Spoke stress relief test

Quoted message said:

Deformation as a result of removing some material does indicate
residual stress. I'm aware of two situations where this is important.
First, machinists know that metal parts with residual stresses can
distort when, say, a surface is milled or ground. The stress balance
(so to speak) is changed, and the part warps.

Exactly what I was thinking about.

Quoted message said:

perhaps a fixture could be constructed to grip the spoke firmly at the
head and just beyond, leaving the curve and long blade free to move.
Then a small amount of the bend's outside could be removed using a
Dremel, being careful to keep everything cool. Assuming the outside of
the bend had compressive residual stresses, the spoke would be seen to
straighten a bit. This could be measureable at the free end, using
Carl's graph paper.

I was thinking along similar lines. But I would like to see the
operation occur with the spoke bathed in cooling fluid (i.e. water). If
that's not feasible, perhaps a slower method of removing material could
be used to avoid heating. In my experience, a Dremel tool creates quite
a lot of heat in the grinding region. Perhaps it could be done with a
lot of patience and a fairly fine grid Dremel attachment, but it might
be easier to do it with a cooling fluid.

I can't think of any tools I have that would allow me to do this with a
cooling fluid. I don't have a tensiometer, so I can't even repeat Carl's
original setup. So I guess that relegates me to the role of armchair
quarterback.

--
Dave
dvt at psu dot edu

dvt said:

Quoted message said:

Deformation as a result of removing some material does indicate
residual stress. I'm aware of two situations where this is important.
First, machinists know that metal parts with residual stresses can
distort when, say, a surface is milled or ground. The stress balance
(so to speak) is changed, and the part warps.

Exactly what I was thinking about.

Quoted message said:

perhaps a fixture could be constructed to grip the spoke firmly at the
head and just beyond, leaving the curve and long blade free to move.
Then a small amount of the bend's outside could be removed using a
Dremel, being careful to keep everything cool. Assuming the outside of
the bend had compressive residual stresses, the spoke would be seen to
straighten a bit. This could be measureable at the free end, using
Carl's graph paper.

I was thinking along similar lines. But I would like to see the
operation occur with the spoke bathed in cooling fluid (i.e. water). If
that's not feasible, perhaps a slower method of removing material could
be used to avoid heating. In my experience, a Dremel tool creates quite
a lot of heat in the grinding region. Perhaps it could be done with a
lot of patience and a fairly fine grid Dremel attachment, but it might
be easier to do it with a cooling fluid.

I can't think of any tools I have that would allow me to do this with a
cooling fluid. I don't have a tensiometer, so I can't even repeat Carl's
original setup. So I guess that relegates me to the role of armchair
quarterback.

Dear Dave and Frank,

I'm gonna be mean about this.

Bend a spoke at the midspan into a tight-radius U-shape.

Measure the bend by marking things on graph paper. Here's a site that
will print squares of whatever size you like:

http://incompetech.com/beta/linedGraphPaper/easy.html

Clamp the bent spoke ends in a vise with the bend sticking up.

Very gently apply a fine-tooth file, pausing between strokes. Heat
will be insignificant.

A flat file will work for the outer bend. A round chainsaw file will
do the inner bend.

Dial calipers will give a rough idea of how much metal is removed.

The stainless steel is soft and it's only 1.8 or 2 mm thick, so you
shouldn't have much trouble removing an eighth or a quarter of its
thickness.

After removing some metal, release the vise, put the spoke back on the
graph paper, and see if the ends have moved.

If the bend changes, I'll see about duplicating your results with a
bent but untensioned spoke and then again with a U-bend spoke whose
tension has been raised on my vise-rig.

I don't know what will happen or what it will mean, since Luns Tee is
still working on what layers lie where, according to the bend radius
versus the spoke thickness.

Cheers,

Carl Fogel

Quoted message said:

Dear Dave and Frank,

I'm gonna be mean about this.

You'll have to try harder.

Quoted message said:

Bend a spoke at the midspan into a tight-radius U-shape.

Measure the bend by marking things on graph paper. Here's a site that
will print squares of whatever size you like:

http://incompetech.com/beta/linedGraphPaper/easy.html

Clamp the bent spoke ends in a vise with the bend sticking up.

This sounds fine so far...

Quoted message said:

Very gently apply a fine-tooth file, pausing between strokes. Heat
will be insignificant.

A flat file will work for the outer bend. A round chainsaw file will
do the inner bend.

Dial calipers will give a rough idea of how much metal is removed.

I guess I should have calipers, but I don't. (insert links to Harbor
Freight here, Carl the Shill 🙂

Quoted message said:

The stainless steel is soft and it's only 1.8 or 2 mm thick, so you
shouldn't have much trouble removing an eighth or a quarter of its
thickness.

