Giant momentum Anleitung Seite 36

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 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 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.
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, and
remove any salt as soon as you can.
SIMPE 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.
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