Carbon Fork Fatigue Failure. Real world experience?
 

I must admit I am a bit skeptical of carbon fiber wear in comparison to steel for a front fork. I have an inexpensive 2005 Trek 1000 that I have ridden approximately 11K miles. Each year that passes I grow less and less confident in the inexpensive carbon fork on the front end of it.

I have read several articles on carbon fiber fork fatigue and most say that it will last longer than metal. Personally I don't buy it. I have no good reason, but I just can't develop the same trust.

I have steel forks on my mountain bikes and feel like I could trust them forever. Are my suspicions unfounded? An article is one thing, but what is being found in the real world? Particularly if you are a bike shop mechanic, are you or have you seen way more catastrophic carbon fork failures compared to steel, or even aluminum?

NOTE: I'm not talking crashes here, I am only talking about fatigue. Most articles indicate that crashing will obviously contribute to carbon fork failure.

i think you are worrying to much.
i have a 92 trek 2300 carbon fiber composite bike, frame tubes are carbon rest of bike alloy. i got the bike in 05 from a friend who was the original purchaser. if i remember correctly he put about 8,000 miles on the bike. the alloy fork was replaced with a carbon one in the year 2000, i think. i put about 3000 miles on the bike in 5 years, started painting it in 2010 haven't finished it yet.the bike has about 11,000 miles on it, the fork about 6,000 miles. there hasn't been any problem with the carbon ever. just quality maintenance.

If you look at fatigue numbers, alloy is a complete deathtrap. It will fail from essentially any level of fatigue, often suddenly, 100% of the time, eventually. But it's not really that big of an issue on bikes, as the continued existence of alloy parts of a lawyer filed world demonstrates. Carbon and steel are both essentially undamaged by normal fatigue levels.

Ride in many airplanes?

Brad

Fynn, I haven't had a reason to worry about any of the carbon forks I've had. I never have had one with a carbon steerer tube so I'll leave that to the experts.

If it makes you feel better, examine the fork for any damage every now and then.

Brad

Everything fails eventually. Even an anvil has a MTBF. But unlike aluminum alloys, which are highly susceptible to stress fatigue, and can fail with next to no warning, properly engineered CF is close to impervious to it. But each individual strand of carbon thread within the weave is an island of strength to itself. So long as it's used within design limits, CF doesn't fail without warning. Just keep an eye on the paint or clearcoat. As long as it feels smooth and shows no cracking, you should be golden.

Don't forget that many modern commercial airplanes are made largely from carbonfiber. The liability to an aircraft manufacturer of using a suspect material would be unfathomable if they weren't dead nutz certain of what they were doing.

Amateurs guess, then slap the stuff together. Then they pray.

Professionals test. And measure. And test some more. Then they have no need of prayer.

In my experience, you can do two things when it comes to carbon bike parts:

1. Worry about them breaking. If this is you, you might as well switch back to metal because worrying about your bike is no fun.
2. Trust them and inspect them for damage periodically. Read reviews before you buy (the first gen Spinergy Rev-X comes to mind.)

In the end, carbon can break. Aluminum can break (and it will eventually, because it has no fatigue limit.) Steel can break. Titanium...maybe, but it's expensive! :)

I sold a carbon mountain bike because I was afraid of breaking it in a crash. Now I have a carbon fork on my cyclocross bike, and I'm considering getting a carbon frame. It takes a while to build trust, especially because early carbon experiments were very failure prone, and there are a million pictures on the internet of broken carbon.

Aircraft also have mandated, thorough airframe checks to ensure structural integrity.

Very true and very expensive. Any cyclist can examine their own bike for any signs of stress.

Brad

So now I have even worse concerns. I have an aluminum frame mountain bike with over 25K miles. Maybe I should be more concerned about that. From what I am reading here, you can visually detect carbon imperfections but aluminum will drop you on the road at a whim.

I have one too and while it never has been rode gently off road including huge table top drops, it shows no signs of failure and I seriously doubt it ever will. I have no idea of the miles on it, but it's now 15 years old and I've had it since new. I also have two '80s Cannondale aluminum road bikes, one that I've crashed on more than one occasion that will no doubt last a lifetime.

Brad

As long as we are comparing miles on carbon forks, I have two bikes with Easton EC 90SLX all-carbon forks (Easton's lightest fork so potentially the most "fragile"). One bike has a bit over 25,000 miles and the other almost 29,000 miles, most of these miles on the finest potholes and pavement lumps PennDOT can provide. So far there have been no signs of cracks or any other defect in either.

You seem to be talking about two different things here, namely the likelihood of a failure versus how dramatic it might be. We all know carbon can fail dramatically.

My 2c is that if your fork has survived 18Mm (why isn't Mm a thing?), then it's proved itself a good one. The only realistic caveat IMO would be the usual beware of impact damage.

I've just ordered a 1" full carbon fork that weighs 330g and I'm riding a broken plastic bike without a worry; it feels tough as nails.

it is
its just not commonly used
but you go ahead and use it all you want

According to one of the newer senses of the term, that means not 'a thing'.