"Very gently apply a fine-tooth file... you shouldn't have much trouble
removing an eighth or a quarter of its thickness." I might give it a
crack this weekend, but I'm not holding out much hope that both of these
will be true.

--
Dave
dvt at psu dot edu

Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)

Quoted message said:

When bent at the elbow, spokes are expected to have residual stresses.

It's often claimed that squeezing spoke pairs together will relieve
these residual stresses by raising the spoke tension above normal.

Unfortunately, before and after stresses have never been shown to
confirm or refute this theory, nor has any formal testing ever been
published to show whether squeezed spokes outlast unsqueezed spokes.
Fatigue testing for spokes is incredibly tedious, taking months on
expensive rigs, so there's little hope in that direction.

A further complication is that squeezing may also improve the wheel
build in other ways, such as seating spoke heads and nipples,
releasing spoke wind-up, and simply encouraging care in other steps.
So even if squeezed spokes do last longer, it may have nothing to do
with stress relief.

Neutron diffraction images are supposed to reveal stresses, but the
neutron diffractor at Fogel Labs has been on the blink for a long
time.

However, Fogel Labs has a propane torch that works just fine. Heat is
generally agreed to relieve residual stresses, and a propane torch
will heat a spoke bend to a cheery orange glow in a few seconds,
relieving any residual stresses. The heated spoke visibly bends to a
new angle as the stresses are relieved.

Alas, initial testing showed an odd problem when old-fashioned carbon
spokes were sacrificed on the altar of stress-relief. Bent in the
middle and toasted, the carbon spokes bent impressively to a new
angle. But they turned out to bend the wrong way, opposite of the
direction in which modern stainless steel spokes bend when toasted.

Hmmm . . . the general opinion in materials circles was that the
carbon steel went through a phase change that overwhelmed the change
in residual stresses. Stainless steel, on the other hand, is not
expected to suffer such phase changes, so testing proceeded with
modern spokes.

The next problem was that factory elbow bends are too tiny, too short,
and too awkward for any useful testing. If they bent after toasting,
the angle was too small to be seen, even with camera magnification.

So a spoke-bending rig was cobbled together:

http://server5.theimagehosting.com/image.php?img=339a_spoke_rig.jpg
or http://tinyurl.com/y9kmws

A U-bend is made in the middle of the spoke by spreading the jaws of
the vise. The vise tension will easily draw the elbow bend straight:

http://server5.theimagehosting.com/image.php?img=340a_spoke_rig_165kgf.jpg
or http://tinyurl.com/ycnr2f

After tensioning, the elbow end must be cut off to remove the spoke:

http://server5.theimagehosting.com/image.php?img=341a_spoke_rig_elbow_cut.jpg
or http://tinyurl.com/yenfz9

At first, the tensioned spoke was squeezed with pliers to mimic the
typical hand-squeezing. But pliers are so powerful that they left
bends in the spokes that were already at about 165 kgf:

http://server5.theimagehosting.com/image.php?img=342a_spoke_squeeze_bends.jpg
or http://tinyurl.com/tdkf3

It turned out to be much simpler to skip the pliers and just use the
vise and tension gauge to raise spoke tension to the desired levels.

Here are four straight 2mm stainless steel Sapim Leader spokes, each
bent in the vise rig and faintly marked at their ends and one side on
2mm graph paper:

http://i12.tinypic.com/2zgtfs5.jpg

From left to right:
Hand-bent, no tension 0 kgf
Elbow straightened, ~76 kgf / 167 lbs (Park gauge mark 21)
Tensioned more, ~121 kgf / 266 lbs (Park gauge mark 25)
Tensioned like crazy, ~179 kgf / 394 lbs (Park gauge mark 28)

Previous experiments found no wheels where tension rose more than
55~65 lbs when opposite pairs of spokes were squeezed with 60 lbs
of force. The spokes bend impressively, giving rise to mistaken
calculations of huge tension increases, but the rim is merely
bending into a faint N-shape that provides slack for the spokes.

After the U-bends were all heated to an orange glow with a propane
torch, all residual stresses were presumably relieved. The faint marks
on the graph paper were enhanced with a red Sharpie:

http://i11.tinypic.com/2qsc507.jpg

The hand-bent spoke shows a noticeable change. In fact, it had to be
set a little to one side to avoid hitting the next spoke. Obviously,
the residual stresses were relieved by heating.

The other three spokes show no significant change after heating.

The test suggests that practically all residual stress at the spoke
elbow is mechanically relieved long before the spokes are brought to
ordinary tension.

That is, just 76 kgf is enough to eliminate residual stresses in 2mm
stainless steel spokes bent into a U-shape.

