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MA3 rim failure, where to now



 
 
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  #21  
Old September 10th 03, 04:09 AM
Tim McNamara
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Default MA3 rim failure, where to now

In article ,
Simon Brooke wrote:

Tim McNamara writes:

But you raise an interesting point, as over the years a number of
British bicyclists in this newsgroup have most vehemently and
even rudely refused to tolerate the notion that the bicycle wheel
stands on its lower spokes. I don't quite know why this notion
seems to be so hard for some to grasp Across The Pond, especially
when it's so simple and so demonstrable.


Simple and demonstrable, yes. Stand the wheel on the spoke, and the
nipple falls out, because the only thing that holds it in is
tension. What you're saying is that _less_ tension somehow
translates into load bearing. It does not and it can not, and, in
particular, the design of spoke nipples would not allow them to
take compression loads even if the design of the wheel allowed them
to. The hub is supported by the tension in the top spokes, which
hang from the rim. This, of course, tends to distort the rim out of
true, and this distortion is resisted by tension in other spokes.
The only spokes which are doing no work at all is the spokes which
_can_ do no work - because they've gone (relatively) slack. To
suggest that the wheel is standing on these spokes is a semantic
perversion of the most peculiar nature.


Erm, yet again- you might want to read the book. And learn something
about the dynamics of pretensioned structures under load.
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  #22  
Old September 10th 03, 05:31 AM
Ted Bennett
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Default MA3 rim failure, where to now

Simon Brooke wrote:

What Simon has missed is that in a spoked wheel there is no compression
in absolute terms. Just a reduction in tension provides the upward
force on the hub.


In other words, it's hanging from the top, not standing on the
bottom. Yup, we all knew that already.


We did? 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.

--
Ted Bennett
Portland OR
  #23  
Old September 10th 03, 09:05 AM
Simon Brooke
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Default MA3 rim failure, where to now

Ian Smith writes:

On Tue, 09 Sep 2003 20:35:02 GMT, Simon Brooke wrote:
Ted Bennett writes:

What Simon has missed is that in a spoked wheel there is no compression
in absolute terms. Just a reduction in tension provides the upward
force on the hub.


In other words, it's hanging from the top, not standing on the
bottom. Yup, we all knew that already.


So, the spokes that have the same stress state whether loaded or
unloaded are doing all teh work,


Yes, they are. Take a bicycle wheel. Cut all the spokes below the
hub. Does the hub move? Apart from some accommodation to the now out
of round rim, no. Turn it the other way up so that all the cut
spokes are above the hub. Does the hub move? Yes, the lower
nipples slide out of their holes and the whole structure slumps.

Note that on a cart wheel exactly the opposite is true, because a cart
wheel is a compression structure and the spokes act in column.

and the spokes in which the stress
changes dramatically are doing nothing at all.


The change is when they _cease_ to do work, not when they _start_ to do
work.

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.


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

;; I'll have a proper rant later, when I get the time.
  #24  
Old September 10th 03, 09:35 AM
Simon Brooke
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Default MA3 rim failure, where to now

(Gary Young) writes:

Simon Brooke wrote in message .uk...
"Just zis Guy, you know?" writes:

"Tim McNamara" wrote in message
...

A bicycle wheel does not support a
load by elongation of the spokes- exactly the opposite, in fact. The
wheel "stands" on the spokes between the hub and the ground, rather
than hanging from the top of the rim as your model would require.

Whoop! Whoop! Flamewar Alert!


Yeah, yeah, spotted it.

This must be why spokes have that significant shoulder for the rim to
sit on - otherwise when the wheel 'stood' on the spoke the nipple
would just slide into the rim tape. It also explains why spokes have
to be thicker in the middle than at the ends, as on cart wheels, so
they won't distort out of column under compression loads.

I'm always impressed by the levels of mathematics, physics and
engineering taught in US colleges. They're so, uhhhmmm, _differently_
educated over there.

WARNING: may contain traces of irony.


Traces, yes, but not enough, surely, to undermine the wholesome and
holy righteousness of American True Knowledge. Far be it from me to
describe anyone as 'wrong'.


I'm not sure I understand why you take comfort in Just zis Guy's
posting. Your explanation of why some wheels are "flippier" depended
on the elongation of spokes. Regardless of whether you want to call
what happens "standing on the bottom spokes," Just zis Guy's posting
seemed to confirm that the bottom spokes shorten. Is your analysis
still valid if the spokes don't elongate to an appreciable degree, and
if so, why?


If they 'shorten' when they get to the bottom, do they 'shorten' at
every revolution? If so, the wheel must logically get smaller and
smaller until, eventually, it disappears with a *pop* of collapsing
credibility. Unless, of course, they 'unshorten' again somewhere else
in the revolution. What do we call 'unshortening', children? Oh,
that's right, 'enlongation'. And if they shorten when they get to the
bottom, where do they enlongate? That's right, not at the bottom. Very
good.

By the way, can you expand on what you mean by "flippier"? It's an
idiom I've never heard applied to wheels.


If the wheel is more rigid, it transfers movement from the rider to
the contact patch more directly (and vice versa). If it's more
compliant, softer, springier, then it damps all movements - both
roadshock coming up and control movement going down. Of course, it
does this as one component in a system, but it contributes to the
overall precision of the whole system.

