In contrast, desmodromic drive uses two cams per valve, each with separate rocker arm (lever tappets). Maximum valve acceleration is limited by the cam-to-tappet galling stress, and therefore is governed by both the moving mass and the cam contact area. Maximum rigidity and minimum contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force; both occur at the base circle, where spring load is minimum and contact radius is largest.