Monday, December 22, 2008

24 Bontrager Carbon Fiber Fork Failure

In my email today, I found the picture of a failed Bontrager fork submitted by Daniel Forrest from Malawi, Africa. Thank you and Merry Christmas from across the Atlantic.

High Res Pic Here


He didn't give me a specific model for the fork. Maybe some of my readers can help me out. Anyway, this is what he wrote to me. 

"I had an accident in May where the actual blades suddenly failed. It was a bontrager carbon fiber fork. The bike was a friend's, and had been ridden quite a bit. It failed right as I got up to sprint. Nonetheless, it was on flat ground, and wasn't involved in a crash."

He asked me specifically what my thoughts are on the failure. Here's a few : 

1. If I were him, I would not take a friend's carbon fiber equipment to use for sprinting, without knowing its history and what it has been through. Perhaps the bike may have sustained his weight but not yours? You may have done things like this as a kid with BMX bikes, but this isn't BMX. Carbon fiber is unsafe to use after a point determined by the possibility of catastrophic failure due to some trigger factors. Infact, very dangerous. Note that you don't need an exceptionally high force to cause carbon fork failure. The ones that cause fatigue are frequently forces of lower magnitude, below those that cause irreversible plastic deformation.

2. There are many variables to consider in trying to assess carbon fiber failure after it occurs. Hence, it is an involved task. I have talked about some of them in a previous post on a broken steerer tube. Here in this case, my first instinct tells me that this failure is a product of delamination. This is the most typical path to failure for a carbon composite. Delamination is a loss of interlaminar shear strength, initiates at the tip of a tiny crack that you may not notice and generally grows with increasing load cycles. Its like a cancer for carbon. In this case, it had to come to a sprint where the cyclic forces acting on the fork, which it could have otherwise handled, amputated it a little above the midsection. Same scenario for both forks. The fact that both forks failed the same way leads me to consider the fact that that this equipment could have gone through a period of harsh use prior to the user getting it in his hands. What that was, we do not know.

3. This fork is junk now. Evaluate the rest of the bike, if its carbon, carefully with a torch light or magnifying lens and use your Private Eye skills. Watch some of Hitchcock's movies to gain inspiration.

4. As for Bontrager, I want to know a bunch of things from their spec sheets. (Like they'll ever give it to us.. :)  )

  • a. What kind of fiber they used : Unidirectional? Bi-directional woven fabric?
  • b. What was the nature of the core?
  • c. What type of resin and fiber/resin ratio did they use? Often, resins are the environmental weak link in a failure. Epoxies have very balanced properties. But some other types of resins are environmentally degraded on contact with high temperatures, corrosive liquids, grease and oils. This is if they are not suitably protected from the environment by a surface film. But an impact, scratch or nick is enough to erode this protective layer of paint/finish/clearcoat off, exposing the underlying material to the environment, moisture, corrosion etc. Especially in its just-cured state, the part must be handled carefully on the manufacturing floor. That is something to consider. [Source of information : ASM Handbook, Volume 21 on Composites, Page 359]
  • d. Where was this job sourced?
  • e. Is there any evidence of past failure of this nature on the same product?
  • f. What does their in-house statistical data on testing of samples indicate?

Solutions? From a quality standpoint, I already ranted about solutions in a previous petition to the bicycle industry.  

I have two others I can think of but may be practical or impractical depending on how you look at it. These are just ideas.

a. Sensor : Can a sensor (strain gage?) be embedded inside critical points of the composite structure such that an unwarranted damage or unusually high load can be detected by a resistance change, which can then be transmitted as a signal to a receiver on the handlebar where it'll then be processed into sensible information? I dreamt of this in a previous post and named the idea "Smart Bicycle". Take a look here

b. Camera : Okay. Maybe more down to earth this time. Failures generally start inside the material that you cannot see. Could a very tiny camera installed with a source of light inside the fork work in conveying useful information? Images could be conveyed to a handlebar monitor so you can get a glimpse of whats happening inside critical hollow structures in the bicycle.You know, like a CCTV for your bike. However, I'm not saying such a surgical technique might not be inexpensive.

