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MA3 rim failure, where to now



 
 
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  #41  
Old September 10th 03, 10:08 PM
Ian Smith
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Default MA3 rim failure, where to now

On Wed, 10 Sep, Just zis Guy, you know? wrote:

It is not reasonable, because it fails to explain the observed fact that
with the spokes on which the hub stands removed, the hub remains supported,


No it doesn't. The wheel collapses.

regards, Ian SMith
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  #42  
Old September 10th 03, 10:34 PM
Tim McNamara
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Default MA3 rim failure, where to now

In article ,
"Pete Biggs" wrote:

Ted Bennett wrote:
If it hangs from the top, then the tension in the top spoke would
increase with load. But it doesn't; the tension in the lower
spoke decreases. A simple test, plucking a few spokes, may help
convince you.


That alone doesn't convince me because rim deformation may be
responsible for the decrease in tension in the lower spokes (for
all I know).


Yes, that is exactly how it works. A the rim, across span of about 4
spokes, flexes slightly towards the hub, which in turn causes a
reduction in the tension of those spokes.

The tension in the rest of the spokes may be
increasing for all I can tell by plucking the spokes because the
load could be spread over so many spokes (not just those right at
the top) that the change in pitch is not enough to notice by ear.


Unless you are tone deaf, you'd hear the change if the tension was
rising enough to hold up the load by hanging off the top spokes. Tiny
changes in tension create audible chanes in frequency- any guitar
player can demonstrate this for you easily.

That's not to say that I'm convinced the rim hangs from the top,
just that you need better arguments.


This has been discussed ad nauseum in hundreds of threads and
thousands of posts over the past 10 years at least. This discussion
has been settled, much like the discussion of whether the Earth is
round has been settled.
  #43  
Old September 10th 03, 11:05 PM
Simon Brooke
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Default MA3 rim failure, where to now

Java Man (Espressopithecus) writes:

In article , bikerider@-
nospam-thanks-rogers.com says...
So the spokes are compressed.

This is understood by analyzing the changes in loading rather than
absolute loads. But it is confusing to many non-technical people. The
spokes at the bottom of a loaded bicycle wheel are "compressed" relative
to their unloaded condition, but are not "in compression".


If they aren't in compression, they aren't compressed, they're just
less extended. Nor are they compressed wrt their unloaded
condition. When the wheel was built they were extended wrt their
unloaded condition, and they will remain so until either they break or
the integrity of the wheel is destroyed.

The spokes
remain in tension--the net axial force acting on the spokes is tensile,
not compressive.


Exactly.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

'Victories are not solutions.'
;; John Hume, Northern Irish politician, on Radio Scotland 1/2/95
;; Nobel Peace Prize laureate 1998; few have deserved it so much
  #44  
Old September 10th 03, 11:35 PM
Simon Brooke
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Default MA3 rim failure, where to now

writes:

David Damerell writes:

I think you (and many other people) are missing the fact that Simon
(and Guy) are disputing the terminology only. No-one is claiming
anything other than that there is a tension change in the bottom
spokes.


Because this is not a semantic difference but a technical one, the
book shows an aluminum a common die-cast moped wheel that looks as
though it might have wire spokes.

http://mopedarmy.com/photos/brand/6/1681/

In such wheels, knowing that they are not tensioned, evokes the
response that "of course, this wheel stands on its bottom spokes."
However, by selective cooling in the die cast process, these spokes
can be tensioned, and the answer becomes unclear. Visually it appears
to be between a wire spoked wheel and a wooden wagon wheel.


This is a poor example. The spokes of this wheel are not free to slide
at either end, either in or out. The spokes of a cart wheel sit in
mortices both at the hub and the rim end. Under tension, they would
slide out of their mortices. Consequently they physically cannot
transmit a tensive load. The rim of a cart wheel is pushed outwards by
its spokes.

The spokes on a bicycle wheel slide through holes in the rim and are
retained by nipples screwed on from the outside. Under compression[1]
they would push the nipple out of the hole in the rim. Consequently
they physically cannot transmit a compressive load. The rim of a
bicycle wheel is pulled inward by its spokes.

The spokes in the example cast wheel are physically part of a unitary
structure. There are no sliding joints in it anywhere. Consequently
it's physically possible that the spokes work under either tension or
compression, or even alternate between tension and compression as the
wheel rotates.

Does prestress of a spoke change its function and that of the wheel?


Clearly it must.

