"CF Bike Shatters" - continued

jim beam wrote:

Chalo wrote:

But since the mechanisms of failure are so different, I think
it's fair to compare the amount of work required to reach brittle
failure for advanced composites and structural metals.

disagree - because the mechanisms are different, we /cannot/ compare them.

I think we can, because catastrophic failure is what we're most
concerned about. The sort of "failure" that lets you avoid a crash,
ride home, or even not know that your frame was damaged until your
mechanic tells you at tune-up time, is a different and preferable
category of failure than what composites display.

You'd have to
seek out an a terribly temperamental metal to find one that is even in
the same ballpark in terms of the small amount of work required to
fracture it. And that's really the practical measure of toughness,
isn't it?

well, composites do have a degree of toughness - because they're
composites, but bike frames are not made to be sustain damage - as the
definition of toughness means.

I have seen many a peened, dented, and scratched top tube on commuter,
messenger, and city bikes demonstrating the principle that some bikes
_do_ need to sustain some damage and keep working.

frames need to /resist/ damage - and for
that, composites that can have much higher strength and much better
fatigue can be a huge benefit.

A steel bike that must be rugged and tolerate impacts and abrasion on
bike racks etc. can simply be made a bit heavier than strictly
necessary for the required strength. If you do this with a CFRP
frame, you wind up with a frame that's stronger and stiffer, but no
more tolerant of dings and abrasions, than a lightweight CFRP frame.
That makes carbon-epoxy acceptable for a vanity bike or a racing bike,
but not so much for a transportational or working bike. And even a
racer or a weekend warrior who would like their bike to be able to
take a blow and keep rolling might want to use a more damage-tolerant
material.

Compare carbon-carbon brake rotors on race cars. They clearly
outperform cast iron rotors, and it would still be a bad idea to stick
them on everybody's Camry.

is wood brittle? [composites are modeled on wood.] it doesn't absorb
energy like a ductile metal does. true, energy absorption may be low,
but they still absorb work during failure so they don't just shatter
like glass. and many high strength metals aren't exactly tough either.
not in the habit of dropping cobalt drill bits are you?

Wood is a pretty seriously compromised material for making bike
frames. So is M42 cobalt high-speed steel.

and even if we /are/ talking
failure mode, we need to compare like with like - saying that 6061
elongates 26% and carbon only 1.5% completely misses the fundamental
point that 24.5% of the aluminum's deformation is plastic, not elastic!
and anything post-elastic is failure in these kinds of applications.

I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.

Chalo

In article .com,
Chalo wrote:


I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.

If the seatpost had been made of CFRP (or if you're conservative, a CFRP
wrap over metal) would it have failed at all?

--
Ryan Cousineau http://www.wiredcola.com/
"I don't want kids who are thinking about going into mathematics
to think that they have to take drugs to succeed." -Paul Erdos

Ryan Cousineau wrote:

wrote:

I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.

If the seatpost had been made of CFRP (or if you're conservative, a CFRP
wrap over metal) would it have failed at all?

That's a worthy question. The fact that this newly bent one is made
of heat-treated 4130 chromoly and weighs about twice what a weight-
weenie type seatpost weighs makes me think it might not be a good idea
to experiment.

The first post that bent was a welded, non-heat-treated chromoly
post. It didn't last long, and it bent very noticeably. The one I
just found to be bent was close enough to straight to make me wonder
if my eyes were fooling me. I used the straight edge of a machinist's
caliper as a reference to determine that the front edge was slightly
bowed and the rear edge slightly rippled.

I've ordered up another of the same kind of seatpost. If it bends
again, then I'll try a Thomson Elite post, the only commercial
seatpost I have used that I've never managed to bend. My hesitance to
use a Thomson is related to my refusal to use a CFRP post-- it's made
of a very high yield strength alloy that doesn't offer much margin
between bending and snapping off. If it doesn't bend, no problem. If
it does, well, that could become a problem.

Chalo

Chalo wrote:
jim beam wrote:
Chalo wrote:
But since the mechanisms of failure are so different, I think
it's fair to compare the amount of work required to reach brittle
failure for advanced composites and structural metals.
disagree - because the mechanisms are different, we /cannot/ compare them.

