We investigate the dependence of net erosion on grain size distribution in experimental debris flows and erodible beds. We systematically and independently varied the composition of each the supply and erodible bed material and the flume inclination angle, ϕ. Then, we demonstrate that there is a unique neutral angle, ϕN (the angle at which erosion is equal to zero), for each bed and supply composition combination. We show that for each system, total net erosion increases roughly linearly with increasing ϕ and can be predicted based on ϕ-ϕN and the geometry of the erodible bed. Our other macroscopic findings include that ϕN is dependent on both the compositions of the bed and the supply; as davg,S increases, ϕN decreases; as davg,B increases, ϕN increases; and as davg,S/davg,B increases, ϕN generally decreases. We then consider particle scale dynamics that drive these macroscopic results including segregation mechanisms, inter-particle collisions, and relative roughness.