Carbon fiber is a very interesting material. The fact that it starts off like a fabric allows designers to be very imaginative with their designs. However, I'm wondering whether some of them are stepping out of bounds and playing with fire. This is what was essentially the subject of my rant in my past petition to the bicycle industry. Any yahoo can build a carbon fiber composite bike in his basement. Its not very difficult. However, making something structurally safe to use is totally another matter. Moreover, my rant was not just considering carbon components. Faulty welding, crappy helmets, breaking pedals, cracking rims, lightweight brakes that fail to function properly etc are all children of poor design, and an over obsessiveness with the lightweight philosophy.

Remember the guy who wanted to break the downhill speed limit record on his bike, and chose a mountain bike made of composite? The videos are all over the web. After he attained a speed of a little above 100mph, the bike disintegerated into two sending him flying over the handlebars like a ragdoll at 100mph. He survived somehow, but what the heck was the need for a lightweight bike in the first place in order to go downhill??

Carbon fiber composite products are generally made for on-the-edge performance. The sacrifice you pay in having purchased such a thing is knowing that durability is way down there, not only in terms of major damage but also minor damage! A tiny nick or suspicious hair line crack is enough to start a bad chain reaction upon further loading. That could inevitably send the material breaking like a biscuit without warning the next time you're on it. The mechanics of this failure is the subject of research in university circles, and often involves complex microstructure dynamics, mechanics and math. Although we won't go to that point, it is upon some of us to find quick preventive measures and solutions to combat this problem before anyone gets seriously hurt. Even dies. Serious. So take care of your equipment and yourself.

24 comments:

  1. Anonymous6:10 PM

    Looks like the Bontrager X-Lite fork.

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  2. Looks like it came from a Trek 1000 or 2100, the two bikes I've ridden. I thought that would have been a picture from my crash last year until 1) I saw the speedometer sensor that wasn't mine and 2) I read the description. Definitely looks like it would have been a Trekkie though. Merry Christmas Ron!

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  3. Anonymous8:55 PM

    Is the rider OK?

    What was the response from Trek regarding this?

    Those ideas about embedding strain gauges and even a micro camera might really benefit the product if they were used during the development process.
    I wonder how difficult it would be to add this stuff to a lay-up?
    - Ryan

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  4. This makes me shudder. I have the same fork on my red Trek fixie (T1), and to be honest, I have beeen taking the bike through hell. One of these days, I may bite the dust hard.

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  5. Oh, and please those who're commenting or reading this post...if you have heard of similar failures let us know. I think a lot of people are riding on this fork as its quite common on the 1000 series.

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  6. Rick : No need to fear. Just take care of your equipment, I believe TREK should have some literature on how to take care of carbon fiber. Did you receive one with the bike? You could also check their website.

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  7. I agree with Ryan. Smart materials are being explored by many researchers. I think this is definitely something that could become a research topic by itself. Maybe they could collaborate with an actual bicycle company so they could learn something from the experience.

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  8. Anonymous10:47 PM

    You mention that carbon composites are used for performance. Excuse me?? This is irony, because its generally in performance that you would want something to go the extra mile. I think it is folly to use something for performance that has very little durability, can fail in many modes and are generally fault intolerant. If designers had some integrity, they would ride these things like the common folk do, and push it to its limits, and understand the variability of usage. Having said that, GO TITANIUM!!!

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  9. Be it titanium or carbon fiber, I think a fork is a fork and has to be well designed. A bicycle fork rarely sees forces in just one direction. It has to cope with in-plane loads, lateral bending and torsion all with a good amount of rigidity.

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  10. I have always wondered why manufacturers wouldn't embed a small piece of metal in their forks, like a steel or aluminum I-beam or tubular structure-- not enough metal to actually bear load, but enough to yield what the guys at Thomson call a "proper mode of failure," i.e. the carbon de-laminates and you have a few seconds to nurse a badly-bent and wobbly fork to the side of the road, instead of it just exploding instantly. I'd buy a product with a safety feature like that engineered in, even if it weighed 100 grams more.

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  11. Jon, good idea but if you're going to have this bonded to carbon it involves its own complexities, one being discontinuities that may act as stress raisers. Is that what you were saying?