I think we can, because catastrophic failure is what we're most
concerned about.

but i think that's misplaced. the "headroom" on quality carbon is
substantial compared to traditional bike materials. there was a graph
on the reynolds for website comparing their fork strength to steel - it
was 3 times stronger for about half the weight iirc.


The sort of "failure" that lets you avoid a crash,
ride home, or even not know that your frame was damaged until your
mechanic tells you at tune-up time, is a different and preferable
category of failure than what composites display.

but a yielding metal tube can cause a crash just like a failing carbon
tube. a fatiguing metal tube can fail at stress much below normal
service. carbon fails much /above/ normal service.


You'd have to
seek out an a terribly temperamental metal to find one that is even in
the same ballpark in terms of the small amount of work required to
fracture it. And that's really the practical measure of toughness,
isn't it?
well, composites do have a degree of toughness - because they're
composites, but bike frames are not made to be sustain damage - as the
definition of toughness means.

I have seen many a peened, dented, and scratched top tube on commuter,
messenger, and city bikes demonstrating the principle that some bikes
_do_ need to sustain some damage and keep working.

my carbon commuter fork is not exactly pristine either. and peened,
dented scratched metal tubes can fatigue.



frames need to /resist/ damage - and for
that, composites that can have much higher strength and much better
fatigue can be a huge benefit.

A steel bike that must be rugged and tolerate impacts and abrasion on
bike racks etc. can simply be made a bit heavier than strictly
necessary for the required strength. If you do this with a CFRP
frame, you wind up with a frame that's stronger and stiffer, but no
more tolerant of dings and abrasions, than a lightweight CFRP frame.

that's not true. can a tick piece of wood take more abuse than a thing
piece? the answer is "of course".


That makes carbon-epoxy acceptable for a vanity bike or a racing bike,
but not so much for a transportational or working bike. And even a
racer or a weekend warrior who would like their bike to be able to
take a blow and keep rolling might want to use a more damage-tolerant
material.

/cost/ makes it inappropriate, not the material's properties.



Compare carbon-carbon brake rotors on race cars. They clearly
outperform cast iron rotors, and it would still be a bad idea to stick
them on everybody's Camry.

no, they don't outperform for normal road cars. carbon brake disks only
really work well above a temperature that normal road cars only seldom
reach.



is wood brittle? [composites are modeled on wood.] it doesn't absorb
energy like a ductile metal does. true, energy absorption may be low,
but they still absorb work during failure so they don't just shatter
like glass. and many high strength metals aren't exactly tough either.
not in the habit of dropping cobalt drill bits are you?

Wood is a pretty seriously compromised material for making bike
frames. So is M42 cobalt high-speed steel.

but the point is, wood's not brittle. and metal can be. an
over-simplification like "metal good, composite bad" shows the speaker
to be completely under-informed.



and even if we /are/ talking
failure mode, we need to compare like with like - saying that 6061
elongates 26% and carbon only 1.5% completely misses the fundamental
point that 24.5% of the aluminum's deformation is plastic, not elastic!
and anything post-elastic is failure in these kinds of applications.

I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.


or not - it could be substantially stronger. unless there are numbers
and testing, failure of one material simply cannot imply failure of another!

Chalo wrote:
Ryan Cousineau wrote:
wrote:
I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.
If the seatpost had been made of CFRP (or if you're conservative, a CFRP
wrap over metal) would it have failed at all?

That's a worthy question. The fact that this newly bent one is made
of heat-treated 4130 chromoly and weighs about twice what a weight-
weenie type seatpost weighs makes me think it might not be a good idea
to experiment.

The first post that bent was a welded, non-heat-treated chromoly
post. It didn't last long, and it bent very noticeably. The one I
just found to be bent was close enough to straight to make me wonder
if my eyes were fooling me. I used the straight edge of a machinist's
caliper as a reference to determine that the front edge was slightly
bowed and the rear edge slightly rippled.

I've ordered up another of the same kind of seatpost. If it bends
again, then I'll try a Thomson Elite post, the only commercial
seatpost I have used that I've never managed to bend. My hesitance to
use a Thomson is related to my refusal to use a CFRP post-- it's made
of a very high yield strength alloy that doesn't offer much margin
between bending and snapping off. If it doesn't bend, no problem. If
it does, well, that could become a problem.

