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Broken steel road bike fork



 
 
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  #21  
Old September 30th 09, 05:04 AM posted to rec.bicycles.tech
Frank Krygowski[_2_]
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Posts: 7,511
Default Broken steel road bike fork

On Sep 29, 10:40*pm, jim beam wrote:
On 09/29/2009 03:15 PM, Jobst Brandt wrote:

Jay Drew wrote:


What is feathering?


A fading out of one cross section into another where they meet:


*http://www.gtgtandems.com/tech/brazing.html


wrong. *feathering is simply filing the edges of the lug.



The sudden step in cross section causes a stress concentration that
leads to cracking.


which is why it's not a straight cross section, it's got curved features
that mitigate - effectively radius reduction, a standard fatigue
mitigation technique.


Not surprisingly, jim beam has things exactly backwards. When
blending a transition from a larger sized piece (like the lug) to a
smaller sized piece (like the fork blade) the idea is to increase, not
decrease the radius. The ultimate in radius _reduction_ would be a
sharp corner, which generates a high stress concentration. Large
radii are the opposite and generate lower stress concentrations.

You can play around with different radius values at
http://www.fatiguecalculator.com/cgi-bin/findkt.pl
to investigate this.

For example, for D=25 mm, d = 23 mm and r=1 mm, the stress
concentration factor is 1.84 - that is, peak stress in the area of the
shoulder will be 1.84 times the stress further away. If you reduce
the radius (as jim seems to recommend) to 0.1 mm, the stress
concentration factor jumps to 3.52

- Frank Krygowski
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  #22  
Old September 30th 09, 05:19 AM posted to rec.bicycles.tech
jim beam[_5_]
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Posts: 941
Default Broken steel road bike fork

On 09/29/2009 09:04 PM, Frank Krygowski wrote:
On Sep 29, 10:40�pm, jim wrote:
On 09/29/2009 03:15 PM, Jobst Brandt wrote:

Jay Drew wrote:


What is feathering?


A fading out of one cross section into another where they meet:


�http://www.gtgtandems.com/tech/brazing.html


wrong. �feathering is simply filing the edges of the lug.



The sudden step in cross section causes a stress concentration that
leads to cracking.


which is why it's not a straight cross section, it's got curved features
that mitigate - effectively radius reduction, a standard fatigue
mitigation technique.


Not surprisingly, jim beam has things exactly backwards. When
blending a transition from a larger sized piece (like the lug) to a
smaller sized piece (like the fork blade) the idea is to increase, not
decrease the radius. The ultimate in radius _reduction_ would be a
sharp corner, which generates a high stress concentration. Large
radii are the opposite and generate lower stress concentrations.


you're a fraud and moral retard krygowski. you knew exactly what i
meant before you tried putting false words in my mouth - the concept of
decreasing radius sharpness is something so simple and obvious, even a
failed phd candidate like you should be able to grasp it.



You can play around with different radius values at
http://www.fatiguecalculator.com/cgi-bin/findkt.pl
to investigate this.

For example, for D=25 mm, d = 23 mm and r=1 mm, the stress
concentration factor is 1.84 - that is, peak stress in the area of the
shoulder will be 1.84 times the stress further away. If you reduce
the radius (as jim seems to recommend) to 0.1 mm, the stress
concentration factor jumps to 3.52


you're a fraud and moral retard krygowski. resign and stop ****ing up
people's careers.


  #23  
Old September 30th 09, 06:01 AM posted to rec.bicycles.tech
[email protected]
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Posts: 3,092
Default Broken steel road bike fork

On Sep 29, 6:52*am, raamman wrote:
On Sep 29, 12:37*am, wrote:

The shimmy was the wheel moving back and forth due to the fork
leg being fatigued. *There hasn't been any accident damage since
I put this fork on the bike. *The crack was almost all the way
through, starting from the BACK of the fork leg.


well, there you go, "steel is real" has been the mantra- but the
phrase " suseptable to rust" could be added. The fork looks very
clean, so I wonder how it happened to escape your notice earlier. I am
glad you escaped any mishap.


It doesn't have anything to do with rust or cleanliness.
There is a bit of surface oxidation inside the fork,
but it's totally superficial.

There is a stress concentration where the fork
crown meets the blade. The joint was possibly
overheated during manufacture leading to a
weakness at the braze. The crack initiated at
the rear of the fork, at the place where the crown
meets the blade, and propagated forward.

