131
PART 2
1. Understanding Metals
Section D. Inspect For Safety
GT Bicycle Owner’s Manual
With that important note, we can tell you that if
the impact is hard enough the fork or frame may
be bent or buckled.
See Figure A on following page. .
On a most all steel bikes, 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 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 fiber. See the
composites 101 section 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.
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 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 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 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.
