4 spokes failed on unridden bike!

I've always done all my maintenance, and commuted 3,000 miles a year for 16
years now; so I've seen quite a bit.

I was cleaning up a couple old bikes we hadn't ridden in a few years,
preparing to donate them. My wife's road bike had a broken spoke, My old
MTB had broken two. All were rear drive-side.

The first job went fine. I hand screwed the spokes into the mountain bike's
wheel and put it in the truing stand. As I was about to start truing it, 4
spokes suddenly snapped! That made 6 spokes on a bike I hadn't ridden! My
only guess was some combination of stress-corrosion cracking (the stainless
spokes had several rusty spots) and over-tightening.

I am rebuilding the wheel with all new spokes (done this before too) but
would sure like to know what happened.


--
Robert Haston
Satellite Beach, FL

Robert Haston wrote:
I've always done all my maintenance, and commuted 3,000 miles a year for 16
years now; so I've seen quite a bit.

I was cleaning up a couple old bikes we hadn't ridden in a few years,
preparing to donate them. My wife's road bike had a broken spoke, My old
MTB had broken two. All were rear drive-side.

The first job went fine. I hand screwed the spokes into the mountain bike's
wheel and put it in the truing stand. As I was about to start truing it, 4
spokes suddenly snapped! That made 6 spokes on a bike I hadn't ridden! My
only guess was some combination of stress-corrosion cracking (the stainless
spokes had several rusty spots) and over-tightening.

I am rebuilding the wheel with all new spokes (done this before too) but
would sure like to know what happened.


without seeing your broken spokes, your suggestions are pretty
reasonable on both counts. another thought is that since the wheels
/had/ been ridden in the past, it's quite possible that they were
already fatigued but that your truing efforts raised tension
sufficiently to cause the stress at the fatigue crack tip to exceed
fracture. almost nothing you can do to prevent that. sometimes also
things like vibation & thermal cycling can cause sudden failure of
something that's not in service.

jim beam:

without seeing your broken spokes, your suggestions are pretty
reasonable on both counts. another thought is that since the wheels
/had/ been ridden in the past, it's quite possible that they were
already fatigued but that your truing efforts raised tension
sufficiently to cause the stress at the fatigue crack tip to exceed
fracture. almost nothing you can do to prevent that. sometimes also
things like vibation & thermal cycling can cause sudden failure of
something that's not in service.


Please explain how temperatures high enough to cause destructive thermal
cycling are reached by bicycle spokes. While you're at it, also explain
how vibration amplitudes high enough to cause significant damage (apart
from the loading on the spokes through rider weight) figure into it.

On Sun, 14 Nov 2004 02:55:45 GMT, Jose Rizal wrote:

jim beam:

without seeing your broken spokes, your suggestions are pretty
reasonable on both counts. another thought is that since the wheels
/had/ been ridden in the past, it's quite possible that they were
already fatigued but that your truing efforts raised tension
sufficiently to cause the stress at the fatigue crack tip to exceed
fracture. almost nothing you can do to prevent that. sometimes also
things like vibation & thermal cycling can cause sudden failure of
something that's not in service.


Please explain how temperatures high enough to cause destructive thermal
cycling are reached by bicycle spokes. While you're at it, also explain
how vibration amplitudes high enough to cause significant damage (apart
from the loading on the spokes through rider weight) figure into it.

Dear Jose,

If a pre-tensioned spoke is already about to break, would
the additional contraction and brittleness of a drop to
sub-zero temperatures be enough to let a spoke break on a
bicycle sitting in an unheated garage?

On several cold mornings, I've found a broken rear spoke and
the rim jammed solid against one brake pad after the wheel
had spun freely while I was cleaning the chain the night
before. Typically, the riding temperature the day before was
around 34-40 F, but the overnight low was 0 to -10 F.

Jobst indicated (I think) that a failing spoke can even
break without any apparent added stress:

"Spokes can break with no additional loading once the
developing crack reduces the cross section to the yield
point. Then the crack grows plastically until failure
without change in load or temperature. I have witnessed
spoke failures more than an hour after parking the
bicycle indoors where the occurrence is conspicuous."

http://groups.google.com/groups?q=%2...c.ne t&rnum=1

My layman's guess is that this means that a tiny crack can
slowly grow while the bike is just sitting there.

