Stiffness vs Strength
Stiffness and Strength — same thing, right?
Stiffness and strength are two of the most commonly confused terms in the bicycling world, and many people use them interchangeably. But these are two different things, and building the best-performing frame requires knowing the difference.
So let's start with the engineering definitions:
Strength:
A measure of the maximum load that can be placed on a material before it permanently deforms or breaks. Engineers often use this as yield stress, σy, as a measure of a material's strength.
Stiffness:
A measure of the amount of deflection that a load causes in a material. Engineers use a value called Young's modulus, E, for stiffness.
“All steels have approximately the same stiffness.”
It is easy to see why these two terms would be confused. Something that is 'flimsy' might break when a small load is placed on it — it has low strength — and it might also deflect a large amount with the same load — it has low stiffness.
But these two terms aren't interchangeable.
A piece of rubber surgical tubing has very low stiffness because it deflects a lot under load, but it is relatively strong. A piece of glass filament is the opposite — it deflects very little under load but might not carry a huge load before it breaks.http://www.vendettacycles.com/vendettacycles/stiffness.htm