You can kind of see this from the pictures

http://www.bitrealm.com/misc/fork/p1000784.jpg

Along the broken surface, at the upper right it's
fairly dark, which indicates it's been cracked
for a while and exposed to atmosphere, dirt etc.
This is where the crack follows the line of the crown.
Along the front of the fork blade, the line of breakage
is more smooth than scalloped, as if it were just
torn away, and the surface looks cleaner. This is
what was still attached until the final break
happened.

If the OP could take an in-focus close up picture
of the fracture surfaces (ideally in side lighting,
with a tripod) it would probably show the differences
in texture of the initial and final fractured surfaces
clearly.

It looks like the fork was cracked almost halfway
around for a while before the final rupture.

Any material can fail if there is a flaw in the design
or manufacture. Steel is no panacea, but neither
is anything else. iIt's probably because it was steel
that the fork was "rideable" for a while even though
cracked, but whether that is good or bad depends
on your ideas about fault detection. (That is,
a piece that cracks instantly is bad, but a piece that
lulls you into still riding it around even though it's
about to finally give way and drop you is also bad.)

Ben
  #24  
Old September 30th 09, 01:55 PM posted to rec.bicycles.tech
jim beam[_5_]
external usenet poster
 
Posts: 941
Default Broken steel road bike fork

On 09/29/2009 10:01 PM, wrote:
On Sep 29, 6:52�am, wrote:
On Sep 29, 12:37�am, wrote:

The shimmy was the wheel moving back and forth due to the fork
leg being fatigued. �There hasn't been any accident damage since
I put this fork on the bike. �The crack was almost all the way
through, starting from the BACK of the fork leg.


well, there you go, "steel is real" has been the mantra- but the
phrase " suseptable to rust" could be added. The fork looks very
clean, so I wonder how it happened to escape your notice earlier. I am
glad you escaped any mishap.


It doesn't have anything to do with rust or cleanliness.
There is a bit of surface oxidation inside the fork,
but it's totally superficial.

There is a stress concentration where the fork
crown meets the blade. The joint was possibly
overheated during manufacture leading to a
weakness at the braze. The crack initiated at
the rear of the fork, at the place where the crown
meets the blade, and propagated forward.

You can kind of see this from the pictures

http://www.bitrealm.com/misc/fork/p1000784.jpg

Along the broken surface, at the upper right it's
fairly dark, which indicates it's been cracked
for a while and exposed to atmosphere, dirt etc.
This is where the crack follows the line of the crown.
Along the front of the fork blade, the line of breakage
is more smooth than scalloped, as if it were just
torn away, and the surface looks cleaner. This is
what was still attached until the final break
happened.


disagree - this is not a typical fatigue fracture. the fact that part
of the crown has come away with the blade is testament to that - highly
unusual.

more likely is a combination of poor brazing leaving a gap between the
crown and the blade. subsequent chroming would leave chromic acid
deposits [and others] which would partially erode both the blade and the
crown, and initiate fatigue. the rest is simply fatigue propagation.



If the OP could take an in-focus close up picture
of the fracture surfaces (ideally in side lighting,
with a tripod) it would probably show the differences
in texture of the initial and final fractured surfaces
clearly.


it's not just the fracture surfaces that is of interest, it's the
initiation point.



It looks like the fork was cracked almost halfway
around for a while before the final rupture.

Any material can fail if there is a flaw in the design
or manufacture. Steel is no panacea, but neither
is anything else.


cfrp is highly fatigue resistant - it would not fail in this manner.


iIt's probably because it was steel
that the fork was "rideable" for a while even though
cracked,


that ben, is utter garbage. any material undergoing fatigue is
"rideable" until it fails.


but whether that is good or bad depends
on your ideas about fault detection. (That is,
a piece that cracks instantly is bad,


that's not fatigue, that's simply fracture.


but a piece that
lulls you into still riding it around even though it's
about to finally give way and drop you is also bad.)


this is what is so dangerous about fatigue and why so many millions of
dollars are spent on detection every year - pre-failure symptoms are not
obvious and need skill, awareness and usually testing gear to detect.
undetected failure can be tragic. everyone should periodically inspect
their bike for this kind of stuff.


