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PostPosted: Wed Nov 23, 2016 9:43 pm 
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http://www.sheldonbrown.com/rinard/fram ... e_test.htm


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PostPosted: Wed Nov 23, 2016 9:48 pm 
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cyclespeed wrote:
I remember when I was into MTB a good 10+ years ago and many handlebars were alu, the 'unwritten rule' that did the rounds was to change the bar every 5 years (even for carbon too).

The handlebar is one of the most stressed components on the bike, especially in MTB and undergoes repeated flexing. No aluminium part can withstand that forever, although exactly what the lifespan is is very hard to tell - depends on many things like, how your ride, how thick is the alu, etc. etc.

Why would you change a carbon bar, carbon components essentially have an infinite fatigue life.


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Posted: Wed Nov 23, 2016 9:48 pm 


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PostPosted: Wed Nov 23, 2016 9:53 pm 
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Only if they have no inclusions, have never been overloaded even slightly, have 100% void free manufacture, have never been scratched or scraped and aren't running against any sort of potential stress risers. (even if they are torqued up to spec!)

But yes. They have an infinite fatigue life.


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PostPosted: Wed Nov 23, 2016 10:11 pm 
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mattr wrote:
http://www.sheldonbrown.com/rinard/frame_fatigue_test.htm

Very interesting. It's always made me laugh when people talk about titanium frames as a 'bike for life' :wink:


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PostPosted: Wed Nov 23, 2016 10:18 pm 
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Well. Titanium is a bit like carbon.
If it's perfect. It'll last pretty much forever (well, for ages anyway). As soon as you weld it or drill it or add a "feature", forever gets shortened massively.


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PostPosted: Wed Nov 23, 2016 11:35 pm 
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Posts: 250
Location: Boganville, Australia
How old is the Sheldon Brown article? How relevant is it now?

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PostPosted: Thu Nov 24, 2016 12:56 am 
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Location: Loveland, CO
mattr wrote:
pdlpsher1 wrote:
So which part on your bike do you plan to throw out due to metal fatigue?
Personally speaking i've thrown out an aluminium frame, a pair of cranks, a stem and 4 or 5 sets of bars. Everything else i've either sold or binned due to incompatibility issues before it failed. Or crashed it.

Even most steel "race weight" frames are so lightly built that they are over the materials endurance limit in normal use, and will (eventually) fail.


I'd gladly take your parts that you plan to trash :P


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PostPosted: Thu Nov 24, 2016 1:13 am 
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mattr wrote:
http://www.sheldonbrown.com/rinard/frame_fatigue_test.htm


Although the test does show which frame is the strongest, the test result itself cannot be translated to a service life. No one will ever spring all out on their bike for two days straight. When the frame is stressed during normal riding the fatigue endurance is increased inversely proportional to the loading stress.


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PostPosted: Thu Nov 24, 2016 4:40 am 
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in the industry

Joined: Thu Jan 08, 2009 12:16 pm
Posts: 161
Location: Melbourne Australia
This is a tragedy for this family that in hindsight could have been avoided.

One of the interests within the cycling industry right now is quality control and non destructive inspection of parts, particularly carbon composite. I am actually presenting at a conference sponsored by the UCI on this topic in a couple of weeks in the US.
Flaws in the manufacturing process can significantly reduce the mechanical properties of a part and increase the likelihood of failure. As parts become lighter the margin for error reduces and being able to predict how the part behaves at load becomes more critical. Flaws make this unpredictable.

People claim that testing and inspection is too expensive, however the cost of failure can be significantly more. Maybe the advertising budgets can take a back seat for a while and they can invest further in quality control.

I have scrapped many many road forks that I have found serious manufacturing flaws in using ultrasound scans, these flaws are not visible on the surface. I have cut up some of these as displays and post them on my website and Instagram.

Riding your bike is fun, why take risks that can be easily avoided.

My condolences to the family.