I'll try to accomodate any suggestions for other tests.

Anyone who wants to pursue such tests can do so with some spokes (the
longer the better), a propane torch, a vise, bolts to stick in the
vise-jaw-plate holes, wire-cutters, and graph paper. A tension gauge
is nice, but not necessary.

Here's the vise, which needed 8x1.25mm bolts:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=5655

It's on sale for about $40, and Harbor Freight often offers discounts
in newspaper ads and fliers.

Here's a nice site with a bad name for printing graph paper pdf's at
any size:

http://incompetech.com/beta/linedGraphPaper/easy.html

Here are long Sapim spokes without nipples, $6.50 per 20:

http://www.biketoolsetc.com/index.cgi?id=663198764619&d=single&c=Components&sc=Wheel-and-Rim&tc=Spokes/Straight-Gauge&item_id=SA-LSG14304

BikeToolsEtc also sells nipples.

Cheers,

Carl Fogel

Some emails have asked about how much spoke tension rises when spokes
are squeezed together, so I cobbled together another demonstration.

Briefly, spoke tension on a bicycle wheel rises only about as much as
the squeeze force, a 1-to-1 ratio. When two pairs of spokes are
squeezed together, some of them rise a litle more, some a little less,
but the differences are insignificant compared to the 1-to-1 pattern.

That is, a 60-lb squeeze force raises spoke tension only about 60 lbs
on a bicycle wheel, despite impressive bending.

The reason is that bicycle rims distort into faint N or Z shapes when
two pairs of spokes are squeezed, one pair on either side of the rim.
The tension increase is nowhere near the huge amount that simple
calculations based on angles predict because even the slight amount of
slack from the rim distortion produces wild bend angles.

An eight-dollar 3/4-inch pipe clamp can be misused to tension straight
spokes. The pipe doesn't distort nearly as much as an aluminum box
rim.

I tested a new 298 mm Sapim straight 14 gauge spoke. If you want to
try it, reverse the dumb-end of the pipe clamp to allow pulling.
Otherwise, the crank-end will just drag it down the pipe.

The pipe clamp had pairs of off-center but still convenient holes that
made it easy to attach the spoke. A washer worked to stop the nipple
end, and a drilled piece of angle-iron worked as a fake hub flange to
preserve the spoke elbow.

Here's the clamp:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=94053

You can see the pairs of holes. The little standoff legs make the rest
of the test much easier, so I spent the extra $4 instead of getting a
cheaper pipe clamp with the finer thread crank. Once you snug the
spokeup, you can use the spoke nipple to tighten things the rest of
the way.

I tightened the spoke to a little past Park mark 24, which means 107
kgf or 235 lbs of tension for a 2 mm stainless steel spoke.

Then I hung a ripping crowbar and four roughly 15 lb weights from the
spoke midspan. The tension rose to roughly Park mark 26.5, which would
be around 147 kgf or 323 lbs, a roughly 90-lb rise.

Here's a picture:

http://i13.tinypic.com/2yo7778.jpg

Notice that the stiff pipe prevents the spoke from bending much. Most
posters can bend their spokes far more with a hand-squeeze. If
anything, the close camera angle exaggerates the bend.

The high 90-to-60 ratio of tension rise to squeeze force (1.5 to 1) is
probably due to the much stiffer spoke bracing provided by the pipe.

(You can true an aluminum rim quite easily with small spoke tension
increases, but you can't true a 3/4 inch steel pipe much with a single
spoke. The pipe wall is thicker than the 2 mm spoke.)

Here's what happened when a bicycle rim was tested with 2 spokes at
around 250 lbs initial tension with weights from 0 to 100 lbs in 5-lb
increments:

http://home.comcast.net/~carlfogel/download/newspok2.jpg

With the rim distorting, a 60-lb squeeze force raised the tension only
about 60 lbs, from 250 to around 310, a 1-to-1 ratio. Even the 100-lb
squeeze force produced only a 1-to-1 ratio, about 90 lbs of tension
rise for 100 lbs of squeeze.

The 60-lb force is used because it's more than most posters are likely
to produce with one hand and because much more force will bend spokes
in bicycle wheels so alarmingly that it leaves permanent bends in the
spoke midspans.

To get an idea of the force involved, squeeze a bathroom scale with
both hands and divide by two, or try to raise a 60-lb weight hung from
a rope or handle with your weak hand's thumb and palm braced against
the edge of a table. Do this 8 or 9 times in a minute or so to mimic
spoke-squeezing a 32 or 36 spoke wheel.

Cheers,

Carl Fogel

dvt said:

Quoted message said:

Dear Dave and Frank,

I'm gonna be mean about this.

You'll have to try harder.

