Carbon Composites Ratio of Stiffness to Density

by Sabinna on January 7, 2011

The World Federation of the Sporting Goods Industry 2011 Handbook arrived in the mail a day or so ago. You can download a copy of it at the WFSGI website. It’s of specific interest due to the section on the bicycle industry. NB also the article on the impact of electric bikes.

One article “Composites at the Leading Edge” written by Jan-Anders Manson of the Swiss Federal Institute of Technology, and Pierre-Etienne Bourban of the Laboratory of Polymer and Composite Technology, Lausanne gives an overview of carbon fiber.

They compare carbon fiber with glass fiber composites, steel, titanium, aluminum, magnesium, wood, and plastic as to the ratio of stiffness to density or “specific stiffness”. The “figure” presented in this, admittedly brief, article line these up in a graph that lacks specific values. The graph is divided into five equal proportions that give us sense of the overwhelming superiority of carbon fiber as against the others. I’ll give them values from 1-5.

Barely making it more than 1/10th of the way into the first is plastic, followed by wood making it 3/4 of the way into #1. Then you have magnesium, aluminum, titanium, steel, and fiber-glass all making it from 10-25% into the second section. They are all dwarfed by carbon fiber composites which hits the 4th level–very high stiffness and low density.

This is interesting also in light of the article that appeared around a month or so ago giving anecdotal evidence that a “carbon” bike is no faster than a steel bike (and check out Mark V of Bike Hugger’s comments). The doctor who carried out this study compared commuting to his workplace on a steel-framed roadbike with a carbon-framed bike. He concluded that a “30% reduction in bicycle weight did not reduce commuting time over a distance of 27 miles (43.5 km).”

I noticed that the doctor mentioned “efficiency” at the beginning of the article as he sets up the context to the study: “No one will tell you how much more efficient one bicycle is over another; they just say it is better.” That’s a good point. There is a bit too much of this-is-better-than-thatism where there is no metric made explicit to provide a solid foundation for making such claims right across a wide range of industries that rely on consumers being dazzled by the technology.

The thing is the doctor does not really follow this up. What struck me when a link the article appeared in my twitter feed was that if the commuting times were constant the greater efficiency of carbon (“specific stiffness”) would mean less energy expenditure. A high-end heart rate monitor with caloric expenditure for each journey would have been a very important variable to throw into the  mix I would have thought. At the least it would have been interesting.

The second part of the doctor’s conclusion, however, that would be hard to argue with is that “a reduction in the weight of the cyclist rather than that of the bicycle may deliver greater benefit and at reduced cost.” It’s a point that comes up from time to time and  is completely rational. A cyclist pays a high premium per gram of weight reduction between bikes that may differ less than 1kg in weight.

But, sure, the emotions and processes behind a consumer’s decision to purchase a cheaper and slightly heaver alloy-framed bike vs a (assumed high-modulus) carbon alternative assure carbon fiber’s position as the pre-eminent high-end material of choice. As the authors of the “Composites” article finish off: the future of carbon fiber will involve greater use of nano-materials, an emphasis on damping and making composites more environmentally friendly ie. bio-based polymers.

This third development would go a long way to solving a key problem with carbon-fiber products after they have finished their useful life. (See one of my posts from last year about what Giant did with a whole batch of frames in the early days of adoption of carbon fiber in frame-making).

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