MA3 rim failure, where to now

On Tue, 23 Sep 2003 13:27:25 -0500, Tim McNamara
wrote:

the point at issue [...] whether it is reasonable to use the
term "stand."

And for the reasons that I and other have delineated, it is reasonable
to use the term "stands" in this context. If you don't like it, you
don't have to use it. As far as your understanding goes, as Mr.
Feynman famously pointed out: if you can't express it in plain simple
language, you don't understand it yourself.

In the opinion of one group it is reasonable, and in the opinion of
another it is not. The issue is not the /ability/ to understand it or
explain it (and by the way, your patronise-o-matic is still switched
on), but the precise terms.

A bicycle wheel is a structure which becomes unstable if all the
spokes do not remain in tension. The hub is supported because the
spokes keep the rim true while transferring the load to the rim and
thence the ground. There is a very interesting effect: the bottom of
the rim deflects under load. That leads to the spokes undergoing a
large change in tension as they pass through the bottom few degrees of
arc; a rapid destressing and restressing which helps to explain the
phenomenon of spoke fatigue and breakage. All of this is very easy to
understand.

Now say the hub stands on the bottom spokes and spend the next three
weeks explaining why the word stand is reasonable for something which
cannot support a compressive load. That seems to be the difficulty
for the "standers" - they are unable to explain this clearly. Maybe,
as Dick Feynman suggests, they don't understand.

ISTM that, given the acknowledged fact that the large change in
tension in the bottom spokes is due to rim deflection, to say that
the hub stands on the bottom spokes is in effect to say that the
hub is supported by the deflection of the rim.

Since those are inseperable, like two halves of the same coin, you are
setting up a false dichotomy here. It's the old ploy of discredit by
misrepresentation. You keep overcomplicating the concept and
confusing yourself.

Nope, I am not in the least confused. The deformation is dependent on
the rim characteristics. Are we saying that the amount to which the
hub stands in the bottom spokes is dependant on the design of the rim?
Part of the problem is that there is no structure in nature which
serves as an analogy. One could consider a tethered balloon, and the
idea that its basket stands on the tether, but that would be a
misrepresentation. So what part of the physics are you saying I don't
understand?

By extension, if the rim is more rigid the distribution of forces
is different. So we have an implied secondary mechanism, with the
balance between the two mechanisms being dependent on rim rigidity.

If you understand engineering as well as you say you do, you know that
you are throwing in red herrings yet again.

No, I don't think so. You are apparently getting bad tempered because
you have failed to put an argument which has convinced me. That may
be a sign that we should wander off to the pub.

The number of spokes
involved in supporting the load changes as the model rim becomes more
or less rigid (more spokes if the rim is less rigid, fewer spokes if
the rim is more rigid). A similar thing happens if we make the model
spokes more or less inelastic by changing their diameter. If the rim
was infinitely rigid, then perhaps the dynamics would change and the
rim would equally hang from the upper spokes and stand on the lower
spokes.

So you will only accept consideration of changes in the rim if at the
same time we change everything else? I don't think that's altogether
reasonable. I have, as I said before, two bikes side by side. One
has relatively shallow lightweight 700c rims and the other has deep V
406 rims. Both have the same number of spokes. We have not done an
FEA on the smaller wheel, but it seems fair to assume that it will be
substantially different - the degree of "standingness" as judged by
the sonic spoke tension tester (ping the spoke) is enormously less in
the smaller wheel.

Given that all these things affect the degree to which the hub
"stands on the bottom spokes" (i.e. the degree of deflection of the
rim), does it still look reasonable to say the hub "stands" on the
bottom spokes?

Yes, since you are just casting about here for ay objection you can
raise.

Cobblers. I havs said fromt he beginning exactly why I think the word
"stand" is misleading: it is redundant as soon as any more detailed
discussion is entered into, and it raises the problem of having to
rescind the Janet-and-John explanation in the light of a fuller
explanation which is not that hard to understand. I have children: I
have been caught out that way before.

Now consider that the whole will be unstable in use if the bottom spokes
reach zero tension. Is it still reasonable to use the word "stand?"

Yes. At least if you understand how pretensioned structures support a
load. Of course, this wheel will shortly collapse and then it won't
be standing on anything.

Which is rather the point of my argument.

And consider that the spokes cannot support a compressive load. And so on
and so on.

But the lower spokes do support a compressive load.