Quoted message said:

Bend a spoke at the midspan into a tight-radius U-shape.

Measure the bend by marking things on graph paper. Here's a site that
will print squares of whatever size you like:

http://incompetech.com/beta/linedGraphPaper/easy.html

Clamp the bent spoke ends in a vise with the bend sticking up.

This sounds fine so far...

Quoted message said:

Very gently apply a fine-tooth file, pausing between strokes. Heat
will be insignificant.

A flat file will work for the outer bend. A round chainsaw file will
do the inner bend.

Dial calipers will give a rough idea of how much metal is removed.

I guess I should have calipers, but I don't. (insert links to Harbor
Freight here, Carl the Shill 🙂

Quoted message said:

The stainless steel is soft and it's only 1.8 or 2 mm thick, so you
shouldn't have much trouble removing an eighth or a quarter of its
thickness.

"Very gently apply a fine-tooth file... you shouldn't have much trouble
removing an eighth or a quarter of its thickness." I might give it a
crack this weekend, but I'm not holding out much hope that both of these
will be true.

Dear Dave,

Yes, you should have calipers.

F'r cryin' out loud, Harbor Freight has a digital caliper on sale for
eight bucks.

How can you split hairs if you can't measure their thickness?

And I'm gonna be mean again.

You'll have to type in harbor freight's secret url and search for
calipers. Click on "retail stores" and there are 12 outlets in
Pennsylvania.

They have files and vises, too.

Cheers,

Carl Fogel

In article <[email hidden]>,

Quoted message said:

Clamp the bent spoke ends in a vise with the bend sticking up.

Very gently apply a fine-tooth file, pausing between strokes. Heat
will be insignificant.

I suggest clamping only one side of the bend, so that the bend
is free to uncurl (or re-curl) as you file. Otherwise, as you're
removing stressed material, the stress distribution in the rest of the
spoke changes, and with the bend restrained from responding, may
change enough to yield material elsewhere.

I think I've unwittingly done this experiment in the past, but
with 12 gauge solid copper wire that'd been bent to go around a screw
terminal. Filing the outside of the loop, the loop opens up at first.

I haven't tried, but expect that if you keep filing deeper (into
unyielded material), the response will invert and the loop starts
tightening up again.

-Luns

Quoted message said:

dvt said:

Quoted message said:

Deformation as a result of removing some material does indicate
residual stress. I'm aware of two situations where this is important.
First, machinists know that metal parts with residual stresses can
distort when, say, a surface is milled or ground. The stress balance
(so to speak) is changed, and the part warps.

Exactly what I was thinking about.

Quoted message said:

perhaps a fixture could be constructed to grip the spoke firmly at the
head and just beyond, leaving the curve and long blade free to move.
Then a small amount of the bend's outside could be removed using a
Dremel, being careful to keep everything cool. Assuming the outside of
the bend had compressive residual stresses, the spoke would be seen to
straighten a bit. This could be measureable at the free end, using
Carl's graph paper.

....

Quoted message said:

Dear Dave and Frank,

I'm gonna be mean about this.

Bend a spoke at the midspan into a tight-radius U-shape....

I didn't think that was _very_ mean, but:

I suggested the rig I did because I'm not positive whether the details
of spoke manufacturing have any unanticipated effect at the head. IOW,
maybe something about their bend is different than your bend. Why not
test the actual item?

OK, here's the "why not": Because it's not as easy. The geometry is
tiny, it's hard to hold the spoke precisely, etc. And yes, your test
rig might give exactly the same general results.

But I thought your interest was in the effect of Jobst's "stress
relieving" technique on the bend in spokes near the head. I think
better data comes from testing exactly that, if possible.

My method will require a bit of machine shop time to produce a fixture,
and I've got other things on my plate at the moment that are more
interesting to me - for example, waiting for the effects of anaesthetic
to wear off. I may attack this in the future, if it rises to the top
of my list. Or - other possibility - I may not.

Oh, FWIW I've owned calipers and mikes for many, many years now.

- Frank Krygowski

On Fri, 10 Nov 2006 03:00:26 +0000 (UTC), [email hidden]

(Luns Tee) said:

In article <[email hidden]>,

Quoted message said:

Clamp the bent spoke ends in a vise with the bend sticking up.

Very gently apply a fine-tooth file, pausing between strokes. Heat
will be insignificant.

I suggest clamping only one side of the bend, so that the bend
is free to uncurl (or re-curl) as you file. Otherwise, as you're
removing stressed material, the stress distribution in the rest of the
spoke changes, and with the bend restrained from responding, may
change enough to yield material elsewhere.

I think I've unwittingly done this experiment in the past, but
with 12 gauge solid copper wire that'd been bent to go around a screw
terminal. Filing the outside of the loop, the loop opens up at first.

