Trust Carbon Fork After Wreck?

I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g
--

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g
--

You cannot and I recommend replacing all carbon that saw an impact.
Carbon fiber fails by breaking, not bending. We had a customer that
fell down, pretty undramatic but she killed he carbon fork, BUT it
wasn't obviously broken, just a front wheel that wasn't centered
anymore...

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly

collar probably over-tightened and cracked the tube.

(I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g

do the squeeze test. [google for details] if it passes, keep on riding
it. carbon is stronger than steel. scratches in the clearcoat mean
nothing. i have carbon forks that have been in impacts severe enough to
give haematomas in the palms on both hands and to smash rims - and both
forks are fine. i'm #205.

jim beam wrote:
bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly

collar probably over-tightened and cracked the tube.

(I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g

do the squeeze test. [google for details] if it passes, keep on riding
it. carbon is stronger than steel. scratches in the clearcoat mean
nothing. i have carbon forks that have been in impacts severe enough to
give haematomas in the palms on both hands and to smash rims - and both
forks are fine. i'm #205.

You gamble with this gents teeth pretty readily. Carbon is stronger
than steel for a given weight. Carbon use is lots lighter than steel,
so not necessariluy 'stronger'. Steel bends, carbon breaks.
We have had customers that have broken carbon handlebars, seatposts,
rims, frames and forks.Many JRA..so MY suggestion is you be careful.

Qui si parla Campagnolo wrote:
jim beam wrote:
bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly
collar probably over-tightened and cracked the tube.

(I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g
do the squeeze test. [google for details] if it passes, keep on riding
it. carbon is stronger than steel. scratches in the clearcoat mean
nothing. i have carbon forks that have been in impacts severe enough to
give haematomas in the palms on both hands and to smash rims - and both
forks are fine. i'm #205.

You gamble with this gents teeth pretty readily. Carbon is stronger
than steel for a given weight. Carbon use is lots lighter than steel,
so not necessariluy 'stronger'. Steel bends, carbon breaks.
We have had customers that have broken carbon handlebars, seatposts,
rims, frames and forks.Many JRA..so MY suggestion is you be careful.

we debated this a while back. care is indeed needed, and quality varies
substantially with manufacturer [VERY bad experience with chinese-made
kestrel forks]. but to remind you, this ratio is not just weight for
weight. reynolds forks had graphs showing their forks /three/ times
stronger than steel for less than half the weight. this is entirely in
accordance with my attempts at destruction. i have bent several steel
forks over the years and relatively low stresses. the two carbon forks
i've "tested" have been much stronger. and frankly, if i was on the
bike at impact levels sufficient to break them, i'd be in the e.r.
regardless.

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g
--

Your question is a good one. You are right to be wondering if there is
a different way to handle these things.

I like to put it this way:

Bicycle frame steel (and other ductile metals such as aluminum but to a
lesser extent) gain their toughness through plastic deformation. You
can overload a part, plastically deform it, but lose no strength
(actually increased the strength). You retain most of the toughness if
the plastic deformation was minor. You lose the toughness that was used
up in the "strengthening" that went on with the overload. Perhaps most
significantly, the rigidity of the part is unaffected. It will have the
same resistance to bending (in the elastic range) as before.

Fibrous composites have what I call "one time toughness." If you have
an overload situation, the end result is a loss of stiffness, and a
loss of most of the toughness, even for a relatively low overload.
What happens is that you either microckrack the resin matrix, or or
crack some fibers, or both. In an extreme overload, you pull fibers out
of the matrix, but then you have failure rather than slight overload.

A fibrous composite part that has been overloaded in to the
pseudoplastic region of stress versus strain will sustain a permanent
loss of bending stiffness, while retaining most of its tensile strength
and showing no permanent set. However that residual tensile strength
comes with very little toughness.

What is thoughneess? It is quite simply the amount of energy that a
structure can absorb before failure. A hi-modulus part with low plastic
deformation to failure will have far less strain engergy absorbed
before failure than a high modulus part that experiences significant
plastic deformation before failure. Composites show very little plastic
deformation to failure--especially carbon fiber composites. They have
less toughness pound for pounbd than steel. Typcially the ratio of
toughness is on the order of 10:1

This loss of stiffness after damage is a really interesting aspect of
composite structures, but one which the LBS is unable to use to
advantage in checking a part for damage. It is much easier to check a
metal part for plastic deformation. You can measure it. In the
composite part, you don't have a permanent set. Rather, you have
internal structural changes that would be visible with ultrasonic
detection (though not an easy task!), but not neccessarily the naked
eye.