No, they support a reduction in tension. They are never in
compression. Compression is tension but in the opposite direction,
and using these words in the opposite of their normal sense is
inherently confusing.

The source of the confusion, I think, is in employing entirely
hub-centric terminology to desribe a situation where there are two
equal and opposite forces separated by the radius of the wheel.

We disagree. And I dare say we will not agree. Is there a point in
continuing to waste our time?

Not as far as I'm aware. The pub's open...

Guy
===
** WARNING ** This posting may contain traces of irony.
http://chapmancentral.demon.co.uk

On Tue, 23 Sep 2003 13:27:25 -0500, Tim McNamara
wrote:

the point at issue [...] whether it is reasonable to use the
term "stand."

And for the reasons that I and other have delineated, it is reasonable
to use the term "stands" in this context. If you don't like it, you
don't have to use it. As far as your understanding goes, as Mr.
Feynman famously pointed out: if you can't express it in plain simple
language, you don't understand it yourself.

In the opinion of one group it is reasonable, and in the opinion of
another it is not. The issue is not the /ability/ to understand it or
explain it (and by the way, your patronise-o-matic is still switched
on), but the precise terms.

A bicycle wheel is a structure which becomes unstable if all the
spokes do not remain in tension. The hub is supported because the
spokes keep the rim true while transferring the load to the rim and
thence the ground. There is a very interesting effect: the bottom of
the rim deflects under load. That leads to the spokes undergoing a
large change in tension as they pass through the bottom few degrees of
arc; a rapid destressing and restressing which helps to explain the
phenomenon of spoke fatigue and breakage. All of this is very easy to
understand.

Now say the hub stands on the bottom spokes and spend the next three
weeks explaining why the word stand is reasonable for something which
cannot support a compressive load. That seems to be the difficulty
for the "standers" - they are unable to explain this clearly. Maybe,
as Dick Feynman suggests, they don't understand.

ISTM that, given the acknowledged fact that the large change in
tension in the bottom spokes is due to rim deflection, to say that
the hub stands on the bottom spokes is in effect to say that the
hub is supported by the deflection of the rim.

Since those are inseperable, like two halves of the same coin, you are
setting up a false dichotomy here. It's the old ploy of discredit by
misrepresentation. You keep overcomplicating the concept and
confusing yourself.

Nope, I am not in the least confused. The deformation is dependent on
the rim characteristics. Are we saying that the amount to which the
hub stands in the bottom spokes is dependant on the design of the rim?
Part of the problem is that there is no structure in nature which
serves as an analogy. One could consider a tethered balloon, and the
idea that its basket stands on the tether, but that would be a
misrepresentation. So what part of the physics are you saying I don't
understand?

By extension, if the rim is more rigid the distribution of forces
is different. So we have an implied secondary mechanism, with the
balance between the two mechanisms being dependent on rim rigidity.

If you understand engineering as well as you say you do, you know that
you are throwing in red herrings yet again.

No, I don't think so. You are apparently getting bad tempered because
you have failed to put an argument which has convinced me. That may
be a sign that we should wander off to the pub.

The number of spokes
involved in supporting the load changes as the model rim becomes more
or less rigid (more spokes if the rim is less rigid, fewer spokes if
the rim is more rigid). A similar thing happens if we make the model
spokes more or less inelastic by changing their diameter. If the rim
was infinitely rigid, then perhaps the dynamics would change and the
rim would equally hang from the upper spokes and stand on the lower
spokes.

So you will only accept consideration of changes in the rim if at the
same time we change everything else? I don't think that's altogether
reasonable. I have, as I said before, two bikes side by side. One
has relatively shallow lightweight 700c rims and the other has deep V
406 rims. Both have the same number of spokes. We have not done an
FEA on the smaller wheel, but it seems fair to assume that it will be
substantially different - the degree of "standingness" as judged by
the sonic spoke tension tester (ping the spoke) is enormously less in
the smaller wheel.

Given that all these things affect the degree to which the hub
"stands on the bottom spokes" (i.e. the degree of deflection of the
rim), does it still look reasonable to say the hub "stands" on the
bottom spokes?

Yes, since you are just casting about here for ay objection you can
raise.

Cobblers. I havs said fromt he beginning exactly why I think the word
"stand" is misleading: it is redundant as soon as any more detailed
discussion is entered into, and it raises the problem of having to
rescind the Janet-and-John explanation in the light of a fuller
explanation which is not that hard to understand. I have children: I
have been caught out that way before.

