Kona bicycle Owner's Manual page 39

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 evi-
dence 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 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.
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 deter-
mine how often your bicycle should be inspected. The more
you fit the "shorten product life" profile, the more frequent
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