So, back to your situation. The prudent thing is to discard the parts.
I don't mean give away. I mean destroy and replace. Nobody should
reinstall them.

Jim Beam mentions the "strength" of his carbon forks. What he doesn't
mention is that he has no way to actually know how much of that
strength is used up in a crash. The hospital comment is dangerously
misleading. You can put tremendous overloads to the bike parts while
doing no damage to your body--and vice versa! All depends on the crash
dynamics.

One more thing worth mentioning, and that is fatigue. Metals and
Composites fatigue completely differently. Interestingly a composite
part is easier to check for fatige, provided that you have the
engineering baseline data (which for bikes you do not). Here's how it
works:

In metals, fatigue is an insidious problem of crack propagation through
an otherwise ductile metal, during load cycling which is entirely
non-plastic (non-overload). In heavily loaded parts (yet still loaded
in the elastic region), no noticable changes in the parts stiffness or
deflection occurr until the crack leads to complete failure, or is
about to fail completely. (I have personally experienced this on a
Raleigh Professional reynolds 531, an Eddie Merckx reynolds 753, an
Easton 7005, a Gios deddacai, and countless campagnolo axles).

In composites, fatigue will gradually reduce the bending stiffness of
the composite part. The amount of fatigue life left can be gaged by the
residual bending stiffness. Unfortunately bicycle components do not
come with a fatigue life guide and so the nature of composite fatigue
cannot be used to advantage. Yet this property is used to advantage for
assessing life on helicopter rotor blades and other parts.

Finally, you have the nexus of overload and fatigue. Again, the metals
are totally different from the composites in this regard. Whereas a
ductile metal that is experiencing fatigue crack growth can actually
have its life *extended* by periodic overloads, a composite part will
have its fatigue life dramatically reduced. This is due to the
*mechanism* of fatigue in composites, namely cracking of the resin
matrix and/or cracking of fibers in the matrix (depends on the ratio of
stiffness of resin versus fiber, elongation to failure of each,
orientation and makeup of the fibrous portion, and the ratio of fibers
to resin, and the diameters of the fibers.) It turns out that the
overload pseudo-plasticity of a composite is the same mechanism as the
fatigue mechanism so that if you overload a part, you have literally
taken the life out of a part that is subjected to cyclic loading.

(In metals, the mechanism of fatigue is not the same as the mechanism
of plasticity. It is more complicated to describe but essentially what
happens is that imperfections in the metal cause cracks to form, and
then the tip of a crack causes the stress to exceed the cleavage
strength locally and so the crack grows on each cycle. The
thermodynamics of surface energy prevent runaway crack propagation
until the average stress passes a threshold).

So, even if your composite forks appear to be ok, they may in fact have
lost most of their fatigue life due to a crash. This is totally
different than in metal forks!

"bill" wrote in message
oups.com...

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent
the
wheel. No obvious damage to the fork or carbon rear stay and I left
it
with the LBS to check out. But absent any visible wrinkles how much
can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of
trusting
the fork.
thanks,
bill g
--

Your question is a good one. You are right to be wondering if there is
a different way to handle these things.

I like to put it this way:

Bicycle frame steel (and other ductile metals such as aluminum but to
a
lesser extent) gain their toughness through plastic deformation. You
can overload a part, plastically deform it, but lose no strength
(actually increased the strength). You retain most of the toughness if
the plastic deformation was minor. You lose the toughness that was
used
up in the "strengthening" that went on with the overload. Perhaps most
significantly, the rigidity of the part is unaffected. It will have
the
same resistance to bending (in the elastic range) as before.

Fibrous composites have what I call "one time toughness." If you have
an overload situation, the end result is a loss of stiffness, and a
loss of most of the toughness, even for a relatively low overload.
What happens is that you either microckrack the resin matrix, or or
crack some fibers, or both. In an extreme overload, you pull fibers
out
of the matrix, but then you have failure rather than slight overload.

A fibrous composite part that has been overloaded in to the
pseudoplastic region of stress versus strain will sustain a permanent
loss of bending stiffness, while retaining most of its tensile
strength
and showing no permanent set. However that residual tensile strength
comes with very little toughness.

