lead to failure. It is very important that you read The basics of metal fatigue below.
Let’s say you hit a curb, ditch, rock, car , a nother c yclist or other object . At
any speed above a fast walk, your body 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 e xpect fro m your met al frame? It depends on m any
comple x factors, which is w hy we te ll 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 fo rk and frame to be be nt or b uckled. Hit harder and the to p tube ma y be broken i n
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 ductilit
y in bent, buckled or folded metal.
It is no w c ommon for the main frame to be made of met al and the fork of carbon fiber . See Section B, Underst anding
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 nothin
g that is used last
s 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 accum ulated 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 lev
el, 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 p art is too weak to carry the lo ad that it could carr y without the cr ack. At that poi nt ther e can be a complete an d
immediate failure of the part.
One can design a part that is so strong that fatigue life is ne
arly 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 i nfinite fatigue life, it would weigh far more than any bicycle sold today. So we all make a
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