Understanding Metals - Cannondale Chase Series Owner's Manual

1. 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
enthusiast customers 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.
The 6000 series aluminum alloys Cannondale
has used for years are commonly used in marine
applications. 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 .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 Metal Fatigue 0
next.
Let's say you hit a curb, ditch, rock, car, fallen
cyclist or other object. First, read the Important
warnings at the beginning of SECTION A: Bicycles
Cannot Protect You. 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 and fork 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 most all steel
bikes, 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.
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