How does stiffness affect experience and performance?
A Wikipedia definition is pretty much applicable. Stiffness is how much things resist bending.

Stiffness is measured by measuring how much stuff bends when lateral force is applied. So, in the example of frame stiffness, you could put one end of the frame in a vise, horizontally, then hang a known weight from the other end and measure how much it sags.

Because bending takes and dissipates energy, the stiffer stuff is, the less energy it robs from the rider. In general, you want everything in the system to be as stiff as possible, except the elements that are specifically designed to be flexible, like tires and shocks. Also, stiffer bikes (or any type of vehicle really) are more predictable in handling, because they have less degrees of freedom, are less prone to vibrations etc.


All that being said, the effect of stiffness is generally felt only by the performance oriented cyclists and is typically secondary to other considerations, like weight. For instance, for years MTB XC racing cyclists used the lightest forks possible like RockShox SID, even though they had the reputation of being &quot;flexible like maccaroni&quot; because weight trumped all else. Only in recent years, and especially in the road world where there is a fixed lower limit for bike weight in competition, manufacturers started to focus on stiffness as one of the main differentiating factors of their components.