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MEC BIKE MANUAL APPENDIX
52
What to look for
Once a crack 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 a crack,
replace the part.
Corrosion 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.
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.
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
tradeo: the wonderful, lightweight performance we want requires that we
inspect the structure.
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