[1] if they did not simply bend (because, unlike cartwheel spokes,
they are not shaped to remain in column under compression)

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

'Victories are not solutions.'
;; John Hume, Northern Irish politician, on Radio Scotland 1/2/95
;; Nobel Peace Prize laureate 1998; few have deserved it so much
  #45  
Old September 11th 03, 02:29 AM
jt
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Default MA3 rim failure, where to now


"Ian Smith" wrote in message
...
On Wed, 10 Sep 2003, Simon Brooke wrote:

Some stuff that's so badly wrong I have trouble believing he actually
meant it. However, just in case he believes what he wrote, I'll go
through it slowly...


Does the following quote from Simon Brooke help?

"It simply amuses me to take the **** out of
people who will earnestly argue that their particular form of words
is 'right' and everyone else (even when they describe exactly the same
phenomenon) is 'wrong'."



  #46  
Old September 11th 03, 10:05 AM
Simon Brooke
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Default MA3 rim failure, where to now

Ian Smith writes:

On Wed, 10 Sep 2003, Simon Brooke wrote:
Consider a tug of war. Two teams heave on a rope, and the hankerchief
stays over the line, because each team is heaving equally hard. Now
suppose the North team go off and get a beer. Their end of the rope
goes slack, and the handkerchief moves. Is this because the North team
are doing more work? That is your argument.


Sorry, no. My argument is that the north team pulls less hard, and
teh handkerchief moves. Which team caused teh handkerchief to move?
You're saying the south team did, but they are doing nothing
different, so it's not sensible (by cause and effect) to say they
caused teh change.


The South team _do_ cause the change. That's simply an observable
fact, and no amount of distortion of reality can alter it. They are
still doing work, whereas the North team are driking beer. If the
South team had dropped the rope at the same time the North team did,
the handkerchief would not move.


--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

'Victories are not solutions.'
;; John Hume, Northern Irish politician, on Radio Scotland 1/2/95
;; Nobel Peace Prize laureate 1998; few have deserved it so much
  #47  
Old September 11th 03, 12:39 PM
David Damerell
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Default MA3 rim failure, where to now

wrote:
David Damerell writes:
I think you (and many other people) are missing the fact that Simon
(and Guy) are disputing the terminology only. No-one is claiming
anything other than that there is a tension change in the bottom
spokes.

Because this is not a semantic difference but a technical one,


No. There is potential for a technical disagreement as to which spokes
actually show a change in tension, but that is not the disagreement here.
The discussion is purely about the semantics used to describe what
everyone agrees happens.
--
David Damerell Kill the tomato!
  #48  
Old September 11th 03, 03:36 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Ian Smith" wrote in message
...

You are saying the effect of a large stress change is caused by
nothing happening.


Nope, I'm saying it's due to the coincidental deformation of a rim which is
not inifinitely rigid.

Meanwhile, no stress change is caused by the spokes carrying teh
weight applied to the wheel.


Nope, "no stress change" is caused by that part of the rim not deflecting.

--
Guy
===

WARNING: may contain traces of irony. Contents may settle after posting.
http://www.chapmancentral.com


  #49  
Old September 11th 03, 03:40 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Java Man (Espressopithecus)" wrote in message
ble.net...

The methaphor "standing on its spokes" is illustrative but does not
fully explain the phenomenon. That is the problem.


It is worse than that: it is actively misleading. Consider a more rigid
rim. Suddenly the change in tension in the lower spokes is significantly
reduced. So on wheels with more rigid rims the hub doesn't stand quite so
hard on the bottom spokes, raising two questions: (a) is "stand" meaningful
if the degree of "standingness" is dependent on the rim rigidity not the
mass applied at the hub and (b) what happens to the component of the mass
which is no longer "standing" on the bottom spokes as a result? Is some of
that now hanging from the upper spokes?

IOW, the "stand" terminology raises more questions than it answers.

The wheel is supported by the spoke nipples pushing on the rim, I say...

--
Guy
===

WARNING: may contain traces of irony. Contents may settle after posting.
http://www.chapmancentral.com


  #50  
Old September 11th 03, 03:42 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Ian Smith" wrote in message
...

Actually, they elongate at teh bottom - alongside teh contact patch.
In fact, the greatest elongation is in the lower half of the wheel.


They have a negative modulus then?

--
Guy
===

WARNING: may contain traces of irony. Contents may settle after posting.
http://www.chapmancentral.com


 




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