Bicycle Stiffness: Why Does It Matter?

What is stiffness?

When talking about stiffness in bicycle frames and most other components found on the bicycle, we are referring to torsional stiffness. Torsional stiffness means the ability of a structure, a bike frame for example, to resist twisting. This is due to the bicycle being exposed primarily to torsional forces, rather than lateral (vertical, planar) forces that would bend the frame side to side for example.

Controlling vertical bending is no longer a priority due to advanced materials and generally sound engineering used to make bicycle products. If anything, too little vertical bending (compliance) may be interpreted by some riders as "harshness".

Why are high torsional stiffness bikes and components good for you?

At the most basic level, stiff components enhance your feeling of confidence when cornering, when making sudden direction changes, and when riding off road and powering through rock gardens, or berms for example. A stiff bike with a suitable geometry is often referred to as having "good handling". The not so stiff, or downright flexible bikes are described as "unsure", "vague", and in severe cases can contribute to the dangerous high speed wobble, often referred to by those who experienced it as "death wobble".

As your riding improves, bike stiffness plays a significant role in your feeling of speed and control. Besides the slight efficiency gains due to reduced energy loss in stiffer frames, advanced riders experience a gratifying instant response when sprinting, or attempting to make a break during a difficult climb. This instant response from a stiff bike leads to increased confidence and the feeling of power, which under extreme strain caused by attempting to sprint uphill is a very important reward factor to inspire you to keep on pushing and to make the break.

Well, every bike brand claims that their bikes are stiff, or "stiffer than before", how can you determine the truth?

Some larger bike brands are now starting to release their internal stiffness testing results which can be considered reliable as long as they measure torsion and are presented in N/mm for bottom bracket stiffness, or N/deg units. You can also easily convert the somewhat less meaningful stiffness to weight ratio expressed in N/deg/kg to N/deg by multiplying the result by the claimed weight of the frame, or other component that was tested.

I feel that the following stiffness numbers are the baseline for what should be considered a stiff frame:

Bottom bracket stiffness: 120 N/mm

Head tube stiffness: 80 N/mm

Any number less than that may work well in practice, but there are alternative frames even from the same manufacturer that deliver greater overall stiffness, and therefore higher performance.

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