I haven't tried, but expect that if you keep filing deeper (into
unyielded material), the response will invert and the loop starts
tightening up again.

-Luns

Dear Luns,

Your idea of clamping just one leg sounds like a better approach.

Even if the difference turned out to be insignificant, clamping only
one leg removes that potential confusion.

It's so good that I'm having to restrain myself from trying it, but
I'm still gonna be mean and insist that someone else spend ten minutes
actually doing a test.

After all, there's no real evidence that I'm not a troop of Girl
Scouts in North Dakota, whacked on meth and having fun with a pirated
photoshop program.

If someone bends a spoke, marks the leg positions on some graph paper,
applies a file, and finds a change, I'll be happy to try a few samples
with some tension applied in a vise-rig and measured with a tension
gauge.

It's amazing how many odd things you can learn with a used tire and
some wheels, a few bucks worth of spokes, a tension gauge, some
weights, a vise, some graph paper, the post office scales, a
speedometer hooked up to a rear wheel, a bike pump with a dial gauge,
a propane torch, some dial calipers, and the stopwatch function of a
wrist watch.

Oh, I forgot the high-tech plastic jar full of 10w-40 motor oil that I
stuffed a piece of inner tube into.

Anyone who thinks that popping plastic bubble wrap is fun should soak
a piece of inner tube in oil for a few months. At first, you think
nothing happened, but then you find that you've turned into Superman
and can tear a butyl rubber inner tube into confetti with your
steel-like fingers:

http://groups.google.com/group/rec.bicycles.tech/msg/520b44c3705d32ea

To lend dignity to the proceedings, I eventually dunked and weighed a
tube to see how much oil it soaked up, then inflated it, with
explosive results:

http://groups.google.com/group/rec.bicycles.tech/msg/5d078751e537efa2

Cheers,

Carl Fogel

Quoted message said:

Quoted message said:

dvt said:

[email hidden] wrote:
> Deformation as a result of removing some material does indicate
> residual stress. I'm aware of two situations where this is important.
> First, machinists know that metal parts with residual stresses can
> distort when, say, a surface is milled or ground. The stress balance
> (so to speak) is changed, and the part warps.

Exactly what I was thinking about.

> perhaps a fixture could be constructed to grip the spoke firmly at the
> head and just beyond, leaving the curve and long blade free to move.
> Then a small amount of the bend's outside could be removed using a
> Dremel, being careful to keep everything cool. Assuming the outside of
> the bend had compressive residual stresses, the spoke would be seen to
> straighten a bit. This could be measureable at the free end, using
> Carl's graph paper.

...

Quoted message said:

Dear Dave and Frank,

I'm gonna be mean about this.

Bend a spoke at the midspan into a tight-radius U-shape....

I didn't think that was _very_ mean, but:

I suggested the rig I did because I'm not positive whether the details
of spoke manufacturing have any unanticipated effect at the head. IOW,
maybe something about their bend is different than your bend. Why not
test the actual item?

OK, here's the "why not": Because it's not as easy. The geometry is
tiny, it's hard to hold the spoke precisely, etc. And yes, your test
rig might give exactly the same general results.

But I thought your interest was in the effect of Jobst's "stress
relieving" technique on the bend in spokes near the head. I think
better data comes from testing exactly that, if possible.

My method will require a bit of machine shop time to produce a fixture,
and I've got other things on my plate at the moment that are more
interesting to me - for example, waiting for the effects of anaesthetic
to wear off. I may attack this in the future, if it rises to the top
of my list. Or - other possibility - I may not.

Oh, FWIW I've owned calipers and mikes for many, many years now.

- Frank Krygowski

Dear Frank,

I may be able to save you some wasted time.

A spoke elbow is a nasty little beast. It's very short, it's
annoyingly rounded, and it's hard to fix in exactly the same plane
twice.

I magnified pictures of toasted elbows to ridiculous sizes and applied
onscreen rulers and protractors, but I couldn't get any clear results.

Here's an elbow with a faint diagonal chisel mark that I thought might
let me see if its bend changed after heating:

http://i13.tinypic.com/33f591e.jpg

Here's the same elbow after heating:

http://i13.tinypic.com/2qjeg05.jpg

Can you tell if that untensioned elbow L-bend opened up after heating
as much as one of my untensioned V-bends?

I couldn't tell, but you may have better equipment, more time, and
superior techniques.

Maybe you have a fixture in mind that will be so precise that it can
get around the measurement problem?

I decided to make my own bends in the middle, mark things on graph
paper, and see what happened. As far as I know, it's the only test
that's been done.

Cheers,

Carl Fogel

Quoted message said:

Quoted message said:

When bent at the elbow, spokes are expected to have residual stresses.

