Understanding Metals - Charge City Owner's Manual

Inspect For Safety
Charge Bicycle Owner's Manual

Understanding Metals

Understanding Metals
Steel is the traditional material for building bicycle frames. It has good characteristics, but in high
performance bicycles, steel has been largely replaced by aluminum and some titanium. The main
factor driving this change is interest in lighter bicycles by cycling enthusiasts.
When a metal bike crashes, you will usually see some evidence of this ductility in bent, buckled or
folded metal. It also is now common for the main frame and other bicycle components to be made
from composite materials, also called carbon fiber components. See "Understanding Composites. "
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.
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 trade off: the
wonderful, lightweight performance we want requires that we inspect the structure.
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.
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