What is thoughneess? It is quite simply the amount of energy that a
structure can absorb before failure. A hi-modulus part with low
plastic
deformation to failure will have far less strain engergy absorbed
before failure than a high modulus part that experiences significant
plastic deformation before failure. Composites show very little
plastic
deformation to failure--especially carbon fiber composites. They have
less toughness pound for pounbd than steel. Typcially the ratio of
toughness is on the order of 10:1

This loss of stiffness after damage is a really interesting aspect of
composite structures, but one which the LBS is unable to use to
advantage in checking a part for damage. It is much easier to check a
metal part for plastic deformation. You can measure it. In the
composite part, you don't have a permanent set. Rather, you have
internal structural changes that would be visible with ultrasonic
detection (though not an easy task!), but not neccessarily the naked
eye.

So, back to your situation. The prudent thing is to discard the parts.
I don't mean give away. I mean destroy and replace. Nobody should
reinstall them.

Jim Beam mentions the "strength" of his carbon forks. What he doesn't
mention is that he has no way to actually know how much of that
strength is used up in a crash. The hospital comment is dangerously
misleading. You can put tremendous overloads to the bike parts while
doing no damage to your body--and vice versa! All depends on the crash
dynamics.

One more thing worth mentioning, and that is fatigue. Metals and
Composites fatigue completely differently. Interestingly a composite
part is easier to check for fatige, provided that you have the
engineering baseline data (which for bikes you do not). Here's how it
works:

In metals, fatigue is an insidious problem of crack propagation
through
an otherwise ductile metal, during load cycling which is entirely
non-plastic (non-overload). In heavily loaded parts (yet still loaded
in the elastic region), no noticable changes in the parts stiffness or
deflection occurr until the crack leads to complete failure, or is
about to fail completely. (I have personally experienced this on a
Raleigh Professional reynolds 531, an Eddie Merckx reynolds 753, an
Easton 7005, a Gios deddacai, and countless campagnolo axles).

In composites, fatigue will gradually reduce the bending stiffness of
the composite part. The amount of fatigue life left can be gaged by
the
residual bending stiffness. Unfortunately bicycle components do not
come with a fatigue life guide and so the nature of composite fatigue
cannot be used to advantage. Yet this property is used to advantage
for
assessing life on helicopter rotor blades and other parts.

Finally, you have the nexus of overload and fatigue. Again, the metals
are totally different from the composites in this regard. Whereas a
ductile metal that is experiencing fatigue crack growth can actually
have its life *extended* by periodic overloads, a composite part will
have its fatigue life dramatically reduced. This is due to the
*mechanism* of fatigue in composites, namely cracking of the resin
matrix and/or cracking of fibers in the matrix (depends on the ratio
of
stiffness of resin versus fiber, elongation to failure of each,
orientation and makeup of the fibrous portion, and the ratio of fibers
to resin, and the diameters of the fibers.) It turns out that the
overload pseudo-plasticity of a composite is the same mechanism as the
fatigue mechanism so that if you overload a part, you have literally
taken the life out of a part that is subjected to cyclic loading.

(In metals, the mechanism of fatigue is not the same as the mechanism
of plasticity. It is more complicated to describe but essentially what
happens is that imperfections in the metal cause cracks to form, and
then the tip of a crack causes the stress to exceed the cleavage
strength locally and so the crack grows on each cycle. The
thermodynamics of surface energy prevent runaway crack propagation
until the average stress passes a threshold).

So, even if your composite forks appear to be ok, they may in fact
have
lost most of their fatigue life due to a crash. This is totally
different than in metal forks!


VERY well put but expect a barrage of retorts from the anecdotal
resident experts. As a manufacturing engineer/consultant with a
background in metallurgy and material science, I've been involved with
research and manufacturing of products made of reinforced carbon fiber
and other types of composites for over 25 years; everything from rocket
engine nozzles to golf club shafts to airliner floorboards.

Products made of carbon fiber composites are very prone to failure from
nicks and scratches. You've done a very good job of pointing out
internal failure modes that are not visible from the outside. Invisible
internal delamination due to overstressing is a common cause of failure
in carbon fiber composite components.

These composite materials should be looked at as fiber reinforced
plastic resins (as in fiberglass reinforced epoxy which was the original
term for fiberglass). The resin encapsulates the fibers and holds them
into a solid form. The bond between the resin and the fibers is
generally not very strong and it's the encapsulating that gives the
product it's strength. On complex forms this requires a lot of
engineering expertise to take advantage of the material's strengths.

