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Steel, like
the paper clip, has good ultimate strength, with a much lower
yield strength. This is good, since it means that a steel frame
will bend well before
it breaks, lessening the chance of a disastrous crash. Unlike the paper clip,
steel used for bikes has pretty good elasticity. This combination of properties
has made steel a longtime favourite of frame builders. Steel's only drawback
is its relatively high weight.
Aluminum has recently become a choice
material for frames, because of its very light weight. However,
aluminum has a yield strength very close to its ultimate strength.
In other words, it is quite brittle, and prone to breaking. This
has many dangerous consequences for the rider of an aluminum
bike, so frame makers have responded by over-building aluminum
bikes with very large tubes and thick welds, to lessen the chance
of frame breakage. The wooden pencil has properties similar to
aluminum: light weight, high strength, and brittleness. Wood,
like aluminum, will only bend slightly before breaking.
Other materials, like carbon fibre and
titanium, are similar in qualities to the plastic comb and pen:
light weight, high elasticity, high ultimate strength, relatively
low yield strength. This means that frames made of these materials
need to be designed well, in order to be stiff enough to resist
pedaling forces. It also means that such frames are extremely
light and resilient. However, these materials are extremely expensive,
putting carbon-fibre or titanium frames out of reach for all
but the wealthy or the fanatic racer.
Design
The most popular frame design
is known as the diamond or double- triangle. This design has
changed very little since the advent of the saftey cycle
in the 1880s. Paolo explained, "It's proven to be a great
use of materials, great for bracing angles, great for strength;
it lends itself to being beat up pretty hard and still being
ridable." The strength of the design comes from the triangle
shapes that make up the diamond design. As Paolo explained, "Structurally,
it's quite impressive. If you look at engineers playing with
structures, they tend to come back to triangles and since the
bike is basically three triangles, it works out to be a pretty
strong structure."
While the
diamond design is the core of most cycles built today, some
frame builders are experimenting with new variations on this
classic design. For example, some carbon-fibre frames are being
made with oval tubing, making the cycle more aerodynamic.
New full-suspension bikes have altered the diamond design to
allow for a large shock to be mounted on the seat stem. However,
most changes to the design are more subtle and have to do with
maximizing performance for different types of terrain or uses.
Single-Track
and Fire Roads
It's easy to see the differences
between a road bike and a mountain bike. However, it's sometimes
harder to notice the differences between different models
of cycles built for the same type of surface. For example
some mountain bikes have a long wheel base making them more
stable at higher speeds but incapable of making tight turns.
Bikes like this would perform well on wide-open fire roads,
but would have difficulty making tight turns on single-track
trails. The length of the wheel base, the slope of the top
tube, and other differences in frame design all play a role
on the road or the trail.
Stress on the
Frame
Cycle frames have to be built
to handle a variety of loads. First, the frame needs to support
itself and other components of the cycle. These are considered
static loads. In addition, the frame needs to be able to
handle the cyclist's weight, the forces of pedaling and braking,
and the effects of the road's surface. These are dynamic
loads; they are the most problematic for a frame builder
since, as the name implies, they move and vary in intensity.
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