MA3 rim failure, where to now

Just zis Guy, you know? wrote:

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 does if you understand that the load-supporting is considered as a
change relative to the unloaded state. Since all sides seem to agree on
the actual tensions and changes this seems like a rather sterile debate
on semantics, but I personally find the 'standing on the lower spokes'
description a more useful and succinct description than the alternative.

James

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). 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.

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

~PB

"James Annan" wrote in message
...


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 does if you understand that the load-supporting is considered as a
change relative to the unloaded state.

But once you take into account the fact that without those spokes which do
nothing the wheel collapses, and without the spokes which do all the work it
stays up, and the mechanism by which the substantial change occurs is simply
the deformation of the rim, the word "stand" ceases to have any meaning or
use.

I refer the hon. gentleman to my earlier answer: the hub is supported by the
spoke nipples pushing on the rim ;-P

--
Guy
===

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

Simon Brooke wrote in message .uk...
(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.


I'm certainly no expert, but once again I don't think your snideness
gets you out of this problem. The fact that the spokes elongate
immediately after they leave the contact patch does not demonstrate
that they are elongated at the top of the wheel, as one would expect
if the "hanging from the top spokes" hypothesis were true.

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

Rick

In article ,
says...
"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.


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

Rick

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.

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

Jobst Brandt

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

I think you need to think about cause and effect a little.

You are saying the effect of a large stress change is caused by
nothing happening.
Meanwhile, no stress change is caused by the spokes carrying teh
weight applied to the wheel.

Curious.

regards, Ian SMith
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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...

Ian Smith writes:

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?

At real levels of load, yes, the wheel collapses due to flexural
failure of the rim in the lower portion. The hub hits the floor.

So, your argument actually falls at teh very first hurdle, however,
let's suppose you want to talk about useless wheels that can't support
much load at all, and thus the rim manages to withstand the flexural
loads.

The argument is still rubbish. You're now not talking about a bicycle
wheel but a totally different structure that carries load in a
different way. Stated more formally: teh wheel is statically
indeterminate, so altering the stiffness of components fundamentally
alters the loads in teh other components. If you remove spokes, you
effectively set their stiffness to zero - as big a change as can be
made. Statical indeterminancy is a tricky concept to grasp, but
there's some discussion and simple examples on my web page.

As an example of why it;s so badly wrong, let's test your method on
another structure. What your method seems to be is to imagine
removing a component, work out if the structure still works, and if it
does, decide the component did not contribute to the working even when
it was there.

Let's apply this theory to teh Eiffel Tower. Suppose we remove the
northmost leg. Does the tower fall over? No - it has three remaining
legs, each with a large footprint, so it continues to stand (albeit
somewhat less stable). So, your argument deduces that the
northernmost leg does not contribute to the tower standing up. Now,
since it's symmetrical in 4 parts, exactly the same argument can be
applied to each leg - anything that works for the north leg must work
for the east leg, for example. Thus, we reach the conclusion,
applying your argument, that the Eiffel Tower stands up without any
contribution from any of teh legs - it simply floats there, hanging
(presumably) from sky-hooks.

This is clearly not the case, so (evidently) your thought experiment
actually doesn't tell you anything useful about how a structurally
indeterminate structure stands up.

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.

Eh? You appear to be saying taht something can resist some load
without any change in its state of stress. That's a fairly novel
concept, and on teh assumption that you're not intent on creating a
whole new understanding of structural mechanics, it might be helpful
to explain what you mean in more detail.

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 North team, on teh other hand, have changed what they are doing,
and as soon as they changed what they were doing, the effect changed.
Your argument says that teh team that didn't change somehow made an
effect happen, but the team that did change had no effect.

Suppose two people have two buttons. Both press button A, and nothing
happens. Both press button A, nothing happens. Both press button A,
nothing happens. Both press button A, nothing happens. Person 1
presses button A, and person 2 presses button B and this time a bell
rings. Your argument is that the person 1, by pressing button A, must
have made the bell ring (even though every previous time, pressing
button A caused nothing to happen). Frankly, that's just barmy - by
occam, the most likely explanation is that pressing button B causes a
bell to ring, and person 2 caused the bell.

Why don't you do the analysis? At teh very least, you could read
Jobst Brandt's book.

regards, Ian SMith
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On Wed, 10 Sep 2003 08:35:05 GMT, Simon Brooke wrote:

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.

Actually, they elongate at teh bottom - alongside teh contact patch.
In fact, the greatest elongation is in the lower half of the wheel.
Jobst Brandt describes this in his book, and it's clearly visible in
my own analysis at http://www.astounding.org.uk/ian/wheel/. The
elongation at this pont (in the bottom half of teh wheel, remember) is
about twice any elongation occurring in teh upper half.

If you're going to be facile, getting your facts right would make you
look less silly.

regards, Ian SMith
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