Tuesday, December 23, 2008

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.


Anonymous said...

Looks like the Bontrager X-Lite fork.

Zach said...

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!

Anonymous said...

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

Rick said...

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.

Rick said...

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.

Anonymous said...

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!!!

jon said...

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.

Ron said...

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?

jon said...

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.

Anonymous said...

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

Anonymous said...

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?

Anonymous said...

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.