Many cyclists seem to have the opinion that any component made of
"carbon" (reinforced carbon fiber composite) is going to be almost fail
proof: "It's lighter and stronger than steel" (and it's the latest and
greatest).

I've been trying to find a web site that I visited a few weeks back that
showed comparisons of quality versus poorly constructed carbon fiber
composite frames and forks. I think that it may have been on Colnogo's
site but they've redesigned it so a lot of information isn't there
anymore.

Poorly designed carbon fiber composite bicycle components can look
stylish but be prone to catastrophic failure at time.

Chas.

..

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent the
wheel. No obvious damage to the fork or carbon rear stay and I left it
with the LBS to check out. But absent any visible wrinkles how much can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of trusting
the fork.
thanks,
bill g
--

It depends on how much excitement do you want in your life. If you want
certainty and safety, replace it. However, if you want to live your
life on the edge, then don't. Do you enjoy the adrenaline rush? Does it
turn you on to think that your fork may give up halfway through the
ride? will it make you go faster just to finish the ride sooner?
This are important questions that you need to ask yourself.
As you can see, there is a split between Peter ana Jim. Maybe, you are
completely safe, but not knowing will make you feel alive. At least
until you get the bill from the orthodontist.

Andres

Peter,
Thanks for reply.
But even if I do replace all the front stuff what about the carbon
stay? It was pretty much getting thunked by the bumper about the same
time
my knee was.
My wife says this is one of those blink moments. Get a new bike. Is it
worth the risk even absent any obvious trauma.
bill g
--

* * Chas wrote:
"bill" wrote in message
oups.com...

bg wrote:
I got hit from the side the other day. Car was going maybe 15mph.
Bumper hit my knee and front wheel. Knocked me off the bike. Bent
the
wheel. No obvious damage to the fork or carbon rear stay and I left
it
with the LBS to check out. But absent any visible wrinkles how much
can
one trust carbon stuff esp the fork bars and stem after a shot like
that? It's got a carbon steerer. I had a StupidLight seat post break
last year unexpectedly (I weigh 140lbs) so I'm kinda leery of
trusting
the fork.
thanks,
bill g
--

Your question is a good one. You are right to be wondering if there is
a different way to handle these things.

I like to put it this way:

Bicycle frame steel (and other ductile metals such as aluminum but to
a
lesser extent) gain their toughness through plastic deformation. You
can overload a part, plastically deform it, but lose no strength
(actually increased the strength). You retain most of the toughness if
the plastic deformation was minor. You lose the toughness that was
used
up in the "strengthening" that went on with the overload. Perhaps most
significantly, the rigidity of the part is unaffected. It will have
the
same resistance to bending (in the elastic range) as before.

Fibrous composites have what I call "one time toughness." If you have
an overload situation, the end result is a loss of stiffness, and a
loss of most of the toughness, even for a relatively low overload.
What happens is that you either microckrack the resin matrix, or or
crack some fibers, or both. In an extreme overload, you pull fibers
out
of the matrix, but then you have failure rather than slight overload.

A fibrous composite part that has been overloaded in to the
pseudoplastic region of stress versus strain will sustain a permanent
loss of bending stiffness, while retaining most of its tensile
strength
and showing no permanent set. However that residual tensile strength
comes with very little toughness.

What is thoughneess? It is quite simply the amount of energy that a
structure can absorb before failure. A hi-modulus part with low
plastic
deformation to failure will have far less strain engergy absorbed
before failure than a high modulus part that experiences significant
plastic deformation before failure. Composites show very little
plastic
deformation to failure--especially carbon fiber composites. They have
less toughness pound for pounbd than steel. Typcially the ratio of
toughness is on the order of 10:1

This loss of stiffness after damage is a really interesting aspect of
composite structures, but one which the LBS is unable to use to
advantage in checking a part for damage. It is much easier to check a
metal part for plastic deformation. You can measure it. In the
composite part, you don't have a permanent set. Rather, you have
internal structural changes that would be visible with ultrasonic
detection (though not an easy task!), but not neccessarily the naked
eye.

So, back to your situation. The prudent thing is to discard the parts.
I don't mean give away. I mean destroy and replace. Nobody should
reinstall them.