It's often claimed that squeezing spoke pairs together will relieve
these residual stresses by raising the spoke tension above normal.

Unfortunately, before and after stresses have never been shown to
confirm or refute this theory, nor has any formal testing ever been
published to show whether squeezed spokes outlast unsqueezed spokes.
Fatigue testing for spokes is incredibly tedious, taking months on
expensive rigs, so there's little hope in that direction.

A further complication is that squeezing may also improve the wheel
build in other ways, such as seating spoke heads and nipples,
releasing spoke wind-up, and simply encouraging care in other steps.
So even if squeezed spokes do last longer, it may have nothing to do
with stress relief.

Neutron diffraction images are supposed to reveal stresses, but the
neutron diffractor at Fogel Labs has been on the blink for a long
time.

However, Fogel Labs has a propane torch that works just fine. Heat is
generally agreed to relieve residual stresses, and a propane torch
will heat a spoke bend to a cheery orange glow in a few seconds,
relieving any residual stresses. The heated spoke visibly bends to a
new angle as the stresses are relieved.

Alas, initial testing showed an odd problem when old-fashioned carbon
spokes were sacrificed on the altar of stress-relief. Bent in the
middle and toasted, the carbon spokes bent impressively to a new
angle. But they turned out to bend the wrong way, opposite of the
direction in which modern stainless steel spokes bend when toasted.

Hmmm . . . the general opinion in materials circles was that the
carbon steel went through a phase change that overwhelmed the change
in residual stresses. Stainless steel, on the other hand, is not
expected to suffer such phase changes, so testing proceeded with
modern spokes.

The next problem was that factory elbow bends are too tiny, too short,
and too awkward for any useful testing. If they bent after toasting,
the angle was too small to be seen, even with camera magnification.

So a spoke-bending rig was cobbled together:

http://server5.theimagehosting.com/image.php?img=339a_spoke_rig.jpg
or http://tinyurl.com/y9kmws

A U-bend is made in the middle of the spoke by spreading the jaws of
the vise. The vise tension will easily draw the elbow bend straight:

http://server5.theimagehosting.com/image.php?img=340a_spoke_rig_165kgf.jpg
or http://tinyurl.com/ycnr2f

After tensioning, the elbow end must be cut off to remove the spoke:

http://server5.theimagehosting.com/image.php?img=341a_spoke_rig_elbow_cut.jpg
or http://tinyurl.com/yenfz9

At first, the tensioned spoke was squeezed with pliers to mimic the
typical hand-squeezing. But pliers are so powerful that they left
bends in the spokes that were already at about 165 kgf:

http://server5.theimagehosting.com/image.php?img=342a_spoke_squeeze_bends.jpg
or http://tinyurl.com/tdkf3

It turned out to be much simpler to skip the pliers and just use the
vise and tension gauge to raise spoke tension to the desired levels.

Here are four straight 2mm stainless steel Sapim Leader spokes, each
bent in the vise rig and faintly marked at their ends and one side on
2mm graph paper:

http://i12.tinypic.com/2zgtfs5.jpg

From left to right:
Hand-bent, no tension 0 kgf
Elbow straightened, ~76 kgf / 167 lbs (Park gauge mark 21)
Tensioned more, ~121 kgf / 266 lbs (Park gauge mark 25)
Tensioned like crazy, ~179 kgf / 394 lbs (Park gauge mark 28)

Previous experiments found no wheels where tension rose more than
55~65 lbs when opposite pairs of spokes were squeezed with 60 lbs
of force. The spokes bend impressively, giving rise to mistaken
calculations of huge tension increases, but the rim is merely
bending into a faint N-shape that provides slack for the spokes.

After the U-bends were all heated to an orange glow with a propane
torch, all residual stresses were presumably relieved. The faint marks
on the graph paper were enhanced with a red Sharpie:

http://i11.tinypic.com/2qsc507.jpg

The hand-bent spoke shows a noticeable change. In fact, it had to be
set a little to one side to avoid hitting the next spoke. Obviously,
the residual stresses were relieved by heating.

The other three spokes show no significant change after heating.

The test suggests that practically all residual stress at the spoke
elbow is mechanically relieved long before the spokes are brought to
ordinary tension.

That is, just 76 kgf is enough to eliminate residual stresses in 2mm
stainless steel spokes bent into a U-shape.

I'll try to accomodate any suggestions for other tests.

Anyone who wants to pursue such tests can do so with some spokes (the
longer the better), a propane torch, a vise, bolts to stick in the
vise-jaw-plate holes, wire-cutters, and graph paper. A tension gauge
is nice, but not necessary.

