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Titanium Use and Abuse
Titanium's amazing strength, light weight and exotic origins have created a bizarre
mythology, and led to its appearance in some odd places. As with any material, there are
good applications and bad applications. The trick is to use titanium in the right place
for the right reason.
Some of 3-2.5 titanium's strengths are:
- Excellent fatigue strength (twice that of 4130 steel)
- High strength-to-weight ratio
- Excellent elongation (ductility) of 15-30%
- Excellent corrosion resistance
Titanium's high fatigue strength gives the designer a wide latitude in choosing how the
bicycle will perform. A frame can be made relatively resilient or very stiff, depending
on the need, simply by modifying the thickness and shape of the tubes.
Unfortunately, there are many areas on a bicycle that have design constraints, due to the
use of standardized components. Most of the geometries used in bicycle tubing were created
to exploit the best properties of the steels available 40 or so years ago. Today, any deviation
from those standards requires an enormous commitment of energy and resources to convince
component manufacturers that a change is necessary, and retail dealers that it is worthwhile
to carry a separate inventory of non-standard replacement parts.
Nevertheless, there is no current frame application that is not well suited to titanium,
assuming the designer has the freedom to specify an appropriate tubing geometry. In areas of
the bike where design latitude is restricted, the advantage is not always as great.
Forks are a good example of an area where geometry restrictions bias the material
application toward steel. Assuming the designer is restricted to a one-inch steerer, and
the goal is to create a titanium steerer as stiff as its steel counterpart, the titanium
steerer will have to weigh over 60% more than the steel equivalent.
Increasing the size of the steerer and headset does not necessarily improve the equation.
With a 1.25-inch headset, a titanium steerer is roughly 25% lighter than a steel steerer
of equal stiffness. However, the 1.25-inch headset is heavier than a 1-inch version, and
the larger head tube required is also heavier. Apart from expense, there is no net gain.
These complications occur because titanium's modulus, or stiffness, is roughly half that
of steel (given identical tube cross sections). To explain the steerer issue another way:
Doubling the wall thickness of a given tube almost doubles its bending stiffness. That is,
the relationship is close to linear. However, doubling the diameter of the same tube-without
altering wall thickness at all-increases the bending stiffness by the third power, or
roughly 800%!
Thus, the most efficient way to increase the stiffness of any metal is to enlarge the
diameter, not the wall thickness. Of course, there is a limit to diameter increases versus
wall thinning; if the ratio between diameter and wall becomes too great, the tube will
collapse under pressure, like an aluminum can.
When designers run up against diameter and wall thickness limitations, they often turn to
shape manipulation as a way to locally strengthen the tube. Flaring, ovalizing, and
tapering are common strategies, but, as we will see in the following section, each has
significant limitations and problems.
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