42
43
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.”
Fatigue Is Not A Perfectly Predictable Science
Fatigue is not a perfectly predictable science, but here are some general factors to
help you and your dealer determine how often your bicycle should be inspected. The
more you fit the “shorten product life” profile, the more frequent your need to inspect.
The more you fit the “lengthen product life” profile, the less frequent your need to
inspect.
Factors that shorten product life:
•
Hard, harsh riding style
•
“Hits”, crashes, jumps, other “shots” to the bike
•
High mileage
•
Higher body weight
•
Stronger, more fit, more aggressive rider
•
Corrosive environment (wet, salt air, winter road salt, accumulated sweat)
•
Presence of abrasive mud, dirt, sand, soil in riding environment
Factors that lengthen product life:
•
Smooth, fluid riding style
•
No “hits”, crashes, jumps, other “shots” to the bike
•
Low mileage
•
Lower body weight
•
Less aggressive rider
•
Non-corrosive environment (dry, salt-free air)
•
Clean riding environment
WARNING: Do not ride a bicycle or component with any crack, bulge or dent,
even a small one. Riding a cracked frame, fork or component could lead to
complete failure, with risk of serious injury or death.
B. Understanding composites
All riders must understand a fundamental reality of composites. Composite materials
constructed of carbon fibers are strong and light, but when crashed or overloaded,
carbon fibers do not bend, they break.
What Are Composites?
The term “composites” refers to the fact that a part or parts are made up of different
components or materials. You’ve heard the term “carbon fiber bike.” This really means
“composite bike.”
Carbon fiber composites are typically a strong, light fiber in a matrix of plastic,
molded to form a shape. Carbon composites are light relative to metals. Steel weighs
7.8 grams/cm
3
(grams per cubic centimeter), titanium 4.5 grams/cm
3
, aluminum 2.75
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 pathway 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.
• SIGNIFICANT SCRATCHES, GOUGES, DENTS OR SCORING
CREATE STARTING POINTS FOR CRACKS. Think about the cut
surface as a focal point for stress (in fact engineers call such
areas “stress risers,” areas where the stress is increased).
Perhaps you have seen glass cut? Recall how the glass was
scored and then broke on the scored line.
SIMPLE RULE 4 : Do not scratch,
gouge or score any surface. If you
do, pay frequent attention to this
area or replace the part.
• SOME CRACKS (particularly larger ones) MAY MAKE
CREAKING NOISE AS YOU RIDE. Think about such a noise as
a serious warning signal. Note that a well-maintained bicycle
will be very quiet and free of creaks and squeaks.
SIMPLE RULE 5 : Investigate
and find the source of any noise.
It may not a be a crack, but
whatever is causing the noise
should be fixed promptly.
