Editor’s note: This story first appeared in Bicycle Times issue #17, published in June 2012. Words and photos by Jeff Archer.
For the third installment in our series of historical looks at frame materials, we will consider carbon fiber. The properties of steel and titanium are well documented and pretty consistent. Carbon fiber’s properties are much more difficult to define and are dependent on many different factors. Carbon fiber is made up of parallel fibers of pure carbon held in place by an epoxy. This structure makes up a carbon fiber ply. As you can imagine, carbon fiber has a very high tensile strength, since we are pulling along the length of a fiber. On the other hand, the compression strength is low; imagine pushing on the end of a string. This structure is also not very strong when impacted from the side and is flexible along the length of the ply. The carbon fiber matrix also performs poorly in elongation tests. It is not very ductile and, when it fails, it tends to fail catastrophically. With all these issues, carbon fiber doesn’t seem to be an ideal frame-building material.
However, since there has been a boom in carbon fiber bikes in the past decade, there has to be more to the story. Many different grades of carbon fiber are available, so it is difficult to apply specific numbers to the material, but we can make some general observations. Carbon fiber is less dense (lighter) than metallic materials, which can result in a lighter-weight frame. As we stated above, carbon fiber has flexibility along its length, which can be used by designers to build in some shock absorption. They can also build up layers of carbon fiber and align the plies to tune the ride. In areas such as the bottom bracket where they want to reduce the flexibility, the plies can be layered at 45 degree angles to make the structure stiff in all directions. The material can also be shaped, which can free up the designer to make a more aerodynamic, flowing frame. There are many more challenges when designing a carbon fiber frame, but when done properly, the result can be stiffer, more comfortable and lighter than frames built from metal.
This Exxon Graftek G1 frame was one of the first commercially available carbon fiber frames. Realizing the potential issues of carbon, the engineers came up with an interesting hybrid construction method. The frame uses thin-wall aluminum tubing wrapped with carbon fiber to take advantage of the torsional strength of aluminum and the longitudinal strength of carbon fiber. These tubes were then bonded into a set of stainless steel lugs. The steel forks also featured a carbon-fiber wrap on the fork blades, which likely made minimal difference but did match the aesthetics of the frame. Advertising touted a 3.5 pound frame weight (compared to a typical 5 pound steel frame of the era) and claimed a 10-to-1 advantage in shock absorption. The pictured bike has a whole batch of lightweight components to match the lightweight frame, including an aluminum freewheel, alloy railed saddle, drilled-out brake levers and a lightweight Hi-E wheelset. The complete bike weighs 17.9 pounds.
While the Graftek didn’t take full advantage of carbon fiber, it was the first step in the process that brought us Tour de France-ready bikes such as the Trek Madone.
This bike can be seen at the Museum of Mountain Bike Art & Technology in historic downtown Statesville, North Carolina. If you can’t visit in person, check out the collection at www.mombat.org.