Giant Bicycles - Page 39

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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 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 denition 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 innite. 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 innite 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.
A few things to think about
• ONCE A CRACKS 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
crack, replace the part.
• CORROSSION SPEEDS DAMAGE. Cracks grow more
quickly when they are in a corrosive environment.
Think about the corrosive solution as further weaken-
ing and extending the crack.
SIMPLE RULE 2 : Clean your
bike, lubricate your bike,
protect your bike from salt,
remove any salt as soon as
you can.
• STAINS AND DISCOLORATION CAN OCCUR NEAR A
CRACK. Such staining may be a warning sign that a
crack exists.
SIMPLE RULE 3 : Inspect and
investigate any staining to
see if it is associated with a
crack.