How much does tension rise on squeezed spokes?

[[email protected]]

On Mon, 8 May 2006 16:01:53 -0700, "Phil Holman"
<piholmanc@yourservice> wrote:

>
><[email protected]> wrote in message
>news:[email protected]...
>> On Mon, 08 May 2006 09:34:51 -0500, Tim McNamara
>> <[email protected]> wrote:
>>
>>>OK: 300 mm spoke displaced 10 mm results in stretching the spokes .66
>>>mm. That's assuming that the rim does not deflect towards the hub.
>>>
>>>How much does a stretch of .66 mm raise the spoke tension?
>>>
>>>If we know the yield point of the spokes, how far to we have to
>>>deflect
>>>them to raise tension enough to achieve stress reliving?
>>
>> Dear Tim,
>>
>> Unfortunately, the whole point of Joe Riel's article is that
>> the rim is not likely to be infinitely stiff and that
>> individual aluminum rims may deflect significantly toward
>> the hub.
>>
>> Private emails from other experimenters have shown a
>> noticeably greater tension rise than I got for the same
>> squeezing force--but they were using shorter spokes, which
>> suggests bigger and stiffer rims.
>>
>> The rim that I used is just a plain rim, not deep, no box
>> section, with 294mm spokes. The other testers mentioned
>> 270mm, 275mm, and 285mm spokes in their emails. I don't know
>> if any of them are going to post their results, which all
>> used squeeze forces around 30-50 pounds.
>
>Here is what I measured and calculated on one of my wheels - Velocity
>deep V rim and 270mm spokes.
>
>Squeeze Load (lbf) 0 4 30
>Lateral Deflection (mm) 0 1 5
>Spoke Tension (lbf) 249 270 405
>
>Phil H

Dear Phil,

I think that suggest that Joe is right about the importance
of the stiffness of the rim.

Your deep rim, shorter spokes, and higher initial tension
produced about 4 times as much tension increase for the same
squeeze.

For a 30-pound squeeze, your tension rose 156 pounds from
249 to 405 pounds.

For a 30-pound squeeze, my tension rose only 37 pounds from
177 to 214 pounds.

Of course, my "squeeze" was arguably confused because at
that level, the stupid lower spoke's tension had dropped 20
pounds, from 150 to 130.

(After dropping with the initial loads, the lower spoke's
tension slowly climbed back up to about where it started as
the loads increased.)

I don't know why the lower spoke behaved oddly, but I assume
that it had something to do with the rope friction as it
went around the upper spoke and the brake pads holding the
wheel steady against the torque of the weight hanging from
the upper spoke.

Cheers,

Carl Fogel

 

[[email protected]]

Jason Krantz writes:

> That's a good point, Jobst. Total deflection (Ut) is equal to rim
> deflection (Ur) plus spoke deflection (Us). Since what we're really
> concerned with is spoke deflection, we can calculate it directly
> from the change in spoke tension. I got so excited about actually
> getting an Ixx for a modern rim that I lost track of what we were
> actually interested in.

We are not interested in spoke deflection but rather spoke tension and
tension is dependent on lateral force applied at midspan. For most
reasonable rims (and we are not all riding on deep cross section aero
rims) spoke deflection arises entirely from rim deflection, spoke
length remaining functionally unchanged during this process.

> Jobst, I'm not sure why you're so dismissive of Hooke's law, as you
> need it to perform the calculations you suggest. I think what you
> meant was "don't get too caught up in the theory when a simple
> equation will do." That's a good point, but even so, I'd rather not
> throw Hooke's law out with the bathwater (as it were).

Reference to Hook's Law only adds unrelated engineering jargon and has
nothing to do with the procedure of stress relieving. Hook's Law does
not apply to steel in the yield stress region. "Ixx" is also fog
jargon and helps no one in this discussion. Just use common English.

Jobst Brandt

 

[[email protected]]

Carl Fogel writes:

>>>> OK: 300 mm spoke displaced 10 mm results in stretching the spokes
>>>> .66 mm. That's assuming that the rim does not deflect towards
>>>> the hub.

>>>> How much does a stretch of .66 mm raise the spoke tension?

>>>> If we know the yield point of the spokes, how far to we have to
>>>> deflect them to raise tension enough to achieve stress reliving?

>>> Unfortunately, the whole point of Joe Riel's article is that the
>>> rim is not likely to be infinitely stiff and that individual
>>> aluminum rims may deflect significantly toward the hub.

>>> Private emails from other experimenters have shown a noticeably
>>> greater tension rise than I got for the same squeezing force--but
>>> they were using shorter spokes, which suggests bigger and stiffer
>>> rims.

>>> The rim that I used is just a plain rim, not deep, no box section,
>>> with 294mm spokes. The other testers mentioned 270mm, 275mm, and
>>> 285mm spokes in their emails. I don't know if any of them are
>>> going to post their results, which all used squeeze forces around
>>> 30-50 pounds.

