Jamis Bicycle Owner's Manual - page 28
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Metals are subject to fatigue. With enough cycles of use, at high enough loads, metals will eventually develop cracks that 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.
Periodic, more detailed inspection of your bicycle is important. How often this more detailed inspection is needed depends upon you.
You, the rider/owner, have control and knowledge of how often you use your bike, how hard you use it and where you use it. Because
your dealer cannot track your use, you must take responsibility for periodically bringing your bike to your dealer for inspection and
service. Your dealer will help you decide what frequency of inspection and service is appropriate for how and where you use your bike.
For your safety, understanding and communication with your dealer, we urge you to read this Appendix in its entirety. The materials
used to make your bike determine how and how frequently to inspect.
Ignoring this WARNING can lead to frame, fork or other component failure, which can result in serious injury or death.
A. 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 by cycling enthusiasts in lighter bicycles.
Properties of Metals
Please understand that there is no simple statement that can be made that characterizes the use of different metals for bicycles. What is true
is how the metal chosen is applied is much more important than the material alone. One must look at the way the bike is designed, tested,
manufactured, supported along with the characteristics of the metal rather than seeking a simplistic answer.
Metals vary widely in their resistance to corrosion. Steel must be protected or rust will attack it. Aluminum and Titanium quickly develop an
oxide film that protects the metal from further corrosion. Both are therefore quite resistant to corrosion. Aluminum is not perfectly corrosion
resistant, and particular care must be used where it contacts other metals and galvanic corrosion can occur.
Metals are comparatively ductile. Ductile means bending, buckling and stretching before breaking. Generally speaking, of the common bicycle
frame building materials steel is the most ductile, titanium less ductile, followed by aluminum.
Metals vary in density. Density is weight per unit of material. Steel weighs 7.8 grams/cm3 (grams per cubic centimeter), titanium 4.5 grams/
cm3, aluminum 2.75 grams/cm3. Contrast these numbers with carbon fiber composite at 1.45 grams/cm3.