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  12. Hey Ron, yeah, that is what I'm saying, and I agree, no doubt this would be a consequential engineering challenge. This would just seem to me to be the sort of engineering challenge that a fork maker would be very interested in, given that catastrophic failure of a fork is so likely to result in such serious injury. And, of course, time was that metal objects in carbon forks (like steerer tubes, crowns, and dropouts) were pretty common.

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  13. Anonymous11:17 AM

    You know, thinking about this all day, I find it very strange that this fork spontaneously failed at roughly the same location on both fork blades.

    Then I remembered that one thing that damages a lot of forks every year is when a care drives into a garage, or some other closed space with a bike on the roof rake.

    I am not saying that is what happened, but a scenario where this did occur and the owner did not think of replacing the fork, because the fork did not display any significant damage would be a likely catalyst to this mode of failure.
    - Ryan

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  14. In that case, the fork should have been professionally inspected. This is what I'd do if any critical carbon component on my bike takes a hit. Better safe than sorry.

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  15. Anonymous10:42 PM

    First of all, I take exception to the rider's actual quote in the email you found.

    "I had an accident in May where the actual blades suddenly failed. It was a bontrager carbon fiber fork. The bike was a friend's, and had been ridden quite a bit. It failed right as I got up to sprint. Nonetheless, it was on flat ground, and wasn't involved in a crash."

    I don't know if he's trying to merely look brave, but I can't be expected to believe that he didn't shed a speck on his skin given that he sprinted and then fork blades failed. Whats the true story here?

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  16. Ron, thanks for the post. I appreciate the response. Sorry to be slow in response, our internet has been down for a of days.

    I was surprised by the first item you had on your post: of course its not a BMX. It was a road bike, which are designed for sprinting. I am not sure what the issue would be.
    I am exactly the same build as my buddy, whose bike it is. (180lb, 5'10"). So I would think that the difference in weight wouldn't do it.

    I am very appreciative of the technical thoughts. Those were all the things I wondered about, but have no training in.

    Now, in response to Anonymous here is the exact story: I was home in Seattle for the weekend and I wanted to do some riding, so I borrowed my buddy's bike. He rides about 200 miles a week on a nice trail with negligible bumps. We were on a trail that cuts through town, and is nicely paved. Another set of cyclists were coming the other way so I got up on my sprint bars to scoot in front of my friend. Right as I did I felt my foot in the middle of the wheel since the fork snapped. Next thing I knew I was slowing down my body with my elbow and wrist.

    Sorry for the confusion about my injury; it wasn't my concern in writing. So, apologies for coming off 'brave', not my intention. When I said that the bike was involved in a crash I simply meant that I didn't run into, or wasn't run into by anything. If your desperate enough to see my ugly mug and the scuffs sustained here's the link (http://bp0.blogger.com/_uDEV4tFXYVM/SCOKTQRedPI/AAAAAAAAAGg/ST9rCfT2DDM/s1600-h/DSCF3294.JPG).

    Thanks for everyone's posts, they have been good and informative.

    As they say here in Malawi "Krisimisi ya Bwino"

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  17. I EXPERIENCED A SIMILAR FAILURE OF FRONT FORKS ON A TREK 2300. I PASSED A MAN WALKING A DOG ON THE BIKE PATH. THE DOG WAS ON ONE OF THOSE LONG, RETRACTABLE LEASHES. I DID NOT SEE THE LEASH. THE MAN WAS ON THE RIGHT SIDE OF THE PATH, THE DOG ON THE LEFT. I PASSED THE MAN ON HIS RIGHT, THEN SWERVED IN TO THE LEFT TO PASS THE DOG. THE LAST THING I REMEMBER IS THINKING "THERE'S THE DOG" AS I PASSED HIM ON MY RIGHT. I WOKE UP IN AN AMBULENCE. THE STATEMENT OF THE MAN SAID THAT THE LEASH HAD BECOME ENTANGLED IN ONE OF MY WHEELS. THE FRONT FORKS APPARANTLY SNAPPED AT WHAT LOOKS LIKE THE EXACT PLACE INDICATED IN THE ATTACHED PICTURE. I MUST HAVE GONE HEAD FIRST INTO THE PAVEMENT. SUSTAINED "SLIGHT" FRACTURE OF C4 VERT AS WELL AS THE BONY COLLAR THAT THE SPINE INSERTS INTO AT THE BASE OF THE SKULL. TORE UP LEFT EAR BUT NO PERMANENT INJURIES HAVE SURFACED SO FAR (15 DAYS LATER). I HAVE BEEN A BIKE COMMUTER SINCE 1988 AND BELIEVE THAT HAVE GOOD SKILLS WHILE AMONGST PEDESTRIANS.
    BIKE HAS BEEN GARAGED SINCE PURCHASE AND RIDEN 12 MILES DAILY TO WORK WITH NO RACING OR TRAILS. I PASSED THE DOG WALKER AT NO MORE THAN 10 MPH. CONCERNED THAT ENOUGH FORCE WAS DEVELOPED TO SNAP THE FORK CLEANLY. MECANIC AT SHOP SAID I MUST HAVE BECOME AIRBORNE AND THEN TOUCHED DOWN ON THE FRONT WHEEL. WHEEL SHOWS NO DEFORMITY.
    COMMENTS?
    MIKE IN PORTLAND, OR.
    mfitz54@yahoo.com

