you are right of course. a lump of steel is 3 times stiffer than a lump of aluminium. but it’s what we do with it that’s important;
Aluminium has very poor fatigue properties, if it flexes, it will fail
(everything flexes).
Steel is great for making springs, at stress levels aproximately half the yield strength a steel spring will last forever
(not even titanium does this).
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Steel frames can be built from skinny tubes which are flexible.
Aluminium frames have to be built from large diameter tubes which are stiffer.
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the stiffness of a tube has as much to do with it’s shape/size as it’s material.
if you built 2 identical frames, one from steel, the other from ally, the steel frame would be 3 times stiffer, but also 3 times heavier.
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We’ve decided that a bicycle frame should weigh 2ish Kg, and with that amount of material, An ally frame is either oversized/stiff, or normal/skinny/flexy and broken through fatigue. whereas a steel frame is either normal/skinny/flexy, or oversized/stiff and paper-thin, and broken though terminal denting.
An aluminium frame WILL fail, eventually. A steel frame might just last forever.
personally, i love steel bikes.