>> Here is what I measured and calculated on one of my wheels -
>> Velocity deep V rim and 270mm spokes.

>> Squeeze Load (lbf) 0 4 30
>> Lateral Deflection (mm) 0 1 5
>> Spoke Tension (lbf) 249 270 405

> I think that suggest that Joe is right about the importance of the
> stiffness of the rim.

> Your deep rim, shorter spokes, and higher initial tension produced
> about 4 times as much tension increase for the same squeeze.

> For a 30-pound squeeze, your tension rose 156 pounds from 249 to 405
> pounds.

> For a 30-pound squeeze, my tension rose only 37 pounds from 177 to
> 214 pounds.

That is not possible. Your assessment of the input force or the
measurement of spoke tension was incorrect. Unless the deflection
angle of the spoke was 45 degrees, 30lb cannot cause only a 30lb
increase in tension. I don't believe you can bend a spoke to 45
degrees without kinking it at the nipple and permanently at midspan.

> Of course, my "squeeze" was arguably confused because at that level,
> the stupid lower spoke's tension had dropped 20 pounds, from 150 to
> 130.

With this description, I have no idea what was occurring but it wasn't
as you describe.

> (After dropping with the initial loads, the lower spoke's tension
> slowly climbed back up to about where it started as the loads
> increased.)

> I don't know why the lower spoke behaved oddly, but I assume that it
> had something to do with the rope friction as it went around the
> upper spoke and the brake pads holding the wheel steady against the
> torque of the weight hanging from the upper spoke.

I think you are safe from being quoted on this. I can't imagine what
you did or what resulted from according to your description.

Jobst Brandt

 

[Joe Riel]

[email protected] writes:

> That is not possible. Your assessment of the input force or the
> measurement of spoke tension was incorrect. Unless the deflection
> angle of the spoke was 45 degrees, 30lb cannot cause only a 30lb
> increase in tension.

No. It is possible. There may be measurement issues, but I cannot
dismiss the data this easily. You are confusing relative tension
(tension increase) with absolute tension. The force equation is

(1) F = 2*Tf*sin(theta),

where

F = applied squeezing force (at mid span);
theta = deflection angle of the spokes;
Tf = final (absolute) tension in spoke.

The tension increase is

(2) DT = Tf - Ti = F/2/sin(theta) - Ti.

For the particular case that Carl measured, with DT = F,
we get

Ti = F*(1/2/sin(theta) - 1).

If we assume that Ti is about 200lbf, then the deflection angle
would be about 3.7 degrees (seems on the low side, to me).

I'm not claiming that Carl's numbers are correct---I'm pretty
sure there are measurement errors there.

Carl, did you measure deflection, I didn't see it in your post?
It would be helpful for checking.

--
Joe Riel

 

[[email protected]]

On Tue, 09 May 2006 01:05:15 GMT, Joe Riel
<[email protected]> wrote:

>[email protected] writes:
>
>> That is not possible. Your assessment of the input force or the
>> measurement of spoke tension was incorrect. Unless the deflection
>> angle of the spoke was 45 degrees, 30lb cannot cause only a 30lb
>> increase in tension.
>
>No. It is possible. There may be measurement issues, but I cannot
>dismiss the data this easily. You are confusing relative tension
>(tension increase) with absolute tension. The force equation is
>
>(1) F = 2*Tf*sin(theta),
>
>where
>
> F = applied squeezing force (at mid span);
> theta = deflection angle of the spokes;
> Tf = final (absolute) tension in spoke.
>
>The tension increase is
>
>(2) DT = Tf - Ti = F/2/sin(theta) - Ti.
>
>For the particular case that Carl measured, with DT = F,
>we get
>
> Ti = F*(1/2/sin(theta) - 1).
>
>If we assume that Ti is about 200lbf, then the deflection angle
>would be about 3.7 degrees (seems on the low side, to me).
>
>I'm not claiming that Carl's numbers are correct---I'm pretty
>sure there are measurement errors there.
>
>Carl, did you measure deflection, I didn't see it in your post?
>It would be helpful for checking.

Dear Joe,

I hung weights from a rope, adding 5 pounds at a time. I'm
baffled by the idea that the weights weren't hanging in
mid-air. They're standard weight-lifting weights, so it's
reasonable to assume that they're fairly close to their
markings.

As for the measurements, I used a Park tension gauge that
gave increasingly higher readings as I added weights. With
5-lb increments, the march of the needle was in baby steps,
so I certainly don't claim better than +/- 0.3 marks on the
scale, but it was clear whether the needle was on the mark,
close to the mark, close to half-way, closer to the far
mark, or on the far mark.