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  18. Anonymous11:34 AM

    This looks to me like maybe a twig or stick or something flipped up from the road and got jammed in the spokes.... effectively snapping the fork in the same spot on both sides. Kinda like what would happen if somebody stuck a broom handle in your spokes and stopped you in your tracks.

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  20. Man, this kind of thing and other carbon failures pictured on bustedcarbon.com are freaking me out a little.

    You would think anything branded from Trek would be fully tested to death - and I bet the design was.

    Who cares if the original owner of the bike rode it hard, there's no way it should have snapped in that fashion afterwards. A steel fork wouldn't have failed like that, it would bent and/or cracked first - not just given away all at once.

    There's millions of carbon forks out there - even lower end bikes now sport them. Makes me wonder what the failure rate is. Even if low, when a fork does break like that - you're going down hard.

    Being old school dude, I have a few steel bikes with steel forks. But I ride my modern carbon bike the most. I sometimes wonder about the carbon steerer tube lurking in the head tube. It currently has about 6000 miles on it. I pull it and check for piece of mind.

    Carbon fiber bikes ride incredibly well, I dig mine a lot. I don't worry about frame failure, but do wonder about the fork at times.

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  21. Dan O : It really won't hurt to inspect your fork from time to time. And if you manage to catch a crack, then what right? If I were you, I'd immediately call up the company that produced the product and tell them about it. But if you're the kind of guy who has confidence in his eyes, you can avoid the above step should you determine that the crack length and the position it is in your bicycle wouldn't have any long term ill-effects, sort of like DIY risk assessment if you know what I mean. Speaking of which, I've wanted to write a post on that topic.

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  22. You should be afraid of a carbon fiber bike. They are actually produced for professional racers who get a new bike every year and have paid mechanics to inspect them every night.

    It is only a matter of time until someone sues a big bike company over this type of thing and ends up owning Trek or cannondale

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  23. Anonymous3:32 PM

    I realize this is an old post, but I just had a fork break like this in a 30mph swerve to avoid a fallen rider. The bike was a 2011 top shelf model (ProTour riders have the same bike). I will keep me and the manufacturer anonymous while we are working things out. At this point, it looks unlikely that the manufacturer will cover the fork or the replacement of the many other broken components. The initial comment on the phone from them was "forks just don't break like that." Mine was new and never had a crash, a drop, a bump, or anything accept normal riding. Heck, I'm not even that big of a guy, and I am not a good sprinter. The whole thing is freaky and bizarre considering that in my incident I never contacted another person, bike, or object other than the ground after the fork came apart. Any comments or suggestions would be appreciated regarding my dealings with the manufacturer.

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  24. I also just had my trek 1.2 fork break. It's a year old bike with daily use and di-weekly sprinting. It has been used a considerable amount yet nevertheless, my fork breaking could easily have been a fatal accident. In this case however I happened only to be doing a meagre 10 mph, but there was no trigger. No warning, nothing it just broke and I found myself unable to breathe for 10 seconds on the tarmac. Luckily I was with 5 other cyclists to verify to the manufacturer the nature of this break.

    I could send pictures if you like, the fork is in worse shape than the one pictured above.

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Thank you. I read every single comment.