In my mind, there is no need for concern over the stress cycles of aluminum bicycle frames. Many automobile wheels have been made out of aluminum (alongside the basic steel wheels) for decades and they see stress patterns that far exceed any bike frame and do not commonly suffer from stress fatigue.

Additionally, aluminum has been used to build airplanes for over a century, and historic aluminum airplanes still see flight today. Again, these materials see stresses far greater than any bicycle frame. Boeing, the major manufacturer of passenger planes that see thousands of flights every day, as well as all other major aircraft manufacturers, think enough of the material that anywhere between two thirds and three fourths of any airplane's weight can be attributed to aluminum, including the frame of the plane itself. Frames over 200 feet long. Still worried about it's durability in a bicycle?

Yes, it's true that aluminum does not respond to bending as well as steel. Thus, the response from the manufacturer is to make the frame thicker so that it is more rigid. The same is true for automobile wheels. Steel wheels are much heavier even though they are thinner in design. The aluminum wheels are much thicker, but lighter, and again do not suffer stress related failures very often. Even when the wheels get very old.

Will the aluminum frame or aluminum automobile wheels eventually snap due to fatigue? Possibly, but it's going to be long after any of us are capable of riding or driving. When considering purchasing a new bike, it's essentially a non-issue.

The only argument that can really be solidly applied to steel vs. aluminum is ride quality. Because steel frames can bend and flex, they can often provide a much smoother ride than a rigid aluminum frame.

Forgot to mention also, that aluminum has been used to send people and sattelites into space since the dawn of the space race...with Sputnik. Titan rockets and the current space shuttle's frame are made out of aluminum, specificially because of it's ability to withstand the stresses during launching. Let me put it this way, if NASA thinks aluminum is good enough for the frame of the space shuttle, I think it's safe to say that you don't have to worry about your bicycle frame being made of aluminum. Buy with confidence.
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