Thursday, February 25

Carbon for the Rest of Us

So Far, this Carbon Seatpost on Buddy Hasn't Turned into Splinters
FOREWORD - THE CHALLENGE
CITIZEN RIDER, in his post here, presented a challenge that I cannot resist. You see, he expressed frustration at a bicycle subject that I deal with professionally, but which never before occurred to me to post about on a bike transportation blog. Carbon. You see, it's often amused me to see what cyclists and their publications, such as Buycycling Magazine, say about carbon bike components. Still, I never really thought to say much about the nonsense I hear every day. I've limited my inputs to occasional comments on blogs, such as here.

You see, as some of you know, I'm an engineer. More significantly, as far fewer know, I have over 30 years of carbon composite experience in flight vehicle structures. These range from flaky homebuilt aircraft up to things that lead me to take offense when people talk about "rocket scientists." It's somewhere upward of 300 composite assemblies over the years.

The world of carbon composites is vast and complex. Some of it is fundamental to making weapons of war, so I'll stay away from things that will cause me to be thrown into prison for violating the "International Trafficking in Arms Regulations." Don't look here if you want me to spill the beans on how to build the world's most advanced fighter jet, or how to sabotage carbon helicopter blades. Put simply, carbon is just another material. Like many, if you abuse it, it can kill you. Aircraft manufacturers have failed with the material (of COURSE my example is Airbus) Carbon is more than just a fashion statement, though it is clearly THAT as well.

If you want to use carbon composites as MORE than a fashion statement, there's a lot of stuff you need to know, and I'll plan on doing a series of articles that, strung together, constitute at least a short story on the subject for regular people - and bike mechanics.

First off, I'll treat a few of the items brought up in CITIZEN RIDER'S excellent challenge. This will preceed the organized treatment:

CR COMMENTS AND OBSERVATIONS
#1 - All those inspection items are not going to do you any good at all, though squid magnotometers at least sound really cool. In aircraft, beyond a visual inspection to see really extreme stuff, it is really only ultrasonic or xray inspection that is used. I think most bike shops and people are not in a position to do xray inspections. Even ultrasonic inspections are not as wonderful as most think. Ultrasonic equipment suitable for a bike shop can be had for $200-$4000 via ebay. The upper end seems more than I'd want to pay. The lower end is a thickness measurer. Maybe just not good enough. For a dedicated bike mechanic, there are other common sense solutions, however.

#2 - It is not wise to abhor carbon fiber, nor to develop a fondness for it. As I said, it's another material, with its proper place. OTOH, here and here, it is just a style statement. I do not plan to talk about composites as design style statements in this series.

#3 - Tapping. Perhaps I'll regret accusing someone who thought tapping on composites was fine as living in the dark ages and practicing witchcraft. Tests have shown that tap testing, despite the impressions of those doing it, are unreliable AT BEST and positively worthless in most cases. Still, I reluctantly admit I do it myself often.OTOH, you CAN do MUCH better.
#4 - Ham, get those thoughts out of your mind - and THIS is why this preface post was made NOW. "Or, do I change is every 4/5 years - 20,000 miles?" HAVE NO DOUBT WHATSOEVER - WHEN CARBON GETS DAMAGED, AND THE LOAD IS TOO HIGH, IT'LL GO THE VERY NEXT TIME THAT LOAD IS TOO HIGH. IT COULD BE TODAY. IT COULD BE NEVER, BUT TIME HAS NOTHING TO DO WITH IT. It's NOT like a crack in aluminum or steel that will grow. It'll either work, or it'll fly apart. NO WARNING. If you have ANY doubts, go read THIS (luckily, the rider only broke his collarbone) or ANY OF THESE. The common thread is time is not something you can use to predict a failure. It'll either be OK, or it'll be gone. Imagine, if you will, what would happen if a front fork suddenly flew apart while you were riding at 20mph towards a corner in heavy traffic. NOT PRETTY.

#5 - Composites are not just sexy black replacements for metal components. Carbon composites are brittle. They are more like plywood or reinforced concrete. Get those thoughts about cracks and bent components completely out of your mind. That is not what these materials do. They either work, or they break. There's not a lot in between. As a cyclist, you want to make sure the ones you use work.

8 comments:

Anonymous said...

I've always loved iron but it rusts too quickly so I settle for beautiful massive heavy shiny steel. A steel bike will last several lifetimes and I don't mind getting a little more exercise from the weight. Also, why do people train for a race on a light bike? Why not train on a heavy bike and go like the wind on the expensive light bike.

Rantwick said...

Thank you for this. Carbon is such a mysterious topic to so many, including me. I'm looking forward to more.

Ed W said...

OK, Steve, 'splain in simple terms how carbon (or any other material) fails when it's stressed too much. And would inspection under UV light reveal delaminations better than visible light? Just a thought.

Ham said...

Very interesting, I do realise the way that carbon fails (I know someone whose seatpost failed). My question about time and wear is more about reducing risk.

Am I right in thinking that with carbon, it is the structure that provides the strength? That, once the outer "skin" is damaged, that can presage structural failure? I know carbon is strong, and when it remains undamaged, I would have thought the chances are that it will retain its strength. But what when you start to scratch and dig at it, as inevitably happens over time commuting in London. Is there a point where replacing a part (fork in my case) minimises a risk?

Steve A said...

Carbon fibers break much like glass strands. They stretch until they just snap. The answer to Ed's other question is simpler - "no." Fortunately, while the subject isn't simple, it's not beyond the capability of what the typical garage contains. This series will break the subject down into understandable hunks.

Bike manufacturers could do a lot more to help bike mechanics in this area.

Steve A said...

Ham,
Carbon is not like a fiber skin, it is more like a combination of plywood layers glued together and reinforced concrete in which the tension capability is provided by the steel rebars.

During this series, you will see that it is more like a helmet - it gets pranged and it's toast. Even in London and without expensive equipment, you can verify your fork remains safe. Luckily we are talking about a bike and not a jet fighter. As I'll describe, you can do a "proof load" test if you have reason to suspect the integrity of your fork. It passes and it remains safe. It breaks and you know it was no good. Simple, and there is no question.

Velouria said...

Just looking at that seat post sends shivers down my spine!...

Ron said...

Steve :

So bike manufacturers could do a lot more to help bike mechanics in this area? Meaning they don't? Ha. I wouldn't be surprised. Check my blog out for failures from time to time.

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