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A — Understanding metals
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 paperclip 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 may 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 ma-
terial and a lot of weight. Any structure that must be light and strong will have a nite fatigue life.
Aircraft, race cars, and 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
tradeo: the wonderful, lightweight performance we want requires that we inspect the structure.
What to look for
• ONCE A CRACK STARTS IT CAN GROW AND GROW FAST. Think
about the crack as forming a pathway to failure. This means that
any crack is potentially dangerous and will only become more
dangerous.
SIMPLE RULE 1: If you nd a
crack, replace the part.
• CORROSION SPEEDS DAMAGE. Cracks grow more quickly when
they are in a corrosive environment. Think about the corrosive
solution as further weakening and extending the crack.
SIMPLE RULE 2: Clean your
bike and protect it from salt,
remove any salt as soon as
you can.
frequency of inspection and service is appropriate for how and
where you use your bike. For your safety, understanding, and com-
munication with your dealer, we urge you to read this Appendix in
its entirety. The materials used to make your bike determine how
and how frequently to inspect. Ignoring this WARNING can lead to
frame, fork, or other component failure, which can result in serious
injury or death.
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