Crank about to break?

jim beam wrote:
Jim Rogers wrote:
I bought some used older (~94) Campy Chorus cranks 2 years ago and
didn't notice any problems with them when I installed them. I've ridden
~2,000 miles on these cranks and weigh 225.

Yesterday I was switching them to another bike and noticed a problem on
the left one. I don't know how to tell if this is just a scratch or a
crack. Here are a couple of pictures:

http://pg.photos.yahoo.com/ph/irene_r_1999/my_photos

Is that crank safe to use?

--Jim Rogers

for the time being, yes. it's just a surface scratch.

i'd continue to use it. i'd check it periodically, but it's unlikely to
be a problem.

Somebody probably put the bike into one of those CyclePro crank arm
bicycle stands and knocked it over. That would account for the
scratch/gouge on the inside of the arm in that location. Or perhaps
something did fall between the BB and the arm, but that would be
unusual since things fall through that area on the left (e.g. no
dropped chain like on the right). -- Jay Beattie.

The one that concerns me most is what appears to be a crack across the base of
the crank at 10:00 to 10:30. This crack curls around the lip to the flat
surrounding the square hole. This aspect is only visible in the full-size image.

Ah yess-- I see what you're talking about now. I'm 99% certain that's
just a surface scratch.

It's too bad you didn't take more pictures of this area. I can also see what
appear to be other cracks initiating in several places around the square hole,
especially near the corner at 9:00. Again, most of this detail is lost in the
reduced-size image.

That face has several of these surface scratches. I'm pretty sure they
are not cracks.

The obvious circumferential scoring at the base of the arm that is grabbing
everyone's attention worries me less. Still, I would not use this crank.

I probably won't use it but I'm waiting to here back fro the guy that
offered to do the x-ray.

--Jim

wrote:
Jim Rogers writes:

This _first_ (1 of 5) image in your album shows what may be a stray
thread (i.e. dust), a surface scratch (harmless), or a fatigue
crack across the base of the crank. My eye wants the crack to
continue to the nearest corner of the square hole, but I can't say
for sure by looking at this one photo. Can you examine that more
closely and confirm what this is?

If I understand you, you are describing what looks like a crack or
thread at about 2 o'clock on the face of the base in photo 1. It
angles down and to the left at about a 45* angle.

If so, that appears to be just a surface scratch.

As was mentioned, scratches have upset edges, being plowed furrows
from which material was displaced. There is no upset edge visible and
no lack of clear alodine finish in the groove or its edges.
Therefore, it is most likely not a surface scratch but rather a
manufacturing flaw of unknown depth.

Jobst, have you seen cracks starting on the back of a crank arm (away
from the bolt hole or spider arm)? All of my crank arm failures
started with cracks on the front -- I would assume due to fatigue from
bending toward the frame. -- Jay Beattie.

Jay Beattie writes:

This _first_ (1 of 5) image in your album shows what may be a
stray thread (i.e. dust), a surface scratch (harmless), or a
fatigue crack across the base of the crank. My eye wants the
crack to continue to the nearest corner of the square hole, but I
can't say for sure by looking at this one photo. Can you examine
that more closely and confirm what this is?

If I understand you, you are describing what looks like a crack or
thread at about 2 o'clock on the face of the base in photo 1. It
angles down and to the left at about a 45* angle.

If so, that appears to be just a surface scratch.

As was mentioned, scratches have upset edges, being plowed furrows
from which material was displaced. There is no upset edge visible
and no lack of clear alodine finish in the groove or its edges.
Therefore, it is most likely not a surface scratch but rather a
manufacturing flaw of unknown depth.

Jobst, have you seen cracks starting on the back of a crank (away
from the bolt hole or spider)? All of my crank failures started
with cracks on the front -- I would assume due to fatigue from
bending toward the frame.

This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were. A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

Most crank failures that I have observed were tensile failures caused
by bending force at the bottom of the pedal stroke. The ones that
didn't fit that description were caused by re-tightening the retaining
bolt, the best of which split a Campagnolo crank lengthwise.