Chalo


what is the extension? did you calculate the strength of the post vs.
the leverage the extension exerts? have you compared the strength of
say an easton carbon post and your steel post?

"jim beam" wrote:
...
can a tick piece of wood take more abuse than a thing piece?...

Huh? Who knows?

--
Tom Sherman - Holstein-Friesland Bovinia

--
Posted via a free Usenet account from http://www.teranews.com

Tom "Johnny Sunset" Sherman wrote:
"jim beam" wrote:
...
can a tick piece of wood take more abuse than a thing piece?...

Huh? Who knows?


yeah, ok. make that:
"can a tHick piece of wood take more abuse than a thin_ piece?"

thanks.

In article om,
Chalo wrote:

Ryan Cousineau wrote:

wrote:

I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.

If the seatpost had been made of CFRP (or if you're conservative, a CFRP
wrap over metal) would it have failed at all?

That's a worthy question. The fact that this newly bent one is made
of heat-treated 4130 chromoly and weighs about twice what a weight-
weenie type seatpost weighs makes me think it might not be a good idea
to experiment.

The first post that bent was a welded, non-heat-treated chromoly
post. It didn't last long, and it bent very noticeably. The one I
just found to be bent was close enough to straight to make me wonder
if my eyes were fooling me. I used the straight edge of a machinist's
caliper as a reference to determine that the front edge was slightly
bowed and the rear edge slightly rippled.

I'm not telling you anything new, but this sounds like you're chasing a
design issue with materials, inasmuch as you really want a shorter
seatpost extension or a larger-diameter seatpost. But I'm assuming that
would mean abandoning an expensive-to-replace frame.

I've ordered up another of the same kind of seatpost. If it bends
again, then I'll try a Thomson Elite post, the only commercial
seatpost I have used that I've never managed to bend. My hesitance to
use a Thomson is related to my refusal to use a CFRP post-- it's made
of a very high yield strength alloy that doesn't offer much margin
between bending and snapping off. If it doesn't bend, no problem. If
it does, well, that could become a problem.

--
Ryan Cousineau http://www.wiredcola.com/
"I don't want kids who are thinking about going into mathematics
to think that they have to take drugs to succeed." -Paul Erdos

Ryan Cousineau wrote:
In article om,
Chalo wrote:

Ryan Cousineau wrote:
wrote:
I just noticed that I had a post-elastic failure in my 29er's steel
seatpost-- the second such event in its short career. The
implication? I have to remember to order up another one sometime. In
the meantime I can continue to use this one. If my seatposts had been
made of CFRP, the situation might have been a bit more problematic.
If the seatpost had been made of CFRP (or if you're conservative, a CFRP
wrap over metal) would it have failed at all?
That's a worthy question. The fact that this newly bent one is made
of heat-treated 4130 chromoly and weighs about twice what a weight-
weenie type seatpost weighs makes me think it might not be a good idea
to experiment.

The first post that bent was a welded, non-heat-treated chromoly
post. It didn't last long, and it bent very noticeably. The one I
just found to be bent was close enough to straight to make me wonder
if my eyes were fooling me. I used the straight edge of a machinist's
caliper as a reference to determine that the front edge was slightly
bowed and the rear edge slightly rippled.

I'm not telling you anything new, but this sounds like you're chasing a
design issue with materials,

well said. it's ridiculously common unfortunately - not unique to chalo
by any means.


inasmuch as you really want a shorter
seatpost extension or a larger-diameter seatpost. But I'm assuming that
would mean abandoning an expensive-to-replace frame.

I've ordered up another of the same kind of seatpost. If it bends
again, then I'll try a Thomson Elite post, the only commercial
seatpost I have used that I've never managed to bend. My hesitance to
use a Thomson is related to my refusal to use a CFRP post-- it's made
of a very high yield strength alloy that doesn't offer much margin
between bending and snapping off. If it doesn't bend, no problem. If
it does, well, that could become a problem.

jim beam wrote:

Tom "Johnny Sunset" Sherman wrote:

"jim beam" wrote:
...
can a tick piece of wood take more abuse than a thing piece?...

Huh? Who knows?

yeah, ok. make that:
"can a tHick piece of wood take more abuse than a thin_ piece?"

thanks.

LOL! I get it now!