Here's the vise, which needed 8x1.25mm bolts:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=5655

It's on sale for about $40, and Harbor Freight often offers discounts
in newspaper ads and fliers.

Here's a nice site with a bad name for printing graph paper pdf's at
any size:

http://incompetech.com/beta/linedGraphPaper/easy.html

Here are long Sapim spokes without nipples, $6.50 per 20:

http://www.biketoolsetc.com/index.cgi?id=663198764619&d=single&c=Components&sc=Wheel-and-Rim&tc=Spokes/Straight-Gauge&item_id=SA-LSG14304

BikeToolsEtc also sells nipples.

Cheers,

Carl Fogel

Some emails have asked about how much spoke tension rises when spokes
are squeezed together, so I cobbled together another demonstration.

Briefly, spoke tension on a bicycle wheel rises only about as much as
the squeeze force, a 1-to-1 ratio. When two pairs of spokes are
squeezed together, some of them rise a litle more, some a little less,
but the differences are insignificant compared to the 1-to-1 pattern.

That is, a 60-lb squeeze force raises spoke tension only about 60 lbs
on a bicycle wheel, despite impressive bending.

The reason is that bicycle rims distort into faint N or Z shapes when
two pairs of spokes are squeezed, one pair on either side of the rim.
The tension increase is nowhere near the huge amount that simple
calculations based on angles predict because even the slight amount of
slack from the rim distortion produces wild bend angles.

An eight-dollar 3/4-inch pipe clamp can be misused to tension straight
spokes. The pipe doesn't distort nearly as much as an aluminum box
rim.

I tested a new 298 mm Sapim straight 14 gauge spoke. If you want to
try it, reverse the dumb-end of the pipe clamp to allow pulling.
Otherwise, the crank-end will just drag it down the pipe.

The pipe clamp had pairs of off-center but still convenient holes that
made it easy to attach the spoke. A washer worked to stop the nipple
end, and a drilled piece of angle-iron worked as a fake hub flange to
preserve the spoke elbow.

Here's the clamp:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=94053

You can see the pairs of holes. The little standoff legs make the rest
of the test much easier, so I spent the extra $4 instead of getting a
cheaper pipe clamp with the finer thread crank. Once you snug the
spokeup, you can use the spoke nipple to tighten things the rest of
the way.

I tightened the spoke to a little past Park mark 24, which means 107
kgf or 235 lbs of tension for a 2 mm stainless steel spoke.

Then I hung a ripping crowbar and four roughly 15 lb weights from the
spoke midspan. The tension rose to roughly Park mark 26.5, which would
be around 147 kgf or 323 lbs, a roughly 90-lb rise.

Here's a picture:

http://i13.tinypic.com/2yo7778.jpg

Notice that the stiff pipe prevents the spoke from bending much. Most
posters can bend their spokes far more with a hand-squeeze. If
anything, the close camera angle exaggerates the bend.

The high 90-to-60 ratio of tension rise to squeeze force (1.5 to 1) is
probably due to the much stiffer spoke bracing provided by the pipe.

(You can true an aluminum rim quite easily with small spoke tension
increases, but you can't true a 3/4 inch steel pipe much with a single
spoke. The pipe wall is thicker than the 2 mm spoke.)

Here's what happened when a bicycle rim was tested with 2 spokes at
around 250 lbs initial tension with weights from 0 to 100 lbs in 5-lb
increments:

http://home.comcast.net/~carlfogel/download/newspok2.jpg

With the rim distorting, a 60-lb squeeze force raised the tension only
about 60 lbs, from 250 to around 310, a 1-to-1 ratio. Even the 100-lb
squeeze force produced only a 1-to-1 ratio, about 90 lbs of tension
rise for 100 lbs of squeeze.

The 60-lb force is used because it's more than most posters are likely
to produce with one hand and because much more force will bend spokes
in bicycle wheels so alarmingly that it leaves permanent bends in the
spoke midspans.

To get an idea of the force involved, squeeze a bathroom scale with
both hands and divide by two, or try to raise a 60-lb weight hung from
a rope or handle with your weak hand's thumb and palm braced against
the edge of a table. Do this 8 or 9 times in a minute or so to mimic
spoke-squeezing a 32 or 36 spoke wheel.

Cheers,

Carl Fogel

This seems like a good place to add a post showing the same pipe-clamp
spoke-tension rig in action, this time with a measuring rod added to
show how little the rig deforms.

Here's an overhead view of the same pipe-clamp rig with the same 2mm
straight spoke tensioned to the same Park mark 24 (107 kgf, or 235
lbs) just like the previous test.

Click on the lower right in Explorer to see the enlarged photo:

http://i11.tinypic.com/30u8spe.jpg

The vise-clamp chain-tool is loose, but wrapped around the spoke and
3/4" steel pipe, whose walls are ~3 mm thick.

