Operation Manual

THIS IS A
BENT
METAL FORK.
THIS IS A
COMPLETELY BROKEN
CARBON FORK.
FIGURE A
68 69
PART II
When all metal bikes are crashed 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 ber. See the composites
101 section below. The relative ductility of metals
and the lack of ductility of carbon ber 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.
Metal Fatigue 101
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 denition 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 same load that, without
the crack, it could carry. 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 innite. This requires a lot of material and
a lot of weight. Any structure that must be light and
strong will have a nite fatigue life. Aircraft, race cars,
motorcycles: all have parts with nite fatigue lives. If
you wanted a bicycle with an innite fatigue life, it
would weigh far more than any bicycle sold today. So
we all make a trade-o: the wonderful, lightweight
performance we want requires that we inspect the
structure.
In most cases a fatigue crack is not a defect. It is a
sign that the part has been worn out, a sign the part
has reached the end of its useful life. When your car
tires wear down to the point that the tread bars are
contacting the road, those tires are not defective.
Those tires are worn out and the tread bar says
“time for replacement.” When a metal part shows a
fatigue crack, it is worn out. The crack says “time for
replacement.”