Thursday, January 31, 2008

5 Broken Steerer Tube : Composites Are Not Perfect

Thanks to some high quality pictures from Erik (I hope I got the name right), we can now visualize a suspect carbon steerer failure in high resolution. The bicycle is a Bianchi Virata, possibly from the company's 2006 lineup. Some pictures of Erik's injuries can be viewed here (since he's made the page public). Lets hope he has fully recovered.





I notice how the metal-cf bond has failed at the headset junction, and the fiber within has just snapped like straw. Honestly, I do not have a lot of composite experience so I cannot really tell you how this fork failed. Only Erik knows best what he did with the bike before this happened. We can sit here and postulate too. But I'll be posing this question to some composites experts. Understanding this culprit is vital for my brain's health.

Composites consist of a combination of materials - a matrix and sheets of short or long fibers. Each of these two have their own characteristic properties and when combined together and designed well, they take care of each other's weaknesses and yield a high strength composite having properties like no other conventional material. This is called complementary nature (sum of parts is greater individual parts).

Some important design factors to consider in dealing with these materials are :

1. Aspect Ratio , defined as length divided by cross sectional diameter. Fibers can be long or short and each of them have advantages and disadvantages.


2. Ply orientation. A ply is basically a strand or layer of carbon fiber and how you orient it in your design will speak of how it'll perform in real world. Orientation depends on structural application for example, a golf club shaft.


Pictures courtesy of a University Webpage.


3. Choice of fiber and matrix.

4. Choice of manufacturing method.

This is why composites are so attractive. You can tailormake them to give specific properties in specific regions. They are also tremendously lightweight and strong anisotropically.

Composites are also a funny material. They are great for on the edge, performance based design. But theydon't show warning signals before failure (unlike metals), and are infamous for snapping catastophically.

The modes of failure
are divided into three :

1. Fiber Failure
2. Resin Failure
3. Delamination

You can read all about composites on the left hand portion of this page.

Generally, I think one would desire the following :

1. As much fiber as you can cram into the piece, but not a whole lot such that dry fiber results.
2. Delamination and failure can also happen without an appropriate percentage by volume of fiber.

The above pictures could reflect a manufacturing defect (such as leaving voids, using less resin or fiber etc), or just outright negligence of design principles such as factor of safety, laterial and longitudunal loading conditions (including impacts and those damn bumps on the road).

Either way, its a failure of engineering ethics.

Morals of the story :

1. Composites are a very specialized field. Good design with composites takes years of experience and learning. So do not buy unbranded bikes or forks from someone you do not know, for example, Ebay.

2. Don't laugh the next time you see a warning label on your handlebar or seatpost. They are there for a reason so READ instructions and tightening values carefully!

3. Check your carbon fiber equipment at all times. Look for any suspicious (if not tiny ones) scratches and nicks. They can be stress risers for the material. Be attentive to weird noises as you ride. Use a coin and gently tap the surface for variations in noise. I know these are archaic methods, but if you can afford the latest ultrasound technology to detect duplicate echoes, go right ahead.

5 comments:

  1. I don't think you're supposed to do that...
    ;^D

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  2. Looking at the failure, it seems likely that he hit something. Also the fork appears to be of the carbon legs, aluminum crown, carbon tube construction, this creates a large stress riser right at the al/c junction where the fork failed. A fork that is carbon monocoque will be stronger. If there was a colision that caused this failure it likely would have yielded a metal fork possibly resulting in less injuries. This highlights the need for proper design and to use things appropriately, ie Clydesdale's shouldnt ride superlight forks

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  3. Cycling Phun : I didn't ride the bike. Its not mine :)

    Carl, I was thinking the same. I'm not very sure what the role of the metal alloy (aluminum?) crown is, but I guess I can say that a stress riser, possibly in the form of a large shear force, led to the damage.

    Manufacturing wise, the bonding wasn't good, or maybe too less fiber/resin or vice versa combination. It could also be that a very low grade carbon fiber was used for the design. Higher grades are stronger but also costlier.

    This kind of damage is not very good, especially since its hidden within the head tube and is not noticeable while riding. Thankfully, the rider survived!

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  4. Thanks for that. I've had a CF fork on my road bike for a year now. I'm changing it over to a new frame this spring and will take a careful look at the carbon steerer.

    I wonder if the failures are different in CF blades to AL steers of if the all CF fail as well?

    I notice while on my trainer and the front wheel is in a block that you can so easily flex the whole setup by twisting. Surprisingly easy to do.
    -B

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  5. Thanks for the post on the failed steerer. I had an accident in May where the actual blades suddenly failed.

    It was a bontrager carbon fiber fork. The bike was a friends, 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.

    Here is a link to a picture of the wheel with the remains of the fork still attached. (http://bp0.blogger.com/_uDEV4tFXYVM/SCOKUQRedQI/AAAAAAAAAGo/C3PDkjyRlzQ/s1600-h/DSCF3295.JPG )

    I'd be curious to hear your thoughts on that.

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