Ensuring the stability and safety of steel storage racks requires the proper sizing of components and connections, which is typically based on their mechanical properties, often determined through experimental testing. The overall stiffness, load distribution, and stability of the structure largely depend on the mechanical properties of the speed-lock beam-to-upright connections. Two key parameters are the design moment and stiffness, both of which are significantly influenced by the initial stiffness. Accurately predicting the initial stiffness is essential to ensure that the design values align with real-world performance. This paper introduces a novel approach for calculating initial stiffness, focusing on the analysis of stiffness evolution based on the interactions between the upright and connector. Stiffness evolution is defined as the curve obtained by deriving the bending moment with respect to rotation over time. This new methodology, termed the Stiffness Evolution Method, was applied to nine configurations, combining three types of uprights and three types of beams with a common connector. Experimental tests were conducted using the simple cantilever test method as specified in the EN15512 standard. During testing, significant variations were observed in the moment-rotation curves for the same configuration. To investigate further, the internal contacts between the upright and connector were analysed both experimentally and through a numerical model. The results revealed that the primary points of contact occur at the lateral connector-to-upright interaction and the first two tab-to-slot interactions. The Stiffness Evolution Method was then compared with other established initial stiffness calculation methods from the literature. It was found that the new method produced results with lower standard deviations, leading to a more standardized and reliable approach for stiffness calculation.