Jobst Brandt

wrote:
Jay Beattie writes:

This _first_ (1 of 5) image in your album shows what may be a
stray thread (i.e. dust), a surface scratch (harmless), or a
fatigue crack across the base of the crank. My eye wants the
crack to continue to the nearest corner of the square hole, but I
can't say for sure by looking at this one photo. Can you examine
that more closely and confirm what this is?

If I understand you, you are describing what looks like a crack or
thread at about 2 o'clock on the face of the base in photo 1. It
angles down and to the left at about a 45* angle.

If so, that appears to be just a surface scratch.

As was mentioned, scratches have upset edges, being plowed furrows
from which material was displaced. There is no upset edge visible
and no lack of clear alodine finish in the groove or its edges.
Therefore, it is most likely not a surface scratch but rather a
manufacturing flaw of unknown depth.

Jobst, have you seen cracks starting on the back of a crank (away
from the bolt hole or spider)? All of my crank failures started
with cracks on the front -- I would assume due to fatigue from
bending toward the frame.

This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were.

if any gouge caused a "plowed furrow", it would be impossible to machine
anything. whether or not any upsetting occurs depends on a number of
factors, including the cutting material, the cut material, surface
treatments, metallography, geometry, speed, lubrication, etc.

A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

not if it were abraded, like would happen with cable rub.


Most crank failures that I have observed were tensile failures caused
by bending force at the bottom of the pedal stroke.

that's ductile yield, not fatigue, what you really mean to describe.
and you can't fudge about specifying "tensile" when talking about
fatigue since materials don't fatigue in compression.

The ones that
didn't fit that description were caused by re-tightening the retaining
bolt, the best of which split a Campagnolo crank lengthwise.

Jobst Brandt

wrote:
This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were. A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

How about shoe rub? Or, as jim beam mentioned, cable rub? I have cranks
marred in both ways, and neither has upset material beside the scratched
or buffed area. I don't see how the absence of upset material rules out
the possibility that the flaw was caused by a scratch.

--
Dave
dvt at psu dot edu

Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)

On Tue, 28 Nov 2006 12:20:53 -0500, dvt wrote:

wrote:
This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were. A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

How about shoe rub? Or, as jim beam mentioned, cable rub? I have cranks
marred in both ways, and neither has upset material beside the scratched
or buffed area. I don't see how the absence of upset material rules out
the possibility that the flaw was caused by a scratch.


It all depends of your definitions. What is a "scratch", and "rub", a
"cut", a "crack"?

Jobst is (I assume) using "scratch" to indicate a feature produced
when surface material is displaced, presumably by a single action, but
is still attached. A "rub" tends to indicate a feature where the
displaced material has become detached (or just possibly burnished),
presumably by multiple repeated actions.

It seems to have become fashionable to take imagined offense and
justify it by claiming that an the authors of an opposing view have
made _fundamental_ errors, errors which are of course of this nature;
neither truly errors nor fundamental to the point at issue. Another
recent example was claiming that two spokes were not "adjacent" when
the rim holes into which their nipples seated were separated by a
third hole. The author of this seemed deliberately oblivious to the
fact that if one were to inspect the other ends of the two spokes in
question they were indeed "adjacent"; and that in reality the nature
of their "adjacency", if determined as the original poster had
intended it be, re-inforced the original poster's point.

On Tue, 28 Nov 2006 17:51:50 GMT, wrote:

On Tue, 28 Nov 2006 12:20:53 -0500, dvt wrote:

wrote:
This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were. A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

How about shoe rub? Or, as jim beam mentioned, cable rub? I have cranks
marred in both ways, and neither has upset material beside the scratched
or buffed area. I don't see how the absence of upset material rules out
the possibility that the flaw was caused by a scratch.


It all depends of your definitions. What is a "scratch", and "rub", a
"cut", a "crack"?