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PostPosted: Thu Nov 24, 2016 8:45 am 
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Bogan wrote:
How old is the Sheldon Brown article? How relevant is it now?
They haven't changed the laws of physics since it was written, so it's no less relevant now than it was when it was published. Only thing that has changed is that the cycling industry has got slightly better at not doing stupid things from a materials science/engineering point of view.


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PostPosted: Thu Nov 24, 2016 8:55 am 
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pdlpsher1 wrote:
I'd gladly take your parts that you plan to trash :P
I guess you don't inspect your parts then? You don't plan to trash them, you inspect those that have had a hard life. So the frame and cranks were actually cracked, frame at the bottom bracket, cranks at the spider/arm stress riser. The handle bars either started creaking (even when off the bike) or started to show surface cracks/crazing and i didn't fancy taking the risk.

Posting them halfway round the world to be put in landfill is probably not the best use of anyones time. Unless you plan to use them, in which case i hope you have good medical insurance.

pdlpsher1 wrote:
Although the test does show which frame is the strongest, the test result itself cannot be translated to a service life. No one will ever spring all out on their bike for two days straight. When the frame is stressed during normal riding the fatigue endurance is increased inversely proportional to the loading stress.
I thought you said that bike parts weren't stressed enough to cause fatigue? :wink: And it's not about having a defined service life, it's about showing that there physically is one, and it might not be as long as you think. A 55 kilo social rider might never reach the limit, an 80 kilo Pro/Elite classics specialist might get to the fatigue limit within a season.


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PostPosted: Wed Nov 30, 2016 4:09 am 
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Lennard Zinn chimed in...

Carbon fiber composites have no fatigue limit, but that is not really the question to ask, since they do not behave like isotropic, hard (i.e., brittle) metals. Steel (titanium, too) has a fatigue limit, so fatigue failure can be avoided with it. As you say, aluminum has no fatigue limit and can thus be more difficult to predict when it might fail under a given cyclic loading. But being a matrix, carbon is less predictable in its failure modes under fatigue.

Carbon fiber composites can indeed fatigue, but they do so very differently from metals. Metals tend to be isotropic (uniform in any direction), even though they can be mildly anisotropic in that they can have a predominant grain direction and, like wood, can be stronger along the grain than across it. By contrast, carbon fiber composites are extremely anisotropic, depending on how the layers of carbon fabric (usually unidirectional fibers) are laid across each other. And this can vary from fork to fork in the same batch, since the fabric pieces are laid up by hand.

If a material has a fatigue limit or “endurance limit,” it means that if the cyclic stresses applied to it are below a certain magnitude (measured in units like pounds per square inch), the structure will not fail due to fatigue. Look at the graph; if the stresses on the steel structure depicted in the graph are below about 28ksi (kilopounds per square inch) – or 28,000psi, the steel will never fail due to fatigue (of course assuming no rusting or impact damage). The fatigue curve for titanium looks similar. But you can see that the fatigue curve for aluminum never flattens out, so even small cyclic loads, if they go on long enough, will eventually cause it to fail.

Carbon fiber composites can indeed fatigue, but they do so very differently from metals. Metals tend to be isotropic (uniform in any direction), even though they can be mildly anisotropic in that they can have a predominant grain direction and, like wood, can be stronger along the grain than across it. (According to the coronial report you sent, this particular aluminum steering tube was not uniform in that it had an “inclusion flaw” in it.) By contrast, carbon fiber composites are extremely anisotropic, depending on how the layers of carbon fabric (usually unidirectional fibers) are laid across each other. And this can vary from fork to fork in the same batch, since the fabric pieces are laid up by hand.