  #25  
Old September 30th 09, 04:12 PM posted to rec.bicycles.tech
Frank Krygowski[_2_]
external usenet poster
 
Posts: 7,511
Default Broken steel road bike fork

On Sep 30, 12:19*am, jim beam wrote:
On 09/29/2009 09:04 PM, Frank Krygowski wrote:

On Sep 29, 10:40 pm, jim *wrote:


The sudden step in cross section causes a stress concentration that
leads to cracking.


which is why it's not a straight cross section, it's got curved features
that mitigate - effectively radius reduction, a standard fatigue
mitigation technique.


Not surprisingly, jim beam has things exactly backwards. *When
blending a transition from a larger sized piece (like the lug) to a
smaller sized piece (like the fork blade) the idea is to increase, not
decrease the radius. *The ultimate in radius _reduction_ would be a
sharp corner, which generates a high stress concentration. *Large
radii are the opposite and generate lower stress concentrations.


you're a fraud and moral retard krygowski. *you knew exactly what i
meant before you tried putting false words in my mouth - the concept of
decreasing radius sharpness is something so simple and obvious...


"jim," I didn't put words in your mouth. I quoted exactly what you
said.

You had it exactly backwards. If you had any class at all, you'd have
simply said "Oops, my mistake. I meant to say..."

- Frank Krygowski
  #26  
Old September 30th 09, 08:12 PM posted to rec.bicycles.tech
someone
external usenet poster
 
Posts: 2,340
Default Broken steel road bike fork

On 30 Sep, 13:55, jim beam wrote:
On 09/29/2009 10:01 PM, wrote:



On Sep 29, 6:52 am, *wrote:
On Sep 29, 12:37 am, wrote:


The shimmy was the wheel moving back and forth due to the fork
leg being fatigued. There hasn't been any accident damage since
I put this fork on the bike. The crack was almost all the way
through, starting from the BACK of the fork leg.


well, there you go, "steel is real" has been the mantra- but the
phrase " suseptable to rust" could be added. The fork looks very
clean, so I wonder how it happened to escape your notice earlier. I am
glad you escaped any mishap.


It doesn't have anything to do with rust or cleanliness.
There is a bit of surface oxidation inside the fork,
but it's totally superficial.


There is a stress concentration where the fork
crown meets the blade. *The joint was possibly
overheated during manufacture leading to a
weakness at the braze. *The crack initiated at
the rear of the fork, at the place where the crown
meets the blade, and propagated forward.


You can kind of see this from the pictures


http://www.bitrealm.com/misc/fork/p1000784.jpg


Along the broken surface, at the upper right it's
fairly dark, which indicates it's been cracked
for a while and exposed to atmosphere, dirt etc.
This is where the crack follows the line of the crown.
Along the front of the fork blade, the line of breakage
is more smooth than scalloped, as if it were just
torn away, and the surface looks cleaner. *This is
what was still attached until the final break
happened.


disagree - this is not a typical fatigue fracture. *the fact that part
of the crown has come away with the blade is testament to that - highly
unusual.

more likely is a combination of poor brazing leaving a gap between the
crown and the blade. *subsequent chroming would leave chromic acid
deposits [and others] which would partially erode both the blade and the
crown, and initiate fatigue. *the rest is simply fatigue propagation.


I have made this kind of fracture at thre other end of the forks when
attempting to correct a build error. You know, cold set the forks.
New fork blades ,I was bending them but they wouldn't give, used
alittle more leverage than I expected and they snapped. Checked with
my trusted framebuilder, and he said "Nio, they shouldn't have done
that. You just cold set the forks after brazing." The coarse
granularity of what was steel was caused by an excess of spelter being
absorbed due to prolonged and/or excessive heat. There was no chrome,
only paint. The 'steel' failed in tension with two applications of
force. The point is the metal would not yield, it could only
fracture. The klutz who had 'brazed' the forks had turned the
advanced steel into a similar metal to cast iron. Only the colour of
fracture was different. This had the slightest tinge of yellow where
one would expect blue or neutral. Cast iron is grey with variations
of shade in the granulation.
  #27  
Old October 1st 09, 02:37 AM posted to rec.bicycles.tech
Michael Press
external usenet poster
 
Posts: 9,202
Default Broken steel road bike fork

In article
,
wrote:

