70
Perspective
70
The lifespan of your bike and its components
Perspective
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 crashworthi-
ness 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 duc-
tile 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 usu-
ally see some evidence of this ductility in
bent, buckled or folded metal.
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 dam-
age, the load the part receives must be
great enough. A crude, often-used exam-
ple is bending a paper clip back and forth
(repeated loading) until it breaks. This sim-
ple defi 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
load that it could carry without the crack.
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 fi nite fatigue life. Aircraft, race
cars, and motorcycles all have parts with
fi nite fatigue lives. If you wanted a bicycle
with an infi nite fatigue life, it would weigh
far more than any bicycle sold today. So
we all make a tradeoff: the wonderful, light-
weight performance we want requires that
we inspect the structure.
