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PART II
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 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 breakage. It is very
important that you read Metal Fatigue 101 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 crash worthiness 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.
See Figure A on page 68. 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.
