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KONA OWNER’S MANUAL
39
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
.
In most cases a fatigue crack is not a defect. It is a sign
that the part has been worn out, a sign the part has
reached the end of its useful life. When your car tires
wear down to the point that the tread bars are contact-
ing the road, those tires are not defective. Those tires are
worn out and the tread bar says “time for replacement.
When a metal part shows a fatigue crack, it is worn out.
The crack says “time for replacement.
Fatigue Is Not A Perfectly Predictable Science
Fatigue is not a perfectly predictable science, but here are
some general factors to help you and your dealer determine
how often your bicycle should be inspected. The more you
fall within the "Factors that shorten product life" column
above, the more frequent your need to inspect. The more
you t within the “lengthen product life” column, the less
frequent your need to inspect
.
WARNING: Do not ride a bicycle or component with
any crack, bulge or dent, even a small one. Riding a
cracked frame, fork or component could lead to com-
plete failure, with risk of serious injury or death.
B) UNDERSTANDING COMPOSITES
All riders must understand a fundamental reality
of composites: composite materials constructed of
carbon bers are strong and light but when crashed or
overloaded, carbon bers do not bend, they break.
What Are Composites?
The term “composites” refers to a part or parts being
made up of different materials. You’ve heard the term
“carbon ber bike.” This really means “composite bike.
Carbon ber composites are typically a strong, light ber
in a matrix of plastic, molded to form a shape. Carbon
composites are lightweight relative to metals. Steel
weighs 7.8 grams/cm3 (grams per cubic centimeter),
titanium 4.5 grams/cm3, aluminum 2.75 grams/cm3. Con-
trast these numbers with carbon ber composite at 1.45
grams/cm3.
The composites with the best strength-to-weight ratios
are made of carbon ber in a matrix of epoxy plastic. The
41


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