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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
pathway to failure. This means that any crack is
potentially dangerous and will only become more
dangerous.
SIMPLE RULE 1 : If you find
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 weakening
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.
• SIGNIFICANT SCRATCHES, GOUGES, DENTS OR
SCORING CREATE STARTING POINTS FOR
CRACKS. Think about the cut surface as a focal point
for stress (in fact engineers call such areas “stress
risers,” areas where the stress is increased). Perhaps
you have seen glass cut? Recall how the glass was
scored and then broke on the scored line.
SIMPLE RULE 4 : Do not
scratch, gouge or score any
surface. If you do, pay
frequent attention to this area
or replace the part.
• SOME CRACKS (particularly larger ones) MAY MAKE
CREAKING NOISE AS YOU RIDE. Think about such a
noise as a serious warning signal. Note that a well-
maintained bicycle will be very quiet and free of creaks
and squeaks.
SIMPLE RULE 5 : Investigate
and find the source of any
noise. It may not a be a crack,
but whatever is causing the
noise should be fixed
promptly.
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