(Jim Plaia) wrote in
If I'm correct about that, frame stiffness is again a function of
modulus and does not include tensile strength at all.

The desirable types of stiffness in a bike frame come from
increasing rigidity in bending and torsion. These increase rapidly
with increases in diameter. Higher tensile strength steels allow
higher diameter to wall thickness ratios. OX Platinum is available in
1.375" diameter that is only 0.016" thick between the butts. In 4130
such a tube would dent or buckle too easily to be practical.

there isn't a steel in existence that matches aluminum or titanium in
modulus to weight ratio.

Steel and Aluminum have almost identical modulus to weight ratios.
Steel's is pretty constant. Aluminums have some variation but the
average is about the same as steel. Titanium lags well behind at
about 87% of steel. Aluminum will build a stiffer frame for a given
weight, only because you can use larger diameters.

Example; If an aluminum tube has a 20% larger OD can have walls twice
as thick as a steel one, for dent and buckling resistance, and still
be 11% stiffer yet weigh only 82% as much.

If OD is constrained, steel wins, as the extra bulk of the aluminum
is closer to the middle of the tube where it is less effective.

Don't get me wrong, I've no desire to move into exotic steels, Al, CF
or Ti. I built a 4130 frame with a 60" wb that weighs 4.5lbs. and is
stiff as granite against pedalling forces, yet nicely compliant
vertically. An Al frame with this much vertical flex would fail rather
soon IMHO.

BTW True Temper has found that their non-air hardening, heat treated
steels retained a higher percentage of their strength after welding
than their non-HT tubes.