In the end, I had a pinata consisting of four 15-lb weights,
two 10-pound weights, and four 5-pound weights, all dangling
from a rope going over the midspan of the upper parallel
spoke to where it was knotted around the midspan of the
lower parallel spoke.

The rear wheel was held motionless in the frame by the brake
pads.

The graph of the data makes a fairly smooth curve, if you
allow for the crudge conversion that leads to
stair-stepping.

The deflection ended up going from the spokes about 75mm
apart to about 31mm apart (that figure is in the notes above
the raw figures, which are dense enough that anyone could
miss them).

In any case, measuring the deflection of the spoke was
obviously not useful. The Park spoke gauge clearly showed
that tension dropped on the stupid lower spoke, despite the
lower spoke showing a slight, but obvious bend.

It could be that the brake pads holding the rim against the
increasing down-force confused things. Or it could be that
this is such a flimsy rim that it behaves oddly. (I put
about 30,000 miles onto it before retiring the whole bicycle
last week in favor of its slightly larger twin brother.)

Some comments have made me wonder if everyone posting has
actually looked at the data in detail, so I'll see about
getting the graphs up as pictures.

Cheers,

Carl Fogel

 

[jim beam]

[email protected] wrote:
> Jason Krantz writes:
>
>
>>>>OK: 300 mm spoke displaced 10 mm results in stretching the spokes
>>>>0.66 mm. That's assuming that the rim does not deflect toward
>>>>the hub.
>
>
>>>>How much does a stretch of 0.66 mm raise the spoke tension?
>
>
>>>>If we know the yield point of the spokes, how far to we have to
>>>>deflect them to raise tension enough to achieve stress reliving?
>
>
>>>Unfortunately, the whole point of Joe Riel's article is that the
>>>rim is not likely to be infinitely stiff and that individual
>>>aluminum rims may deflect significantly toward the hub.
>
>
>>>Private emails from other experimenters have shown a noticeably
>>>greater tension rise than I got for the same squeezing force--but
>>>they were using shorter spokes, which suggests bigger and stiffer
>>>rims.
>
>
>>>The rim that I used is just a plain rim, not deep, no box section,
>>>with 294mm spokes. The other testers mentioned 270mm, 275mm, and
>>>285mm spokes in their emails. I don't know if any of them are going
>>>to post their results, which all used squeeze forces around 30-50
>>>pounds.
>
>
>>The exact deflection of a given aluminum (i.e., isotropic) rim under a
>>spoke-squeezy loading condition would be easy to predict with FEA. All
>>we would need is the moment of inertia of the rim cross-section in the
>>plane of the wheel. If someone could provide me with that, I'd be happy
>>to run a model using beam elements.
>
>
> It's far simpler than that. Rim deflection is lateral spoke
> deflection divided by the tangent function from spoke deflection and
> length, something easy to measure. All deflection comes from rim
> deflection, the spokes not changing length.

rubbish. spokes change length on load/deflection change. period.

> To hell with isotropic

eh?

> or
> Hook's Law and the like.

wow.

> Besides, no one needs to know rim
> displacements.

you seem pretty damned interested in rim displacements when it comes to
trivializing road shock transmission.

> Only the forces involved make any difference to stress
> and stress relief in spokes.

jobst, you don't know what the hell you're talking about. you dismiss
all relevant fact, and try to use inappropriate simplifications as a
basis for an advanced fatigue elimination theory? you're an
uneducatable idiot.

>
> Jobst Brandt

 

[[email protected]]

On 09 May 2006 00:09:02 GMT, [email protected]
wrote:

>Carl Fogel writes:
>
>>>>> OK: 300 mm spoke displaced 10 mm results in stretching the spokes
>>>>> .66 mm. That's assuming that the rim does not deflect towards
>>>>> the hub.
>
>>>>> How much does a stretch of .66 mm raise the spoke tension?
>
>>>>> If we know the yield point of the spokes, how far to we have to
>>>>> deflect them to raise tension enough to achieve stress reliving?
>
>>>> Unfortunately, the whole point of Joe Riel's article is that the
>>>> rim is not likely to be infinitely stiff and that individual
>>>> aluminum rims may deflect significantly toward the hub.
>
>>>> Private emails from other experimenters have shown a noticeably
>>>> greater tension rise than I got for the same squeezing force--but
>>>> they were using shorter spokes, which suggests bigger and stiffer
>>>> rims.
>
>>>> The rim that I used is just a plain rim, not deep, no box section,
>>>> with 294mm spokes. The other testers mentioned 270mm, 275mm, and
>>>> 285mm spokes in their emails. I don't know if any of them are
>>>> going to post their results, which all used squeeze forces around
>>>> 30-50 pounds.
>
>>> Here is what I measured and calculated on one of my wheels -
>>> Velocity deep V rim and 270mm spokes.
>
>>> Squeeze Load (lbf) 0 4 30
>>> Lateral Deflection (mm) 0 1 5
>>> Spoke Tension (lbf) 249 270 405
>
>> I think that suggest that Joe is right about the importance of the
>> stiffness of the rim.
>
>> Your deep rim, shorter spokes, and higher initial tension produced
>> about 4 times as much tension increase for the same squeeze.
>
>> For a 30-pound squeeze, your tension rose 156 pounds from 249 to 405
>> pounds.
>
>> For a 30-pound squeeze, my tension rose only 37 pounds from 177 to
>> 214 pounds.
>
>That is not possible. Your assessment of the input force or the
>measurement of spoke tension was incorrect. Unless the deflection
>angle of the spoke was 45 degrees, 30lb cannot cause only a 30lb
>increase in tension. I don't believe you can bend a spoke to 45
>degrees without kinking it at the nipple and permanently at midspan.

