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, another cyclist 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 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
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