Now consider that the whole will be unstable in use if the bottom spokes
reach zero tension. Is it still reasonable to use the word "stand?"

Yes. At least if you understand how pretensioned structures support a
load. Of course, this wheel will shortly collapse and then it won't
be standing on anything.

Which is rather the point of my argument.

And consider that the spokes cannot support a compressive load. And so on
and so on.

But the lower spokes do support a compressive load.

No, they support a reduction in tension. They are never in
compression. Compression is tension but in the opposite direction,
and using these words in the opposite of their normal sense is
inherently confusing.

The source of the confusion, I think, is in employing entirely
hub-centric terminology to desribe a situation where there are two
equal and opposite forces separated by the radius of the wheel.

We disagree. And I dare say we will not agree. Is there a point in
continuing to waste our time?

Not as far as I'm aware. The pub's open...

Guy
===
** WARNING ** This posting may contain traces of irony.
http://chapmancentral.demon.co.uk

Ian Smith writes:

On Wed, 24 Sep 2003, Simon Brooke wrote:

For the thirteen millionth time, no-one is arguing about the physics
or the engineering. We all agree about the physics and the
engineering. We're arguing about the abuse of English.

Err, no we don't. Guy for example persists in describing the
compressive strain of teh lower spokes as a secondary effect brought
on by rim deformation.

Others maintain that the bottom spokes don't see greatest strain.

Clearly they don't see greatest strain, but not even the floggers
think they do. They see greatest _change_ of strain. They see _least_
actual instantaneous strain, as a strain guage will easily show you.

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

Age equals angst multiplied by the speed of fright squared.
;; the Worlock

Ian Smith writes:

On Wed, 24 Sep 2003, Simon Brooke wrote:

For the thirteen millionth time, no-one is arguing about the physics
or the engineering. We all agree about the physics and the
engineering. We're arguing about the abuse of English.

Err, no we don't. Guy for example persists in describing the
compressive strain of teh lower spokes as a secondary effect brought
on by rim deformation.

Others maintain that the bottom spokes don't see greatest strain.

Clearly they don't see greatest strain, but not even the floggers
think they do. They see greatest _change_ of strain. They see _least_
actual instantaneous strain, as a strain guage will easily show you.

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

Age equals angst multiplied by the speed of fright squared.
;; the Worlock

On Wed, 24 Sep 2003 20:05:02 GMT, Simon Brooke wrote:
Ian Smith writes:

Others maintain that the bottom spokes don't see greatest strain.

Clearly they don't see greatest strain, but not even the floggers
think they do. They see greatest _change_ of strain. They see _least_
actual instantaneous strain, as a strain guage will easily show you.

Nonsense on two counts. Firstly, strain is by definition a change
from a reference state. Secondly a strain gauge cannot easily show
you what you claim it will, since they are not easily connected to a
spoke before it is assembled into a wheel and subsequently loaded.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

On Wed, 24 Sep 2003 20:05:02 GMT, Simon Brooke wrote:
Ian Smith writes:

Others maintain that the bottom spokes don't see greatest strain.

Clearly they don't see greatest strain, but not even the floggers
think they do. They see greatest _change_ of strain. They see _least_
actual instantaneous strain, as a strain guage will easily show you.

Nonsense on two counts. Firstly, strain is by definition a change
from a reference state. Secondly a strain gauge cannot easily show
you what you claim it will, since they are not easily connected to a
spoke before it is assembled into a wheel and subsequently loaded.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

Ian Smith wrote:

But, let's continue. Add a spoke to the hub/rim vertically down
from the hub, attached as per normal to the hub but under no
tension. What has changed? There is an additional force on the hub
from the weight of the lower spoke which is also in fact also
hanging from the rim via the hub but the lower spoke in no way acts
to support the hub.

Here's where your experiment starts to go wrong - your proto-wheel has
(potentially dramaticaly) different length spokes. Bicycle wheels
don't.


Ok, full version has the wheel laid down (or the load/hub suported
externally) and two equal length untensioned spokes installed, wheel is then
put upright and lo and behold, you see a tension in the top spoke and none
in the bottom spoke.

Lie the wheel down again/ Remove the load. Tension each spoke equally, stand
up again and lo and behhold the top spoke shows a highe tension and the
lower spoke a lower tension



A couple of other things:

1: thought experiments can easily give an impossible conclusion (that
cat - is it alive or is it dead? no, it's neither).

