37
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 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 trade-off: the
wonderful, lightweight performance we want requires that we inspect the structure.
What to look for:
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.”
• 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.
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