I enjoy the challenge of this vague, context-dependent sort of language : )

the M prefix litterally is a thing
although not a thing in the sense you meant

so its up to you to make it a thing

from now on
express all numbers in terms of M
so 1m = 1.0E-6 Mm
and 100 kPa = 100E-3 MPa

Please note, this is all simply information base on my opinions...I have to say that so I don't find a burning cross on my front lawn!

While any cyclist can exam their carbon fiber bike and or fork we lack the xray or ultrasound equipment needed to find damage that often occurs to carbon fiber not on the outside where we can see it but INSIDE the tubing.
http://www.bike-manual.com/brands/fi...bon_fiber.html
http://thecycleway.com/?p=111

But then you have the added problem of cheap generic CF that has gotten spotty quality control during the manufacturing process and are sent out the door to be sold to unexpecting customers. http://www.newsomelaw.com/blog/2013/...sudden-failure

Then there's this from a bike shop: http://www.rideyourbike.com/carbonfiber.shtml

Then of course you can watch my videos below for more info about the relative strength of various materials, I don't find CF superior to the others. The videos are here:
http://www.youtube.com/watch?v=nvk63bmVpck
http://www.youtube.com/watch?v=xk98yvozq1g
http://www.youtube.com/watch?v=09WC8iPGM08


Now having said all of that, my last bike I got 2 or so months ago came with a CF fork, there was no getting around it. So I opted to get the only CF fork made in America from a company called Enve, then I also opted to get their strongest fork the 2.0 rated for a 350 pound rider instead of their 1.0 rated for 220 or 240 pound rider even though I only weigh 164. I figured if I was going to have to trust my well being to carbon fiber than I'll do by riding on something over engineered for my weight!! The odd thing is, the weight penalty of the superior fork is only 98 grams.

Catastrophic failure is the worry with all materials. Don't worry, be happy.

Really, carbon is tough stuff, but it does have a limited life when exposed to blunt force/impact. Micro fractures cannot be detected by the naked eye. Aluminum has similar characteristics, but will display fractures nicely. If you are crashing a lot, use steel. Otherwise don't worry, be happy.

All materials used in industries like aircraft manufacturing are thoroughly researched and have well understood stress limits and life expectancies. Materials don't have to be totally fail-safe (nothing is totally fail-safe) but they need to be able to calculate how long every piece will last and how soon it needs to be replaced.

Actually it's not just crashes, they've had plenty of failures that had no reason for the CF to break, get on the internet and you can find those stories, that why there are lawsuits against CF manufactures for such a thing. Most failures however are due to China's lack of quality control, you don't hear about that stuff from American manufactures of CF frames and forks...of course American made CF is only found on top of the line bikes.

I need to clarify.

Fatigue is defined as repeated application of high stress. Everything is susceptible to fatigue - aluminum, steel, even carbon. However, the key is the definition of "high stress". Fatigue occurs when applied stress is comparable to ultimate strength of the material (that is, stress that would cause immediate failure), and life expectancy grows exponentially with the decrease in stress. For carbon fiber, life expectancy is essentially infinite if stress is less than ~60% of ultimate strength. For steel, it's 50%, and aluminum has a bad rep because its effective fatigue limit is as low as 30-40%. (I say "effective" because aluminum does not have a true fatigue limit, but, by 30%, life expectancy is into tens of millions of cycles. If you hit an aluminum object once per second with the stress of 30% of ultimate strength, it will last several months of continuous abuse before it fails.)

None of this is particularly relevant to bicycle forks, because they are built with a lot of excess strength in them. They are built to be stiff, and that automatically makes them far stronger than you'd possibly need. You can ride your bike off a curb and your fork will not explode. Therefore, the stress that occurs when you jump a curb is less than the ultimate strength of the fork material. And normal everyday stresses of riding over rough pavement are much smaller than that stress. Yield strength of aluminum and carbon fiber are on the order of 50 to 70 _thousand_ pounds per square inch. Cross section area of a road fork is right on the order of 1 square inch, which means that you need to generate 50 thousand pounds of force to break the fork and some nontrivial fraction of that amount to cause fatigue. And even if you do, spokes in the wheel will be the first ones to go, since they can't handle nearly as much stress.

Concerning CF forks, there's this: http://www.rivbike.com/product-p/carbonomas.htm

There has been a lot of great info posted in this thread. However, it doesn't appear that anyone has had any "real world" experience with a CF fork breaking, either personally or through witnessing it. My somewhat recently formed opinion is that as others have mentioned, everything will fail eventually. The prospect of my CF fork giving out is not a good one.

Recently I had an experience where two egg beater pedals broke from simple wear and tear. One of these episodes left me crashing into the ground. That got me thinking about whatever component I might have in my stable that are waiting to let loose.

Doesn't that Trek have an aluminum steerer anyway? I ask because most of the carbon fork failures I've heard recounted ( I've never seen one...) have been due to improper tightening of the stem steerer clamp(s). Inspect it periodically if you must (I do so on all my bikes during their regular wipe-downs), and then just ride the thing... :thumb:

You mean like this one?

http://bikeforums.net/attachment.php...hmentid=348641