Thursday, 10 December 2009

Steel frame is softer than aluminium one?

Modulus of elasticty, brittleness, strength, stiffness, elastic limit, are standard terms of the theory of elasticity. They are, however, more often than not misinterpreted when used in "technical" debates about bicycles. Many times they are abused as a "scientific evidence" of some natural feel like: "steel is softer then aluminium". Softness, by the way, is not a standard quantity in the theory of elasticity. It can be interpreted as a reciprocal quantity to stiffness. The measure of stiffnes of a material is its modulus of elasticity, or Young modulus, E, which is defined as a ratio of stress vs. strain. In that sense the common bicycle myth that steel is softer than aluminium is a total nonsense. It is not true that steel has relatively low modulus of elasticity, i.e. is a "soft material". In fact its modulus of elasticity (Young modulus) is the highest of all commonly used engineering materials: it is 210 GPa, which is 3 times greater than aluminium (with 69 GPa), 1.75 times greater than titanium (120 Gpa) and at least 1.4 greater than carbon fiber (150 GPa). You can find these figures in a table on Wikipedia page: Steel is the stiffest material to make a frame of, unless you are considering a tungsten or a diamond one.

It is perfectly OK with me if someone feels a steel frame to be softer than aluminium. But please don't try to prove this by misinterpretation of some half-understood theory.

P.S. I am not a "steel-hater". Steel is the best engineering material for most applications because of its strength, stiffness, ductility, fatigue strength, weldability and price - but certainly not because of it's softness.


  1. In material terms, yes, aluminum is much more flexible than steel. If one bike frame were made with tubes of steel and another were made with tubes of aluminum in tubes of exactly the same diameter and thickness, then the aluminum frame would be much more flexible. It would also fail much sooner, since aluminum is both weaker and has no defined fatigue limit. In order to go around these liabilities, builders make aluminum frame with much larger diameter tubes. This, in general, makes aluminum frames considerably stiffer than steel frames. The material weight matters, but so does the implementation.

  2. The above comment says it all really. It is not the inherent elasticity of the materal that determines the 'softnessss' of the ride, but the structural quality of the frame. Given its greater strength steel can be drawn with thinner diameter walls and with narrower tubing. The thicker tube dameter needed for aluminium increases stiffness by the power of 3. Any tube diameter thickness radically increases stiffness.

  3. Yes, but the bicycle frame is - in engineering terms - structurally essentially a truss and not a frame. That means that axial forces are much more important then bending forces. And tube diameter doesn't affect resistance of a tube against axial force (other then its buckling strength).

  4. Even so, assuming they are each designed to withstand the same amount of stress (via thicker aluminium tubes), steel can still withstand ~3x the strain.

    The same principles (albeit for bending not axial loads) are what make carbon fibre or fibre glass feasible, by reducing the cross sectional area of a normally stiff material you can make it bendy.