Operation Manual

66 67
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
dierent 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
lm that protects the metal from further corrosion.
Both are therefore quite resistant to corrosion. The
aluminum alloys GT 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
ber 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.