// MANUAL
failure. It is very important that you read the basics of metal fatigue below. Let’s say you hit a
curb, ditch, rock, car, another cyclist or other object. At any speed above a fast walk, your body
will continue to move forward, 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 metal frame? It
depends on many complex factors, which is why we tell you that crashworthiness cannot be
a design criteria. With that important note, we can tell you that if the impact is hard enough
the fork or frame may be bent or buckled. On a steel bike, the steel fork may be severely bent
and the frame undamaged. Aluminum is less ductile than steel, but you can expect the fork
and frame to be bent or buckled. Hit harder and the top tube may be broken in tension and
the down tube buckled. Hit harder and the top tube may be broken, the down tube buckled and
broken, leaving the head tube and fork separated from the main triangle. When a metal bike
crashes, you will usually see some evidence of this ductility in bent, buckled or folded metal.
It is now common for the main frame to be made of metal and the fork of carbon ber. See
Section B, Understanding composites below. The relative ductility of metals and the lack of
ductility of carbon ber means that in a crash scenario you can expect some bending or bu-
cking in the metal but none in the carbon. Below some load the carbon fork may be intact even
though the frame is damaged. Above some load the carbon fork will be completely broken.
The basics of metal fatigue:
Common sense tells us that nothing that is used lasts forever. The more you use something,
and the harder you use it, and the worse the conditions you use it in, the shorter its life.
Fatigue is the term used to describe accumulated damage to a part caused by repeated
loading. To cause fatigue damage, the load the part receives must be great enough. A crude,
often-used example is bending a paper clip back and forth (repeated loading) until it breaks.
This simple denition will help you understand that fatigue has nothing to do with time or
age. A bicycle in a garage does not fatigue. Fatigue happens only through use. So what kind of
“damage” are we talking about? On a microscopic level, a crack forms in a highly stressed area.
As the load is repeatedly applied, the crack grows. At some point the crack becomes visible to
the naked eye. Eventually it becomes so large that the part is too weak to carry the load that it
could carry without the crack. At that point there can be a complete and immediate failure of
the part. One can design a part that is so strong that fatigue life is nearly innite. This requires
a lot of material and a lot of weight. Any structure that must be light and strong will have a
nite fatigue life. Aircraft, race cars, motorcycles all have parts with nite fatigue lives. If you
wanted a bicycle with an innite fatigue life, it would weigh far more than any bicycle sold
today. So we all make a tradeoff: the wonderful, lightweight performance we want requires
that we inspect the structure.
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