The thin ruler is attached with yellow tape to the left side of the
pipe-clamp, as close as possible to the spoke. The ruler's right end
is lying free on the right-hand pipe-clamp. It's a 132-column printer
bail, marked in 0.1" increments.

The red pipe-clamp's right-hand inner-edge is at about 12.15" on the
ruler (between marks 121 and 122). If the pipe-clamp deforms when the
spoke is squeezed, the ruler will show the movement.

Next is a side view of the spoke pulled downward with considerable
force by the vise-grip with a chain attachment:

http://i10.tinypic.com/4gryvqu.jpg

Again, click on the lower right in Explorer to see the enlarged photo.
The spoke goes into the solid red clamp on the left, not the slanted
shiny steel plates.

The small displacement and gentle bend angle are the modest result
when a spoke is attached to two very stout anchor-points instead of
starting at a bicycle hub, bending around another spoke at the
crossing, and finishing up at a relatively flimsy rim. The dramatic
bending of spoke-pairs squeezed in a wheel is greatly reduced.

And here's the view downward again, showing that the Park gauge
indicates around mark 26.5 (as in the previous test), and that the
pipe-clamp rig has not noticeably deformed in relation to the free end
of the ruler. The inner edge of the right-hand pipe-clamp still sits
at about 12.15" (between marks 121 and 122) on the free end of the
ruler attached to the left-hand clamp:

http://i10.tinypic.com/2qd39cg.jpg

Both times, the tension gauge rose from Park mark 24 (107 kgf, or 235
lbs) to Park mark 26.5 (147 kgf, or 323 pounds), a rise of ~90 pounds
for a ~60 pound squeeze force.

The first time, the force was provided by a measured but awkward 60-lb
weight that would have gotten in the way of any ruler showing how much
the pipe-clamp deformed.

The second time, the force was provided by the vise-grip chain-tool,
which isn't as precise as a measured weight, but which left room for
the ruler--which showed that the ends of the pipe clamp didn't change
0.05" (~1mm) relative to each other.

This pipe-clamp rig is as close as we'll get to the idealized two
fixed-anchor points that beguile some posters who try to calculate
spoke tension changes by applying pure theory instead of gauges.

It is unlikely that any spoked bicycle wheel is this rigid, and
bicycle spokes bend around each other at the crossing. The rim moves
up to 6 mm sideways when spokes are squeezed by hand, the crossing
slides up to 10 mm, and the second bend's tension is ignored by
idealized calculations.

Under the best possible conditions (the pipe-clamp), we see only a
1.5-to-1 rise for spoke tension versus squeeze force, with a 90-lb
rise for a 60-lb squeeze.

Not a 5-to-1 rise from 250 to 400 pounds for a 30-lb squeeze force.

Nor even a 2-to-1 rise of 100 lbs for a 49-lb squeeze force.

So far, all real wheels measured with tension gauges deform so much
when their spokes are squeezed that the tension rises only 55~65
pounds for a 60-pound squeeze-force, noticeably less than predicted by
idealized calculations.

It's a good example of why theoretical calculations need to be tested.

Anyone can repeat this experiment with a ~50-cent spoke, an $8 pipe
clamp, a $4 piece of pipe, a ~$60 Park tension gauge, some weights, a
clamp, some tape, and a thin ruler (or even just another spoke with a
piece of tape as a marker).

As for accuracy, elsewhere the Park tension gauge indicated ~187
pounds of tension for ~190 pounds of weights hanging on a spoke.

Cheers,

Carl Fogel

Quoted message said:

On Thu, 09 Nov 2006 15:37:41 -0700, [email hidden] wrote:

Quoted message said:

This seems like a good place to add a post showing the same pipe-clamp
spoke-tension rig in action, this time with a measuring rod added to
show how little the rig deforms.

Not really, by tacking it to the end of a thread that's been idle for
almost 3 months, I missed it completely.

Quoted message said:

Under the best possible conditions (the pipe-clamp), we see only a
1.5-to-1 rise for spoke tension versus squeeze force, with a 90-lb
rise for a 60-lb squeeze.

Not a 5-to-1 rise from 250 to 400 pounds for a 30-lb squeeze force.

Nor even a 2-to-1 rise of 100 lbs for a 49-lb squeeze force.

So far, all real wheels measured with tension gauges deform so much
when their spokes are squeezed that the tension rises only 55~65
pounds for a 60-pound squeeze-force, noticeably less than predicted by
idealized calculations.

It's a good example of why theoretical calculations need to be tested.

If you would provide the as-tested spoke length and deflection
distances, perhaps I could perform the calculations.