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Marin Bikes Manual

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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
carbon ber. See Section B, Understanding composites below. The relative
ductility of metals and the lack of ductility of carbon ber 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.
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 lm
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 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 failure. It is very
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