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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
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