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.

So how does this apply to bikes?

First, all bikes must be strong enough to avoid damage during heavy use. But this requirement for strength can be met with different levels of stiffness through the selection of different steel alloys and other frame design choices.

It is not a surprise that different steel alloys have different yield stress values. AISI 1040 plain carbon steel has a yield stress of 353 MPa, while Columbus Niobium alloy tubing has a yield stress of 750 MPa — more than twice the strength. Because fancy alloy is twice as strong, tubes with half the thickness will have the same strength. These thinner tubes create a lighter, more responsive bike with no compromise in strength — alloys are cool!

But here is the mind-blowing surprise: All steels have approximately the same stiffness.

Plain carbon steel and the Columbus alloy tubing both have a Young's modulus of approximately 200 GPa. This means that a bicycle made with those thin, but strong Columbus tubes will have 1/2 the stiffness of a bicycle made with our thick plain carbon ones.

Of course different levels of stiffness are required in different areas of the bicycle frame to optimize handling, efficiency, and ride quality.

At Vendetta Cycles, we use our knowledge of materials and structures to determine tubing shape, thickness, alloy, and curvature to create a custom bicycle that meets the rider's needs for both stiffness and for strength.