(Image credit: Will Jones) Frame stiffness, both in terms of lateral stiffness and vertical compliance, is an incredibly important consideration when it comes to engineering a bike frame. It is the special sauce that defines how well a bike rides. So much so that when brands are designing their latest flagship race bikes, it's given the same importance as aerodynamics and weight. Some will chase new stiffness targets, but others will have these figures set in stone as immovable targets from the beginning, as they aim to retain the DNA of the platform while improving its performance elsewhere. What is stiffness and compliance? It's common for bikes to launch with claims of being "laterally stiff" and "vertically compliant," but what does that actually mean? Starting with 'laterally stiff', this refers to the frame’s resistance to side-to-side deflection at the bottom bracket shell when under the 'load' of the rider's pedalling force.…