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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.
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. Aircraft, race cars, motorcycles all have
parts with finite fatigue lives. If you wanted a bicycle with an infinite fatigue life,
it would weigh far more than any bicycle sold today. So we all make a tradeoff:
the wonderful, lightweight performance we want requires that we inspect the
structure.
What to look for
• ONCE A CRACKS STARTS IT CAN GROW AND
GROW FAST. Think about the crack as forming a path-
way to failure. This means that any crack is potentially
dangerous and will only become more dangerous.
SIMPLE RULE 1 : If you find
crack, replace the part.
• CORROSSION SPEEDS DAMAGE. Cracks grow
more quickly when they are in a corrosive environment.
Think about the corrosive solution as further weakening
and extending the crack.
SIMPLE RULE 2 : Clean
your bike, lubricate your bike,
protect your bike from salt,
remove any salt as soon as
you can.
• STAINS AND DISCOLORATION CAN OCCUR NEAR
A CRACK. Such staining may be a warning sign that a
crack exists.
SIMPLE RULE 3 : Inspect and
investigate any staining to see
if it is associated with a crack.
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.
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