Bonj, you really are coming out with some total guff.
Both steel and aluminium alloys are suitable materials for frame construction. Different types of frames have different requirements. E.g.a track racing frame might be made to be extremely stiff for power transfer and an audax bike springier for long distance comfort.
As far as your comment about flexing – all materials flex. This is a function of their modulus of elasticity. Fatigue life is not directly due to flexing but due to cyclical stresses (which may come from flexing - bending but could equally come from tension / compression).
The feel of a bike is much more down to its geometry than the material it is made from. Large diameter tubing will be much stiffer than skinny tubes.
However, as User482 has already pointed out, there are distinct differences in the mechanical properties of steel – particularly modulus of elasticity, density, corrosion resistance and the fatigue limit.
Aluminium alloys have a third the density of steel alloys. So you might expect al frames to be a third the weight of steel frames. But this is not the case. Why? Because al is a much less strong material (lower modulus of elasticity) and so cannot withstand the same stresses as steel frames without failing. Also steel frames typically have very thin tube walls and al frames with this thickness tubing would dent easily.
So al frames are made with thicker walls and using oversized tubing (to limit the internal stresses in the frame). So al frames have more material in them than steel ones, hence them only being slightly lighter than good steel frames despite being made of a much lighter material.
Both al and steel frames corrode. Aluminium is actually a very reactive metal and corrodes very rapidly but it has the advantage that its oxide is very adherent so after the initial surface corrosion then rate of corrosion slows down to almost nothing. High alloys steels are less corrosion resistant for general use. However, aluminium oxide is not stable in the presence of salts and will suffer badly from localised corrosion. So it is important that you keep all bikes clean!
Finally, steel has a ‘fatigue limit’ whereas aluminium doesn’t. The springs eluded to before are operated below their fatigue limit stress and so can flex forever without breaking. The paperclips are operated above their fatigue limit stress and will fail rapidly. A properly designed steel frame will operate below its fatigue limit so will NEVER fail through fatigue. Aluminium frames will ALWAYS have the potential to fail through fatigue as they have no fatigue limit and every fatigue cycle they endure brings them closer to their final failure. However, if you design the al frame so that the stress it sees is kept to a minimum (through OS tubes, thick tubes) then it may be that you can extend the frame’s fatigue life such that it will be replaced due to fashion or crashing before it fails due to fatigue.
In summary, you can make a bike out of anything.
Steel frames will probably be slightly heavier than al frames, have thinner section tubes, probably thinner diameter tubes, be springier, corrode more easily unless looked after (eg painted and frame sealed) and will never fail due to fatigue.
Aluminium alloy frames will probably be slightly lighter than steel frames, they will be much stiffer as they need oversized tubes with thick walls for any sort of longevity, very corrosion resistant if looked after (washed regularly) and will eventually always fail due to fatigue if not replaced beforehand.
If you are interested in this sort of stuff there is quick a good chapter on it in the book Bicycling Science.
