The Lifespan of Your Bicycle
& its Components
cont’d
It is now common for the main frame to be made of metal and the fork of carbon fiber.
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 buckling in the metal but none in the carbon fiber.
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.
C
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
lifespan.
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 paperclip 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 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 trade-off: the wonderful, lightweight performance we want requires that we inspect
the structure.
What To Look For
ONCE A CRACK 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.
RULE OF THUMB #1: If you find a crack, replace
the part.
CORROSION 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.
RULE OF THUMB #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.
RULE OF THUMB #3: Inspect and investigate
any staining to see if it is associated with a
crack.
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Summary of Contents for KC720-220
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