[snip]

Dear Jobst,

I hung 5-lb weights from a rope around a spoke. What do you
think was incorrect about that?

I used a Park tension gauge. Do you think that I mis-read it
over 40 times on two spokes?

In short, what do you think was incorrect?

Maybe you should consider the effect of the rim stiffness?

After all, you do have a history of announcing that things
are impossible, such as chains coming off worn front
sprockets.

Cheers,

Carl Fogel

 

[jim beam]

[email protected] wrote:
> Jason Krantz writes:
>
>
>>That's a good point, Jobst. Total deflection (Ut) is equal to rim
>>deflection (Ur) plus spoke deflection (Us). Since what we're really
>>concerned with is spoke deflection, we can calculate it directly
>>from the change in spoke tension. I got so excited about actually
>>getting an Ixx for a modern rim that I lost track of what we were
>>actually interested in.
>
>
> We are not interested in spoke deflection but rather spoke tension and
> tension is dependent on lateral force applied at midspan. For most
> reasonable rims (and we are not all riding on deep cross section aero
> rims) spoke deflection arises entirely from rim deflection, spoke
> length remaining functionally unchanged during this process.

contradictory drivel. hookes law, which you don't seem to know,
understand, or care about, would tell you this.

>
>
>>Jobst, I'm not sure why you're so dismissive of Hooke's law, as you
>>need it to perform the calculations you suggest. I think what you
>>meant was "don't get too caught up in the theory when a simple
>>equation will do." That's a good point, but even so, I'd rather not
>>throw Hooke's law out with the bathwater (as it were).
>
>
> Reference to Hook's Law

that's hooke jobst, hooke.

> only adds unrelated engineering jargon and has
> nothing to do with the procedure of stress relieving.

damned right, but not for reasons you evidence understanding.

> Hook's Law does

hooke

> not apply to steel in the yield stress region.

it applies all the way up to yield, and on relaxation from yield. but
you mistakenly think springback, i.e. relaxation from yield obeying
hooke's law, is evidence of residual stress, so you really ought to pay
attention to this stuff.

> "Ixx" is also fog
> jargon and helps no one in this discussion. Just use common English.
>
> Jobst Brandt

 

[[email protected]]

Carl Fogel writes:

>>>>>> OK: 300 mm spoke displaced 10 mm results in stretching the
>>>>>> spokes 0.66 mm. That's assuming that the rim does not deflect
>>>>>> toward the hub.

>>>>>> How much does a stretch of 0.66 mm raise the spoke tension?

>>>>>> If we know the yield point of the spokes, how far to we have to
>>>>>> deflect them to raise tension enough to achieve stress
>>>>>> reliving?

>>>>> Unfortunately, the whole point of Joe Riel's article is that the
>>>>> rim is not likely to be infinitely stiff and that individual
>>>>> aluminum rims may deflect significantly toward the hub.

>>>>> Private emails from other experimenters have shown a noticeably
>>>>> greater tension rise than I got for the same squeezing
>>>>> force--but they were using shorter spokes, which suggests bigger
>>>>> and stiffer rims.

>>>>> The rim that I used is just a plain rim, not deep, no box
>>>>> section, with 294mm spokes. The other testers mentioned 270mm,
>>>>> 275mm, and 285mm spokes in their emails. I don't know if any of
>>>>> them are going to post their results, which all used squeeze
>>>>> forces around 30-50 pounds.

>>>> Here is what I measured and calculated on one of my wheels -
>>>> Velocity deep V rim and 270mm spokes.

>>>> Squeeze Load (lbf) 0 4 30
>>>> Lateral Deflection (mm) 0 1 5
>>>> Spoke Tension (lbf) 249 270 405

>>> I think that suggest that Joe is right about the importance of the
>>> stiffness of the rim.

>>> Your deep rim, shorter spokes, and higher initial tension produced
>>> about 4 times as much tension increase for the same squeeze.

