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Forces on spokes



 
 
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  #1  
Old August 28th 06, 01:23 PM posted to rec.bicycles.tech
bicycle_disciple
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Posts: 247
Default Forces on spokes

Hi all.

Just wanted to clear a little question. Thinking of a wheel spoke as a
prismatic member, what is the nature of normal forces acting on it. Is
it all in tension, all in compression or a mix of both?

Thanks.

Ron

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  #2  
Old August 28th 06, 04:48 PM posted to rec.bicycles.tech
[email protected]
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Posts: 7,934
Default Forces on spokes

On 28 Aug 2006 05:23:47 -0700, "bicycle_disciple"
wrote:

Hi all.

Just wanted to clear a little question. Thinking of a wheel spoke as a
prismatic member, what is the nature of normal forces acting on it. Is
it all in tension, all in compression or a mix of both?

Thanks.

Ron


Dear Ron,

Forces on a pre-tensioned wheel loaded at the axle:

http://www.astounding.org.uk/ian/wheel/index.html

Cheers,

Carl Fogel
  #3  
Old August 28th 06, 06:22 PM posted to rec.bicycles.tech
Jeff
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Posts: 71
Default Forces on spokes


wrote:
On 28 Aug 2006 05:23:47 -0700, "bicycle_disciple"
wrote:

Hi all.

Just wanted to clear a little question. Thinking of a wheel spoke as a
prismatic member, what is the nature of normal forces acting on it. Is
it all in tension, all in compression or a mix of both?

Thanks.

Ron


Dear Ron,

Forces on a pre-tensioned wheel loaded at the axle:

http://www.astounding.org.uk/ian/wheel/index.html

Cheers,

Carl Fogel


An interesting and thorough analysis at that link, and yet I am not
sure that I believe
the final result. He concludes that the load is supported almost
exclusively by the
bottom few spokes (the ones pointing down toward the road) which are
strongly in
compression. However, long slender members such as spokes cannot
support large compressive loads because of their tendency to buckle
(bend). Also, much of the
strength of a wheel comes from the fact that all the spokes contribute
to the load at
all times. I suspect that he has not accounted fully for the
pretensioning of the spokes.
Jeff

  #4  
Old August 28th 06, 06:55 PM posted to rec.bicycles.tech
[email protected]
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Posts: 7,934
Default Forces on spokes

On 28 Aug 2006 10:22:51 -0700, "Jeff"
wrote:


wrote:
On 28 Aug 2006 05:23:47 -0700, "bicycle_disciple"
wrote:

Hi all.

Just wanted to clear a little question. Thinking of a wheel spoke as a
prismatic member, what is the nature of normal forces acting on it. Is
it all in tension, all in compression or a mix of both?

Thanks.

Ron


Dear Ron,

Forces on a pre-tensioned wheel loaded at the axle:

http://www.astounding.org.uk/ian/wheel/index.html

Cheers,

Carl Fogel


An interesting and thorough analysis at that link, and yet I am not
sure that I believe
the final result. He concludes that the load is supported almost
exclusively by the
bottom few spokes (the ones pointing down toward the road) which are
strongly in
compression. However, long slender members such as spokes cannot
support large compressive loads because of their tendency to buckle
(bend). Also, much of the
strength of a wheel comes from the fact that all the spokes contribute
to the load at
all times. I suspect that he has not accounted fully for the
pretensioning of the spokes.
Jeff


Dear Jeff,

Actually, Ian's whole article is about accounting fully for the
pre-tensioning of the spokes.

It's a subject that's been covered repeatedly. That's the nicest
online, detailed explanation that I know of.

You can find pretty much the same engineering analysis and conclusions
in "The Bicycle Wheel" by Jobst Brandt, any edition.

And you can see experimental strain gauge confirmation in figures 10
and 11 Professor Gavin's paper he

http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf

The icicle-shapes on the graphs show the pre-tensioned spoke losing
and then regaining a large amount of tension as it rolls under the
loaded axle.

Until all the pre-tension is used up, even a string will "support" a
compressive load, which is why emergency repair spokes can be made of
kevlar string and why whole wheels can and have been made of them.

Cheers,

Carl Fogel
  #5  
Old August 28th 06, 06:59 PM posted to rec.bicycles.tech
[email protected]
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Posts: 3,751
Default Forces on spokes

Jeff Thomas writes:

Just wanted to clear a little question. Thinking of a wheel spoke as a
prismatic member, what is the nature of normal forces acting on it. Is
it all in tension, all in compression or a mix of both?


Forces on a pre-tensioned wheel loaded at the axle:


http://www.astounding.org.uk/ian/wheel/index.html

An interesting and thorough analysis at that link, and yet I am not
sure that I believe the final result. He concludes that the load is
supported almost exclusively by the bottom few spokes (the ones
pointing down toward the road) which are strongly in compression.


I didn't see that there was any compression in the analysis but rather
a reduction in tension. The only spokes that experience a significant
change in length are the ones at the bottom. This analysis was done
after the publication of "the Bicycle Wheel" in which this matter is
discussed at length to avoid any misunderstanding. Because this is
appears to be such an unusual perspective, no analysis of the wheel
was published before "the Bicycle Wheel".