Jobst is (I assume) using "scratch" to indicate a feature produced
when surface material is displaced, presumably by a single action, but
is still attached. A "rub" tends to indicate a feature where the
displaced material has become detached (or just possibly burnished),
presumably by multiple repeated actions.

It seems to have become fashionable to take imagined offense and
justify it by claiming that an the authors of an opposing view have
made _fundamental_ errors, errors which are of course of this nature;
neither truly errors nor fundamental to the point at issue. Another
recent example was claiming that two spokes were not "adjacent" when
the rim holes into which their nipples seated were separated by a
third hole. The author of this seemed deliberately oblivious to the
fact that if one were to inspect the other ends of the two spokes in
question they were indeed "adjacent"; and that in reality the nature
of their "adjacency", if determined as the original poster had
intended it be, re-inforced the original poster's point.

Dear J.,

Elsewhere in that thread, the deliberately oblivious author is
struggling not to ask another poster how one removes cross-ties from a
monorail.

He still thinks that "adjacent" means "adjacent", just as he thinks
that one differs from two.

Cheers,

C*** F****

Jim Rogers wrote:
I bought some used older (~94) Campy Chorus cranks 2 years ago and
didn't notice any problems with them when I installed them. I've ridden
~2,000 miles on these cranks and weigh 225.

Yesterday I was switching them to another bike and noticed a problem on
the left one. I don't know how to tell if this is just a scratch or a
crack. Here are a couple of pictures:

http://pg.photos.yahoo.com/ph/irene_r_1999/my_photos

Is that crank safe to use?

--Jim Rogers

It looks like nothing. I, like others her, would ignore it. However,
you can buy a left crank arm for peanuts.

Andres

On Tue, 28 Nov 2006 11:02:03 -0700, wrote:

On Tue, 28 Nov 2006 17:51:50 GMT, wrote:

On Tue, 28 Nov 2006 12:20:53 -0500, dvt wrote:

wrote:
This is not a crack caused by fatigue and doesn't even look like a
gouge or scratch because there is no upset material adjacent to the
"plowed furrow" as it were. A cur as clean as this one would require
a sharp tool with the proper angle of incidence (rake) and even then a
machined track would be visible.

How about shoe rub? Or, as jim beam mentioned, cable rub? I have cranks
marred in both ways, and neither has upset material beside the scratched
or buffed area. I don't see how the absence of upset material rules out
the possibility that the flaw was caused by a scratch.


It all depends of your definitions. What is a "scratch", and "rub", a
"cut", a "crack"?

Jobst is (I assume) using "scratch" to indicate a feature produced
when surface material is displaced, presumably by a single action, but
is still attached. A "rub" tends to indicate a feature where the
displaced material has become detached (or just possibly burnished),
presumably by multiple repeated actions.

It seems to have become fashionable to take imagined offense and
justify it by claiming that an the authors of an opposing view have
made _fundamental_ errors, errors which are of course of this nature;
neither truly errors nor fundamental to the point at issue. Another
recent example was claiming that two spokes were not "adjacent" when
the rim holes into which their nipples seated were separated by a
third hole. The author of this seemed deliberately oblivious to the
fact that if one were to inspect the other ends of the two spokes in
question they were indeed "adjacent"; and that in reality the nature
of their "adjacency", if determined as the original poster had
intended it be, re-inforced the original poster's point.

Dear J.,

Elsewhere in that thread, the deliberately oblivious author is
struggling not to ask another poster how one removes cross-ties from a
monorail.

He still thinks that "adjacent" means "adjacent", just as he thinks
that one differs from two.

Cheers,

C*** F****

Oh, I forgot--if "one were to inspect the other ends of the two spokes
in question," one would find, alas, that they are not adjacent on a
cross-3 hub.

They're actually separated by four or six other spoke heads.

One might remove this objection by providing imaginary radial lacing
for the real wheel.

Cheers,

C*** x x x x x x F****
or
C*** x x x x F****