>>> For a 30-pound squeeze, your tension rose 156 pounds from 249 to
>>> 405 pounds.

>>> For a 30-pound squeeze, my tension rose only 37 pounds from 177 to
>>> 214 pounds.

>> That is not possible. Your assessment of the input force or the
>> measurement of spoke tension was incorrect. Unless the deflection
>> angle of the spoke was 45 degrees, 30lb cannot cause only a 30lb
>> increase in tension. I don't believe you can bend a spoke to 45
>> degrees without kinking it at the nipple and permanently at
>> midspan.

> [snip]

> I hung 5-lb weights from a rope around a spoke. What do you think
> was incorrect about that?

You mention the rope going around a pair of spokes elsewhere and I
cannot visualize how this is done. You don't need a pair. I only
mention a pair because when done with force, it could misalign the
freshly trued wheel if only one side were stressed. I think that the
method you are pursuing is ideal but the friction of the rope around a
pair of spokes makes it not work. Just hang the load on one spoke so
no friction =can get involved.

> I used a Park tension gauge. Do you think that I mis-read it over
> 40 times on two spokes?

I'm sure that is adequate to rule out repeatability problems.

> In short, what do you think was incorrect?

Yes.

> Maybe you should consider the effect of the rim stiffness?

As I said, the only thing that matters in this pursuit is the force
and the deflection angle of the s[poke (to determine how much a stress
relieving force increases tension).

> After all, you do have a history of announcing that things are
> impossible, such as chains coming off worn front sprockets.

Let's not drag in the baggage. This is heavy enough.

Jobst Brandt

 

[Joe Riel]

Joe Riel <[email protected]> writes:

My final computation was incorrect. The ratio of the normal force at
the rim to the applied lateral force is 1 when tan(theta) = 1/2, which
is at angle of 26 degrees. I don't know what 1:1 ratio at 45 degrees
Jobst is referring to.

--
Joe Riel

 

[[email protected]]

On Sat, 06 May 2006 15:49:51 -0600, [email protected]
wrote:

>Elsewhere we've wandered off into how to calculate and test
>the rise in tension when a spoke is squeezed. That thread
>(as usual) is getting a bit large and awkward for people to
>follow, so here's a new thread.
>
>Joe Riel has a page with calculations indicating that rim
>stiffness plays a large role in the rise in tension when a
>tensioned spoke is displaced sideways:
>
>www.k-online.com/~joer/cycling/spoke-tension.pdf
>
>Joe recently commented:
>
>"I think it's pretty clear from the final graph that there
>is a large variation in the final tension, dependent upon
>the rim stiffness---that was the point of the article."
>
>Testing the rise in tension is a little trickier than you'd
>expect. The gauge is so large that it gets in the way, you
>could use another pair of hands, it's hard to do two spokes
>on one side with your left hand and two on the other with
>your right for balance, and everything will vary with the
>next tester--initial tension, strength of squeeze, kind of
>spoke, stiffness of rim.
>
>But I took a stab at it with an old 36-spoke 700c rim,
>straight 2mm spokes, and probably less initial tension than
>it ought to have.
>
>Unsqueezed, the spoke registered 15 on my Park gauge, which
>is probably around a pathetic 41 kgf (the Park conversion
>chart doesn't go below 17 for such spokes.)
>
>Squeezed as hard as I could manage with a rope, the spoke
>tension increased to 20 on the Park gauge, 68 kgf.
>
>(In both measurements, I squeezed and released the gauge
>three times--without that, it would have stayed on 15
>instead of rising to 20.)
>
>I think that this roughly 30 kgf (65-lb) tension increase is
>far less than most people expect from spoke squeezing, but I
>hasten to add that a greater increase might be obtained with
>better techniques--which is what this post is meant to
>provoke.
>
>A stiffer rim, hauling harder on the rope, more initial
>tension, and perhaps a more sensitive gauge might improve
>the results.
>
>The rim is plain and not boxed, so it may not be very stiff.
>
>The rope squeeze may not be as good as one hand, even though
>it looks pretty good. The rope is right in the midspan of
>the spoke, but the area squeezed is much smaller than a
>hand, which might affect leverage. (With the gauge on the
>spoke, I can't get my whole hand in to squeeze.)
>
>The initial tension was embarrassingly low, which could
>confuse the rim stiffness and other factors.
>
>The Park gauge is said to use a lot of force itself (you can
>see the bending within the gauge in the picture) and might
>confuse things).
>
>Here's a Park tension gauge showing 15 on the target spoke:
>http://www.filelodge.com/files/room19/497501/spoke1.jpg
>
>Here it shows 20 with the spoke squeezed with a rope:
>http://www.filelodge.com/files/room19/497501/spoke2.jpg
>
>A blurry picture of gauge and rope setup:
>http://www.filelodge.com/files/room19/497501/spoke3.jpg
>
>Better focus of gauge and rope setup:
>http://www.filelodge.com/files/room19/497501/spoke4.jpg
>
>Closer view of gauge and rope, showing bending of spoke:
>http://www.filelodge.com/files/room19/497501/spoke5.jpg
>
>Here's Park's 0-50 scale conversion chart for 2mm straight
>round steel spokes:
>
>scale_marking
> Park_kgf
> estimate_kgf
>13 --
>14 --
>15 -- (41?) <--initial tension
>16 -- (46?)
>17 51
>18 56
>19 62
>20 68 <--squeezed tension
>21 76
>22 85
>23 95 <--recommended tension for most wheels
>24 107
>25 121
>26 137
>27 156
>28 178
>29 ---
>30 ---
>
>I hope that this provokes pictures from people with better
>cameras, wheels, grips, and testing techniques. If nothing
>else, I learned that the rope will stay in place, unknotted,
>under heavy tension because the spokes are so thin that the
>loop jams solid.
>
>If anyone lacking a gauge can suggest a different way to do
>it, I'll give it a try, with the obvious warning that my
>experimental skills are already pushing their limits.
>
>Cheers,
>
>Carl Fogel

