Handleiding Kona 2014 (pagina 39 van 42) (Engels)

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KONA OWNER’S MANUAL

will continue to move forward, momentum carrying you

over the front of the bike. You cannot and will not stay on

the bike, and what happens to the frame, fork and other

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 evi-

dence 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 ber. See Section B, Understanding

composites below. The relative ductility of metals and

the lack of ductility of carbon ber 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 denition 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 innite. This requires a lot of material and a lot of

weight. Any structure that must be light and strong will have a

nite fatigue life. Aircraft, race cars, motorcycles all have parts

with nite fatigue lives. If you wanted a bicycle with an innite

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

In most cases a fatigue crack is not a defect. It is a sign

that the part has been worn out, a sign the part has

reached the end of its useful life. When your car tires

wear down to the point that the tread bars are contact-

ing the road, those tires are not defective. Those tires are

worn out and the tread bar says “time for replacement.”

When a metal part shows a fatigue crack, it is worn out.

The crack says “time for replacement.”

Fatigue Is Not A Perfectly Predictable Science

Fatigue is not a perfectly predictable science, but here are

some general factors to help you and your dealer deter-

mine how often your bicycle should be inspected. The more

you t the “shorten product life” prole, the more frequent

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