If the whole system is taut - which includes hard tires and short
angles as well as more rigid wheels, but more rigid wheels are an
important component in this - the response of the bike to control
input is more immediate and more precise. The downside is of course
that you get more roadshock transmitted back up.

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

;; I'll have a proper rant later, when I get the time.
  #25  
Old September 10th 03, 12:29 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Ian Smith" wrote in message
...


So, the spokes that have the same stress state whether loaded or
unloaded are doing all teh work, and the spokes in which the stress
changes dramatically are doing nothing at all.



As is readily understood by considering the limiting case where tension = 0
in the bottom-most spoke, quite right.

--
Guy
===

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


  #26  
Old September 10th 03, 12:37 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Benjamin Weiner" wrote in message
...
Simon Brooke wrote:


This explanation would suggest that the deformation of the
rim from a circle occurs at the top of the wheel.


Which is why I refer people to the FEA, showing deformation at the bottom.

Testing the spoke tension in a loaded wheel
by plucking spokes also shows that it is the lower spokes
whose tension changes most significantly. This is a really simple
experiment which anyone interested in the issue should try.


And considering the limiting case where the bottom spoke has zero tension,
so could be removed without affecting the system in any way at all, readily
reveals the absurdity of describing this behaviour as "standing" on the
bottom spokes.

That is not to dispute the physics or the mechanism by which the wheel bears
load.

But is is reasonable to suggest, as this argument effectively does, that one
can stand on a piece of string provided it is part of a system which ensures
that it still remains in tension while supporting your weight? I think not.

--
Guy
===

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


  #27  
Old September 10th 03, 12:41 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Chris B." wrote in message
...

Having reviewed a finite element analysis of a bicycle wheel under load,
what happens is that the bottom of the rim deflects, reducing the tension

in
the bottom spokes,


So the spokes are compressed.


Provided you extend the meaning of the word compressed to mean "in reduced
tension, but still requiring that there is a structure which maintains them
in a state of not less than zero tension, otherwise the wheel will fail."
Which is, to quote another poster, a semantic perversion.

But to describe it as standing on the bottom spokes is like saying you

can
stand on a piece of string provided it's kept under tension by some other
structure.


I'll bet most people don't know or understand that either.


Your money is safe. Most people think weight is measured in kilogrammes,
after all. Apart from the Post Office, who think it's in Kelvin gramme
seconds ;-)

--
Guy
===

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


  #28  
Old September 10th 03, 12:50 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Tim McNamara" wrote in message
...

FEA analysis, as you no doubt know already, shows that the spokes
which change tension under a load applied to the axle- as is the case
for a wheel in a frame- are only those spokes between the hub and the
ground. Therefore it is reasonable to say that the wheel stands on
those spokes


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,
whereas with the spokes whioch "do nothign" removed the hub will not be
supported. It is therefore misleading.

I have nothing against a simplistic description of a complex system, as long
as it makes reasonable sense, but describing a hub as "standing" on spokes
which patently cannot support a net compressive load does not provide any
kind of illumination.

It's also easily demonstrable to the average person. Pluck a few
spokes at the top and bottom and sides; then put a load on the bike
and pluck the same spokes again. Unless the wheel is so horribly
undertensioned so that the bottom spokes go slack, the only spokes
that will change in pitch are those at the bottom of the wheel.


Now remove the bottom spokes and see what happens.

People just love to overcomplicate this discussion.


Inventing a Janet and John explanation which has to be discarded almost
immediately any detail is introduced would seem to me to be needlessly
confusing, since the fuller explanation isn't that hard to understand in the
first place.

--
Guy
===

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


  #29  
Old September 10th 03, 12:58 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"Ian Smith" wrote in message
...

to describe it as standing on the bottom spokes is like saying you can
stand on a piece of string provided it's kept under tension by some

other
structure.


Whereas your proposal is that the hub hangs from something below it?


No, Ian, as you are aware I am from the "none of the above" school. The
wheel works because all the spokes are in tension. If the rim were
infinitely rigid there would be no deflection and the bototm spokes would
have the same change in tension under load as the top spokes. If the side
spokes were missing the rim would squash. If the bototm spokes were absent
the wheel would stay up. If spokes which "do nothign" were taken out, the
hub would crash to the ground. So the wheel can only work if all the spokes
are in tension all the time.

Once that is understood, the larger cyclic change in the bottom spokes due
to rim deflection becomes an incidental though significant curiosity.

That's like saying you hang from the seat you're sitting on, which is
even greater nonsense.


Except it's not what I said and it's not the sense of what I said either.

Actually, what holds the wheel up is the shoulders of the nipples pressing
on the rim.

So there we have it: the wheel is held up by the shoulders of the spoke
nipples pressing on the rim, and by the neck of the spoke pressing outwards
on the hub.

Can we all go home now?

--
Guy
===

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


  #30  
Old September 10th 03, 01:10 PM
David Damerell
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Default MA3 rim failure, where to now

Benjamin Weiner wrote:
This explanation would suggest that the deformation of the
rim from a circle occurs at the top of the wheel.


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.
--
David Damerell flcl?
 




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