Metals tend to eventually fail under fatigue at a single crack. Carbon fiber composites do not do this — they tend to degrade under fatigue throughout the entire volume of the structure. Composite materials fail due to fatigue in four basic ways: cracking of the matrix (i.e., the resin that holds the fibers together), delamination (peeling apart of one layer of fabric from another), breakage of fibers, and debonding of individual fibers from the resin. You may be able to hear delamination, debonding, and cracking by tapping a coin along the fork; where the matrix is intact, it will make a nice “clack” sound, whereas the sound will be deadened if layers have peeled away from each other or if fibers are cracked or no longer stuck together. Cracking of the fibers and/or of the matrix can be deep within the layers and may not be visible from the surface. Fatigue cracks in metal structures always propagate from the surface (the trick is finding the cracks when they are small).


Long answer to your question, but carbon forks can be subject to fatigue. However, in the absence of crashes or impacts, high-quality carbon forks tend to be highly resistant to fatigue. If your fork goes through a hard crash or receives a hard impact, you ought to replace it right away. In the absence of crashes and/or impact damage, it still makes sense to replace full-carbon forks every now and then. Unless you weigh over 200 pounds, however, you can probably figure a good fork will last a long time (10 years or so?) before it needs replacing. The heavier you are, the shorter the interval between replacements. A sub-130-pound rider can probably assume a full-carbon fork will last them a lifetime unless they go through a crash or suffer an impact.

The issue of course is how a bike is used over the years, since 10 years of use for different riders will be different mileage and riding surfaces and tire pressures. To truly monitor fatigue, you would need have an odometer on it and a way to measure the magnitude of the stresses it receives. I don’t see this appearing on a smartphone app anytime soon.

I have been talking about forks from trusted sources, too. As you can see from this article, if you get a super deal on a brand name, it may be too good to be true, and you could be endangering your life.

When it comes to forks, handlebars and stems, breakage due to fatigue is catastrophic, as it was in this instance, since there is no way to control the bike. When it comes to replacement of bike parts, it’s always advisable to err on the side of caution.

Source www.velonews.com/2016/11/bikes-and-tech ... gue_425464


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PostPosted: Wed Nov 30, 2016 4:49 am 
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I think there is some confusion among casual readers due to the term "fatigue limit". Zinn explains it well and I hope that's the end of it.

This was a freak accident. There aren't any conclusions to be drawn except, stuff happens.


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PostPosted: Wed Nov 30, 2016 10:59 am 
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Posts: 100
Having recently seen you video's and pics, it was certainly an eyopener on the carbon bike industry, and made me take a step back and think about the stuff I am riding. i think we all need to put a bit of pressure on the bike industry to get it where it should be in terms of quality off the shelf.



whodesigns wrote:
This is a tragedy for this family that in hindsight could have been avoided.

One of the interests within the cycling industry right now is quality control and non destructive inspection of parts, particularly carbon composite. I am actually presenting at a conference sponsored by the UCI on this topic in a couple of weeks in the US.
Flaws in the manufacturing process can significantly reduce the mechanical properties of a part and increase the likelihood of failure. As parts become lighter the margin for error reduces and being able to predict how the part behaves at load becomes more critical. Flaws make this unpredictable.

People claim that testing and inspection is too expensive, however the cost of failure can be significantly more. Maybe the advertising budgets can take a back seat for a while and they can invest further in quality control.

I have scrapped many many road forks that I have found serious manufacturing flaws in using ultrasound scans, these flaws are not visible on the surface. I have cut up some of these as displays and post them on my website and Instagram.

Riding your bike is fun, why take risks that can be easily avoided.

My condolences to the family.


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Posted: Wed Nov 30, 2016 10:59 am 


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PostPosted: Wed Nov 30, 2016 11:51 am 
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Joined: Tue Jun 07, 2005 3:32 pm
Posts: 595
This carbon tech guru has been getting lots of views on Youtube lately.

"voids in the fork steerers are not isolated to one brand. Here is a small sample."
https://twitter.com/raoul_luescher

Image


Last edited by User Name on Wed Nov 30, 2016 12:32 pm, edited 3 times in total.

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