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different components or materials. You’ve heard the term “carbon fiber bike.”
This really means “composite bike.”
Carbon fiber composites are typically a strong, light fiber in a matrix of plastic,
molded to form a shape. Carbon composites are light relative to metals. Steel
weighs 7.8 grams/cm3 (grams per cubic centimeter), titanium 4.5 grams/
cm3, aluminum 2.75 grams/cm3. Contrast these numbers with carbon fiber
composite at 1.45 grams/cm3.
The composites with the best strength-to-weight ratios are made of carbon
fiber in a matrix of epoxy plastic. The epoxy matrix bonds the carbon fibers
together, transfers load to other fibers, and provides a smooth outer surface.
The carbon fibers are the “skeleton” that carries the load.
Why Are Composites Used?
Unlike metals, which have uniform properties in all directions (engineers call
this isotropic), carbon fibers can be placed in specific orientations to optimize
the structure for particular loads. The choice of where to place the carbon fibers
gives engineers a powerful tool to create strong, light bicycles. Engineers may
also orient fibers to suit other goals such as comfort and vibration damping.
Carbon fiber composites are very corrosion resistant, much more so than
most metals.
Think about carbon fiber or fiberglass boats.
Carbon fiber materials have a very high strength-to-weight ratio.
What Are The Limits Of Composites?
Well designed “composite” or carbon fiber bicycles and components have
long fatigue lives, usually better than their metal equivalents.
While fatigue life is an advantage of carbon fiber, you must still regularly
inspect your carbon fiber frame, fork, or components.
Carbon fiber composites are not ductile. Once a carbon structure is
overloaded, it will not bend; it will break. At and near the break, there will be
rough, sharp edges and maybe delamination of carbon fiber or carbon fiber
fabric layers. There will be no bending, buckling, or stretching.
If You Hit Something Or Have A Crash, What Can You Expect From Your
Carbon Fiber Bike?
Let’s say you hit a curb, ditch, rock, car, other cyclist or other object. At
any speed above a fast walk, your body will continue to move forward, the
momentum carrying you over the front of the bike. You cannot and will not
stay on the bike and what happens to the frame, fork and other components is
irrelevant to what happens to your body.
What should you expect from your carbon frame? It depends on many
complex factors. But we can tell you that if the impact is hard enough, the fork
or frame may be completely broken. Note the significant difference in behavior
between carbon and metal. See Section 2. A, Understanding metals in this
Appendix. Even if the carbon frame was twice as strong as a metal frame, once
the carbon frame is overloaded it will not bend, it will break completely.
