MA3 rim failure, where to now

On Tue, 9 Sep, Just zis Guy, you know? wrote:
"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!

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, but to describe this as "standing" on the bottom spokes
is misleading because if their tension ever below zero during the load cycle
the wheel becomes unstable and rapidly fails.

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.

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

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

Incidently, if anyone else wants to review an FEA of a wheel, you can
find one at http://www.astounding.org.uk/ian/wheel/ (something like
that, anyhow, certainly it's on my site somewhere).

regards, Ian SMith
--
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|o o|
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In article ,
Simon Brooke wrote:

"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.

Certainly no flames on my part. If you choose to descend to that,
that's your call.

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.

Umm, "no" on all counts. I'll again mention that book I referenced
previously- which includes several finite element analyses of bicycle
wheels to quantify how the wheel supports a load. Given that the size
of the compression is on the order of the thickness of a sheet or two
of typewriter paper, the phenomenon you describe can't happen unless
the wheel is so overloaded that it collapses.

And of course if you care to peruse some of the many previous threads
on this topic, which occurs about every 3 to 4 months, you can save
yourself and us a lot of time and tedium. Just go to Google, click on
"groups" and navigate to rec.bicycles.tech and type in "wheel Mitke
Brandt" as the search terms.

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

You'll have to take that up with the engineer who did the FEA of
bicycle wheels. He posts to this newsgroup sometimes. He can also
give you an idea of what training a mechanical engineer receives in
the US. It was good enough for Porsche to hire him to design
suspension for race cars...

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.

Ted Bennett writes:

Simon Brooke wrote:

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.

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.

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

;; I'll have a proper rant later, when I get the time.

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.

But then we are, as others have pointed out before me, two nations
separated by a single language - and, it often seems, a single
education system.

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

;; I'll have a proper rant later, when I get the time.

On Tue, 09 Sep, Simon Brooke wrote:

tension. What you're saying is that _less_ tension somehow translates
into load bearing. It does not and it can not,

It can and does, and frequently does in all sorts of prestressed
structures.

The hub is supported
by the tension in the top spokes, which hang from the rim.

Nonsense. The chnage in tension in teh top spokes accounts for a
vanishingly tiny degree of uplift to the hub.

Do teh analysis if you don't believe it.

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.

Actually, many of the spokes act to pull the hub down. In fact, the
spoke that shows the greatest change in tension effect is one towards
the bottom of the wheel (just to the side of the contact patch).

That is, not only do some spokes do no work at all, many of them are
working in the oppositte direction, ie doing 'harm' to the overall
uplift force applied to the hub.

Do the analysis if you don't believe it.

regards, Ian SMith
--
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|o o|
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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, and the spokes in which the stress
changes dramatically are doing nothing at all.

Curious. It seems not only bicycle wheel hubs, but also causality is
suspended in your universe.

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

Simon Brooke wrote in message .uk...


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


It's the standard BA (Bachelor of Art, loosely translated from the
Latin some of the degrees are still written in) from a liberal arts
college that prepares students to become lawyers and such. No math
requirement, and one science course (if any) that can be satisfied
with the non-mathematical course work of Physics for Poets 101, or
Geology or Archeology 101.

Even some science graduates from these schools who become doctors,
psychologists, et al., do not know the definitions of basic
engineering terms.

Graduates of bona fide U.S. engineering schools have no such problems.

Bike shop wrenches either learn on the job, or from trade schools.
On-the-job and vocational training of this sort is not a substitute
for an education in basic engineering terms and principles. There are
exceptions; the smartest bike mechanic I knew went on to get an
advanced degree in math and is a now professor at a major university.
But he also became a motor cycle mechanic along the way, because it
paid more.

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?

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

Simon Brooke wrote:

... 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.

This explanation would suggest that the deformation of the
rim from a circle occurs at the top of the wheel. Jobst Brandt's
finite element analysis shows that the rim deforms at the bottom
of the wheel. 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.

This, by the way, is a FAQ in rec.bicycles.tech, and you may search
Google for previous debates on the subject. It is, however, declasse
to bring it up before the end of Daylight Savings Time.

But then we are, as others have pointed out before me, two nations
separated by a single language - and, it often seems, a single
education system.

I sometimes wonder if the minority of Britons who resort to these
snobbish digs at American education realize how they are playing into
a stereotype, or how insufferable they sound. I've also noticed that
the cross-pond jabs at education or culture tend to come out when
one's argument is on thin ice on the merits.

BTW, one of my PhD advisors was British and he didn't see the need
to lay on this kind of attitude. But then, he preferred the lack of
Oxbridge attitude at a US university.

Ben

In article ,
Simon Brooke wrote:

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

Nope. But at this point methinks you are just trolling.