Atomic weights do not equate to densities of metals. What controls the density of metal is valence in a pure crystal and crystal structure, neither of which appear on the periodic table. While it is correct to assume Lead is denser than Magnesium, Zinc is less dense than Iron, Cerium is much less dense than Gold, rather than a little....


Right:

Carbon: Close enough for an entry level understanding. Some newer thermoplastic matrix composites are demonstrating far superior fracture toughnesses to the older epoxy systems, but still essentially brittle and stiff for their density.

Aluminium: Pure aluminium has moderate fracture toughness and poor mechanical strength. It's work hardening rate is low. Aluminium alloys fall into two camps. The only ones that cyclists are likely to see have to be two-stage heat treated. They can be readily joined by fusion wedling techniques in the majority. Their primary benefit is their low density, which enables the contruction of larger diameter tubes at the same mass and wall thickness, leading to greater longitudinal and rotational stiffness in the tube. Aluminium frame is likely to be more rigid (not stiffer - stiffness is a material parameter, not a geometrical parameter). This has given aluminium alloys a reputation for harsh rides.
Aluminium alloys also do not rust. This does not mean that they do not corrode. Indeed, their corrosion rate in salt water is higher than steels. This performs both an aesthetic and structural function in aluminium frames.

Steels: Two thousand years of development cannot be addressed in a paragraph. Highest stiffness, highest toughness, highest strength of all the alloy families used in frames. Downsides - rusts, just under three times as dense as aluminium alloys, so frames of equal stiffness are heavier than aluminium. Can be joined by conventional and esoteric fusion welding, filler-metal soldering and thermal bonding processes. Metallurgy is still contantly being evolved as levels of impurities have least effect on steels, so ever-superior refining techniques lead to ever-greater mechanical properties. Almost all manufacturers of steels for bicycle frame tubing use the same ranges of alloys, contrary to some beliefs. They are either based on the iron-carbon-chromium-molybdenum system or the iron-carbon-manganese-molybdenum system. With the exception of stainless steels, all alloys are derived from either one or the other.

I have seen frames in all three break. There is no such thing as an indestructible frame, but the severity of the failure and the warning you get before it varies between materials.

No such thing. Fatigue life is rated at a stress, and as such varies above that stress. And titanium alloys do not have higher fatigue lives than steels.


As simple as I could make it