Owner`s manual

30
Cervélo Owner’s Manual
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 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.
A FEW THINGS TO THINK ABOUT:
• 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.
SIMPLE RULE 1: If you find a crack, replace the part.