Originally Posted by
Kimmo
Is it just too hard to build a fork light and strong out of aluminium? I'm sure it's possible, but why haven't I seen one, isn't it feasible?
It's possible, but thay wouldn't be very durable.
Aluminum alloys do not have a
fatigue limit. Even the slightest cyclic stresses in aluminum structures add up, so that eventually fatigue failure occurs. Designers compensate for this physical property of the material by beefing up the structure, but that makes it heavy. Lack of fatigue limit is the reason pressurized aluminum airframes are retired after a specified number of pressurization/depressurization cycles.
Ferrous alloys (steels) and titanium alloys do have fatigue limits; a virtually infinite number of cyclic stresses that are below the fatigue limit amplitude can be imposed on structures made of these materials and they won't fail from fatigue.
Since forks are continuously subjected to cyclic stresses when the bike is being ridden, aluminum as a material for fork blades is a poor choice where light weight is a requirement.