Here's a crude graph for the upper spoke's fairly steady
tension increase in 21 steps from 0 to 100 pounds as 5-lb
weights were added:

http://home.comcast.net/~carlfogel/download/spoke1a.jpg

The spoke tension rose from around 170 to around 350 pounds.

The stair-stepping is noticeable because the Park tension
gauge marks were read (as best as possible) in tenths of a
mark and then crudely converted to 1/10th of the difference
between each integer mark. (See longer explanations
elsewhere.)

Here's a crude graph for the lower spoke's odd tension drop
and rise at the same time as the 5-lb weights were added in
21 steps from 0 to 100 pounds:

http://home.comcast.net/~carlfogel/download/spoke2b.jpg

The lower spoke, to which the rope was knotted, visibly bent
as soon as the first weight was added--but the Park gauge
showed a strange drop instead of rise up to about 50 pounds,
where it rose back toward its original tension.

By the time that 100 pounds was hanging from the rope looped
over the upper spoke, the gap between the two spokes at
midspan had shrunk from about 75mm to 31mm, with an alarming
angle at the upper spoke's nipple.

This may be just an unusually flexible rim or some odd
effect of hanging weights on horizontal spokes with the
wheel anchored by the brake pads on an upside-down bicycle
clamped in a workbench vise, but the weights were hanging in
mid-air with the rope touching nothing but the two spokes
and the Park tension gauge gave fairly consistent readings.

(Look carefully at the raw data, and you'll see a spot or
two where it didn't seem to budge with added weight and I
irritably recorded the data as such. And I was certainly
baffled when I added weight after weight and the lower spoke
lost tension instead of gaining it as I expected.)

Cheers,

Carl Fogel l



http://home.comcast.net/~carlfogel/download/spoke1a.jpg

 

[Joe Riel]

[email protected] writes:

> The deflection ended up going from the spokes about 75mm
> apart to about 31mm apart (that figure is in the notes above
> the raw figures, which are dense enough that anyone could
> miss them).

Now I see it, thanks. It appears that the deflection of the rim may
have screwed up some of the measurements. Supporting it might help.

The left and right initial tensions in the (rear) wheel seem close
together: 150lbf and 177lbf, respectively. How little offset does
that wheel have?

--
Joe Riel

 

[Joe Riel]

[email protected] writes:

> (Look carefully at the raw data, and you'll see a spot or
> two where it didn't seem to budge with added weight and I
> irritably recorded the data as such. And I was certainly
> baffled when I added weight after weight and the lower spoke
> lost tension instead of gaining it as I expected.)

Presumably that was caused by the rim deflecting downwards,
effectively shortening the path for the bottom spoke. Supporting the
rim so it cannot deflect laterally (downwards) should stop that.

--
Joe Riel

 

[[email protected]]

On 09 May 2006 04:49:24 GMT, [email protected]
wrote:

[snip]

>> In short, what do you think was incorrect?
>
>Yes.

[snip]

Dear Jobst,

?

Cheers,

Carl Fogel

 

[[email protected]]

On Tue, 09 May 2006 05:21:50 GMT, Joe Riel
<[email protected]> wrote:

>[email protected] writes:
>
>> The deflection ended up going from the spokes about 75mm
>> apart to about 31mm apart (that figure is in the notes above
>> the raw figures, which are dense enough that anyone could
>> miss them).
>
>Now I see it, thanks. It appears that the deflection of the rim may
>have screwed up some of the measurements. Supporting it might help.
>
>The left and right initial tensions in the (rear) wheel seem close
>together: 150lbf and 177lbf, respectively. How little offset does
>that wheel have?