However, long slender members such as spokes cannot support large
compressive loads because of their tendency to buckle (bend). Also,
much of the strength of a wheel comes from the fact that all the
spokes contribute to the load at all times. I suspect that he has
not accounted fully for the pretensioning of the spokes.


I suggest you review what was written and possibly look at the full
analysis in the book.

http://sheldonbrown.com/harris/books.html#brandt

Jobst Brandt
  #6  
Old August 28th 06, 07:41 PM posted to rec.bicycles.tech
[email protected]
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Posts: 261
Default Forces on spokes


Jeff wrote:
An interesting and thorough analysis at that link, and yet I am not
sure that I believe
the final result. He concludes that the load is supported almost
exclusively by the
bottom few spokes (the ones pointing down toward the road) which are
strongly in
compression. However, long slender members such as spokes cannot
support large compressive loads because of their tendency to buckle
(bend). Also, much of the
strength of a wheel comes from the fact that all the spokes contribute
to the load at
all times. I suspect that he has not accounted fully for the
pretensioning of the spokes.


Someone did a test of a bicycle with a tensiometer providing constant
telemetry of spoke tension and found that the spokes under the axle
lost tension, the spokes above the axle stayed relatively close, and
the spokes at +-90o from those under the axle increased. My ignorant
conclusion based on this data was that all the spokes except those
directly under the axle contributed to sharing the load, and that the
load was shared (this part is even more controversial) by the tendencey
of the rim to distort ovally. Others on this ng will now proceed to
dismiss this data as insignificant and insist that because the spokes
under the axle are tensioned, they are able to support the weight of
the bike until the load becomes great enough that they go slack, and
they don't really bother to explain convincingly (for me) why the
greatest tension rise is seen in the spokes that are _parallel_ to the
road surface.

For me personally, the tensioned spoke theory would be plausible if the
spoke nipple were somehow fixed in the rim, but because the nipple is
not fixed, there is no way for the spoke (tensioned or not) to
significantly act acgainst the rim to provide support of the weight of
the bicycle when the spoke is directly under the axle/hub.

  #7  
Old August 28th 06, 08:00 PM posted to rec.bicycles.tech
[email protected]
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Posts: 7,934
Default Forces on spokes

On 28 Aug 2006 11:41:45 -0700, wrote:


Jeff wrote:
An interesting and thorough analysis at that link, and yet I am not
sure that I believe
the final result. He concludes that the load is supported almost
exclusively by the
bottom few spokes (the ones pointing down toward the road) which are
strongly in
compression. However, long slender members such as spokes cannot
support large compressive loads because of their tendency to buckle
(bend). Also, much of the
strength of a wheel comes from the fact that all the spokes contribute
to the load at
all times. I suspect that he has not accounted fully for the
pretensioning of the spokes.


Someone did a test of a bicycle with a tensiometer providing constant
telemetry of spoke tension and found that the spokes under the axle
lost tension, the spokes above the axle stayed relatively close, and
the spokes at +-90o from those under the axle increased. My ignorant
conclusion based on this data was that all the spokes except those
directly under the axle contributed to sharing the load, and that the
load was shared (this part is even more controversial) by the tendencey
of the rim to distort ovally. Others on this ng will now proceed to
dismiss this data as insignificant and insist that because the spokes
under the axle are tensioned, they are able to support the weight of
the bike until the load becomes great enough that they go slack, and
they don't really bother to explain convincingly (for me) why the
greatest tension rise is seen in the spokes that are _parallel_ to the
road surface.

For me personally, the tensioned spoke theory would be plausible if the
spoke nipple were somehow fixed in the rim, but because the nipple is
not fixed, there is no way for the spoke (tensioned or not) to
significantly act acgainst the rim to provide support of the weight of
the bicycle when the spoke is directly under the axle/hub.


Dear SSTW,

Figure 10 in Professor Gavin's paper shows strain gauge results for a
spoke on an actual wheel as it was being ridden, with 8 revolutions in
2.0 seconds:

http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf

The huge icicle spikes show the loss of tension as the spoke rolls
under the loaded axle. They're about ten times the size of the other
spikes.

Like the huge spikes, the tiny spikes vary, reflecting real world
conditions.

Figure 11 shows the averaged spoke strain profile from three road
tests. The huge icicle spike is about ten times the size of the other
variation.

It would be very difficult to argue that any significant changes are
seen in the experimental data except when the spoke rolls under the
axle and its tension drops like a rock.

Experiment seems to confirm theory.

Cheers,

Carl Fogel
  #9  
Old August 28th 06, 08:09 PM posted to rec.bicycles.tech
[email protected]
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Posts: 261
Default Forces on spokes


wrote:
Experiment seems to confirm theory.


The experiment confirms that the spokes do indeed go slack as they pass
under the hub. It doesn't in anyway prove that they are supporting the
wheel through compressive loading before they go slack.

 




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