Specialized BICYCLE Owner's Manual page 49

in bent, buckled or folded metal.
It is now common for the main frame to be made of metal and the fork of
carbon fiber. See Section B, Understanding composites below. The relative
ductility of metals and the lack of ductility of carbon fiber means that in a crash
scenario you can expect some bending or bucking 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 definition 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 infinite. This
requires a lot of material and a lot of weight. Any structure that must be light
and strong will have a finite fatigue life. Aircraft, race cars, motorcycles all have
parts with finite fatigue lives. If you wanted a bicycle with an infinite 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.
What to look for
• ONCE A CRACKS STARTS IT CAN GROW AND
GROW FAST. Think about the crack as forming a path-
way to failure. This means that any crack is potentially
dangerous and will only become more dangerous.
• CORROSSION 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.
• STAINS AND DISCOLORATION CAN OCCUR NEAR
A CRACK. Such staining may be a warning sign that a
crack exists.
SIMPLE RULE 1 : If you find
crack, replace the part.
SIMPLE RULE 2 : Clean
your bike, lubricate your bike,
protect your bike from salt,
remove any salt as soon as
you can.
SIMPLE RULE 3 : Inspect and
investigate any staining to see
if it is associated with a crack.
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