Dear Joe,

I think that you may be misunderstanding which spokes.

Both spokes are on the non-drive side, a parallel pair, not
one on the left and one on the right.

I picked them solely because they were about 90 degrees from
the valve hole, in case that affected rim stiffness. They're
on the rear wheel of a bicycle retired about a week ago
after over 30,000 miles.

I've mentioned that the odd behavior of the lower spoke
being pulled up by the rope baffled me and that it might be
due to the brake pads anchoring the wheel on the upside-down
bicycle.

The rope looped over the upper spoke and knotted onto the
lower spoke doesn't mimic a single hand squeezing perfectly,
but I'm not sure what a single support under the wheel would
do (or even where to put it).

I'll try to sketch a diagram of the setup and post it, since
a picture large enough to show everything doesn't show the
details very well.

Cheers,

Carl Fogel

 

[Joe Riel]

[email protected] writes:

> Joe Riel writes:
>
> So you say you cannot tell me what the tension in a wire is if I hang
> a known weight at its midspan and it deflects 5 degrees from its
> normally straight line alignment because you don't know how tight it
> was before the load was hung on it?

You cannot tell the *change* in tension with that technique,
only the final tension. To determine the change you'll have to make
at least two measurements. The 30lbf was the *change*, not the
final tension.

> In that case, how can a tensiometer work when it does exactly what is
> being attempted but in reverse. It puts a known load and determines
> the deflection to get tension.

A tensiometer measures the final tension, but since the deflection is
small, it is close enough to the initial tension. It doesn't measure
the change in tension.

--
Joe Riel

 

[[email protected]]

Joe Riel writes:

> My final computation was incorrect. The ratio of the normal force
> at the rim to the applied lateral force is 1 when tan(theta) = 1/2,
> which is at angle of 26 degrees. I don't know what 1:1 ratio at 45
> degrees Jobst is referring to.

That would be the force in line with the straight path f the spoke
before applying a side load. The wire tension would be as you say
when tan=0.5. 1:1 would be the sides of a square with the spoke a
diagonal, however, spoke tension is not parallel to the sides so the
sine and tangent got in there again for me.

Jobst Brandt

 

[[email protected]]

Joe Riel writes:

>> (Look carefully at the raw data, and you'll see a spot or two where
>> it didn't seem to budge with added weight and I irritably recorded
>> the data as such. And I was certainly baffled when I added weight
>> after weight and the lower spoke lost tension instead of gaining it
>> as I expected.)

> Presumably that was caused by the rim deflecting downward,
> effectively shortening the path for the bottom spoke. Supporting
> the rim so it cannot deflect laterally (downward) should stop that.

Oooh! I'm gradually getting the picture. The bicycle is lying on its
side and there is a a rope around a pair of spokes at their midpoint
pulling down.

The load is bending the wheel as a whole which loosens the low side
spokes in that region and tightens the high side spokes. Therefore,
we are not seeing the force increasing spoke tension in the sense that
grasping a pair of spokes and squeezing them against each other, but
rather a combined effect of wheel bending and spoke midpoint
displacement.

This does not model what occurs when manually stress relieving spokes.
As you suggested. supporting the wheel laterally might help but this
is all so removed from the free standing model in which no forces act
on the wheel other than between two "parallel" spokes.

Jobst Brandt

 

[[email protected]]

On Sat, 06 May 2006 15:49:51 -0600, [email protected]
wrote:

