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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.
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 crash
worthiness 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. 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.
Appendix F
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