Giant Bicycles Owner's Manual page 38

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 in bent, buckled or folded metal.
It is now common for the main frame to be made of metal and the fork of
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