Come to think of it, now I'm wondering whose spokes Jobst
witnessed breaking an hour after being parked indoors.

Carl Fogel

Quoth Carl Fogel: (For some reason my spell checker wants to change his=20
name to "Fogey")

If a pre-tensioned spoke is already about to break, would
the additional contraction and brittleness of a drop to
sub-zero temperatures be enough to let a spoke break on a
bicycle sitting in an unheated garage?

Dear Carl,

No, no way cold could do this, since the rim would contract too. In=20
fact, an aluminum rim would contract twice as much as the steel spokes,=20
since aluminum has twice the coefficient of expansion that steel does.

Sheldon "That's Not It" Brown
+------------------------------------------------+
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| November 12, 13, 14 and 18, 19, 20, 21 |
| http://web.mit.edu/gsp/www |
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+------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
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:

On Sun, 14 Nov 2004 02:55:45 GMT, Jose Rizal wrote:

jim beam:
sometimes also
things like vibation & thermal cycling can cause sudden failure of
something that's not in service.


Please explain how temperatures high enough to cause destructive thermal
cycling are reached by bicycle spokes. While you're at it, also explain
how vibration amplitudes high enough to cause significant damage (apart
from the loading on the spokes through rider weight) figure into it.

Dear Jose,

If a pre-tensioned spoke is already about to break, would
the additional contraction and brittleness of a drop to
sub-zero temperatures be enough to let a spoke break on a
bicycle sitting in an unheated garage?

Thermal cycling means a repetitive load, not a load applied once to a
weakened structure that causes breakage.

A normal spoke under tension that is subjected to climatic sub-zero and
warm daytime temperatures will not experience high enough thermal cyclic
amplitudes to cause failure by itself.

If the case is that a spoke is about to break due to existing tension,
it means the load-bearing cross-section of the spoke has been reduced to
below that required to support the load. The reduced cross section will
keep reducing as the crack propagates until the spoke breaks. The
tension on a spoke is orders of magnitudes larger than any contraction
that sub-zero weather temperatures will cause. Climatic sub-zero
temperatures are also not low enough to cause "additonal brittleness" to
a stainless steel spoke.

On several cold mornings, I've found a broken rear spoke and
the rim jammed solid against one brake pad after the wheel
had spun freely while I was cleaning the chain the night
before. Typically, the riding temperature the day before was
around 34-40 F, but the overnight low was 0 to -10 F.

It's doubtful that temperature due to weather by itself caused those
spoke failures. You need to look at the quality of the wheel build.

Jobst indicated (I think) that a failing spoke can even
break without any apparent added stress:

"Spokes can break with no additional loading once the
developing crack reduces the cross section to the yield
point. Then the crack grows plastically until failure
without change in load or temperature. I have witnessed
spoke failures more than an hour after parking the
bicycle indoors where the occurrence is conspicuous."

http://groups.google.com/groups?q=%2...c.ne t&rnum=1

Which is what I stated above. Note again the part of your quote where
it says "without change in load or temperature". This has nothing to do
with thermal cycling.

My layman's guess is that this means that a tiny crack can
slowly grow while the bike is just sitting there.

Yes, when it's reached the point of failure. But again, nothing to do
with thermal cycling.

On Sat, 13 Nov 2004 23:29:15 -0500, Sheldon Brown
wrote:

Quoth Carl Fogel: (For some reason my spell checker wants to change his
name to "Fogey")

If a pre-tensioned spoke is already about to break, would
the additional contraction and brittleness of a drop to
sub-zero temperatures be enough to let a spoke break on a
bicycle sitting in an unheated garage?

Dear Carl,

No, no way cold could do this, since the rim would contract too. In
fact, an aluminum rim would contract twice as much as the steel spokes,
since aluminum has twice the coefficient of expansion that steel does.

Sheldon "That's Not It" Brown

Dear Sheldon,

Hmmm . . . I think that you're right, but using the wrong
figures.

Unlike sluggish ordinary steels, stainless steel has roughly
the same coefficient of thermal expansion as lively
aluminum, a ratio about about 11 to 12 (not 1 to 2).

Where X is the coefficient of thermal expansion (inch/inch F
10-6) . . .

