Specialized Roubaix Comp Road Manual De Instrucciones página 50

When a metal bike crashes, you will usually see some evidence 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.
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.
• 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.
• STAINS AND DISCOLORATION CAN OCCUR NEAR
A CRACK. Such staining may be a warning sign that a crack
exists.
• 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 1 : If you find
crack, replace the part.
SIMPLE RULE 2 : Clean your
bike, lubricate your bike, protect
your bike from salt, remove any
salt as soon as you can.
SIMPLE RULE 3 : Inspect and
investigate any staining to see if it
is associated with a crack.
SIMPLE RULE 4 : Do not
scratch, gouge or score any
surface. If you do, pay frequent
attention to this area or replace
the part.
45