So, I've been noticing that the front brakes of my road bike
were acting "grabby" in that the front would shimmy pretty
badly just as I'm coming to a stop. The rims felt a little
sticky, maybe some Gatorade got on them and it hasn't really
rained in a while. I cleaned the wheels last night and rode
into work today. It was still bad, but didn't seem as bad.
This has been going on for a week, I didn't think anything of
it. I decided to not ride after work and just come home, coming
down the driveway, the grabby-ness was really pronounced, so
I figure that the brake pads must be contaminated with something.
When I took the wheel off, my heart skipped a beat. Without much
effort, this is the result:

http://www.bitrealm.com/misc/fork/p1000783.jpg
http://www.bitrealm.com/misc/fork/p1000784.jpg
http://www.bitrealm.com/misc/fork/p1000785.jpg

It was hanging by no more than 2mm of steeel. I shudder when I
think of the roads I was about to go on, including one really
bad bump at the bottom. I don't think it would have held together
and when you lose the front like this, it's going to be bad.

The shimmy was the wheel moving back and forth due to the fork
leg being fatigued. There hasn't been any accident damage since
I put this fork on the bike. The crack was almost all the way
through, starting from the BACK of the fork leg.


I am happy for you. Now Mr. Mom. If I said it once, I
said it a dozen times, when your bike acts wonky,
dismount and determine the cause. Do not ride an
un-diagnosed wonky bike.

--
Michael Press
  #28  
Old October 1st 09, 09:26 AM posted to rec.bicycles.tech
[email protected]
external usenet poster
 
Posts: 3,092
Default Broken steel road bike fork

On Sep 30, 5:55*am, jim beam wrote:
On 09/29/2009 10:01 PM, wrote:

Any material can fail if there is a flaw in the design
or manufacture. *Steel is no panacea, but neither
is anything else.


cfrp is highly fatigue resistant - it would not fail in this manner.

iIt's probably because it was steel
that the fork was "rideable" for a while even though
cracked,


that ben, is utter garbage. *any material undergoing fatigue is
"rideable" until it fails.


Ah, Beamer. It's always good to have you disagree
with me. When you follow up a post of mine with a
message of agreement, I feel the need to recheck
my calculations.

CFRP is highly fatigue resistant - if it's strong enough
and doesn't have any construction flaws. On the other
hand, so is steel. Saying a CFRP fork wouldn't fail in
this manner is a bit meaningless because the fork
failed from fatigue initiated by a construction error and
worsened by a stress riser
(a brazing problem most likely, possibly worsened by
chroming as you argue, but most chromed forks
don't fail, so that brings us back to an initial assembly
error).

CFRP parts obviously don't fail from brazing
errors, but they certainly can fail from construction
errors and stress risers. My impression is that CF
bike parts tend to have a fairly large reserve strength
because they'll break quickly once they start breaking,
so manufacturers want to avoid the start of breakage
at all. (The exception might be superlight wheels, but
those tend to get banged up in use, and I'm excluding
crash damage from this discussion.)

Any part with a fatigue crack or flaw is rideable until it fails,
the question is how long it takes from the crack becoming
significant until full failure. For example, this fork was
cracked halfway around and still rideable (sort of) and
it sounds like that was going on for several days. An
aluminum part that was cracked halfway around probably
wouldn't last that long before failing completely. This
is conjecture on my part since I have never cracked
an aluminum fork like that, but it seems to me that
with aluminum frames and parts, they make the transition
from cracked to doomed very quickly. This is by no
means an argument against aluminum parts - you won't
catch me riding with steel cranks.


but whether that is good or bad depends
on your ideas about fault detection. *(That is,
a piece that cracks instantly is bad,


that's not fatigue, that's simply fracture.


I meant a piece that transitions from fatigue crack
to total failure quickly. You don't want that because
by the time you figure out something feels wrong,
you're already on the deck. Bad old titanium BB
spindles were probably in this category. Not
necessarily because titanium is evil, it's just not
good in that application.


but a piece that
lulls you into still riding it around even though it's
about to finally give way and drop you is also bad.)


this is what is so dangerous about fatigue and why so many millions of
dollars are spent on detection every year - pre-failure symptoms are not
obvious and need skill, awareness and usually testing gear to detect.
undetected failure can be tragic. *everyone should periodically inspect
their bike for this kind of stuff.


Well, yes, though in this case the rider did notice something
funny beforehand. It's always hard to know what's just
voodoo and what's a real problem, but when the bike starts
acting odd it's time to start taking pieces off and wiggling
things. If nothing falls apart in your hands, you can always
put it back together.

