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MA3 rim failure, where to now



 
 
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  #1  
Old September 8th 03, 01:31 PM
jt
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Default MA3 rim failure, where to now


"Simon Brooke" wrote in message
. uk...

But against that a three cross wheel is substantially springier than a
radial.


I'd like to see some backup for this, please.

Assuming you mean what is commonly meant by "springier" and
"substantially" - i.e deflection per unit force and a difference of, say,
10%.


Numbers please.


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  #2  
Old September 8th 03, 03:35 PM
Simon Brooke
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Default MA3 rim failure, where to now

"jt" writes:

"Simon Brooke" wrote in message
. uk...

But against that a three cross wheel is substantially springier than a
radial.


I'd like to see some backup for this, please.

Assuming you mean what is commonly meant by "springier" and
"substantially" - i.e deflection per unit force


Yup...

and a difference of, say,
10%.


Probably.

Take a radial and a three cross and bounce 'em off the workshop floor,
and you'll see what I mean.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/
;; Our modern industrial economy takes a mountain covered with trees,
;; lakes, running streams and transforms it into a mountain of junk,
;; garbage, slime pits, and debris. -- Edward Abbey
  #3  
Old September 9th 03, 01:19 AM
Benjamin Weiner
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Default MA3 rim failure, where to now

Simon Brooke wrote:

Take a radial and a three cross and bounce 'em off the workshop floor,
and you'll see what I mean.


If you do that with tires on and inflated, I predict any
difference you see will be due to the tires (Try 20psi vs 100psi
to see an exaggerated example).

If you do that without a tire on the rim, I predict you will
hear a loud clang and learn nothing about the properties of
the wheel lacing.

Ben
I'm pretty certain about the loud clang

  #4  
Old September 9th 03, 04:07 AM
Tim McNamara
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Default MA3 rim failure, where to now

In article ,
Simon Brooke wrote:

Take a radial and a three cross and bounce 'em off the workshop
floor, and you'll see what I mean.


So, umm, perhaps you can figure out a way to prove your point without
us bashing perfectly good wheels into the ground?
  #5  
Old September 9th 03, 11:05 AM
Simon Brooke
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Default MA3 rim failure, where to now

Tim McNamara writes:

In article ,
Simon Brooke wrote:

Take a radial and a three cross and bounce 'em off the workshop
floor, and you'll see what I mean.


So, umm, perhaps you can figure out a way to prove your point without
us bashing perfectly good wheels into the ground?


OK, first I confess I'm just repeating what I was taught when I learned
to build wheels. You build touring wheels more crossed to make them
springier and more compliant; you build racing wheels less crossed to
make the bike flippier and more precise. Since I like my bikes
flippier I've always tended to build my own wheels less crossed, and I
don't really have that much 'seat of the pants' experience of whether more
crossed really gives a more compliant ride.

But consider it as a problem of geometry. On a radial wheel pressure
in the plane of rotation can only be resisted by the direct
enlongation of spokes, each forming a simple radius of the circle (and
to a lesser extent by the out-of-round distortion of the rim, but
that's a constant wrt the problem). On semi-tangent wheels, each spoke
is longer than the radius of the circle, possibly by the 10% quoted
earlier. Furthermore, because they're tangential, movement of the rim
doesn't only stretch them, it moves them (slightly) in their
web. Finally, unit movement of a point on the rim wrt the hub does not
translate into unit enlongation of any one spoke; it translates to
slightly less than unit movement. So given that the spokes are equally
springy, it seems (to me at least) logical that longer spokes which
move less will allow greater movement per unit force than shorter
spokes which move more.

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

;; MS Windows: A thirty-two bit extension ... to a sixteen bit
;; patch to an eight bit operating system originally coded for a
;; four bit microprocessor and sold by a two-bit company that
;; can't stand one bit of competition -- anonymous
  #6  
Old September 9th 03, 04:47 PM
Just zis Guy, you know?
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Default MA3 rim failure, where to now

"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.

It is true to say that the spokes undergo most of the cyclic change in
tension in the bottom few tens of degrees of arc.

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.


--
Guy
===

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


  #7  
Old September 9th 03, 04:53 PM
James Hodson
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Default MA3 rim failure, where to now

On Tue, 9 Sep 2003 16:47:07 +0100, "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!


Haven't we been here before?

James

--
"Sorry mate, I didn't see you" is not a satisfactory excuse.
  #8  
Old September 9th 03, 06:05 PM
Simon Brooke
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Default MA3 rim failure, where to now

"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'.

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

;; I'll have a proper rant later, when I get the time.
  #9  
Old September 9th 03, 07:39 PM
Chris B.
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Posts: n/a
Default MA3 rim failure, where to now

On Tue, 9 Sep 2003 16:47:07 +0100, "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,


So the spokes are compressed.

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.


It is possible to compress something that is in a net state of tension
(and stays thusly).

It is true to say that the spokes undergo most of the cyclic change in
tension in the bottom few tens of degrees of arc.

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.

--
Chris Bird
  #10  
Old September 9th 03, 07:42 PM
Chris B.
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Posts: n/a
Default MA3 rim failure, where to now

On Tue, 09 Sep 2003 17:05:07 GMT, 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.

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.


They dun lurn dere fizziks fraam wheelbuilders, dat's fer darn shure!

--
Chris Bird
 




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