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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