Originally Posted by VT to CA
Since you're looking at a new bike with a 1" headtube, I'm betting it's steel- if you're looking at steel, then stiffness is kind of out the window anyway- great material, very strong- but flexy by nature.
There is a material characteristic knows as the Modulus of Elasticity which defines how stiff a material is. Steel is twice as stiff as Ti and three times aluminum so basically it should be said that steel is "stiff by nature".
The stiffness of a frame tube is a combined factor where the Modulus of Elasticity of the material is multiplied by a
shape factor known as the Moment of Inertia. The stiffness goes up greatly as the diameter increases. Bottom line is that steel frames can be either stiff or flexi - depends on the Moment of Inertia of the tubes the builder chooses.
The reason many steel frames on on the compliant side of the equation is because a) they can be (unlike aluminum) and b) the material is density challenged which drives builders to use the thinnest and lightest tubes available thus making the frame more flexible.
Nothing inherently flexible about steel.