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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 carbon fiber. See Section B,
Understanding composites below. The relative ductility of metals and the lack of ductility of carbon fiber
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.
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