Rear Wheel Spokes Breaking -- Interesting Pattern

I've got a Bianchi Pista--cheap machine built wheels. I've had an
interesting problem with broken spokes. I've broken 4 spokes (one at a
time, at different times) on the rear wheel. I was looking at it the other
day (in preparation for re-lacing it with name brand spokes) and notice that
all 4 of the spokes that broke were
- on the non-drive side
- were "pushing spokes", not "pulling spokes".

Now this is a track hub set up for cogs on both sides, so the wheel is not
dished.

Any theories (hopefully reasonable) on why only the non-drive pushing spokes
are breaking?

-Kalukis

PS - Love the Pista -- recommend everybody give fixed-gear a try--it's cheap
and fun!

pull out a few spokes and take a gander with a magnifier and strong
light-
see any surface cracks?
try teflon wax dripped onto the hub-spokes inboard and out-
'like' place the bike on one side then the other, allowing for wax set
up.
this is a partial cure
but DT with teflon goes one better. non-DT or wheelsmith run a wide
range of poor to good within one batch.
it possible to set up a wheel with generic spokes from kaluzamoolee
smelters, run a hundred yards, and goeth sprongggg!
which off course is 'absolutely infuriating.'
but doesn't beet having that go on 2-3 more times b4 throwing the batch
in the can.

Kalukis wrote:
I've got a Bianchi Pista--cheap machine built wheels. I've had an
interesting problem with broken spokes. I've broken 4 spokes (one at a
time, at different times) on the rear wheel. I was looking at it the other
day (in preparation for re-lacing it with name brand spokes) and notice that
all 4 of the spokes that broke were
- on the non-drive side
- were "pushing spokes", not "pulling spokes".

Now this is a track hub set up for cogs on both sides, so the wheel is not
dished.

Any theories (hopefully reasonable) on why only the non-drive pushing spokes
are breaking?

hard to say without seeing the failures. trying to reason that head-in
spokes being at a more severe angle than the head-out doesn't apply
because the wheel's undished. same goes for any assumption about
"stress relief" unless someone managed to "relieve" one side but not the
other - something of a credibility test. drive torque [loading or
unloading] is less on that side than the drive side due to torsion of
the hub, so that's unlikely to be unloading those spokes more than drive
side.

so, of all the possibilities, i think it's more likely to be something
simple like physical damage to the spokes. check them for any scoring
or scratches. the elbows are where spokes usually break, so head in
[elbow out] spokes are more vulnerable to damage initiating fatigue in
this region.


-Kalukis

PS - Love the Pista -- recommend everybody give fixed-gear a try--it's cheap
and fun!

indeed. great machines.

Kalukis asks- I've got a Bianchi Pista--cheap machine built wheels. I've had
an
interesting problem with broken spokes. I've broken 4 spokes (one at a
time, at different times) on the rear wheel. I was looking at it the other
day (in preparation for re-lacing it with name brand spokes) and notice that
all 4 of the spokes that broke were
- on the non-drive side
- were "pushing spokes", not "pulling spokes".



I'd say probably the tension is too low thruout and the left/pushing side
spokes are doing the 'bend a coat hanger back and forth until it breaks" kinda
thing.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"

Qui si parla Campagnolo wrote:
Kalukis asks- I've got a Bianchi Pista--cheap machine built wheels. I've had
an

interesting problem with broken spokes. I've broken 4 spokes (one at a
time, at different times) on the rear wheel. I was looking at it the other
day (in preparation for re-lacing it with name brand spokes) and notice that
all 4 of the spokes that broke were
- on the non-drive side
- were "pushing spokes", not "pulling spokes".




I'd say probably the tension is too low thruout and the left/pushing side
spokes are doing the 'bend a coat hanger back and forth until it breaks" kinda
thing.

no. that failure mode, although it's called "fatigue" in the "faq"'s,
is entirely different from the failure mechanism seen in spoke failures.
it's gross yielding causing the material to essentially "run out of
ductility". there's no cracking & there's no crack tip stress
concentration.

the mechanism that is fatigue in spoke failures is the progression of a
crack at stress well below yield, and unaccompanied by gross yielding.
this crack grows minutely with each stress cycle. once it's grown to
the point where crack tip stress concentration exceeds the fracture
stress of the material, it suddenly fails. a wholly different process.
spoke fatigue cracking is typically initiated at surface flaws such as
mandrel bending marks or "orange peel" marks left by the forming process.

confusing the two different mechanisms is like attributing all rim
cracking to anodization - sounds plausible if you don't know any better,
but wrong all the same.


Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"

On Fri, 14 Jan 2005 06:52:06 -0800, jim beam
wrote:

Qui si parla Campagnolo wrote:

[snip]

I'd say probably the tension is too low thruout and the left/pushing side
spokes are doing the 'bend a coat hanger back and forth until it breaks" kinda
thing.

no. that failure mode, although it's called "fatigue" in the "faq"'s,
is entirely different from the failure mechanism seen in spoke failures.
it's gross yielding causing the material to essentially "run out of
ductility". there's no cracking & there's no crack tip stress
concentration.

the mechanism that is fatigue in spoke failures is the progression of a
crack at stress well below yield, and unaccompanied by gross yielding.
this crack grows minutely with each stress cycle. once it's grown to
the point where crack tip stress concentration exceeds the fracture
stress of the material, it suddenly fails. a wholly different process.
spoke fatigue cracking is typically initiated at surface flaws such as
mandrel bending marks or "orange peel" marks left by the forming process.

confusing the two different mechanisms is like attributing all rim
cracking to anodization - sounds plausible if you don't know any better,
but wrong all the same.

Dear Jim,

I take it that stress fractures occur when the metal is bent
repeatedly below yield, never taking a set, and are cracks,
while the paper clip that just took longer to break than I
expected is doing something else?

I think that I see the difference--I bent that stupid paper
clip out of shape so much that it would have stayed bent,
unlike a spoke flexing only slightly.

But what is "running out of ductility" versus cracking? The
paper clip seemed to fail suddenly. Would its broken ends
look different under a microscope than a stress-broken
spoke?

Carl Fogel

Back when I used to break spokes all the time, it was always
an overdrilled hub (too much ``countersink'' on the hub holes)
and you can get the same effect with too-long elbow ends on the
end of the spokes.

The ``countersink'' supports the bend in the spoke and keeps it
from flexing once per rotation when it's sized properly. If it's
too drilled out, or the spoke end is too long for it, the spoke
flexes and you break a spoke every thousand miles or so.

Quick fix is put a tiny washer under the end of replacement spokes
when you put them in, to draw up their bend into the countersink,
anyway that's what I did and it worked.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

wrote:
On Fri, 14 Jan 2005 06:52:06 -0800, jim beam
wrote:


Qui si parla Campagnolo wrote:


[snip]


I'd say probably the tension is too low thruout and the left/pushing side
spokes are doing the 'bend a coat hanger back and forth until it breaks" kinda
thing.

no. that failure mode, although it's called "fatigue" in the "faq"'s,
is entirely different from the failure mechanism seen in spoke failures.
it's gross yielding causing the material to essentially "run out of
ductility". there's no cracking & there's no crack tip stress
concentration.

the mechanism that is fatigue in spoke failures is the progression of a
crack at stress well below yield, and unaccompanied by gross yielding.
this crack grows minutely with each stress cycle. once it's grown to
the point where crack tip stress concentration exceeds the fracture
stress of the material, it suddenly fails. a wholly different process.
spoke fatigue cracking is typically initiated at surface flaws such as
mandrel bending marks or "orange peel" marks left by the forming process.

confusing the two different mechanisms is like attributing all rim
cracking to anodization - sounds plausible if you don't know any better,
but wrong all the same.


Dear Jim,

I take it that stress fractures occur when the metal is bent
repeatedly below yield, never taking a set, and are cracks,
while the paper clip that just took longer to break than I
expected is doing something else?

yes. two different mechanisms.


I think that I see the difference--I bent that stupid paper
clip out of shape so much that it would have stayed bent,
unlike a spoke flexing only slightly.

yes, the fatigue you're looking at in a spoke happens well below yield.


But what is "running out of ductility" versus cracking?

one is where the material has yielded either once or several times,
becomes full work hardened & simply exceeds its ability sustain more
work. the other is exactly as the name says - the material cracks under
the slow, sub-yield high cycle repetition of stress, and certianly well
below any ductility limit that may be caused by plastic deformation.
once that crack exceeds a critical length, the material experiences
"brittle" failure due to stress concentration at the crack tip.

The
paper clip seemed to fail suddenly. Would its broken ends
look different under a microscope than a stress-broken
spoke?

under the microscope, they'd look similar to a metal that had yielded to
failure, but would look very different to one that had failed due to
high cycle fatigue.


Carl Fogel