Spoke "elongation"

Serious bike tech/engineering question:

In ordinary use, do spokes (nonelastically, i.e. permanently) elongate
under normal use?

For years I *assumed* the answer was NO. That is, I believed that under
normal use, spokes would stretch under tension but - if detensioned -
return to their original length without any "creep", excepting a small,
permanent deformation at the J-bend that we often intentionally impose
while "setting" the spokes during a build.

[Also, we can get some inelastic changes when we intentionally
overstress / "stress relieve" the spokes at build time, again I think
mostly at the J-bend, but this - I believed - didn't occur during *use*.]

Then I looked up "yield strength" and calculated that, say, a 1.8mm
spoke under 120kgf of tension (cross-sectional area pi(.0009)^2 m^2,
and a tension of 1176 N) experiences 462 MPa of (what do we call this?
Pressure?)

.... and this is close to or above the published yield strength of some
stainless steels.

So now I'm confused. Engineers? Help?

Mark J.

On 4/11/2021 4:02 PM, Mark J. wrote:
Serious bike tech/engineering question:

In ordinary use, do spokes (nonelastically, i.e. permanently) elongate
under normal use?

For years I *assumed* the answer was NO.Â* That is, I believed that under
normal use, spokes would stretch under tension but - if detensioned -
return to their original length without any "creep", excepting a small,
permanent deformation at the J-bend that we often intentionally impose
while "setting" the spokes during a build.

[Also, we can get some inelastic changes when we intentionally
overstress / "stress relieve" the spokes at build time, again I think
mostly at the J-bend, but this - I believed - didn't occur during *use*.]

Then I looked up "yield strength" and calculated that, say, a 1.8mm
spoke under 120kgf of tension (cross-sectional area pi(.0009)^2 m^2,
and a tension of 1176 N) experiences 462 MPa of (what do we call this?
Pressure?)

... and this is close to or above the published yield strength of some
stainless steels.

So now I'm confused.Â* Engineers? Help?

Mark J.

PS - I just looked up both Jobst Brandt and John Allen's archived work
on Sheldon Brown's site; Jobst notes that "spokes are usually tensioned
no higher than 1/3 their yield stress" and Allen's "ear tensioning"
guide gives pitches to attain "approximately 1/3 of the yield strength
of good-quality spokes." So it wasn't my imagination that we should
have a good margin before reaching nonelastic elongation. But the
numbers I cited???

Mark J.

On 4/11/2021 6:02 PM, Mark J. wrote:
Serious bike tech/engineering question:

In ordinary use, do spokes (nonelastically, i.e.
permanently) elongate under normal use?

For years I *assumed* the answer was NO. That is, I
believed that under normal use, spokes would stretch under
tension but - if detensioned - return to their original
length without any "creep", excepting a small, permanent
deformation at the J-bend that we often intentionally impose
while "setting" the spokes during a build.

[Also, we can get some inelastic changes when we
intentionally overstress / "stress relieve" the spokes at
build time, again I think mostly at the J-bend, but this - I
believed - didn't occur during *use*.]

Then I looked up "yield strength" and calculated that, say,
a 1.8mm spoke under 120kgf of tension (cross-sectional area
pi(.0009)^2 m^2,
and a tension of 1176 N) experiences 462 MPa of (what do we
call this? Pressure?)

... and this is close to or above the published yield
strength of some stainless steels.

So now I'm confused. Engineers? Help?

Mark J.

I'm not an engineer but yes and no.
Yes, the bend at the head straightens or deforms to some extent.
No, the actual spoke material doesn't have tensile
elongation aside from head shape change.

see also 'chain stretch' which is a misnomer despite the net
effect of the thing being longer overall. The problem is in
semantics not physics.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On Sun, 11 Apr 2021 16:44:37 -0700, "Mark J."
wrote:

On 4/11/2021 4:02 PM, Mark J. wrote:
Serious bike tech/engineering question:

In ordinary use, do spokes (nonelastically, i.e. permanently) elongate
under normal use?

For years I *assumed* the answer was NO.* That is, I believed that under
normal use, spokes would stretch under tension but - if detensioned -
return to their original length without any "creep", excepting a small,
permanent deformation at the J-bend that we often intentionally impose
while "setting" the spokes during a build.

[Also, we can get some inelastic changes when we intentionally
overstress / "stress relieve" the spokes at build time, again I think
mostly at the J-bend, but this - I believed - didn't occur during *use*.]

Then I looked up "yield strength" and calculated that, say, a 1.8mm
spoke under 120kgf of tension (cross-sectional area pi(.0009)^2 m^2,
and a tension of 1176 N) experiences 462 MPa of (what do we call this?
Pressure?)

... and this is close to or above the published yield strength of some
stainless steels.

So now I'm confused.* Engineers? Help?

Mark J.

PS - I just looked up both Jobst Brandt and John Allen's archived work
on Sheldon Brown's site; Jobst notes that "spokes are usually tensioned
no higher than 1/3 their yield stress" and Allen's "ear tensioning"
guide gives pitches to attain "approximately 1/3 of the yield strength
of good-quality spokes." So it wasn't my imagination that we should
have a good margin before reaching nonelastic elongation. But the
numbers I cited???

Mark J.

Well, as Sherlock said, When you have eliminated the impossible,
whatever remains, however improbable, must be the truth.
--
Cheers,

John B.

On 4/11/2021 7:52 PM, AMuzi wrote:
On 4/11/2021 6:02 PM, Mark J. wrote:
Serious bike tech/engineering question:

In ordinary use, do spokes (nonelastically, i.e.
permanently) elongate under normal use?

For years I *assumed* the answer was NO.Â* That is, I
believed that under normal use, spokes would stretch under
tension but - if detensioned - return to their original
length without any "creep", excepting a small, permanent
deformation at the J-bend that we often intentionally impose
while "setting" the spokes during a build.

[Also, we can get some inelastic changes when we
intentionally overstress / "stress relieve" the spokes at
build time, again I think mostly at the J-bend, but this - I
believed - didn't occur during *use*.]

Then I looked up "yield strength" and calculated that, say,
a 1.8mm spoke under 120kgf of tension (cross-sectional area
pi(.0009)^2 m^2,
and a tension of 1176 N) experiences 462 MPa of (what do we
call this? Pressure?)

... and this is close to or above the published yield
strength of some stainless steels.

So now I'm confused.Â* Engineers? Help?

Mark J.

I'm not an engineer but yes and no.
Yes, the bend at the head straightens or deforms to some extent.
No, the actual spoke material doesn't have tensile elongation aside from
head shape change.

I agree with Andrew. I think the part that's troubling you, Mark, is the
strength of the SS alloy. I don't know offhand what alloy is typical for
stainless steel bike spokes, but I'd assume it's very high quality stuff.

And it's certainly cold drawn, which can induce very high strength. Cold
drawing is the main reason why cables made of many strands (for brakes
or derailleurs or suspension bridges) are so much stronger per square
inch than solid steel rods of comparable diameter.


--
- Frank Krygowski