The short answer is:
Any properly designed bike will last long enough that you don't need to
worry about it.
The long answer is:
There are two factors that need to be considered: yield stress and
fatigue life. The yield stress is the point where the frame undergoes
a force that is sufficient to cause a permanent deformation (an example
would be crashing and bending the frame). Once the yield stress has
been exceeded, the frame has been permanently damaged. A steel frame
can sometimes be bent back into shape without adverse effects, however,
bending aluminum back into shape is not advisable.
Since normal bicyle operation does not to exceed the yeild stress, the
only other concern is fatigue life. As a material is flexed within its
elastic limit (ie the stresses are below the yield point), it can
eventually fail due to fatigue. Steel has a fatigue limit which is a
level of stress below which fatigue failure will take an essentially
infite amount of time. Aluminum does not have such a limit. However,
well-made bicycles (steel or aluminum) should be designed such that
fatigue failures should not occur within the bicyle's lifetime under
reasonable operating conditions - if you ride 100,000kms/yr, you might
eventually be able to fatigue a frame.
Once a frame fails by fatigue, it does not go "soft", despite what many
cycling publications that I have seen claim. The stiffness of a frame
depends only on the materials and geometry - time and excessive use
have absolutely no effect on stiffness.
Clear? :-)
-Adrian
(Aerospace Engineering IV, Carleton U)
--- Perry McKenna <[EMAIL PROTECTED]> wrote:
> Hello all,
>
> I would like to know what is the life expectancy of the different
> frames material. I have been getting conflicting info. One book
> (the new Lance Armstrong) says that aluminum is only good for 5
> years. I asked my bike shop and they told me that Aluminum can fail
> after a while, but that it depends on how hard and how much you ride
> but that you should be able to get a lot more than five years.
>
> They also informed me that steel, will also weaken after about five
> years(based on riding about 5000 km a year) and will no longer be
> very stiff after that point. I always thought that steel would last a
> lot longer than that.
>
> Does anyone know the facts?
>
> Perry
>
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