Stainless steel (as used in spokes) is around 11X:

http://www.engineeringtoolbox.com/14_283.html

You have to scroll down to see it. Nearby is a link to this
table:

http://www.engineeringtoolbox.com/24_95.html

.. . . which shows the thermal expansion for aluminum (as
used in rims) is about 12X (and ordinary steel about 7x).

I'm not sure if it matters when the temperature drops that
the aluminum rim is already shrunk (in compression) by the
elongated stainless steel spokes (in tension).

But at an 11/12 ratio, I suspect that the small difference
in what would be a tiny contraction would end up negligible.

Reassuringly, "Stainless steel (type 304 or 316) does not
exhibit any ductile-to-brittle transition at low
temperatures (even down to –400°F), and is an excellent
material for frigid temperatures."

www.b-line.com/engineer/TPS/BLTPS-08.pdf

So maybe it's just coincidence that several spokes
apparently failed during unusually cold nights. I hope so,
since it's snowing lightly right now.

Carl Fogel

Hmm.. sudden temperature change maybe? Did you take the bikes out from
somewhere, and then bring them into a different temperature environment to
start truing them?

Just a weird guess

Mike
http://mikebeauchamp.com

"Robert Haston" wrote in message
link.net...
I've always done all my maintenance, and commuted 3,000 miles a year for
16 years now; so I've seen quite a bit.

I was cleaning up a couple old bikes we hadn't ridden in a few years,
preparing to donate them. My wife's road bike had a broken spoke, My old
MTB had broken two. All were rear drive-side.

The first job went fine. I hand screwed the spokes into the mountain
bike's wheel and put it in the truing stand. As I was about to start
truing it, 4 spokes suddenly snapped! That made 6 spokes on a bike I
hadn't ridden! My only guess was some combination of stress-corrosion
cracking (the stainless spokes had several rusty spots) and
over-tightening.

I am rebuilding the wheel with all new spokes (done this before too) but
would sure like to know what happened.


--
Robert Haston
Satellite Beach, FL

On Sat, 13 Nov 2004 23:29:15 -0500, Sheldon Brown
wrote:

Quoth Carl Fogel: (For some reason my spell checker wants to change his
name to "Fogey")

If a pre-tensioned spoke is already about to break, would
the additional contraction and brittleness of a drop to
sub-zero temperatures be enough to let a spoke break on a
bicycle sitting in an unheated garage?

Dear Carl,

No, no way cold could do this, since the rim would contract too. In
fact, an aluminum rim would contract twice as much as the steel spokes,
since aluminum has twice the coefficient of expansion that steel does.

Sheldon "That's Not It" Brown

Dear Sheldon,

Drat!

After agreeing with you (but pointing out that stainless
steel actually shrinks at almost the same rate as aluminum),
I was suddenly troubled by the vague memory that Jobst has
mentioned that cast control wheels (whose rims and spokes
have identical material) have wiggly spokes because
otherwise they'd crack when they cooled at different rates
and shrank:

"Now consider that die castings have cooling stresses caused
by sequential cooling and that the thicker part of the
spokes near the hub and the hub itself cool last causing
shrink. This puts the die cast spokes in tension. That
castings do this is known and that is why S-shaped spokes
are used on cast iron hand wheels of old to prevent cracks
that occur if they were not S-shaped. All cast railway
wheels also have such ribs on the inside."

http://groups.google.com/groups?selm... utput=gplain

Now I need to be reassured that the thin spokes with their
huge surface area don't cool and contract significantly
faster than the fat rims insulated by pneumatic tires.

This was a lot simpler with wooden ox-carts.

Befuddled,

Carl Fogel

Might the bikes been exposed to fumes of hypochlorite? Just a leaky
bottle of ordinary bleach would be enough over time to kill 300
stainless steel if the pH goes low. In solution with water, presence
of the Na ion keeps the pH high enough to prevent Cl induced SCC. ClO
vapors without the Na ion allows the pH to drop, attacking austinitic
SS, of which spokes are made. With the oxygen free radical, chlorine
induced SCC can happen at much lower temperatures like that of a garage
or store room during the summer.

I have seen SS spokes as brittle as uncooked spaghetti from ClO vapor
after a few months of storage near calcium hypochlorite pool
chemicals.

Just my materials experience as applied to cycling...


--
Weisse Luft