Ben
  #29  
Old October 1st 09, 10:54 AM posted to rec.bicycles.tech
someone
external usenet poster
 
Posts: 2,340
Default Broken steel road bike fork

On 1 Oct, 09:26, " wrote:
On Sep 30, 5:55*am, jim beam wrote:

On 09/29/2009 10:01 PM, wrote:


Any material can fail if there is a flaw in the design
or manufacture. *Steel is no panacea, but neither
is anything else.


cfrp is highly fatigue resistant - it would not fail in this manner.


iIt's probably because it was steel
that the fork was "rideable" for a while even though
cracked,


that ben, is utter garbage. *any material undergoing fatigue is
"rideable" until it fails.


Ah, Beamer. *It's always good to have you disagree
with me. *When you follow up a post of mine with a
message of agreement, I feel the need to recheck
my calculations.

CFRP is highly fatigue resistant - if it's strong enough
and doesn't have any construction flaws. *On the other
hand, so is steel. *Saying a CFRP fork wouldn't fail in
this manner is a bit meaningless because the fork
failed from fatigue initiated by a construction error and
worsened by a stress riser
(a brazing problem most likely, possibly worsened by
chroming as you argue, but most chromed forks
don't fail, so that brings us back to an initial assembly
error).

CFRP parts obviously don't fail from brazing
errors, but they certainly can fail from construction
errors and stress risers. *My impression is that CF
bike parts tend to have a fairly large reserve strength
because they'll break quickly once they start breaking,
so manufacturers want to avoid the start of breakage
at all. *(The exception might be superlight wheels, but
those tend to get banged up in use, and I'm excluding
crash damage from this discussion.)

Any part with a fatigue crack or flaw is rideable until it fails,
the question is how long it takes from the crack becoming
significant until full failure. *For example, this fork was
cracked halfway around and still rideable (sort of) and
it sounds like that was going on for several days. *An
aluminum part that was cracked halfway around probably
wouldn't last that long before failing completely. *This
is conjecture on my part since I have never cracked
an aluminum fork like that, but it seems to me that
with aluminum frames and parts, they make the transition
from cracked to doomed very quickly. *This is by no
means an argument against aluminum parts - you won't
catch me riding with steel cranks.

but whether that is good or bad depends
on your ideas about fault detection. *(That is,
a piece that cracks instantly is bad,


that's not fatigue, that's simply fracture.


I meant a piece that transitions from fatigue crack
to total failure quickly. *You don't want that because
by the time you figure out something feels wrong,
you're already on the deck. *Bad old titanium BB
spindles were probably in this category. *Not
necessarily because titanium is evil, it's just not
good in that application.



but a piece that
lulls you into still riding it around even though it's
about to finally give way and drop you is also bad.)


this is what is so dangerous about fatigue and why so many millions of
dollars are spent on detection every year - pre-failure symptoms are not
obvious and need skill, awareness and usually testing gear to detect.
undetected failure can be tragic. *everyone should periodically inspect
their bike for this kind of stuff.


Well, yes, though in this case the rider did notice something
funny beforehand. *It's always hard to know what's just
voodoo and what's a real problem, but when the bike starts
acting odd it's time to start taking pieces off and wiggling
things. *If nothing falls apart in your hands, you can always
put it back together.

Ben


Aluminium, its alloys and steel all have the advantage that they will
yield before fracture. Faulty brazing may remove this property. I
have experienced this period of yield with aluminium handlebars which
were drooping from the stem, one side tore on the top and the bar
dropped quite some way before I had transferred sufficient weight to
stop the discection. The bars had obviously fatigued but they had
also yielded at the same time and during the final give. There is no
time to take action when a CFRP part is in its final stage of failure,
modification with cellulose fibre such as flax may help to mitigate
the speed of failure.
  #30  
Old October 1st 09, 05:06 PM posted to rec.bicycles.tech
Frank Krygowski[_2_]
external usenet poster
 
Posts: 7,511
Default Broken steel road bike fork

On Oct 1, 5:54*am, someone wrote:
... *There is no
time to take action when a CFRP part is in its final stage of failure,
modification with cellulose fibre such as flax may help to mitigate
the speed of failure.


Hey, I see a long term strategy for Rivendell! Instead of carbon
fiber frames, Grant can build frames of flax fiber!

Wool, beeswax, canvas and flax. It fits!

- Frank Krygowski

 




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