neither breaks.

isn't that what i said? you need to map the whole landscape.

Quoted message said:

Quoted message said:

be careful with the term "fatigue limit" that generally implies the
stress at which a given alloy [without an endurance limit] will endure "n"
cycles. it's not the same as an endurance limit, the stress at which an
alloy will endure an infinite number of cycles.

Geez, you and Jastrzebski have exactly opposite terminology. I will stick
with him until I review your qualifications. Jastrzebski sez: "Fatigue limit
is defined as the maximum stress that can be applied repeatedly an infinite
number of times without fracturing the material."

as i said before, it's common to confuse the two terms. just because
something is written, doesn't make it so. i can think of another book,
regarding bicycle wheels, that contains several such examples.

Quoted message said:

Quoted message said:

Quoted message said:

In actual use, I would hope that most spokes are cycled at loads below
the fatigue limit. The fatigue limit can vary due to alloy, treatment and
surface condition. You could reduce the fatigue strength of a polished
spoke by roughing it up with fine sandpaper.

that's correct - surface finish has a significant impact on fatigue life.
but if you want to sabotage your opponents spokes, make sure your
scratches are perpendicular to the stress axis, otherwise they will have
little effect.

Quoted message said:

jim beam said:

Quoted message said:

I think you are right. Ferrous metals typically have a true fatigue limit or
a stress that can theoretically be cycled forever without failure.

most quality spokes are stainless steel which has no fatigue endurance
limit.

So the quoted fatigue endurance limits for such common stainless alloy
steels as 301,302,303,304,307,309,310,316,321,347, and so on are
manufacturer's goofs?

Or are the steel companies in league with Jobst?

again, it's typical and pervasive for this mistake to be made. example,
even this manufacturer's web site:

http://www.sandmeyersteel.com/316-316L.html

contains the words:
"...fatigue strength or endurance limit..." at the beginning of their
fatigue paragraph. fatigigue strength is the arbitrary number function,
like the strength at 10^7 cycles, but that is not the same as endurance
limit, the stress at which an infinite number of cycles can be applied.

see my post to dan for a description of the different factors
responsible for differing behavior.

Quoted message said:

Quoted message said:

say with the sapim experts in here
would one tell
what the SAPIM NIPPLE LOCK DEVICE is and if it works?

Dear Gene,

It's basically a Nylock nut in the shape of a spoke nipple,
just some semi-soft plastic to lock the threaded end of the
spoke.

not all of them. the sapim nipples just have the threads internally
coated with a type of loctite - there's no nylon bushing.

Quoted message said:

The idea is to prevent the unscrewing that some people
complain about (not everyone likes the taste of linseed oil
or Loc-Tite) and to give the rest of us something to debate.

to answer gene's question, they work fine afaik. at least, i've not had
to true any of my wheels built with them.

Quoted message said:

Carl Fogel

jim beam said:

unfortunately, it's therefore also common for people to assume that when
discussing "steel", anything made of any form of steel has an endurance
limit like low alloy carbon steel. that's not the case. it's a
function of crystal chemistry and carbon content. ferritic steels with
a moderate carbon content exhibit fatigue endurance because carbon can
diffuse in the comparatively open packed lattice and lock into
dislocations halting their progress, and fatigue with it. austenitic
steels, for example stainless steels, don't have the same crystal
chemistry and don't have the same carbon content, so there is no
mechanism by which dislocations can be arrested and fatigue can prosper
regardless.

A couple comments:

First, the phrase "austenitic steels, for example stainless steels" is
a bit misleading. There are many stainless steels that are not
austenitic.

Second: I use DT spokes. I assume they're austenitic SS? (They're
not very magnetic, anyway). But the reason I use them is they have a
reputation for excellent fatigue resistance, borne out in my
experience. Some spokes are ferritic (or at least magnetic), but IME
their fatigue resistance isn't as good.

Does anyone know for sure the type of alloy in DT spokes? Any other
comments?

- Frank Krygowski