Bedienungsanleitung Cervelo Multi-speed racing bicycles (Seite 34 von 40) (Englisch)

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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.

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Cervelo Multi-speed racing bicycles Seite 34

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