>Elsewhere we've wandered off into how to calculate and test
>the rise in tension when a spoke is squeezed. That thread
>(as usual) is getting a bit large and awkward for people to
>follow, so here's a new thread.
>
>Joe Riel has a page with calculations indicating that rim
>stiffness plays a large role in the rise in tension when a
>tensioned spoke is displaced sideways:
>
>www.k-online.com/~joer/cycling/spoke-tension.pdf
>
>Joe recently commented:
>
>"I think it's pretty clear from the final graph that there
>is a large variation in the final tension, dependent upon
>the rim stiffness---that was the point of the article."
>
>Testing the rise in tension is a little trickier than you'd
>expect. The gauge is so large that it gets in the way, you
>could use another pair of hands, it's hard to do two spokes
>on one side with your left hand and two on the other with
>your right for balance, and everything will vary with the
>next tester--initial tension, strength of squeeze, kind of
>spoke, stiffness of rim.
>
>But I took a stab at it with an old 36-spoke 700c rim,
>straight 2mm spokes, and probably less initial tension than
>it ought to have.
>
>Unsqueezed, the spoke registered 15 on my Park gauge, which
>is probably around a pathetic 41 kgf (the Park conversion
>chart doesn't go below 17 for such spokes.)
>
>Squeezed as hard as I could manage with a rope, the spoke
>tension increased to 20 on the Park gauge, 68 kgf.
>
>(In both measurements, I squeezed and released the gauge
>three times--without that, it would have stayed on 15
>instead of rising to 20.)
>
>I think that this roughly 30 kgf (65-lb) tension increase is
>far less than most people expect from spoke squeezing, but I
>hasten to add that a greater increase might be obtained with
>better techniques--which is what this post is meant to
>provoke.
>
>A stiffer rim, hauling harder on the rope, more initial
>tension, and perhaps a more sensitive gauge might improve
>the results.
>
>The rim is plain and not boxed, so it may not be very stiff.
>
>The rope squeeze may not be as good as one hand, even though
>it looks pretty good. The rope is right in the midspan of
>the spoke, but the area squeezed is much smaller than a
>hand, which might affect leverage. (With the gauge on the
>spoke, I can't get my whole hand in to squeeze.)
>
>The initial tension was embarrassingly low, which could
>confuse the rim stiffness and other factors.
>
>The Park gauge is said to use a lot of force itself (you can
>see the bending within the gauge in the picture) and might
>confuse things).
>
>Here's a Park tension gauge showing 15 on the target spoke:
>http://www.filelodge.com/files/room19/497501/spoke1.jpg
>
>Here it shows 20 with the spoke squeezed with a rope:
>http://www.filelodge.com/files/room19/497501/spoke2.jpg
>
>A blurry picture of gauge and rope setup:
>http://www.filelodge.com/files/room19/497501/spoke3.jpg
>
>Better focus of gauge and rope setup:
>http://www.filelodge.com/files/room19/497501/spoke4.jpg
>
>Closer view of gauge and rope, showing bending of spoke:
>http://www.filelodge.com/files/room19/497501/spoke5.jpg
>
>Here's Park's 0-50 scale conversion chart for 2mm straight
>round steel spokes:
>
>scale_marking
> Park_kgf
> estimate_kgf
>13 --
>14 --
>15 -- (41?) <--initial tension
>16 -- (46?)
>17 51
>18 56
>19 62
>20 68 <--squeezed tension
>21 76
>22 85
>23 95 <--recommended tension for most wheels
>24 107
>25 121
>26 137
>27 156
>28 178
>29 ---
>30 ---
>
>I hope that this provokes pictures from people with better
>cameras, wheels, grips, and testing techniques. If nothing
>else, I learned that the rope will stay in place, unknotted,
>under heavy tension because the spokes are so thin that the
>loop jams solid.
>
>If anyone lacking a gauge can suggest a different way to do
>it, I'll give it a try, with the obvious warning that my
>experimental skills are already pushing their limits.
>
>Cheers,
>
>Carl Fogel

Some posts have led me to wonder whether my powers of
description are ever feebler than I thought, so here's a
crude diagram of the setup:

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

Imagine the small circle representing weights to be as many
as ten weights dangling half-way to the floor like a bunch
of bananas.

The rotary vise was completely beyond my pictorial skills,
so just imagine a monster bench vise that can be rotated to
clamp a seat post.

The question is whether the weight hanging down is going to
give a strikingly different result than a single hand
squeeze.

The weight trying to turn the wheel is resisted by the brake
pads, so maybe the rim deforms much more (or in a different
direction) than it would if we could use a calibrated
squeeze-grip of some kind.

I can use another pair of spokes, or just a single spoke for
a comparison, if someone has an idea. Joe Riel has suggested
supporting the rim (which has a 110 psi tire on it), but I'm
not sure where the support would go.

Cheers,

Carl Fogel

 

[[email protected]]

On 09 May 2006 05:55:48 GMT, [email protected]
wrote:

>Joe Riel writes:
>
>>> (Look carefully at the raw data, and you'll see a spot or two where
>>> it didn't seem to budge with added weight and I irritably recorded
>>> the data as such. And I was certainly baffled when I added weight
>>> after weight and the lower spoke lost tension instead of gaining it
>>> as I expected.)
>
>> Presumably that was caused by the rim deflecting downward,
>> effectively shortening the path for the bottom spoke. Supporting
>> the rim so it cannot deflect laterally (downward) should stop that.
>
>Oooh! I'm gradually getting the picture. The bicycle is lying on its
>side and there is a a rope around a pair of spokes at their midpoint
>pulling down.

[snip]

Dear Jobst,

Sorry, the bike is not on its side. As I keep saying, it's
upside down.

Here's a sketch that I just posted:

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

But after you see how it's working, maybe you'll see
whatever is leading to unexpected results.

Joe Riel has wondered if supporting the wheel might help,
but I have no idea if the forces are downward or circular or
even affecting the result.

The brake pads stop the wheel from turning when the weight
hangs from the horizontal spokes.

Joe Riel has wondered if supporting the wheel might help,
but I have no idea if the forces are downward or circular or
both or even affecting the result.

Cheers,

Carl Fogel