Jim Beam mentions the "strength" of his carbon forks. What he doesn't
mention is that he has no way to actually know how much of that
strength is used up in a crash. The hospital comment is dangerously
misleading. You can put tremendous overloads to the bike parts while
doing no damage to your body--and vice versa! All depends on the crash
dynamics.

One more thing worth mentioning, and that is fatigue. Metals and
Composites fatigue completely differently. Interestingly a composite
part is easier to check for fatige, provided that you have the
engineering baseline data (which for bikes you do not). Here's how it
works:

In metals, fatigue is an insidious problem of crack propagation
through
an otherwise ductile metal, during load cycling which is entirely
non-plastic (non-overload). In heavily loaded parts (yet still loaded
in the elastic region), no noticable changes in the parts stiffness or
deflection occurr until the crack leads to complete failure, or is
about to fail completely. (I have personally experienced this on a
Raleigh Professional reynolds 531, an Eddie Merckx reynolds 753, an
Easton 7005, a Gios deddacai, and countless campagnolo axles).

In composites, fatigue will gradually reduce the bending stiffness of
the composite part. The amount of fatigue life left can be gaged by
the
residual bending stiffness. Unfortunately bicycle components do not
come with a fatigue life guide and so the nature of composite fatigue
cannot be used to advantage. Yet this property is used to advantage
for
assessing life on helicopter rotor blades and other parts.

Finally, you have the nexus of overload and fatigue. Again, the metals
are totally different from the composites in this regard. Whereas a
ductile metal that is experiencing fatigue crack growth can actually
have its life *extended* by periodic overloads, a composite part will
have its fatigue life dramatically reduced. This is due to the
*mechanism* of fatigue in composites, namely cracking of the resin
matrix and/or cracking of fibers in the matrix (depends on the ratio
of
stiffness of resin versus fiber, elongation to failure of each,
orientation and makeup of the fibrous portion, and the ratio of fibers
to resin, and the diameters of the fibers.) It turns out that the
overload pseudo-plasticity of a composite is the same mechanism as the
fatigue mechanism so that if you overload a part, you have literally
taken the life out of a part that is subjected to cyclic loading.

(In metals, the mechanism of fatigue is not the same as the mechanism
of plasticity. It is more complicated to describe but essentially what
happens is that imperfections in the metal cause cracks to form, and
then the tip of a crack causes the stress to exceed the cleavage
strength locally and so the crack grows on each cycle. The
thermodynamics of surface energy prevent runaway crack propagation
until the average stress passes a threshold).

So, even if your composite forks appear to be ok, they may in fact
have
lost most of their fatigue life due to a crash. This is totally
different than in metal forks!


VERY well put but expect a barrage of retorts from the anecdotal
resident experts. As a manufacturing engineer/consultant with a
background in metallurgy and material science, I've been involved with
research and manufacturing of products made of reinforced carbon fiber
and other types of composites for over 25 years; everything from rocket
engine nozzles to golf club shafts to airliner floorboards.

Products made of carbon fiber composites are very prone to failure from
nicks and scratches.

Which really makes me wonder about the wisdom of CFRP crankarms, one of
the most frequently nicked and scratched parts on a bicycle. As in some
other things, this is not an issue to a high level, sponsored pro
looking for every advantage. Nicked? Scratched? Here's a new one,
Francois. But for a recreational rider?

You've done a very good job of pointing out
internal failure modes that are not visible from the outside. Invisible
internal delamination due to overstressing is a common cause of failure
in carbon fiber composite components.

These composite materials should be looked at as fiber reinforced
plastic resins (as in fiberglass reinforced epoxy which was the original
term for fiberglass). The resin encapsulates the fibers and holds them
into a solid form. The bond between the resin and the fibers is
generally not very strong and it's the encapsulating that gives the
product it's strength. On complex forms this requires a lot of
engineering expertise to take advantage of the material's strengths.

Many cyclists seem to have the opinion that any component made of
"carbon" (reinforced carbon fiber composite) is going to be almost fail
proof: "It's lighter and stronger than steel" (and it's the latest and
greatest).

I've been trying to find a web site that I visited a few weeks back that
showed comparisons of quality versus poorly constructed carbon fiber
composite frames and forks. I think that it may have been on Colnogo's
site but they've redesigned it so a lot of information isn't there
anymore.

Poorly designed carbon fiber composite bicycle components can look
stylish but be prone to catastrophic failure at time.

Chas.

.