That's quantum mecahnics not bike mechanics - in the quantum world
indeterminate states or mutually exclusive states can co-exist. Cats don't
exist at quantum scales ....

2: you've apparently forgotten that it's a statically indeterminate
structure. Your thought experiment addresses a structure that looks a
little like a bicycle wheel, but isn't.

I think you are confusing the mechanical engineering with the physics. While
in engineering calcualtional terms it is valid to "cancel out" two opposign
equal tensions and theby in the calculation regard a reduction in tension as
a compression, that is not the physical reality. Spokes to not work in
compression and cannot support the load from below


pk

Ian Smith wrote:

But, let's continue. Add a spoke to the hub/rim vertically down
from the hub, attached as per normal to the hub but under no
tension. What has changed? There is an additional force on the hub
from the weight of the lower spoke which is also in fact also
hanging from the rim via the hub but the lower spoke in no way acts
to support the hub.

Here's where your experiment starts to go wrong - your proto-wheel has
(potentially dramaticaly) different length spokes. Bicycle wheels
don't.


Ok, full version has the wheel laid down (or the load/hub suported
externally) and two equal length untensioned spokes installed, wheel is then
put upright and lo and behold, you see a tension in the top spoke and none
in the bottom spoke.

Lie the wheel down again/ Remove the load. Tension each spoke equally, stand
up again and lo and behhold the top spoke shows a highe tension and the
lower spoke a lower tension



A couple of other things:

1: thought experiments can easily give an impossible conclusion (that
cat - is it alive or is it dead? no, it's neither).

That's quantum mecahnics not bike mechanics - in the quantum world
indeterminate states or mutually exclusive states can co-exist. Cats don't
exist at quantum scales ....

2: you've apparently forgotten that it's a statically indeterminate
structure. Your thought experiment addresses a structure that looks a
little like a bicycle wheel, but isn't.

I think you are confusing the mechanical engineering with the physics. While
in engineering calcualtional terms it is valid to "cancel out" two opposign
equal tensions and theby in the calculation regard a reduction in tension as
a compression, that is not the physical reality. Spokes to not work in
compression and cannot support the load from below


pk

Simon Brooke wrote:
"AndyMorris" writes:

Simon Brooke wrote:

I think you probably meant to say 'the bottom spokes undergo more
strain than the top ones'; if so, it's _not_ true, and is just
precisely the misunderstanding this 'stands on' nonsense leads to.
The bottom spokes at any given moment - those immediately above the
contact patch - experience the _least_ strain of all the spokes -
they're doing _least_ work. But they do, dynamically, experience the
greatest _change_ of strain as they move from being normally loaded
at about half-past-four to relatively highly loaded at five o'clock
to relatively unloaded at 6 o'clock to relatively highly loaded at 7
o'clock to normal again by half-past-seven.


No, I meant what I said.

What you said was:

To say that a hub stands on the bottom spokes suggests that the
bottom spokes undergo more strain than the bottom ones, this is
true.

If you meant this, there's no help for you.

Yes but I was talking out of my arse.


--
Andy Morris

AndyAtJinkasDotFreeserve.Co.UK


Love this:
Put an end to Outlook Express's messy quotes
http://home.in.tum.de/~jain/software/oe-quotefix/

Simon Brooke wrote:
"AndyMorris" writes:

Simon Brooke wrote:

I think you probably meant to say 'the bottom spokes undergo more
strain than the top ones'; if so, it's _not_ true, and is just
precisely the misunderstanding this 'stands on' nonsense leads to.
The bottom spokes at any given moment - those immediately above the
contact patch - experience the _least_ strain of all the spokes -
they're doing _least_ work. But they do, dynamically, experience the
greatest _change_ of strain as they move from being normally loaded
at about half-past-four to relatively highly loaded at five o'clock
to relatively unloaded at 6 o'clock to relatively highly loaded at 7
o'clock to normal again by half-past-seven.


No, I meant what I said.

What you said was:

To say that a hub stands on the bottom spokes suggests that the
bottom spokes undergo more strain than the bottom ones, this is
true.

If you meant this, there's no help for you.

Yes but I was talking out of my arse.


--
Andy Morris

AndyAtJinkasDotFreeserve.Co.UK


Love this:
Put an end to Outlook Express's messy quotes
http://home.in.tum.de/~jain/software/oe-quotefix/